' 1982

c . 2

NBS BUILDING SCIENCE SERIES 142

Load-Displacement Characteristics

of Shallow Soil Anchors

U.S. DEPARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS °' S
NATIONAL BUREAU OF STANDARDS

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the Institute for Computer Sciences and Technology.

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NBS BUILDING SCIENCE SERIES 142

Load-Displacement

Characteristics of

Shallow Soil Anchors

Felix Y. Yokel

Riley M. Chung

Frank A. Rankin

Charles W. C. Yancey

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for the

Office of Policy Development and Research

U.S. Department of Housing and Urban Development

Washington, DC 20410

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1982

IIATIONAL BUKSAV

or rrANDAKDB

LIBRART JUL 1 9 1982

t\ot

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no. luy

19
«<«!;A»»MATJ! Iff.

Library of Congress Catalog Card Number: 82-600509

National Bureau of Standards Building Science Series 142

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 142, 163 pages (May 1982) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, B.C. 20402

Price $6.50

(Add 25 percent for other than U.S. mailing)
ABSTRACT

Tests on shallow soil anchors, conunonly used by the mobile home industry,

Including 6-in single helix and 4-in double helix anchors as well as three

types of swivel anchors, were conducted on three sites: a silty site, a sandy

site, and a clay site. Test variables included direction of anchor installation; direction of loading; anchor depth; size of anchor plate; and cyclic

load effects. The effect of these test variables on load-displacement characteristics, measured at the anchor head, is investigated. It is concluded that on most sites the anchor types tested, when installed in accordance with present

industry practice for mobile home tiedown systems, did not deliver the anchor

performance required in present standards. It is recommended that minimum load

capacity requirements for anchors be waived; that all anchors be preloaded to

1.25 times the design load; and that one anchor per mobile home, or three

anchors per site if soil conditions are uniform, be preloaded to 1.5 times the

design load.

Keywords: anchors; cyclic loading; field testing; flood forces; foundations;

load capacity, mobile homes; soil anchors; soil mechanics;

stiffness; wind forces.

COVEH: MobAJio. home, damagzd by Mind, Ancho^O-d mobiZn homQj> in

tko, vldviity 6a{^^2AQA onty minon. damage,.

iii
PREFACE

This report is part of a study which was sponsored by the Office of Policy Development and Research of the U.S. Department of Housing and Urban Development.

The overall objectives of this study were: to determine wind and flood forces

acting on mobile homes; to study the performance characteristics of soil

anchors; and to develop performance criteria for mobile home foundations with

particular emphasis on the t-iedown system. In previous stages of this work,

measurements were made of the wind forces acting on mobile homes, the state-ofthe-art in anchoring technology was studied, and the forces acting o^ tiedown

systems were determined. The work was published in references [13], [12], and

[20], respectively. This report deals with the results of experimental and

analytical studies of the load capacity of soil anchors used to tie down mobile

homes and with methods to insure adequate performance of soil anchors. Initial

results of this work were presented in reference [19], pp. 3-20.

This study was performed by the Geotechnical Engineering Group of the Center

for Building Technology.

Numbers in brackets refer to the literature references in section 7.

Iv
EXECUTIVE SUMMARY

Two hundred and thirty-two anchor tests were conducted on three sites: a silty

site; a sandy site; and a clay site. Anchors tested included 6-in single helix,

A-in double helix, and 3-in single helix anchors, and 6-in triangular, 10 1/4

length X 1 3/4-in o.d. pipe, and 6 1/2 length x 1 1/4-in o.d. pipe swivel

("fluke") anchors. Loading conditions included coaxial and noncoaxial (inclined

pull on vertical and inclined anchors installed to their full depth, and coaxial

pull on anchors installed at various depths ranging from 1 ft to 4 ft. Modes of

loading included monotonic tests, monotonic tests with several intermediate cycle

of unloading and reloading, and cyclic tests. The tests were carried to complete

withdrawal and graphs of load vs. anchor-head displacement were electronically

recorded. Several anchor tests were carried out under submerged conditions. The

tests were correlated with determinations of soil conditions by in-situ and laboratory tests. In-situ tests included soil test probe readings, standard penetration tests, and measurement of the anchor installation torque. It was con- cluded that

1. The anchors tested did not deliver the anchor performance required by ANSI

Standard A119.3 [2]

.

2. The virgin load-displacement characteristics of anchors are a unique

function of installation depth, loading, and soil conditions and are not

substantially altered by intermediate unloading and reloading cycles unless

a great number of cycles of load close to the load capacity are applied.

3. The initial resistance to displacement of preloaded anchors in all loading

modes is much higher than that in the first loading cycle and far exceeds

the performance required by ANSI Standard A119.3.

4. Coaxially loaded inclined anchors have smaller load capacities than

coaxially loaded vertical anchors but their initial resistance to

displacement is similar to that of coaxially loaded vertical anchors.

5. Vertical anchors subjected to inclined loads have higher load capacities

than coaxially loaded vertical anchors, but their initial resistance to

displacement (if they are not preloaded) is much less than that of coaxially

loaded vertical anchors.

6. Swivel ("fluke") anchors can deliver satisfactory performance if properly

seated and adequately preloaded.

7. Helix anchors lose their protective paint coat during installation and thus

have no corrosion protection.

8. Helix anchors experience bending of the helix in all loading modes and

bending of the shaft in noncoaxial loading.

V
9. Helix anchor hardware tended to have adequate load capacity but was vulnerable in the weld between the shaft and the helix, particularly when

subjected to noncoaxial cyclic load.

It is recommended to:

1. Eliminate the minimum load capacity requirements for anchors in present

standards and stipulate instead required anchor resistance per mobile

home, so that the number of anchors used can be determined in accordance

with site conditions.

2. Require that every anchor be preloaded in the direction of the anticipated

service load to 1.25 times the working load during installation, and that one anchor per mobile home, or three anchors per site where soil conditions

are uniform, be preloaded to 1,5 times the required working load; and that

the required working load be the load calculated for the design wind

pressure without the increase for foundations presently required in the

Federal standard [9].

3. Require that anchors be adequately protected against corrosion by galvanizing

or other means; that the corrosion protection be effective for the service

life of the mobile home, and remain effective if anchor deformation

anticipated under the preload or the service load occurs.

It is noted that if anchors are to be included as part of a permanent mobile

home foundation, they should be durable enough to retain their structural

integrity throughout the service life of the mobile home; they should be

preloaded; and consideration should be given to potential effects of frost heave.

vi
TABLE OF CONTENTS

Page

ABSTRACT iii

PREFACE iv

EXECUTIVE SUMMARY v LIST OF FIGURES x LIST OF TABLES xii

LIST OF SYMBOLS xlii

SI CONVERSION xv

1. INTRODUCTION 1

2. SCOPE 7

3. TEST SETUP AND PROGRAM 9

3.1 Description of Test Sites 9

3.1.1 General 9

3.1.2 Test Site A, Silty Soils 10

3.1.3 Test Site B, Sandy Soils 10

3.1.4 Test Site C, Clayey Soils 11

3.2 Test Specimens and Procedures 11

3.2.1 Anchors Tested 11

3.2.2 Test Apparatus 12

3.2.3 Testing Procedure 17

3.3 Test Program 18

3.3.1 General 18

3.3.2 Test Variables 18

3.3.3 Summary of Test Program 21

4. ANALYSIS OF TEST RESULTS 23

4.1 Presentation of Results 23

4.2 Effect of Loading Rate 26

4.3 Static Load-Displacement Characteristics of 6-in Single

Helix Anchors 28

4.3.1 Monotonic Tests 28

4.3.2 Unloading and Reloading Cycles 28

4.3.3 Anchors Installed at an Angle and Pulled Coaxially .... 31

4.3.4 Anchors Installed Vertically and Pulled at an Angle ... 31

4.3.5 Effects of Loading Configuration 31

4.3.6 Effects of Inclination and Depth on the Load Capacity

of Coaxially Loaded Anchors 39

4.3.7 Comparison of Coaxially Loaded Inclined Anchors and

Vertical Anchors Subjected to Inclined Loads 44

4.4 Comparison between Different Anchors Types 44

4.4.1 4-in Double Helix Anchors 44

4.4.2 3-in Single Helix Anchors 49

4.4.3 Self-seating Swivel Anchors 50

4.5 Effect of Soil Characteristics 54

4.5.1 Coaxially Loaded Vertical Anchors 54

vii
TA.BLE OF CONTENTS (Continued)

Page

4.5.2 Vertically Installed Anchors Pulled at an Angle 54

4.5.3 Effect of Depth and Anchor Inclination of Coaxial

Load Capacity 58

4.5.4 Reloading Moduli 58

4.6 Prediction of Anchor Load-Capacity 61

4.6.1 General 61

4.6.2 Correlation of Anchor Capacity with In Situ Tests 63

4.6.3 Theoretical Determination of Anchor-Load Capacity 73

4.6.4 Determination of Load Capacity on the Basis of Pullout

Tests in Similar Conditions 80

4.6.5 Effect of Submerged Conditions 81

4.7 Cyclic Tests 82

4.7.1 Cyclic Tests on Silty Site 82

4.7.2 Cyclic Tests on the Sandy Site 82

4.7.3 Cyclic Tests on the Clay Site 85

4.8 Comparison of Anchor Performance with Present Standard

Requirements 90

4.9 Performance of Anchor Hardware 91

5. SUMMARY OF CONCLUSIONS 93

5.1 General 93

5.2 Virgin Load-Displacement Curves 94

5.3 Reloading Characteristics 94

5.4 Effects of Loading Configuration 94

5.5 Effects of Soil Type 95

5.6 Prediction of Anchor-Load Capacity by In-Situ Tests 95

5.7 Theoretical Prediction of Anchor-Load Capacity 95

5.8 Prediction of Anchor-Load Capacities on the Basis of Tests on Similar Sites 96

5.9 Anchors Subjected to Cyclic Load 96

5.10 Comparison of Anchor Performance with Present Standard

Requirements 96

5.11 Performance of Anchor Hardware 96

5.12 Use of Soil Anchors in Permanent Mobile Home Foundations 97

6. RECOMMENDATIONS 99

6.1 Required Load Capacity 99

6.2 Installation Requirements 100

6.3 Corrosion Protection 103

6.4 Anchor Hardware Capacity 103

7. REFERENCES 105

8. ACKNOWLEDGMENTS 107

viii
TABLE OF CONTENTS (Continued)

Page

APPENDIX A TEST SITES

A.l Introduction 109

A. 2 Soil Exploration Reports 114

APPENDIX B TEST RESULTS

B. l Introduction 137

B.2 Symbols Used in the Tables 137

ix
LIST OF FIGURES

Page

Figure 1.1 Typical mobile home tiedown systems 2

Figure 1.2 Single and double helix anchors 4

Figure 3.1 Self seating swivel anchors (triangular and pipe) used in

the test program 13

Figure 3.2 Test rig 14

Figure 3.3 Details of test rig 15

Figure 4.1 Plot of test C-7, vertical 6-in single helix anchor,

vertical pull 25

Figure 4.2 Effect of loading rate on load-displacement

characteristics 27

Figure 4.3 Monotonic loading test in silt 29

Figure 4.4 Pullout test with unloading and reloading cycles in sand .. 30

Figure 4.5 Coaxial pullout test on inclined 6-in single helix

anchor in sand 32

Figure 4.6 Inclined pullout test on a vertically installed 6-in

single helix anchor in sand 33

Figure 4.7 Anchor deformation in inclined test on vertically installed

4-in double helix anchor 34

Figure 4.8 Effect of loading configuration on load-displacement

characteristics of anchors 35

Figure 4.9 Effects of load inclination on the load-displacement

characteristics of vertically installed anchors 36

Figure 4.10 Load-displacement characteristics of inclined anchors

subjected to non-coaxial loads on the silty site 38

Figure 4.11 Load-deflection curves for coaxially loaded 6-in single

helix anchors installed at various angles 40

Figure 4.12 Effects of embedment depth on the load capacity of

vertically installed 6-in single helix anchors subjected

to coaxial load 41

Figure 4.13 Comparison between inclined and vertical coaxially pulled

6-in single helix anchors 42

Figure 4.14 Comparison between the load-displacement characteristics

of 6-in single helix and 4-in double helix anchors 45

Figure 4.15 Comparison of load-displacement characteristics of

vertically installed 4-in double helix anchors pulled at

different angles 47

Figure 4.16 Comparison of load-displacement characteristics of

coaxially loaded 4-in double helix anchors installed at

different angles 48

Figure 4.17 Tests of 3-in single helix anchors in silty soils 51

Figure 4.18 Load-displacement characteristics of self seating swivel

anchors in silty soil 52

X
LIST OF FIGURES (Continued)

Page

Figure 4.19 Load-displacement characteristics of coaxially loaded 6-in

single helix anchors on the sand, silt and clay sites 55

Figure 4,20 Vertically installed 6-in single helix anchors pulled

at a 40° angle on the sand, silt, and clay sites 56

Figure 4.21 Correlation between Soil Test Probe readings and coaxial

load capacity of vertically installed 6-in single helix

anchors 64

Figure 4.22 Correlation between Soil Test Probe readings and coaxial

load capacity of vertically installed 4-in double helix

anchors 66

Figure 4.23 Correlation between Soil Test Probe readings averaged over

a 2.5 ft depth and the load capacity of coaxially loaded

4-in double helix anchors 67

Figure 4.24 3 ft deep 6-in single helix anchor after pullout on the

sandy site 68

Figure 4.25 Relationship between STP measurements and the shear

resistance of soil 69

Figure 4.26 Relationship between installation torque and pullout

strength for vertical, coaxially loaded 6-in single helix

anchors 71

Figure 4,27 Relationship between installation torque and pullout

strength for vertical, coaxially loaded 4-in double helix

anchors <> 72

Figure 4,28 Correlation between SPT blowcount and STP torque readings

for the silt site 74

Figure 4.29 Effect of anchor plate size on q in the silt and clay

sites 76

Figure 4.30 Cyclic load tests in silty soils 83

Figure 4.31 Cyclic load tests in sandy soils 84

Figure 4.32 Cyclic load tests in clay 86

Figure 4.33 Comparison of the preloading curves of tests C26 and C27 .. 87

Figure 4.34 Results of the cyclic loding test of specimen C33 on

the clay site 89

Figure 4.35 Typical anchor hardware failures 92

Figure 6.1 Illustration of the recommended preloading requirement and

the resulting anchor performance 101

Figure 6,2 Suggested preloading procedures for diagonally loaded

anchors 102

Figure A,l Anchor tests and boring locations on Site A 110

Figure A, 2 Anchor test and boring locations on Site B,l Ill

Figure A. 3 Anchor test and test pit locations on Site B.2 112

Figure A. 4 Anchor test and boring locations on Site C 113

xi
LIST OF TABLES

Table 3.1 Test Sites Selected 10

Table 3.2 Summary of Test Program 21

Table 4.1 Reloading Moduli (Rgs) for the Tests Shown

in Figure 4.9 37

Table 4.2 Comparison of the Strength of Coaxially Loaded Anchors

Installed at Various Depths 43

Table 4.3 Effect of Anchor Inclination on the Load Capacity of

Coaxially Loaded 4-in Double Helix Anchors in Silty Soils . . 49

Table 4.4 Comparison of Load Capacities of 3-in and 6-in Single

Ifelix Anchors 49

Table 4.5 Load Capacity of Self-Seating Swivel Anchors Tested in

Silty Soils 53

Table 4.6 Re-loading Moduli of Swivel Anchors Tested in Silty Soils .. 54

Table 4.7 Comparison of the Effects of Load Inclination on the Load

Capacities of Vertically Installed Anchors on Sites, A, B, and C 57

Table 4.8 Effect of Anchor Depth and Inclination on the Load Capacity

of Coaxially Loaded Single Helix Anchors 59

Table 4.9 Effects of Anchor Inclination on the Load Capacity of 4-in

Double Helix Anchors 60

Table 4.10 Comparison of Reloading Moduli Measured on the Three

Test Sites 62

Table 4.11 Uplift Capactiy Factors for Full-Depth Anchors on the Sandy

Site 79

Table 4.12 Uplift Capacity Factors for 6-in Single Helix Anchors

Installed to Less than Their Full Depth on the Sandy Site .. 80

Table 4.13 Range, Mean, and Coefficients of Variation of the Load

Capacities of the Full Depth Anchors 80

Table 4.14 Comparison Between Regular and Submerged Anchor Tests on

the Silty Site 81

Table B.l Static Test Results on Site A (Silty Soils) 140

Table B.2 Cyclic Test Results on Site A (Silty Soils) 143

Table B.3 Static Test Results on Site B (Sandy Soils) 144

Table B.4 Cyclic Test Results on Site B (Sandy Soils) 145

Table B.5 Static Test Results on Site C (Clayey Soils) 146

Table B.6 Cyclic Test Result on Site C (Clayey Soils) 147

xii
LIST OF SYMBOLS

A Projected surface area of anchor plate, ft^

AH-6 6-in triangular (arrowhead) anchor

B Width (or diameter) of anchor plate, ft

c Cohesion, Ib/ft^

D Average depth of anchor plate below ground surface, ft

Dy Average depth of anchor plate below ground surface for full-depth

vertical anchor, ft

D-4 4-in double helix anchor

H-3 3-in single helix anchor

H-6 6-in single helix anchor

n Number of specimens listed or number of load cycles

N Blow count in Standard Penetration Test, blows/ft

Uplift capacity factor for cohesive soils

Nqu Uplift capacity factor for granular soils

Pc Cyclic load, lb

P^^ Load at 4-in horizontal displacement, lb

Pp Preload, lb

P2y Load at 2-in vertical displacement, lb

P^ Working load, lb

P-6 6 1/2x1 1/4 in pipe anchor

P-10 10 x 1 3/4 in pipe anchor

q Anchor load capacity per unit anchor plate area, Ib/in^

Qu Ultimate load capacity of anchor, lb

xiii
LIST OF SYMBOLS (Continued)

t Ultimate load capacity determined by pullout after completion of

cyclic test, lb

Ultimate load capacity of full-depth vertical anchor, lb

R^o Reloading modulus of anchor after 10 load cycles, lb/in

R35 Reloading modulus of anchor at 85 percent of Q^, lb/in

STP Soil Test Probe

SPT Standard Penetration Test

s Shear strength of soil Ib/ft^

s Average shear strength of soil, Ib/ft^

S Anchor shaft resistance, lb

t Torque reading in STP test, in-lb

t Torque reading in STP test, averaged over anchor depth, in-lb

T Anchor installation torque, ft-lb

V Coefficient of variation of a sample. (The coefficient of variation

of the population will differ considerably if the sample is small.)

Angle of applied load with horizontal, degrees

a2 Angle of anchor shaft with horizontal, measured in same direction and

the opposite quadrant of a]^ , degrees

Y In place unit weight of soil, Ib/ft-^

Anchor head displacement in direction of pull at ultimate load, in

xlv
SI CONVERSION UNITS

In view of present accepted practice in the U.S. mobile home Industry, common

U.S. units of measurement are used throughout this report. The table below Is

presented to facilitate conversion to SI Units.

To Convert From To Multiply by

ft m 0.305

In mm 25.4

ft2 m2 9.29 x 10-2

ln2 mm2 645.16

lb (force) N 4.45

lb/ft2 (psf) Pa 47.88

lb/ln2 (psl) kPa 6.89

lb/ft3 Kg/m3 16.02

XV
FACING PAGE: hhbllz homz anchoh^ puZZzd ouut by llood. Tkd

bzYvt anckox 6ka{jt^ Indicato, that la/Lge. honlzowtal

loK.c.<L compomnti acted on thz anchon^.

xvi
1. INTRODUCTION

A common foundation type presently used to support mobile homes consists of

pairs of piers, about 8 to 10 feet on center, which support the chassis beams

of the mobile home unit. In addition, the mobile home is attached to soil

anchors by transverse over-the-roof ties and transverse diagonal ties attached

to the chassis beam (see figure 1.1). The loads acting on this type of foundation have been studied by Yokel et al. [20]. The horizontal component of the

wind load is resisted by the diagonal ties and by whatever horizontal-load

resistance is provided by the piers. Since piers are not normally designed

1
DIAGONAL TIE

VERTICAL TIE

GROUND LINE

H

k-Xt

(a) - Near Tie Connection

GROUND LINE

H

4

Xt

(b) - Far Tie Connection

Figure 1.1 Typical mobile home tiedown systems
to resist horizontal loads, the diagonal ties must provide the necessary

horizontal-load resistance. The vertical over-the-roof ties are provided to

resist uplift and overturning. It has been shown [20] that if the diagonal

ties are attached to the chassis beams adjacent to the anchor [(figure l,l(a)],

the vertical ties are not essential. However, the vertical over-the-roof ties

help hold the mobile home together. The vertical ties are also engaged by

uplift forces resulting from flooding and they must be used to resist windloads

if the diagonal ties are attached as shown in figure 1.1(b). Thus, soil anchors

must provide effective resistance to horizontal as well as vertical forces.

Present anchor technology was studied by Kovacs and Yokel [12] . The anchors

most frequently used by the mobile home industry are single helix, 6-in

diameter anchors installed to a 4-ft maximum depth and double helix 4-in

diameter anchors installed to a 2 ft - 9 in maximum depth (figure 1.2). Other

types of anchors are also available, but not extensively used at the present

time. Miscellaneous hypotheses have been developed which correlate the load

capacity of anchors to the shear strength of soils, which in turn can be measured by various in-situ and laboratory tests. However, it was concluded on

the basis of available data that the correlation between calculated and measured

anchor-load capacities, particularly in granular soils, tends to be poor [12].

In part, our inability to make reliable predictions of anchor-load capacity on

the basis of the shear strength of soils is attributable to our inability to

make reliable measurements of the in-situ shear strength, particularly that of

granular soils. This measurement problem is even more severe at shallow depths,

where soils are subjected to many disturbances, such as freezing and thawing,

changing moisture content and the effect of root systems and organic matter.

Moreover, the most commonly used in-situ test, the Standard Penetration Test

(ASTM D 1586) [4] is difficult to interpret at a shallow depth because of the

short drill-stem length used [15].

In present practice, the load capacity of mobile home anchors is estimated on

the basis of in-situ soil test probe (STP) measurements, coupled with predictions based on the results of pull-out tests conducted in soils with characteristics similar to that of the site. Guidance for this procedure is provided

in ANSI Standard A119.3 [2] and in miscellaneous charts published by industry

[7]. ANSI Standard A119.3 stipulates in section 4.5.1 that a ground anchor,

when installed, shall be capable of resisting an allowable working load at

least equal to 3,150 lb in the direction of the tie, plus a 50 percent overload (4,725 lb total) without failure. Failure is defined as an anchor movement of 2 inches at 4,725 lb in the direction of the vertical tie. Anchors

which are designed for loads other than "direct withdrawal" (coaxial loads)

shall resist an applied design load of 3,150 lb at 45° from the horizontal

without displacing the anchor more than 4 inches horizontally at the point

where the tie is attached to the anchor. Anchors designed for connection of

multiple ties shall be designed to resist the combined working load and overload consistent with the intent expressed in section 4.5.1. The magnitude of

the stipulated load capacity in ANSI A119.3 is entirely predicated on the load

capacity of presently used steel straps with no regard to whether existing

soil anchor technology can provide the stipulated load resistance.

3
SINGLE HELIX ANCHORS

DOUBLE HELIX ANCHORS

ALL DIMENSIONS IN INCHES

Figure 1.2 Single and double helix anchors
There is evidence that in present practice withdrawal tests are conducted in

accordance with the first part of this provision, namely coaxially, and that

the anchor capacity is then determined for a 2 in withdrawal. Most available

data are limited to this test condition, and therefore do not provide much

information on anchor capacity under larger displacements or under inclined

loads which have a component normal to the axis of the anchor.

Provisions for mobile home anchors are generally enforced by the States. These

provisions are not uniform throughout the United States; some States have no provisions, while others, like Texas [17] require that the anchor resist a load

"in the direction of the expected applied loads" of 4,725 lb for single headed

anchors and 6610 lb for double headed anchors. In the Texas provision, "failure" is defined as a movement of 3 inches in the direction of the axis of the . anchor. This relaxes the more stringent limitation of 4 inches on horizontal

movement and 2 inches on axial movement provided in ANSI 119.3. Proof of com- pliance with the various State provisions can generally be provided by anchor

manufacturers or installers by documenting the results of withdrawal tests con- ducted in soils with characteristics similar to those of the site. Some States

will accept the results of tests in artificially prepared soils. Implicit in

the use of withdrawal tests as proof of compliance in similar soil conditions

are two assumptions:

1. That soil conditions can be characterized well enough so that anchor

capacity on a given site can be predicted from test results on similar

sites

.

2. That anchors will have satisfactory load-displacement characteristics in

the horizontal, as well as the vertical direction.

The objective of the test program presented herein was to study the performance

characteristics of the most common types of mobile home anchors and to determine

how adequate performance can be assured.

5
FACIWG PAGE: K-V n.zc.oh.d(2A u&2.d to plot anchor. t2^t ^e^uZti

Iyl thz {^tdtd.
2. SCOPE

Two hundred and thirty-two anchor tests were conducted on the three sites:

a sandy site; a silty site; and a clay site. Two hundred and nineteen of

these tests were conducted with single and double helix anchors and 13 tests

were conducted with self-seating swivel anchors (triangular and pipe) . Of

these tests, 179 were pullout tests using vertical and inclined axial pulls

and inclined pulls at an angle to the anchor shaft; 53 were cyclic tests

using several hundred equal loading cycles of vertical coaxial pull or inclined pull at an angle to the anchor shaft.

7
The soil characteristics of the test sites were determined by two types of

in-situ tests: the Standard Penetration Test (ASTM D 1586); and the Soil Test

Probe (refer to ANSI A119.3). In addition, disturbed and undisturbed soil

samples were analyzed and tested in the laboratory.

Test results were recorded electronically by an x - y plotter as a plot of

applied load vs. displacement of the point where the tie is attached to the

anchor. Most static tests were carried to complete anchor withdrawal with

several intermediate cycles of unloading and reloading in order to provide

information of the load-displacement characteristics to the point of incipient

loss of load capacity. Cyclic tests were conducted at various load levels and

in most instances carried to a point where the probable trend of response to

additional load cycles is apparent. Throughout the test program emphasis was placed on the effect of pre-loading on anchor response. Several tests were conducted under submerged conditions in order to study the effect of flooding

on anchor capacity.

In this report all the test results are presented in tabular form. In the

analysis of the test results, anchor load capacity is correlated with soil

strength as measured by in-situ and laboratory tests and an assessment is made

of our ability to predict anchor-load capacity. Anchor performance characteristics in various soil types are compared and studied and methods of insuring

adequate anchor performance are recommended.

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8
3. TEST SETUP AND PROGRAM

3.1 DESCRIPTION OF TEST SITES

3.1.1 General

There was prior evidence that performance characteristics of soil anchors

depend on the type of soil in which the anchors are embedded [12]. Thus, it was important to test anchors on sites with a variety of soil characteristics.

It was therefore decided to choose three different sites: a silty site (A), a
sandy site (B), and a clay site (C). The three sites selected are listed in

table 3.1. Plans showing anchor test and soil-boring locations, as well as test reports containing boring logs and soil test data are included in

appendix A. Pertinent site characteristics are described hereafter.

Table 3.1 Test Sites Selected

Site Predominant Soil Type Location

A Silt NBS grounds, Gaithersburg , MD

B Sand Odenton, MD

C Clay Upper Marlboro, MD

3.1.2 Test Site A, Silty Soils

Test Site A was explored by eleven 5-ft deep test borings. The borings

generally indicate 2 ft of fill consisting of local silty material

overlying residual silty soil and quartz-rich schist of the Wissahickon Formation. No groundwater accumulated in the borings during, and up to 5 hours

after, the drilling. On the basis of visual observation and laboratory analysis, the soil can be described as brown stiff clayey silt with some fine sand

and quartz fragments, and classified as Group ML in accordance with the Unified

Soil Classification System (ASTM D 2487 [5]).

Laboratory tests indicate that the soils consisted of 62 to 83 percent by

weight of materials passing No. 200 sieve (particle size smaller than 0.074 mm).

The liquid limit and plasticity index were 39 and 12 percent, respectively.

The SPT "N" values ranged from 7 to 19 blows per foot. Laboratory strength

tests determined an unconfined compressive strength of 4000 psf at 4 percent

strain. Direct shear tests provide some additional information. Measured

natural wet density was 119 Ib/ft-^ and dry density was 98 Ib/ft-^.

3.1.3 Test Site B, Sandy Soils

Test Site B is located at the bottom of an excavated borrow pit and was explored by four 10 ft deep test borings. No water accumulated in the borings to the depth where the holes caved, which ranged from 5 to 6.5 ft. The

deposits on this site can be traced to the Potomac Group which generally consists of interbedded sands and silty clay layers of cretaceous origin which

characteristically are overconsolidated . The sands in this formation tend to

be medium dense to dense and the silty clays stiff to hard. The borings indicate a 6 to 9 ft thick layer of moist fine-to-course clean sand which rests on sandy silty clay. On part of the test site the above-mentioned sand layer is

covered by a 0.5 to 1.0 ft thick crust of dense silty sand fill with gravel.

Most of the anchor tests were performed in locations that were not covered by

10
the dense crust. The 6 to 9 ft sand layer can be described as light brown

medium dense fine-to-coarse sand with a trace of silt and gravel and classified

as Group SP in accordance with ASTM D 2487.

Laboratory tests indicate that the sand deposit had 95 percent by weight of

material in the range between No. 4 and No. 200 sieves (particle sizes between

4.699 and 0.075 mm), a natural moisture content of 2 to 4 percent and a natural

dry density of 92 to 95 Ib/ft^. The SPT "N" values over a 5 ft depth below

ground ranged from 10 to 21 blows per ft. Consolidated undrained triaxial

compressive strength tests on reconstituted samples yielded an angle of shearing resistance «J = 31°. The 4> value obtained from direct shear tests was approximately 29°.

The dense crust had natural dry densities from 105 to 108 Ib/ft^ and SPT "N"

values from 20 to 40 blows per foot.

3.1.4 Test Site C, Clayey Soils

Test site C was explored by four test borings from 5 to 5.5 ft deep below

ground. Groundwater was observed in all the borings upon completion of the

drilling at depths ranging from 1 to 4 ft. While no long-term groundwater

observations were made, it was noticed during the field testing that the groundwater table was near the surface. The site is a wooded tract and root systems

were encountered on some of the tests. The deposits on Site C are believed to

be pleistocene river terrace deposits of the Western Branch of the Patuxent

River and generally consist of silty clays overlying sands. On the basis of

field observation and in-situ and laboratory tests, the soil is described as a grey, medium stiff to stiff silty clay, with traces of sand and organic matter

and classified as Group CL in accordance with ASTM D 2487.

Laboratory tests indicate 70 to 90 percent of material passing No. 200 sieve

(particle size smaller than 0.074 mm) and a natural moisture content of 23

percent. Natural dry density varied from 92 to 100 Ib/ft^. The liquid limit

varied from 27 to 39 percent and the plasticity index varied from 7 to 19 percent. Standard Penetration Test "N" values varied from 2 to 17 blows per foot.

Unconsolidated undrained triaxial compression tests on an undisturbed sample

yielded a c of 700 psf and a ^ of 19° . An unconf ined compression test yielded

an unconfined compression strength of 1930 psf at 6.7 percent strain.

3.2 TEST SPECIMENS AND PROCEDURES

3.2.1 Anchors Tested

Most anchors tested were of the helical type with 6- in single or 4- in double

helixes welded to nominal 5/8 in or 3/4 in shafts. A few tests using three

inch single helixes welded to a 3/8 in shaft were used to investigate size

effects. See figure 1.2 (page 4) for typical sizes.

11
Fourteen self-seating swivel (fluke) anchors were also tested. Ten of these

were pipe segments of two different sizes (10.25 in long by 1.75 in outer

diameter and 6.5 in long by 1.25 in outer diameter) and four were triangularshaped (arrowhead) plate anchors with 6 in side lengths. See figure 3.1 for

details. All the anchors used were commercially available anchors furnished

by industry.

3.2.2 Test Apparatus

The anchor tests were performed by the test rig developed for the project as shown in figure 3.2 which has the capability to exert a 10,000 lb vertical or inclined pull against the anchor head. The pulling force can be exerted at an angle to the horizontal of 15° or steeper. The test rig consists of an aluminum

tripod with extendable legs (one 4 ft long segment and one 3 ft long segment;

in the figure the legs are fully extended). At the apex of the tripod a pulley

is installed on a removable axle which is attached to the main leg of the

tripod. The anchor is pulled by a chain which passes over the pulley and is

attached at one end to the anchor head and at the other end to a pair of

push-pull rams which are connected back to back to achieve a long stroke.

The plunger ends of the rams are fitted with a clevis eye and a chain hook,

respectively. The clevis eye is attached to a bracket welded to the main leg

of the tripod and the chain hook is grabbing the most convenient link of the

pulling chain. The rams are designed for a maximum pressure of 10,000 psi

which develops a pulling force of 9,800 lb. The 1/2-in thick pulling chain

with electro-welded links has a load capacity of 15,000 lb. It was used for

pulling because it was readily available and had convenient accessories which

made it easy to make length changes as needed.

The front legs of the tripod can be attached to the main leg in two places:

they can be attached to the removable pulley axle shown in figure 3.2, or to

another location 1 ft down on the main leg which is shown in figure 3.3. This

second configuration, which projects the pulley on a 1 ft cantilever, together

with the shorter (4 ft) leg length can be used to pull anchors installed under

a mobile home. When attached to the axle, the front legs are spread at a fixed 90° angle. The angle between the front legs and the main leg can be

adjusted by rotation, about the axle.

The tripod is designed to withstand a 10,000 lb pull when the legs are spread

so wide that the pulley axle is only 1 ft above ground. In any other position

the load capacity would be greater. Except for the pulley and axle, the tripod

is fabricated from 6061 T6 high strength aluminum alloy. The legs are made of

4 in diameter tubing. The front legs have a 1/8 in wall thickness and the main

leg has a 1/2 in wall thickness. The yoke carrying the pulley and axles, as well as the end sections of the legs are made of solid aluminum.

The legs are restrained from spreading by a 3/8 in thick galvanized aircraft

cable and an 8-ft length of chain. The restraining cable that connects the

front legs has a fixed length, since the angle between the front legs is fixed

12

Figure 3.2 Test rig

14
Figure 3.3 Details of test rig

15
at 90° . The restraining chain with another segment of cable connects the main

leg to the front legs, and is adjustable to accommodate any desired angle

between the main leg and the front legs. The restraining chain and cables are designed to resist the forces generated by a 10,000 lb pull in the most

unfavorable (lowest) tripod position.

Bearing plates of 1/2 in thick aluminum, 9 in wide by 19 in long,

are used to support the legs of the tripod. The plates used under the front

legs have a 2 x 2 x 1/4 in aluminum angle welded to the long edge of the top

surface of the plate to resist sliding when the tripod is used for inclined

pulling. Two aluminum stakes are driven into the ground in front of each

bearing plate to provide resistance to lateral forces in angular pulls.

The hydraulic pressure is generated by one of two pumps, depending on the type

of testing. For the slow rate (monotonic) testing, a double-acting hand pump

is used that has 10,000 psi capability and a useable oil capacity of 126 in^.

A bourdon tube type gage is used for visually monitoring hydraulic pressure.

The cyclic testing is done using an electrically driven hydraulic pump mounted

on pneumatic tires for rough terrain use. It has a pressure capability of

10,000 psi and a five gallon useable oil supply. The cyclic application of

hydraulic pressure is controlled by a program control center having five variations of programmed load application, holding, and removal. A continuous cycling program is used that steps through load application to a given load level,

holds the load for a given time, retracts the ram to a position that insures

complete removal of the load, counts the cycle and then repeats the cycle.

Suitable signals are provided to the control center by an adjustable pressure

switch, a timer, a snap-action limit switch, and an automatic cycle counter.

The electricity necessary for running the pump motor, the controller and the

instrumentation is provided by a gasoline powered generator mounted on a van which is used to transport the equipment and to serve as a field laboratory

that houses the electronics and data recording equipment.

The electronics used consists of a pressure transducer for measuring hydraulic

pressure, a 10,000 lb universal load cell, a pair of linear potentiometers

and/or a pair of constant-tension cable position transducers to measure anchorhead displacements, an x-y recorder, a volt-ohm meter and a recorder checker.

The pressure transducer has a 0-10,000 psi range and is the high output semiconductor instrumented diaphragm type with an output of 25mv/volt full scale.

This device is used primarily in the monotonic tests to measure the hydraulic

pressure that translates into pulling force. The load cell is used in tension

to measure the pulling force between the chain and the head of the anchor and

is used in the cyclic tests. Its output is 3mv/volt, full range. The linear

potentiometers have a 10 in travel that varies the voltage linearly through the

stroke. Electronically the resistances of the two potentiometers are combined

to indicate average movement in case the anchor head tilts or rotates. The

cable position transducers are also used in the averaging mode for the same reason. These devices are ten-turn precision rotary potentiometers that have

a known-diameter pulley mounted on their shafts. A constant tension cable

16
take-up assembly was used in conjunction with the potentiometers by having the

cable make one turn around the pulley before exiting the mounting housing.

These instruments have many advantages: 1. The stroke can be chosen by changing drive pully diameter; 2. Constant tension makes calibration easy by simply

inserting a calibrated length t ension link; 3. The constant tension cable

device is not influenced by imprecise alignment. The diameter of the pulley

used produces a 25 in maximum travel in the test set-up. The recorder checker

is used to produce precise voltage for calibration of the x-y recorder.

3.2.3 Testing Procedure

The anchors were installed in a grid pattern in a predetermined area having a a 5 foot grid line spacing (see appendix A). Vertically-installed anchors

were spaced 5 ft apart and anchors installed at an angle were spaced at 10 ft.

The helical anchors were turned into the ground by an electrically-driven installation tool that turned at nine revolutions per minute. Two 18 in handles were

provided on opposite sides of the installing tool to enable two operators to

react against the turning torque. Final installation torque was measured with

a 0-600 ft lb torque wrench and recorded on the data sheet. The self-seating

swivel anchors were installed with an automatic hammering device and a drive

rod. Soil test probe readings (STP) were taken at 1-ft depth intervals to the

full depth of the probe (48 in). Test probe readings were taken midway between

every second pair of anchors. Generally, only enough anchors for one day's

testing activity were installed at one time. The tripod was then brought into

the position that produced a pull in the desired direction. The bearing pads

were placed under the ends of the legs and on angular pulls the stakes were driven into the soil to counteract the pulling force. Two weighted ring stands

with an interconnecting rod were used to support the position transducers used

to measure anchor movement (see figure 3.2). These position transducers were mounted on opposite sides of the pulling chain in such a way that they measured

movement in the direction of the pull. The attachment of the chain to the head

of the anchor simulated the typical mobile home tie-down attachment as closely

as possible. An appropriate yoke was placed on the anchor head or on the shaft

just under the head, protruding from opposite sides of the anchor, to accommodate the attachment of the position transducers. The hydraulic pressure and

return lines were attached by means of quick-disconnect couplers. Signal cables

from the pressure transducer (or load cell in the cyclic tests) and the position

transducers were then connected by mating couplers to the x-y recorder and power

supply in the van. Sound powered headphones were used for communicating between

the pump operator and the recording technician. After a short calibration

check, the test was started.

The monotonic tests were performed at a loading rate of 600 lb per minute

until the load capacity of the anchor dropped. At this time, the anchor was pulled out at any. convenient speed. Movement of the head of the anchor and

the corresponding load were recorded as a load deflection curve plotted on an

x-y recorder.

17
The cyclic tests were performed using the same type anchors, installation

procedures and pulling configurations as the raonotonic tests. The equipment

used was described in section 3.2.2. Most cyclic tests were preceeded by one static load cycle to 83 percent of ultimate load capacity (this level is herein called the preload level.) The ultimate load capacity was taken as the

average of previously performed adjacent static tests with the same loading

configuration. The load level was then adjusted to two-thirds of the ultimate

load (the estimated "design" load) or to other predetermined load levels, and

200 to 300 load cycles were applied. The rise time of a load cycle was generally 30 seconds and the peak load was maintained for 2 1/2 seconds. The

specimen was unloaded after each load cycle. The preloading cycle and the

first five or ten cycles were recorded on the x-y recorder and subsequent cycles were visually monitored with the recording pen lifted off the paper. Periodic

cycles (every 20th, every 50th) or significant events such as increasing creep

or incipient failure were recorded by returning the pen to the paper. This

practice allowed recording of the rate of creep and important events without

covering the paper with repetitious lines.

3.3 TEST PROGRAM

3.3.1 General

Anchor performance is affected by many variables and it was realized early in

the project that thousands of tests would be needed to obtain a statistically

significant number of tests for each condition. It was therefore decided to

conduct a more extensive test program on the silt site (Site A) and keep the

number of tests on the other two sites to a minimum.

3.3.2 Test Variables

The test variables considered were: anchor type; anchor size; anchor depth;

loading conditions; load orientation; soil type; and soil conditions. These

variables are discussed hereafter.

(1) Anchor Type ;

Several anchor types suitable for mobile home application are commerically

available [12] . Presently the most frequently used anchors are the 6 in diameter single-helix and the 4 in diameter double-helix anchor. Very few data

were available for these anchors, and the available data provided only limited

information [12]. Another type of shallow anchor which could be adapted for

mobile-home use is the self-seating swivel anchor. These anchors were extensively used by the Armed Forces and, as a consequence, more test data were

available. It was decided to conduct most tests with single- and double-helix

anchors and a limited number of tests with self-seating swivel anchors.

18
(2) Anchor Size:

Anchor size has a major effect on load capacity. It is generally assumed [12]

that anchor-load capacity is proportional to the area of the anchor

plate which provides resistance to pullout. To study this parameter, several

tests were conducted with 3 in diameter single helix anchors. The result of

these tests can be compared with those obtained from tests on 6 in single helix

anchors. The 3 inch anchor is not used for mobile homes because of its

inadequate load capacity.

( 3) Anchor Depth ;

It has been determined in previous investigations [12] that there are two types

of failure mechanisms for soil anchors: the so-called "shallow" anchors fail

by pulling with them a body of soil (cylinder, truncated cone or other) which

extends to the ground surface; the so-called "deep" anchors fail without causing a substantial disturbance at the ground surface, since the failure (slip)

surface surrounding the anchor does not extend to the ground surface (one

study also identifies "intermediate" anchors [8]). As a consequence, the change

of load capacity with depth differs for these two types of anchors, and different models for predicting load capacity have to be used. The ratio of anchor

depth to anchor diameter (D/B) at which an anchor becomes a "deep" anchor has

been estimated to vary from about 4 to 8 [14] and it is therefore not clear

whether mobile home anchors can be classified as shallow or deep. Several

comparative tests were performed to investigate this parameter, using anchor

depths from 1 to 4 ft.

(4) Loading Conditions :

Three loading types were under consideration:

1. Monotonic load cycles, sustained for relatively short periods of

time

2. Cyclic loads

3. Long-term substained loads

Under actual field conditions, wind would cause one or two cycles of some maximum load sustained for one or several seconds and many cycles of lesser

load; floods would probably cause one load cycle with a relatively slow rise

time and sustained for some period of time, ranging from several minutes to

several hours; long-term sustained loads could be caused by swelling soil

conditions or frost heave.

Since swelling and frost heave effects can not be quantified, and it is also

reasonable to anticipate that periodic relaxation and re-tightening of straps

as a maintenance procedure would be necessary if such effects are experienced,

it was decided not to conduct tests for this loading condition.

19
To assess windload effects, it is important to get an appreciation of the

cyclic-load effects associated with a windstorm. An examination of the stripchart recording of Hurricane Frederick^ indicates that the maximum wind velocity at the gaging station (86 mph) occurred twi^ce during the 7-hour period of

high winds; a wind pressure equal to 75 percent of the maximum was exceeded

10 times ; a wind pressure equal to 60 percent of the maximum pressure was exceeded 86 times ; and a wind pressure equal to 50 percent of the maximum

pressure was exceeded 162 times . Thus, it was decided that test data from 100

to 200 successive load cycles would provide adequate information to assess the

probable effects of a major hurricane. Cyclic loads applied ranged from 50 to

75 percent of the ultimate-load capacity of the anchor. Initially, it was planned to perform monotonic and cyclic-load tests. Later in the program,

several cycles of unloading and reloading were included in the monotonic tests

in order to evaluate strain-hardening effects.

(5) Load Orientation ;

Most anchor-test data available are for pullout in the direction of the anchor

shaft. However, as shown in the NBS load study [20], the most important function of mobile-home anchors is to resist horizontal loads. Since most helix

anchors are installed vertically or near vertically, and the horizontal load

is transmitted to the anchor by a strap which is installed at an angle ranging from 15° to 60° to the horizontal, it is necessary to study anchor performance under this loading condition. To explore the full range of conditions

that could be encountered, vertical anchors were subjected to vertical, as well as inclined loads, and anchors installed in an inclined position were subjected to loads in the direction of the shaft, as well as loads normal to

the direction of the shaft.

(6) Soil Types ;

A great number of soil types are encountered in nature, and oversimplification

cannot be avoided if an attempt is made to condense these into a few typical

cases. There is evidence [12] that there is a fundamental difference between

anchor performance in granular and cohesive soils, since these soils have different strength, drainage, and strain hardening characteristics. The three

soil types selected for this project were discussed in section 3.1.

(7) Soil Condition ;

Soil condition is an important variable, since moisture content changes

seasonally on many sites. In this project two conditions were investigated;

the in-situ condition, which did involve some minor fluctuation in water content during the duration of the tests, and the submerged condition which was

The records were taken on September 13, 1979, at Mobile, Alabama, Muncipal

Airport in open terrain and obtained from the National Weather Service

Station in Mobile, Alabama.

20
either obtained by existing site conditions, or artificially induced by flooding. The submerged condition is important since it is necessary to determine

whether anchor capacity is reduced under flood conditions.

3.3.3 Summary of Test Program

Table 3.1 provides a summary of the test program. To avoid complexity, the

parameters explicitly noted were anchor type, soil type, loading condition and

load orientation. Anchor size, anchor depth, and soil condition were omitted.

These parameters are identified in appendix B where all the test results are presented

.

TABLE 3.1 SUMMARY OF TEST PROGRAM

STATIC LOADING CYCLIC LOADING

\^ Test vCondition

/ /

Anchor \

Type \

« >

< / < t <

Silty

Soil Single Helix

Double Helix

Other

38

12

4

12

12

*

6

15

12

3

9 11

2

Sandy

Soil Single Helix

Double Helix

Other

15

6

3

6

3

3

4

2

4

8

Clayey

Soil $ingle Helix

Double Helix

Other

8

3

3

3

3

3

4

7

4

4

These anchors were self-seating swivel anchors. The anchor itself was inserted

vertically. However, when the test load is applied, the connecting cable aligns

itself in the direction of the load.

21
FACIWG PAGE: InitaZtlng voAXlcal anchon. on SiXo. A.

22
4. ANALYSIS OF TEST RESULTS

4.1 PRESENTATION OF RESULTS

The test results are presented in appendix B. The data were electronically

recorded in the field by an x-y plotter in the form of load versus displacement. The resolution of these plots permitted an estimate of displacement to

the nearest 0.01 in and loads to the nearest 0.01 kip. This resolution was compatible with the accuracy of the measurements. Since it would be impractical to reproduce the resulting 232 plots in the report, data points that

23
were derived from the plots are presented in tabular form in tables B.l through

B.6.

Results from the monotonic loading tests are given in tables B.l., B.3 and B.5.

Each test is identified by letters and a number. The letters identify the site:

ST = silt, SD = sand and C = clay. The test identification is followed by test

location coordinates. The corresponding locations are identified in appendix A.

The third column identifies the anchor type. H-6, D-4 and H-3 mean 6 in single

helix, 4 in double helix and 3 in single helix, respectively, AH means arrowhead (triangular) anchor and P 10 and P 6 mean 10 1/4 and 6 1/2 in long pipe

anchors. (The arrowhead and pipe anchors are self seating swivel anchors.)

The anchors are further identified by make (only identified by letter). The

pull direction is identified as A-axial or I-inclined. The angle of load and

anchor inclination to the horizontal are identified by ai and a2 , respectively.

The "depth" identified is the vertical depth from the ground surface to the

anchor tip. To get the depth of a helix plate the distance from the tip to

the center of the helix (3 in for most anchors tested) must be subtracted (see

figure 1.2). The loading is identified as SM-static monotonic and SUR-static

with unloading and reloading cycles (cyclic loading tests are identified

separately). The soil condition is identified as M(natural moisture content)

W-wet or S-submerged. Subsequently, the soil test probe reading - STP and the

installation torque-T^ are identified.

Figure 4.1 shows a typical plot of a load-displacement curve for an anchor

pulled vertically and installed vertically on Site C (clay). The data points

recorded for the monotonic tests with reloading cycles are shown in figure 4.1

and explained hereafter. Note that most "monotonic" tests contained 2 cycles

of unloading and re-loading at each 1 kip increment of load and 1 cycle of

unloading and re-loading near the point of maximum load (Q^)* The following

data points are recorded:

P2v is the anchor load at 2 in vertical displacement; (P4h» the anchor

load at 4 in horizontal displacement is used for anchors installed vertically and pulled at an angle) . For anchors installed at an angle to the

vertical and pulled axially, the loads recorded under P2v and P4^^ are the

loads for the 2 in vertical and 4 in horizontal displacement components,

respectively. In some instances these 2 and 4 in displacements include

residual displacements from unloading and reloading cycles.

is the "ultimate" load (the maximum load attained by the anchor during

pullout)

Ay is the displacement at ultimate load measured in the direction of the

initial pull. For anchors installed vertically and pulled at an angle oli to the horizontal, the horizontal displacement component, A^j^, can be

approximately calculated by the equation A^^^ = A^/cos aj^, since for these

anchors the anchor head displacement tended to be horizontal with only a minor vertical component. For anchors pulled axially or near axially A^

is an axial displacement. For the few anchors, where the pull was in a

24
5 I

1 r —r

1 r

DISPLACEMENT, IN.

Figure 4.1 Plot of test C-7 , vertical 6-in single helix anchor, vertical pull

25
direction normal to the shaft, is the displacement in the direction

of the pull.

R35 is the secant re-loading modulus at 85 percent of the load before

unloading, measured at the highest load at which unloading and re-loading

was performed (not necessarily as in figure 4.2) expressed as a ratio

of load to displacement (lb/in).

The results of the cyclic tests are given in tables B.2, B.4 and B.6. The

symbols for the data shown in the cyclic tests are explained hereafter:

P(,/Q^ is the ratio of the cyclic load to the estimated ultimate load

obtained from monotonic tests in an adjacent location

N is the number of load cycles used

Pp/Qu ratio of preload (if any) to the estimated ultimate load

A^, Aj^Q, Aq^qq, and A^ are the total displacements in the first, tenth,

hundredth and last load cycle. (They are the sum of the displacement

caused by the applied load in the last load cycle and the cumulative

residual displacements from all previous load cycles.)

RlQ is the secant re-loading modulus in the 10th load cycle

t is the ultimate load actually obtained when the anchor was pulled

out after completion of the cyclic loading.

4.2 EFFECT OF LOADING RATE

The effect of the loading rate on the characteristics of the load-displacement

curve was investigated early in the test program in order to decide on the

loading rate to be used in the tests. The fastest rate at which load could be

applied in the monotonic tests was limited by the pumping capacity of the

manually operated hydraulic system and in each case also depended on the ram displacement associated with a particular load increment. On the average, the

fastest initial loading rate was approximately 4,000 lb. per minute. This rate

has a tendency to decrease as the ultimate load is approached. Thus it was not

possible to investigate dynamic load effects.

The solid curve in figure 4.2 is the record of a test in which load increments

were applied at a fast rate, and after each load increment the load was held

until no measurable creep was recorded over a 15-minute period. For the load

increments up to 4.25 kip, creep virtually ceased after 5 minutes. In the last

two load increments creep continued for 20 minutes.

The shaded area in figure 4.2 is the estimated range of creep effects that

could be anticipated. The upper bound of this range represents the most rapid

load application possible with the available equipment, and the lower bound a

26
Figure 4.2 Effect of loading rate on load-displacement characteristics

27
load-displacement relationship that would not be altered by further decreasing

the loading rates (except that further creep would probably occur if a load

increment would be held for a very long period of time, such as several days).

On the basis of this, and several other observations, the load in the raonotonic

tests was applied at a rate of 600 lb/minute. While this loading rate would

not produce the lower-bound curve in figure 4,2, the test results are assumed

to be close to the lower-bound curve.

Cyclic loads were applied at a much faster rate, since these were intended to

simulate windloads. However, the 30 second rise time for the load was much

slower than the typical windload. The loading rate was limited by the capacity

of the oil pump.

4.3 STATIC LOAD-DISPLACEMENT CHARACTERISTICS OF 6-in SINGLE HELIX ANCHORS

4.3.1 Monotonic Tests

Figure 4.3 shows the result of a typical monotonic loading test on a 6-in single

helix anchor on Site A (silt). The test is a vertical pullout test which was carried to full withdrawal. Vertical displacements of the anchor head in inches

are plotted against applied load in kip. Note that the initial portion of the

load displacement curve is rather steep and there is a break at point A, at a load of about 1.1 kip. This break was characteristic of many, though not all

the tests. There is a gradual, but not very drastic decrease in stiffness until

the anchor yields at a load of approximately 4.8 kip. The anchor subsequently

maintained its load resistance during an additional 10 in withdrawal, and a reduced load resistance over an even larger range of displacements which is not

recorded. Ductile behavior was characteristic of soil anchors tested on Sites

A and C (silt and clay), even though the range of displacements over which the

load is maintained varied with the soil type.

4.3.2 Unloading and Reloading Cycles

Figure 4.4 shows a vertical pullout test on a 6-in single helix anchor in sand

(Site B) installed vertically to its full 4-foot depth. Two cycles of unloading and re-loading were conducted at 1 kip intervals in order to assess the

characteristics of pre-loaded anchors. The re-loading curves are generally

much steeper than the initial "virgin" loading curve, indicating substantial

strain-hardening effects. The characteristics of the curve are interpreted as follows: Whenever load is applied, the soil is compacted, and up to the

applied load, its load-displacement characteristics are modified. As soon as the applied load exceeds the pre-load, the load-displacement curve follows the

shape of the virgin curve which would be obtained in monotonic loading except

that some displacement of the curve will have occurred as a result of the

unloading and reloading cycles. The initial break in the virgin curve, which

was observed at point "A" on figure 4.3, can probably be attributed to preconsolidation of the soil which was about equivalent to a 1.1 kip anchor pull.

28
6

ST.13

4

-

3

Silty soils

6" diameter single helix anchor

4 ft. deep, moist soil

1 -

6 8

DISPLACEMENT, IN.

10 12 14

Figure 4.3 Monotonic loading test in silt

29
VERTICAL DISPLACEMENT, IN.

Figure 4.4 Pullout test with unloading and reloading cycles in sand

30
4.3.3 Anchors Installed at an Angle and Pulled Coaxially

Figure 4.5 shows the load-displacement curve for an approximately coaxial pull on an anchor which was installed on Site B (sand) at an angle of 45° to the

horizontal. Note that this curve is similar to the one shown in figure 4.4,

except that the load capacity is much lower because of the reduced anchor depth

due to the 45° installation angle. The break in the virgin curve is very

pronounced and occurs at about 0.5 kip.

4.3.4 Anchors Installed Vertically and Pulled at an Angle

Figure 4.6 shows the load-displacement curve for a vertically installed anchor

on Site B, pulled at an angle of 40° to the horizontal. The initial stiffness

of this anchor is very low (only 1.2 kip capacity at a 4-inch displacement),

since the 5/8-inch thick shaft developes very little lateral soil resistance as

it is pulled horizontally into the soil. However, as the shaft is bent in the

direction of the pull the soil resistance increases and the ultimate pullout

resistance exceeds that for a vertical pull. The initial, flatter slope of the

re-loading curves is attributable to the elastic rebound of the anchor shaft

which occurs before the soil resistance is engaged. Otherwise the re-loading

curves show characteristics similar to those in the previously discussed

tests. The anchor deformation in these tests is illustrated in figure 4.7

which shows a 4-in double helix anchor which was exposed by excavation after

completion of a similar test.

4.3.5 Effects of Loading Configuration

The reconstructed virgin curves for the tests shown in figures 4.4 through 4.6

which are for tests performed in similar soil conditions, are plotted in figure

4.8. When these envelope curves were drawn, displacement caused by the unloading and re-loading cycles were estimated and subtracted from the total displacement. The figure illustrates the difference in the performance characteristics.

Note the large displacement required to develop load resistance in a vertically

installed anchor subjected to diagonal load, the most commonly encountered

situation associated with present mobile home anchoring technology.

The load capacity (ultimate load) of the coaxially pulled inclined anchor (test

SD 29) was about 50 percent of the load capacity developed in test SD 25.

Anchor SD 25 was installed at a depth of 4 ft and anchor SD 29, because of its

inclination, at a depth of 2.8 ft. Thus a 30 percent increase in embedment

depth caused a 50 percent increase in load capacity. Anchor SD 30 developed a higher load capacity than anchor SD 25, however a much larger displacement

occurred before the full load capacity was developed. Much of this displacement is attributable to the bending of the anchor shaft (see figure 4.7). The

load capacity of anchor SD 30 was 6 kip and that of SD 29 was 5 kip. Thus load

capacity increased by 20 percent. This trend was consistently observed, regardless of soil conditions. The trend is further illustrated in figure 4.9, which

shows a comparative plot of tests on vertically installed anchors performed at

different load inclinations. These tests were performed on Site A (silty

31
Figure 4.5 Coaxial pullout test on inclined 6-in single helix

anchor in sand

32
012345 6789 10

DISPLACEMENT, IN.

Figure 4.6 Inclined pullout test on a vertically installed

6-in single helix anchor in sand

33
Figure A. 7 Anchor deformation in inclined test on vertically

installed 4-in double helix anchor

34
SD-30

DISPLACEMENT, IN.

Figure 4.8 Effect of loading configuration on load-displacement

characteristics of anchors

35
I

DISPLACEMENT, IN.

Figure 4.9 Effect of load inclination on the load-displacement

characteristics of vertically installed anchors

36
soils). Note that as angle ai decreases from 90° to 15°, the load-deflection

slope (stiffness) of the virgin load-displacement curve decreases, but the load

capacity of the anchor increases (at the flatter angles the full load capacity

could not be realized because of failure of the anchor hardware). It is interesting to note that while the virgin load-displacement curves show great differences in stiffness, the re-loading characteristics are quite similar for the

various load inclinations and resemble those illustrated in figure 4.6. This

is shown in table 4.1.

Table 4.1 Reloading Moduli (Rgs) for the Tests

Shown in Figure 4.9

Test Specimen a

^ ° ^85, lb/ in

ST 13 90

ST 96 60 3980

ST 91 45 3610

ST 83 30 5020

ST 81 15 3500

There is no re-loading curve for test ST 13 since the importance of the

re-loading characteristics was only realized at a later stage in the test program. However, data from test ST 122, a vertical pull-out test conducted at

the same site, indicate a re-loading modulus of 17,000 lb/ in.

Note from figure 4.6 that the re-loading curves can be divided into two segments.

Initially, there is a displacement of the order of about 1/2 in where the re- loading curve is relatively flat. This part of the re-loading curve is attributable to the bending of the anchor shaft which rebounds elastically when it

is unloaded. Subsequently, the re-loading curves are very steep with moduli

similar to the one observed for test ST 122 (see above). The two segments of

the re-loading curve are combined when ^85 is determined. It is important to

note that the re-loading moduli observed in the inclined tests of verticallyinstalled anchors far exceed those that would be required to satisfy present

standards and regulations [2, 17].

Another loading configuration that was tested on the silty site is non-coaxial

loading on inclined anchors (a2 = 135°, = 15°, 45° and 60° - see sketch in

figure 4.10). Similar configurations occur in practice, since it is difficult

to insert vertical soil anchors under the outer walls of an installed mobile

home. Typical test results are shown in figure 4.10. Because of the large

displacements associated with this loading configuration the displacement scale

is compressed. Note that, as in the case of vertical anchors with inclined

loading, the stiffness of the anchor decreases as the angle of the load with

the horizontal decreases. Test ST 91 was plotted in figure 4.10 for comparison.

Note that the performance of anchors ST 102, 105 and 108 is very poor when

compared with vertical anchors with inclined loading, which also experience

considerable displacements before developing their load capacity. It is obvious

37
DISPLACEMENT, IN.

Figure 4.10 Load-displacement characteristics of inclined anchors subjected

to non-coaxial loads on the silty site.

38
from the test results that this is the least desirable loading configuration

tested. Thus if anchors are tilted away from the mobile home during

Installation their stiffness and load capacity are likely to decrease.

4.3.6 Effects of Inclination and Depth on the Load Capacity of Coaxially

Loaded Anchors

The effect of the angle of load and anchor inclination on coaxially loaded

inclined anchors is illustrated in figure 4.11. As expected, the load capacity

decreases as the angle of anchor installation becomes flatter. Anchor capacity

in this case may be influenced by two opposing effects: As the installation

angle becomes flatter, the depth of the anchor decreases since the embedment

length of approximately 4 ft remains the same. On the other hand, some test

results and load capacity hypotheses [14, 11] indicate that, for a given depth,

anchor capacity increases as the installation angle becomes flatter. However,

these observations are not corroborated by other investigators. For instance

Harvey and Burley [10] found that coaxial pull-out capacity for shallow anchors

in sand for the same depth of embedment is approximately the same for vertically

installed and inclined anchors. The test results obtained in this project provide some information that can be compared with the above discussed data.

Figure 4.12 shows the results of pull-out tests of anchors installed at various

depths on Site A. Note that there was no significant difference between the

anchors installed at 3 and 4 ft depths. This is an indication that anchors

deeper than 3 ft experienced local failure (the failure surface did not extend

to the ground surface). The depth to diameter (D/B) ratio for the 3 ft deep

anchors is 5.5 and that for the 4 ft deep anchors is 7.5, and the observation

that the 3 ft deep anchors acted like deep anchors would be in agreement with

data obtained by others [8, 12].

Two interesting observations can be made from the comparative plots in figure

4.12: 1. As the anchors become shallower, their peak capacity is reached at an increasingly smaller displacement and their "ductility" decreases. This is

probably related to the failure surface developing as the anchor is withdrawn.

2. The load capacity of an anchor, embedded at a given depth is not unique and

depends on the original embedment depth. Thus the anchor which was initially

embedded 3 ft resisted more than 4.5 kip after it was withdrawn 13 inches and

was 1 ft 11 in deep. At the same depth the initially 2-ft deep anchor resisted

only a 3 kip load. This phenomenon can be explained by the development of a unique failure surface for each anchor depth, which is associated with the virgin load-deflection curve. The soil mass within this surface is compacted by

the applied load. This compaction accounts for the strain hardening effect

evident in the re-loading curves. When the anchor is pulled out to a shallower

depth the compacted soil mass within the slip surface moves up with it, and the

shape of the slip surface does not substantially change.

Figure 4.13 shows a comparison between the pullout strengths of co-axially

loaded vertical and coaxially loaded inclined anchors for various installation

depths. For the 1 ft depth the inclined anchor had substantially higher

strength than the vertical anchor. However, for the other depths the results

39
DISPLACEMENT, IN.

Figure 4.11 Load-deflection curves for coaxially loaded 6-in single

helix anchors installed at various angles

40
6 I

1 1 1 r

DISPLACEMENT, Ni

Figure 4.12 Effects of embedment depth on the load capacity of vertically

installed 6-in single helix anchors subjected to coaxial load

41
Figure 4.13 Comparison between inclined and vertical coaxially

pulled 6-in single helix anchors

42
of the inclined-anchor tests fall into the same pattern as those of the

vertical-anchor tests when anchor depth is correlated with load capacity, and

there is no evidence that the load capaity of inclined anchors is greater than

that of vertical anchors installed at the same depth. To ascertain whether the

test results are significant, all the test results are tabulated in table 4.2.

Table 4.2 Comparison of the Strength of Coaxially Loaded Anchors

Installed at Various Depths

Test No. Depth, ft Inclination ^2 Qu, lb ?u, lb v

ST 40 1 90° (vertical) 700

ST 41 1 90° 800 840 0.19

ST 42 1 90° 1020

ST 49 1.02 15° 1280

ST 50 1.02 15° 1610 1447 0.11

ST 51 1.02 15° 1450

ST 43 2 90° 3120

ST 44 2 90° 3300 3200 0.03

ST 45 2 90° 3180

ST 52 2 30° 3420

ST 53 2 30° 3120 2880 0.24

ST 54 2 30° 2090

ST 55

ST 56

ST 57

2.83

2.83

2.83

45°

45°

45°

4220

4000

4520

4247 0.06

ST 46

ST 47

ST 48

90°

90°

90°

5250

5600

4800

5217 0.08

ST 58

ST 59

ST 60

3.46

3.46

3.46

60°

60°

60°

5280

4300

4110

4563 0.14

Qu = average load capacity, lb

V = coefficient of variation of the sample

43
It can be seen from table 4.2 that with the exception of tests ST 54, ST 59 and

ST 60 the trend Is reasonably consistent. This leaves the question why the 15°

anchors have a consistently higher load capacity than the 1 ft deep vertical

anchors, while in all other instances the load capacity of inclined anchors was approximately equal to that of vertical anchors of the same depth. The explanation is probably in the fact that the pull exerted by the 15° anchors is predominantly horizontal creating a failure surface the geometry of which differs

substantially from that of the 1 ft deep vertical anchor. The field notes indicate that the failure of the 1 ft deep vertical anchors created a 24 in diameter

soil mound and that of the 15° anchors a 22 x 32 in mound "along the axis of

the anchor".

4.3.7 Comparison of Coaxially Loaded Inclined Anchors and Vertical Anchors

Subjected to Inclined Loads

Anchors which have to resist inclined loads (containing a horizontal load

component) are in present practice installed vertically. However, they could

also be installed at an angle in order to resist the load in coaxial pull. The

drastic difference between these two conditions was shown in figure 4.8. The

general load-displacement characteristics of the virgin curves and the re- loading curves have been previously discussed. The question arises how the

great difference in load capacity (ultimate strength) between these two conditions can be explained. Part of the explanation is related to anchor depth.

An anchor installed at an angle of 45° will have only 70 percent of the full

depth, while the vertical anchor is installed to its full depth. It has been

previously shown for coaxially loaded anchors that except for very flat angles

inclined anchors have about the same load capacity as vertical anchors of equal

depth. On the other hand, vertical anchors subject to an inclined pull consistently developed higher load capacities than axially pulled vertical anchors

of equal depth. It is believed that the moment transmitted to the helix plate

and the lateral pressure transmitted by the anchor shaft to the soil play a major part in this increased load capacity by subjecting the soil mass on the

load side of the anchor to compression. The compressive load in turn will tend

to increase the shear strength of the soil mass on the load side of the anchor.

An examination of figure 4.7 reveals evidence of the compression of the soil mass on the load side of the anchor.

4.4 COMPARISON BETWEEN DIFFERENT ANCHOR TYPES

4.4.1 4-in Double Helix Anchors

Figure 4.14 shows a comparison between the load deflection curves for 6-in

single helix and 4-in double helix anchors on the silty site. The broken lines

are for 6-in single helix anchors and include coaxial tests on vertical and 45°

inclined anchors (ST 13 and ST 56) and a 45° pull on a vertically installed

anchor (ST 91). The tests on the 4-in double helix anchors are shown by the

solid lines. Test ST 16 corresponds to test ST 13, ST 69 to ST 56 and ST 89 to

ST 91. Two trends are obvious. The 6-in single helix anchors develop higher

44
Figure 4.14. Comparison between the load-displacement characteristics of

6-in single helix and 4-in double helix anchors

45
load capacities and also tend to exhibit more ductility (except for test ST 91

which may have resulted in a hardware failure).

Several factors combine to produce the difference in performance: 1. The

embedment depth of the helix of the 6-in single helix anchor is deeper (3.7 ft vs 2.6 ft); 2. The 6-in single helix plate has a larger area (2.25 times the

area) and 3. The 4-in double helix anchor has two helixes. Many authors claim

that anchor capacity is proportional to the area of the anchor plate [12]. For

the tests plotted on figure 4.14 the capacity ratios are 1.73 for coaxially

pulled vertical anchors, 2 for coaxially pulled anchors inclined 45° and 2.7

for vertical anchors pulled at 45°. This can be compared with the area ratio

of 2.25. It should be noted that the correlation between the load capacities

of the two anchor types is probably affected by all the factors mentioned above

(areas of helix plate, anchor depth, and the presence of the second helix in

the 4 in anchors).

Another aspect of anchor performance that can be compared are initial stiffness

and ductility. Anchor ST 13 was very ductile, while its 4-in single helix

counterpart, anchor ST 16 rapidly lost load capacity after the maximum load

was attained. The most likely explanation for this difference is that the

failure mechanism of anchor ST 13 made it a deep anchor (the slip surface did

not extend to the ground surface) while anchor ST 16 acted as a shallow anchor.

There was a considerable difference in stiffness between anchors ST 91 and ST

89, namely, the 6 in single helix anchor had smaller lateral displacement than

the 4-in double helix anchor. This difference was consistenly observed in all

the anchor tests and was not anticipated, since it was thought that the long

slender shaft of the 6 in single helix anchor would provide less resistance to

lateral displacement.

In figure 4.15 load-displacement curves for vertical 4-in double helix anchors

pulled at various angles are compared with each other. The trend observed is

similar to that shown in figure 4.9, namely, the stiffness decreases and the

load capacity tends to increase as the angle of pull decreases from 90° to 15°

As previously noted for the 45° pull, the stiffness as well as the load capacity of the 4-in double helix anchors are smaller than those of the 6-in single

helix anchors.

In figure 4.16 tests on coaxially loaded inclined 4-in double helix anchors,

installed at various angles to the horizontal (a^^), are compared. This figure

should be compared with figure 4.11 for 6-in single helix anchors. Note that

in figure 4.11 the vertical anchor and the 60° inclined anchor had similar load

capacities. After comparison with pullout tests at various depths this phenomenon was taken as an indication that both of these anchors acted as deep anchors,

and thus their load capacity did not significantly diminish with depth. In the case of the 4 in double helix anchors figure 4.16 gives a clear indication that

all the anchors acted as shallow anchors. A tabulation of the depth ratio

versus the load capacity ratio is given in table 4.3, to show the average trend

of the test results. The load capacity of most of the inclined anchors is

roughly proportional to their depth. As in the case of the 6-in single helix

46
7

DISPLACEMENT, IN.

Figure 4.15. Comparison of load-displacement characteristics of vertically

installed 4-in double helix anchors pulled at different angles

47
0 L I I I I I 0 2 4 6 8 10

DISPLACEMENT, IN.

Figure 4.16. Comparison of load-displacement characteristics of coaxillay

loaded 4-in double helix anchors installed at different angles

48
anchors the load capacity of the 4-ln double helix anchors at the 15°

inclination was higher than that predicted by the overall trend.

Table 4.3 Effect of Anchor Inclination on the Load Capacity of Coaxially

Loaded A-in Double Helix Anchors in Silty Soils

02 n ^u, lb V D/Dv Ou/Ouv

90° 6 2733 0.13 1 1 60° 3 2677 0.02 0.87 0.98

45° 3 2030 0.15 0.71 0.74

30° 3 1517 0.09 0.50 0.56

15° 3 1307 0.07 0.26 0.48

a2 - angle of anchor shaft with horizontal

_n = number of test performed

= average load capacity, lb

V = coefficient of variation

D/Dv = ratio of depth of inclined anchor to depth of vertical _ _ anchor

Qu/Quv = ratio of average load capacity of inclined anchors to that

of vertical anchors.

4.4.2 3-inch Single Helix Anchors

The 3-inch single helix anchors were tested in order to evaluate size effects.

In practice, these anchors do not have sufficient load capacity to be useable

for mobile home tiedowns. This anchor is like a scaled-down 6-inch single helix

anchor and should therefore afford a good comparison. The depth of the helix

plate of a vertically-installed 3-inch helix anchor is approximately 25 inches,

which gives an D/B ratio of 8.3. Thus, the vertical anchors probably acted

like "deep" anchors. Table 4.4 gives a comparison of the load capacities of

6-inch and 3-inch anchors

Table 4.4 Comparison of Load Capacities of 3-in and 6-in

Single Helix Anchors

Anchor Size Loading No. Tested Qu lb v

5212 0.12

1650 0.17

4247 0.06

1102 0.02

6-inch Vertical 8

3-inch Vertical 5

6-inch 45° Coaxial 3

3-inch 45° Coaxial 3

49
In accordance with the above tabulation, the ratio of load capacities between

the 6-inch and the 3-inch anchor was 3.16 for the vertical anchors and 3.85 for

the 45° anchors. The ratio of the helix areas is 4. Thus, the load capacity

of the 3-inch anchors was higher than the capacity that would be predicted on

the basis of the ratios between the helix areas. This phenomenon will be

further discussed under "prediction of anchor-load capacity."

Typical 3-inch single helix anchor tests are shown in figure 4.17. Note that

the vertical scale was expanded because of the small failure loads. The behavior of these anchors was rather ductile which is taken as another indication

that they acted as deep anchors.

4.4.3 Self-seating Swivel Anchors

In figure 4.18, the load-displacement characteristics of the self-seating swivel

anchors tested are compared with those of 6-inch single helix anchors. Because

of the large displacement associated with the virgin load-displacement curves

for self-seating swivel anchors, the displacment scale was compressed. Note

that extremely large displacements are required to develop the load capacity

of the swivel anchors. Part of this displacement is attributable to the fact

that the anchor must be rotated (upset) before it develops substantial resistance. Note that for both, the vertically-pulled and diagonally-pulled swivel

anchors, a displacement of about 15 inches was required to develop a 4 kip load

capacity. Upon unloading, these anchors had re-loading characteristics similar

to those of the helix anchors, and in the case of the 45° pull, the re-loading

characteristics of the swivel anchors were superior to those of the 6-in single

helix anchors. (Note that these anchors have cables, and that under coaxial

pull, these cables will extend (elongate) much more than the 5/8 in or 3/4 in

anchor stems of the helix anchors.)

Hereafter are some comparisons which give an indication of the effect of anchor

area and direction of pull on load capacity.

50
0 I ' ' I I \ \ \ I 0 2 4 6 8 10 12 14 16

DISPLACEMENT, IN.

Figure 4.17 Tests of 3-in single helix anchors in silty

51
Figure 4.18. Load-displacement characteristics of self seating swivel

anchors in silty soil

52
Table 4.5 Load Capacity of Self-Seating Swivel Anchors Tested in Silty Soils

Anchor Type

Proiectpd Arp^

in2 n

aiJ. ^^11 ^ Ul 1 h J J. u

Pipe 17 .9 2 Vert

.

S J 400 "T" V/ V-* Pipe 17.9 3 60° 5 ,100

Pipe 17.9 2 45" 3,250*

Pipe 8.5 Vert. 3,300

Pipe 8.5 60° 2,800

Pipe 8.5 45° 2,500

Triangular 15.6 Vert. 5,100

Triangular 15.6 60° 5,300

Triangular 15.6 45° 5,000

* One of these anchors was not fully seated during the pull. The

other anchor had a load capacity of 4,200 lb.

Table 4.5 represents very few specimens and hence there is not enough evidence

to establish definitive conclusions. However, the table reveals some consistent

trends

1. Load capacity increases with the projected area of the anchor. This

relationship seems to hold even when two entirely different anchor types are compared, and will be further discussed under "prediction of load

capacity.

2. Vertically-pulled pipe anchors seem to have a higher load capacity than

those pulled at an angle and load capacity seems to decrease when the

angle of pull with the horizontal decreases. (All pipe anchors were installed vertically. An initial pull was needed to orient the cable in

the direction of the pull.) •

3. The triangular anchors' load capacity does not seem to change with the

angle of pull.

It is also of interest to note that one of the pipe anchors pulled was not

fully seated by the load.

Another important characteristic that needs to be examined is the re-loading

modulus. These moduli do not seem to be affected by the angle of pull and

appear to be rather consistent. They are tabulated in table 4.6. It is

obvious from table 4.6 that properly seated, pre-loaded swivel anchors could

provide load-displacement characteristics superior to those required in the

ANSI A. 119. 3 standard, provided that they develop the required load capacity.

53
Table 4.6 Re-loadlng Moduli of Swivel Anchors Tested in Silty Soils

Anchor

Type

Projected

Area No. of

Tests

Coefficient of

Variations of

ini ^85 lb/in R85

Pipe

Pipe

Arrowhead

17.9

"8.1

15.6

5

3

3

10,400

6,500

7,300

0.1

0.07

0.05

4.5 EFFECT OF SOIL CHARACTERISTICS

4.5.1 Coaxlally Loaded Vertical Anchors

Figure 4.19 shows a comparison between the load-displacement characteristics of

coaxial pullout tests on 6-inch single helix anchors tested on the sand, silt

and clay site. Test ST13 is from the silty site (Site A), Test SD25 from the

sandy site (Site B) and Test C7 from the clay site (Site C) . Note that tests

ST13 and C7 exhibit considerable ductility, while anchor SD25 rapidly lost its

load capacity after the peak resistance was developed. The characteristics

illustrated by the figure were typical for most tests on the three sites. The

difference in ductility between the anchors on the sandy site, and those on

the silty and clay sites is attributable to the shear-strength characteristics

of these soils. The clay derives most of its strength from cohesion which does

not substantially decrease with shear strain or minor decrease in depth. The

results of the unconfined compression and direct shear tests for the silt indicate that this material also has substantial cohesive strength as well as frictional shearing resistance. The sand, on the other hand, derives its strength

from frictional shearing resistance. To the extent that there is cohesive

strength in the sand, it is derived from cementation and thus would disappear

as soon as a slip surface develops. Thus the shear strength of the sand depends

primarily on confining pressures which, in turn, are a function of present

overburden pressure and overconsolidation (a stress history of higher vertical

pressures in the past). Present overburden pressures are a function of depth

and thus tend to decrease as the anchor is pulled out. Increased confining

pressures due to overconsolidation are relieved as shear deformations become

large, and thus will rapidly disappear as the anchor is withdrawn. There is

evidence that the sand deposits on Site B are overconsolidated. The site

itself was on the bottom of a borrow pit from which 20 to 30 ft. of material

were removed. In addition, it has been determined that these sand deposits

were overconsolidated during their geologic history. It will be shown later

in this report that the magnitude of the load capacity of the anchors on Site

B gives further corroborative evidence of overconsolidation.

4.5.2 Vertically Installed Anchors Pulled at an Angle

Figure 4.20 compares the load-displacement characteristics of vertical anchors

installed on the three test sites and pulled at a 40° angle. Note that the

54
0

1 1 1 i 1

JST-13 Silt

SD-25 Sand ^^^"^

C-7 Clay

f/

6" diameter single helix anchors

r 4 ft. deep

1

1 1 1 1 1 4 6 8 10

DISPLACEMENT, IN

12 14

Figure 4.19 Load-displacement characteristics of coaxially loaded 6-in single

helix anchors on the sand, silt and clay sites.

55
DISPLACEMENT, IN.

Figure 4.20 Vertically installed 6-in single helix anchors pulled

at a 40° angle on the sand, silt and clay site

56
initial anchor stiffness on the sandy site was less than that on the silt and

clay sites. However, the stiffness on the sandy site increased rapidly with

increasing loads and the peak resistance was reached at a smaller displacement

than that in the silt and clay site. It is noteworthy that while on the sandy

and silty site the load capacity of vertical anchors pulled at an angle tended

to be higher than that of axially-pulled vertical anchors, a similar increase

in load capacity did not occur on the clay site. This can be explained by the

fact that the compressive forces exerted in this loading mode on part of the

soil mass surrounding the anchor (see section 4.3.7) substantially increased

the shear strength of the sands and silts, which increases with increasing con- fining pressures, but not that of the clays, which entirely depends on cohesion

and thus tends to be independent of confining pressures. Table 4.7 summarizes

the observed trends.

Table 4.7 Comparison of the Effects of Load Inclination on the Load

Capacities of Vertically Installed Anchors on Sites, A, B,

and C

Soil Anchor Quv»lb V Qui. lb V Qui/Quv

6" S.H. 5170 0.10 7930 0.02 1.53

Silt 4" D.H. 2730 0.13 3623 0.07 1.33

6" S.H. 5290 0.08 6190 0.05 1.17a/

Sand 4" D.H. 1610^/ 0.08 2740 0.16 1.70

6" S.H. 3430 0.16 3270 0.21 0.95

Clay 4" D.H. 1930 0.03 2130 0.16 1.10

Quv = Average ultimate load capacity in vertical pull, lb

Qui = Average ultimate load capacity in inclined pull (45°), lb

V = Coefficient of variation

a/ Helix in the anchor broke off

b/ Tests SD2 and SD3 were excluded from average because they were influenced

by a dense 1 ft. crust overlying the sand. Inclusion of those tests

would inrease Q to 2,300 lb. and v to 0.41

57
4.5.3 Effect of Depth and Anchor Inclination of Coaxial Load Capacity

It is important to determine whether the trends which were observed on Site A

also occurred on Sites B and C. The observed trends are summarized in the

following tables.

In table 4.8a, is the average load capacity and v is the coefficient of

variation. Whenever v is not given, the average is an average of only two

tests. Even though the number of tests was limited, some trend can be

recognized from table 4.8b.

In the silt, the anchors probably acted as "deep anchors" from a depth somewhere

between 3 and 4 ft. Above this "critical" depth, load capacity seems to be

roughly proportional to depth. On the clay site, load capacity is roughly proportional to depth, if depth is expressed as a fraction of the full (45 in.)

depth.

For the sand, load capacity decreases more rapidly with decreasing depth. Thus

when the depth was 50 percent of the full 45-in depth, the load capacity was only 38 percent of the full load capacity, and at 70 percent of the depth, the

load capacity was 50 percent. This trend is consistent with the shear strength

characteristics of the sands. When shear strength is primarily a function of

cohesive strength, the strength change with depth will be more moderate than

for the case where shear strength depends on confining pressures.

Except for the case of the 15° pull which was noted in the discussion of the

tests on the silty site, table 4.8 gives no indication that inclined anchors

have higher load capacities than vertical anchors installed to the same depth.

Effects of anchor inclination on load capacity of coaxially loaded 4-inch double

helix anchors are shown in table 4.9.

The data give no indication that the trend which emerges for the silt site is

also valid for the sand and clay sites. Unfortunately data for the latter two

sites are not sufficient to establish any trends.

4.5.4 Reloading Moduli

The re-loading characteristics recorded are Rgs , the secant re-loading modulus

at 85 percent of the load before unloading for the static tests, and Rio» the

secant re-loading modulus in the tenth load cycle for the cyclic tests. Table

4.10 gives the range of measured re-loading moduli for the three sites. Since

the importance of re-loading characteristics was not recognized at the outset

of this testing program, only limited data are available for the silty site.

These data include only one static coaxial test on a helix anchor but many

static tests on helix anchors which were installed vertically and pulled at an angle, for which the re-loading modulus included the rebound of the anchor

shaft which generally accounts for most of the displacement. There is a possibility that the re-loading moduli on the clay site, where the soil was

58
Table 4.8 Effect of Anchor Depth and Inclination on the Load Capacity

of Coaxially Loaded 6-in Single Helix Anchors

Table 4.8a: Summary of Load Capacity

Silt Sand Clay

Depth «2 V Qu. lb V Q^,, lb V

1' 90° 840 0.19 910

1.02' 15° 1450 0.11

2' 90° 3200 0.03 2020 1800

2' 30° 2880 0.24

2.83' 45° 4250 0.06 2650 0.05 2530 0.06

3' 90° 5220 0.08 4480 2980

3.46' 60° 4560 0.14

4' 90° 5170 0.10 5290 3430 0.16

Table 4.8b: Load Reduction Ratios

Silt Sand Clay

Depth^/ D/D^^/

V 90° 0.20 0.16 0.27

1.02' 15° 0.26 0.28

2' 90° 0.47 0.62 0.38 0.52

2' 30° 0.50 0.56

2.83' 45° 0.71 0.82 0.50 0.74

3' 90° 0.73 1.01 0.85 0.87

3.46' 60° 0.87 0.88

4' 90° 1.00 1.0 1.0 1.0

/ "Depth" is the total depth to the tip of the anchor.

^ Depth ratio D/D^ is the actual depth to the center of the helix divided by

the depth of the vertical anchor to the center of the helix.

^ Qu average ultimate load of tests.

Quv ~ average ultimate load in vertical pull of anchors installed to their

full depth.

59
Table 4.9 Effects of Anchor Inclination on the Load Capacity of

Coaxially Loaded 4-in Double Helix Anchors

Table 4.9a. Summary of Load Capacities

Silt Sand Clay

02 D/D^ Qu, lb V lb v Q^, lb v

90° 1 2730 0.13 1610^/ 0.08 1930 0.03

60° 0.87 2680 0.02

45° 0.71 2030 0.15 1813 0.24 1780

30° 0.5 1500 0.08

15° 0.26 1310 0.07

Table 4.9b. Load Reduction Ratios

Silt Sand Clay

02 Quv/Qu

90° 1 1 1 1

60° 0.87 0.98

45° 0.71 0.74 1.13 0.92

30° 0.50 0.55

15° 0.26 0.48

^/ Tests conducted on the part of the sand site overlain by the dense

crust are not included in this average.

60
saturated, are influenced by the buildup of porewater pressure gradients. If

this was the case, the moduli may be substantically lower in very slow tests.

Some general conclusions can be drawn from table 4.10.

1. In all instances, the re-loading modulus exceeded the stiffness requirements

of ANSI Standard A119.3 by a substantial margin (see section 4.8).

2. The re-loading moduli in the coaxial tests tended to be much higher than

those in the non-coaxial tests.

3. The re-loading moduli in the cyclic tests tended to exceed those in the

static tests with the same loading conditions.

4. The re-loading moduli of the helix anchors varied over a considerable

range; those of the swivel anchors tended to be quite predictable and

increased with increasing anchor-plate (projected anchor area) size.

4.6 PREDICTION OF ANCHOR-LOAD CAPACITY

4.6.1 General

Three methods have been used to predict anchor-load capacity:

1. Correlation of anchor-load capacity with in situ tests [in particular,

correlation with Soil Test Probe (STP) measurements are widely used by

industry]

;

2. Determination of load capacity on the basis of measured or estimated shear

strength characteristics of the soil and some analytical model which assumes

a failure mechanism;

3. Determination of load capacity on the basis of pullout tests in similar

soil conditions.

Method 1 is used by industry with some success. Many theoretical studies have

been conducted in conjunction with method 2; however, it is diffult, and relatively expensive to determine the shear strength of the soil, and the characteristics of soils at shallow depths increase these difficulties. Method 3 is

used by many states to certify anchors; however, its validity is questionable

when tests performed at one site are used to predict anchor capacity at another

site where the shear strength of the soil may be different.

61
4.10 Comparison of Reloading Moduli Measured on the Three Test Sites

Soil Type Anchor Type a2 RgS lb/in R^g lb/in

SILT

1 7 nnn 9n snn — Mn nnn

DU 9 "^nn H-6 45° 90° 2,700 - 3,600

30° 90° 2,100 - 5,000

15° 90° 3

',500 - 7 ',800

90° 90° >50,000

60° 90° 2,000 - 3,700

D-4 45° 90° 3,400 - 6,200

ou Qn° yyj 9 nnn — 9 A in

1 S° Ann - s 9nn

90° 90° 7,100

60° 90° 7,700

45° 90° 7,100

P-lOa/

90° 90° 10,200

60° 90° 9,000 - 11,300

45° 90° 11,300

P-6a/

90°

60°

45°

90°

90°

90°

6,400

6,200

7,100

SAND H-6

90°

40°

45°

90°

45°

90°

10,000 - 24,000

14,200 ->50,000

5,900 - 6,200

12,400 - 20,600

13,500 - 20,700

D-4

90°

45°

45°

90°

40°

90°

11,700 - 42,000

>50,000

10,100 - 16,400

>50,000

7,400 - 23,000

CLAY

H-6

90°

45°

45°

90°

45°

90°

7,600 - 13,700

13,600 - 21,200

8,500 - 9,300

9,000 - >50,000

12,000 - 40,000

D-4

90°

45°

45°

90°

45°

90°

13,500 - 42,500

8,500 - 21,300

1,700 - 14,700

20,000 - >50,000

3,500 - 5,700

a 3 In the swivel anchors which were installed vertically and pulled at an angle,

the cable aligned itself in the direction of the pull.

62
4.6.2 Correlation of Anchor Capacity with In Situ Tests

(1) General

Three in-situ test methods were used to determine soil properties: the Soil

Test Probe (STP); the installation torque of the anchor; and the Standard

Penetration Test (SPT).

(2) Soil Test Probe (STP)

Correlations between STP readings and coaxial pullout tests on vertical 6-in.

single helix anchors installed to their full depth are given in figure 4.21.

The STP torque correlated with the test results is the reading taken with the

tip of the instrument at 4 ft. depth, which is influenced by the shear strength

of the soil between the depths of 3 and 4 ft. It should be noted that while in

the silty soil the readings between 2 ft and 4 ft tip elevation did not tend to

increase very much, there was a steady increase in the torque with depth in the

sand.

The solid points in the figure indicate ultimate strength (Q^) and the open

points anchor load at 2 in withdrawal (P2v)* Load plotted because it

corresponds to the "load capactiy" as defined in ANSI Standard A119.3. Round

points are for silt, triangles for sand and squares for clay. Note that there

is a definite correlation between STP reading and pullout strength. For the

tests plotted, average can be estimated by the equation

Qu = 2300 + lit (eq. 4.1)

where: = average pullout strength in lb.

t = STP torque at 4 ft tip penetration in in. -lb.

A reasonable lower bound for is given by:

Qu > 1300 + lit (eq. 4.2)

Tests ST26 and ST25 were not considered in deriving eqs. 4.1 and 4.2. The loaddisplacement characteristics of Test ST26 were different from those of other

anchors, indicating that perhaps a root or some other object impeded the withdrawal. The STP reading for Test ST25 showed a sudden drastic increase in

torque at the 4 ft level, while the torque at other depths was relatively low.

Thus, it is reasoned that the probe hit an obstruction or bedrock. This condition would not increase anchor capacity. Note that in figure 4.21 all soil

types fall into the same pattern.

A reasonable lower bound for P2v is given by:

P2v > 600 + lit (eq. 4.3)

63
6" single helix anchors

•ST26

Average Qu

slower bound for Qn

• ST25

^-6

o o

Silt Sand Clay

Qu • A

P2V o

± J. 100 200 300 400

SP TORQUE, t, AT 4' TIP DEPTH, in-lb

500

Figure 4.21 Correlation between Soil Test Probe readings and coaxial load

capacity of vertically installed 6-in single helix anchors

64
The correlation between pullout tests of 4-in double helix anchors and

corresponding STP readings Is shown in figure A. 22. Note that the STP readings

were low for clay and high for sand when compared with the silt readings. The

explanation for the low readings in clay is that the readings were affected by

pumping action resulting from excess pore water pressure buildup and perhaps

sensitivity of the clay (see also discussion of shear strength prediction on page 70). The readings in sand were high, since the 4-in double helix is

basically a shallow anchor and its load capacity is influenced by the shear

strength of the soil between the depths of 0 and 2.5 ft (the load capacities

of the 6-in single helix anchors which are deep anchors is more closely related

to the shear strength of the soil near the helix). The STP readings were taken with the tip at 3 ft and the STP helix between 2 and 3 ft. This position

perhaps best characterizes the shear strength close to 3 ft depth. To get a better correlation, the STP reading was averaged over the 2.5 ft depth of the

lower anchor helix. This correlation is plotted in figure 4.23, and it can

be seen that the sand and silt tests fall into a consistent pattern. A reasonably conservative prediction could be made by this equation:

Since there is a definite correlation between the STP and anchor capacity, the

question arises whether the STP can also be used to measure the shear strength

of soils. This question was investigated using the "shallow" anchor tests,

i.e., those anchor tests which gave evidence that the failure surface extended

to the ground surface. These include anchors up to 3 ft deep (refer to figure

4.13 and table 4.8b). To calculate average shear strength, a cylindrical failure surface was assumed, extending from the helix plate to the ground surface.

Even though it has been shown that the actual failure mechanism is more complex

(for instance Balla [6]), the assumed surface is a possible mechanism, and the

shear resistance thus computed would therefore be equal to, or smaller than, the

shear strength of the soil and constitute a lower bound for the shear strength.

There is field evidence that the body of soil initially displaced may have been

greater than the assumed cylinder (refer to notes on soil mounds formed in

tables in appendix B.) However, as can be seen from figure 4.24, that experimental evidence does not preclude the assumed cylindrical surface as the primary

mechanisms

.

Figure 4.25 shows a plot of average shear resistance on the assumed cylindrical

failure surface against average STP readings. Tests C6 , SD24 and SD47 which

are for 4 ft deep anchors were included since there is evidence that, unlike

in the silt, the 4 ft deep, 6-in single helix anchors in the sand and clay

were on the borderline between deep and shallow anchors. A reasonable lower

bound for the shear resistance, which in turn is a lower bound for the in-situ

shear strength of the soil is given by the equation:

Ou >; 13t (eq. 4.4)

where: t is the torque averaged over the anchor depth

s = 5t (eq. 4.5)

65
100 200 300 400

STP TORQUE, t, AT 3' TIP DEPTH, in-lb

Figure 4.22 Correlation between Soil Test Probe readings and coaxial load

capacity of vertically installed 4-in double helix anchors

66
100 200 300

AVERAGE SIP READING, t, in-lb

Correlation between Soil Test Probe readings averaged over a 2.5 ft depth and the load capacity of coaxially loaded 4-in double

helix anchors

67
Figure 4.24 3 ft deep 6-in single helix anchor after pullout on

the sandy site

68
to 1500

CO^ 1000

g 500

4" double helix

6" single helix

Silt Sand Clay •

O A

SD47A' 'ASD24

^°C6

s=5t

JL 100 200

AVERAGE STP TORQUE, % in-lb

300

Figure 4.25 Relationship between STP measurements and the shear

resistance of soil

69
where: s = shear strength in psf

t = STP torque in in-lb

A statistical analysis of the results gives the following values:

for 10 tests in silt:

s = 6.4t; V = 0.20 (eq. 4.6)

for 6 tests in sand:

s = 4.82t; V = 0.13 (eq. 4.7)

for the combined sand and silt tests (16 tests)

s = 5.81t; V = 0.22

where: s = average shear strength in psf

t = STP reading in in-lb

V = coefficient of variation of shear strength

The results for the clay site are inconsistent, with s/t ranging from 4.88 to

22.67. The extremely low torque readings on 2 out of the 3 tests on the clay

site are attributed to porewater pressure buildup and resulting pumping action,

and perhaps sensitivity of the clay. Thus the STP may not be a good tool in

saturated clays.

The s/t ratio for the sand tended to be lower than that for the silt. A

possible explanation of this phenomenon is the fact that in the silt torque

did not change much between the depths of 1 and 3 ft. In the sand the torque

steadily increased with depth. The shear strength of the soil affects the STP

torque in two ways: by resistance at the lower tip of the STP; and by skin

friction exerted on the helix of the instrument. If the measurement is primarily affected by tip resistance, then the shear strength measured when the tip

of the STP is at 4 ft may be characteristic for the depths from 3.5 to 4.5 ft,

rather than for the depths from 3 to 4 ft as was assumed herein. This would

result in a lower s/t ratio if the soil strength increases with depth and the

average is calculated by the method used herein.

Further studies will be required to refine the use of the STP for the in situ

measurement of the shear strength of soils.

(3) Installation Torque, T

The correlation between installation torque and anchor strength is shown in

figures 4.26 and 4.27, for 6-in single helix and 4-in double helix anchors,

respectively. For the 6-in single helix anchors the scatter is considerable

70
100 200 300

INSTALLATION TORQUE, T, ft-lb

Figure 4.26 Relationship between installation torque and pullout strength

for vertical, coaxially loaded 6-in single helix anchors

71
100 200

INSTALLATION TORQUE, T, ft-lb

ure 4.27 Relationship between installation torque and pullout strength

for vertical, coaxially loaded 4-in double helix anchors

72
and there is no observed trend related to soil type. A reasonable lower bound

is given by the equation

Qu > 15T (eq. 4.9)

where: T = installation torque measured at maximum anchor penetration in ft-lb.

For the 4-in double helix there is a distinct difference between sand on one side, and silt and clay on the other side. A similar phenomenon was observed

for the Soil Test Probe (figure 4.22) where the difference was eliminated when

torque readings were averaged over the depth of the anchor. In the case of

installation torque such a procedure would not be practical. Thus installation torque may be misleading as a strength measure for shallow anchors in

soils in which shear strength increases rapidly with depth.

Equations for the lower bounds of T vs . 0^ are shown in figure 4.27. It should

be noted that manufacturers recommend the equation:

Qu <. lOT (eq. 10)

where is in lb and T in ft-lb

which is considered conservative for the data presented herein.

(4) Standard Penetration Test (SPT)

The Standard Penetration Test is generally considered to correlate well with

the shear strength of granular soils. However, in this instance, the exploration is shallow and drill stem lengths are therefore very short. It has been

shown [15] that for drill stem lengths less than 10 ft. the energy delivered to

the split spoon is extremely sensitive to the drill stem length. Thus, for this

shallow exploration, one should expect erratic results from the SPT. The

quantity of tests taken in this project does not permit a comparison of SPT

counts with the strength of individual anchors. However, a comparison between

STP readings and SPT blowcounts was made and is shown in figure 4,28. The

scatter in the figure is considerable and no useful correlation can be derived.

4.6.3 Theoretical Determination of Anchor-Load Capacity

(1) General

Several hypotheses have been advanced which correlate anchor-load capacity with

the in-situ shear strength and unit weight of the soil. All those hypotheses

distinguish between "deep" and "shallow" anchors. In deep anchors the failure

(slip) surface does not extend to the ground surface. In general, the ratio of

depth below the surface to anchor-plate width (D/B) is used to determined

whether an anchor is deep or shallow. The anchors tested in this project have

D/B ratios at full penetration depth which puts them close to the dividing

point between deep and shallow anchors. This somewhat complicates data

interpretation

.

73
600

500

400

S

^ 300

CO

^

200

100

0

III!

Silty soils

OCD

o

o

o o

8o

o

o

o

o

o

°

o

oo

— oo

<P o

o

o o

oO

o

o

1

1

1

1 5 10 15 20

STANDARD PENETRATION TEST, N, blows

25

Figure 4.28 Correlation between SPT blowcount and STP torque

readings for the silt site

74
Since all anchors are 4 ft or less, their strength is determined by the shear

strength and unit weight of the soil between 0 and 4 ft depth. As already

noted, soil shear strength in this depth range varies rapidly with depth and

is difficult to measure. This further complicates the problem of comparing

the test results with theoretical models.

(2) Comparison of Test Results with Uplift Capacity Equations

(a) Cohesive Soils

All the full-depth anchors will be considered in this section, even though it

appears that all the 4-in double helix anchors (because of the upper helix)

acted like shallow anchors, and the 6-in single helix anchors on the clay site

may have been on the borderline between deep and shallow anchors.

The following equation was proposed to calculate the load capacity of anchors in

cohesive soils [11]:

Qu = N^cA + S (eq. 4.11)

where A = projected anchor plate area

c = cohesive strength of soil

= an uplift capacity factor

S = resistance of anchor shaft

S is assumed to be very small and therefore can be neglected. Strictly speaking,

only the clay on Site C would act like a cohesive soil. The silt derives only

part of its shear strength from cohesion (or apparent cohesion). The other

part would be attributed to frictional resistance. However, due to the fact

that the deepest anchors are only 4 ft deep the confining pressures and thus

the frictional resistance should be small. This is further corroborated by the

characteristics of the depth vs. shear strength profile evident from the STP

readings and by the great ductility of the anchors tested in silt (confining

pressures caused by overconsolidation would be relieved as the anchor is pulled

out.)

The value of is generally assumed to increase with depth until the anchor is

a deep anchor and then to remain essentially constant. Typical values proposed

for are summarized by Davie and Sutherland [8, figure 7]. These values

range from 5 to 10 and tend to become constant for D/B ratios greater than 6

(3 ft depth for a 6-in single helix anchor).

Implicit in equation 4.11 with a constant value of is the assumption that

for D/B ratios greater than 6 the anchor capacity should be essentially proportional to the area of the anchor plate (barring some shape factors related

different plate geometries). In figure 4.29, the average load capactiy per

unit area of anchor plate in psi is plotted against the size of the anchor

plate in in^ for the coaxial tests on full-depth vertical anchors and the tests

on the self seating swivel anchors for the silt and the clay sites. These values

75
0 5 10 15 20 25 30

ANCHOR PLATE AREA, A, in^

Figure 4.29 Effect of anchor plate size on q in the silt and clay sites

76
should be constant in accordance with accepted hypotheses. However, as can be seen from the figure, there is a consistent trend for q to increase with

decreasing anchor-plate size. The trend is definitely not attributable to

changes in the soil profile. The swivel anchors were all installed at the same depth of approximately 50 inches and had D/B ratios of 8 or more. The smaller

helix anchors were shallower than the larger ones. If anything, this should

produce the opposite effect, since soil strength tended to increase with depth.

The fact that the 4-in double helix anchors acted like shallow anchors should

also produce the opposite effect.

There is at this time no satisfactory explanation for the trend observed in

figure 4.29. Similar trends have been observed by Tsangarides [18], pg. 186,

for anchors in sand. However, in that case, plate diameters were 2 in or smaller. Another interesting trend that can be derived from figure 4.29 is

that the value of for the swivel anchors is greater than that for the helix

anchors, and that the size effects for the swivel anchors are more pronounced.

Even though the shear strength of the soil changed with depth as well as location, it is of interest to try to determine values for the anchors in

the silt and clay sites.

On the basis of the laboratory tests, the c value of the clay on Site C is

between 700 (U-U triaxial test) and 965 psf (unconfined compression test).

Values calculated from the pullout tests of shallow anchors, using the simplified cylindrical surface are 570 psf for the 1 and 2 ft depths (6 in anchors),

720 psf for a 2.5 ft depth (4 in anchors), and 620 psf for a 3 ft depth (6 in

anchors). These values are reasonably consistent with each other.

Based on 700 psf shear strength in the 3 to 4 ft depth range, the following

values are calculated:

Anchor C-6 = 2800 lb., = 20

Anchor C-7 = 3800 lb., = 28

Anchor C-8 = 3650 lb., 27

These values of N^, as well as the trend for to increase with decreasing

anchor plate area are not consistent with accepted anchor capacity hypotheses.

There are two factors which may have increased anchor capacity: suction effects

(negative porewater pressures) associated with the large pullout displacements

and which did not dissipate during the test because of the low permeability of

the clay (such effects have been observed by others [1]); and root systems in

the soil,

i For the silt site, the laboratory test results are not as consistent as those

for the clay site. The unconfined compressive strength was 4000 psf, which

i would indicate a shear strength of 2000 psf. Shear strengths obtained from

j direct shear tests ranged from 400 to 800 psf. Lower bound shear strengths

calculated from the pullout tests, using a cylindrical failure surface ranged

77
from 430 to 1200 psf. Thus, It may be misleading to use any one value. Hierefore, is calculated in two ways:

1. Using 2000 psf on the basis of the unconfined compression test; and

2. Using a value of s = 6.4t for tests where t was measured.

The following results are obtained:

Using test probe readings:

The average value of N^j for the 6-in single helix anchors is 19 with a coefficient of variation of 0.2.

Using s = 2000 psf:

The average value of for the 6-in single helix anchors is 12.6 with

a coefficient of variation of 0.09. The average value of N^ for the 3-in

single helix anchors is 16.8 with a coefficient of variation of 0.17 and

the N^ values for the swivel anchors are 27.9 for the 6 1/2-in pipe, 23.6

for the 6 in arrowhead and 20.5 for the 10-in pipe.

If a cylindrical failure surface is assumed to be the failure mechanism it can be shown that

Nu = 4D/B* (eq. 4.12)

where D/B* is the D/B ratio at which the failure surface ceases to extend to

the ground surface.

It has been previously shown that for the 6-in single helix anchors on the

silt site the critical dep th3/ where the anchors cease to be shallow anchors

is between 2.83 ft and 3 ft (see figure 4.13) thus D/B* is somewhat smaller

than 5.5, and Nm calculated by eq. 4.12 would be somewhat less than 22.

Note that the values for the silt site are not inconsistent with those

obtained for the clay site. However, as in the case of the clay site, they are not consistent with hypotheses and data presented by others [8]. It should be

noted, however, that in accordance with available data from engineering studies

in the area the silt may have an angle of shearing resistance of as much as 30°, and thus the pullout capacity is not adequately predicted by eq. 4.11.

3/ Actually the concept of a clear demarcation between "deep" and "shallow"

anchors has been questioned. Davie and Sutherland [8] distinguished

three zones of D/B ratios: shallow - 0 < D/B < 2; intermediate:

2 < D/B < 4.5; deep: D/B > 4.5.

78
(b) Granular Soils

Anchor capacity on the sandy site should be compared with the pullout capacity

equation proposed for sands [11]:

Qu = YDNqu A

where y = in-situ unit weight of soil

D = depth of anchor plate below surface

Nqu = uplift capacity factor for granular material which is a function

of the angle of sheaing resistance (cj)) and the D/B ratio.

The only "deep anchors" tested in sand were the 6-in single helix anchors.

Thus size effect cannot be effectively explored. There is evidence [3] that

increases with depth at least to a D/B ratio of 14. Thus, there is no sharp dividing line between "shallow" and "deep" anchors.

The Nqu values calculated on the basis of the test results are given in

table 4.11 (tests conducted in the area overlain by the hard crust were not

considered):

Table 4.11 Uplift Capacity Factors for Full-depth Anchors on the Sandy Site

Range Coefficient of

Anchor Type D/B Number of Tests of Nq^ (Average) Variation of Nq^

H-6 7.5 5 74-84 75.6 0.07

D-4 7.5 3 71-84 77.5 0.08

The values in table 4.10 are quite consistent and the scatter is not very

great. The Nq^ values are high compared with other available data [3] (A cj) value of 31° was used for the comparison). However, there is considerable

scatter in the available data. The relatively high load capacity on the site

is attributed to overconsolidation which increases the shear strength by

increasing confining pressures (there was approximately 20 ft overburden

which was recently removed). The rapid loss of load capacity as anchors are pulled out is also attributed to overconsolidation.

Nqu ratios were also calculated for the shallow anchor tests and are given in

table 4.12. The values in table 4.11 can be compared with those for the fulldepth anchors. All the results are for 6-in single helix anchors on the sandy

site.

79
Table 4.12 Uplift Capacity Factors for 6-in Single Helix Anchors Installed

to Less Than Their Full Depth in the Sandy Site

Anchor Depth D/B No. of tests Range of Nq^ Average Nq^

2 ft 4 2 48-61 54

3 ft 6 2 72-88 80

3.75 ft 7.5 5 74-84 76

Unfortunately, there are not enough tests to determine whether the size effects

observed on the silt and clay sites also occur in sands. However, the consistency of the Nq^ values when comparing the full-depth 6-in single helix and 4-

in double helix anchors indicates that there were probably no size effects for

the anchors tested.

4.6.4 Determination of Load Capacity on the Basis of Pullout Tests in Similar

Conditions

The tests presented herein were performed on reasonably uniform sites.

Nevertheless, there were considerable variations in pullout strength on any one

site. Much greater variations should be expected if an anchor is certified

generically for some soil condition occurring over a larger region. The fulldepth vertical coaxial pullout test results are summarized in table 4.13 below

for the three sites to give an overview of the variability of test results

encountered. All the numbers are for in lb.

Table 4.13 Range, Mean, and Coefficient of Variation of the Load Capacities

of the Full-Depth Anchors

Site Anchor Type No. of Tests Range , lbMean , lb

Coefficient of

Variation

Silt 18 2800-6000 4740 0.18

D-4 12 1900-3200 2700 0.18

Sand H-6 10 2750-6825 5100 0.23

D-4 6 1530-3890 2390 0.40

Clay H-6 3 2800-3850 3430

D-4 3 1900-2000 1930

Table 4.13 was compiled without regard to special local conditions such as the

stiff crust covering part of the sand site and submerged areas, since such con- ditions should be expected to occur in practice. It can be seen that in most

instances, even for one site which was considered uniform, there is considerable

80
strength variation. The effects of the strength variation were encountered

during the cyclic tests on the clay site which was considered uniform. Cyclic

load levels were set in advance at what was thought to be 75 percent of the load

capacity as derived from adjacent static tests. However, in many instances the

anchors failed before these load levels were reached. Typically, the coefficient of variation for various test results tended to be about 0.2. It can be seen that it increased to as much as 0.4 when local variations within the site

are disregarded.

4.6.5 Effect of Submerged Conditions

The clay site was saturated, and therefore submerged conditions would not have

had much effect on load capacity. On the other two sites, effects of submergence

were explored. On the silt site, this was done in an area which was permanently

under water. On the sandy site an area was temporarily submerged during some

of the anchor tests. Results for the silty site are summarized in table 4.14,

Table 4.14 Comparison Between Regular and Submerged Anchor Tests

on the Silty Site

Anchor Type Condition No. of Tests Range Average V

of Qu, lb Qu. lb

H-6 Unsubmerged 12 4200-6000 5090 0.11

H-6 Sumberged 6 2800-5700 3980 0.25

D-4 Unsubmerged 6 2250-3100 2730 0.13

D-4 Sumberged 6 1900-3200 2660 0.29

H-3 Unsubmerged 5 1300-2050 1650 0.17

H-3 Submerged 5 1000-2625 1895 0.36

It can be seen from the above summary that only the average strength of the

6-in single helix anchors was reduced by submergence. For all three anchor

types, however, there was greater variation in the submerged test results and

some individual submerged tests showed substantially reduced strength. It is

suspected that the shear strength of the soil was actually reduced by submergence, but that some individual anchors had increased resistance because of the

presence of some boulders in this area, and also possibly because of suction

effects resisting the pullout.

81
The submergence tests in the sand did not result in strength reduction because

it was impossible to submerge a large enough area to eliminate seepage forces

(piezometric heads at anchor plate elevation extended only 0.5 ft above the

anchor plate).

It is assumed that submergence should substantially reduce the resistance of

anchors in granular soil, but that it does not necessarily affect the cohesive

strength. However, no consistent trend emerges from the test data presented

herein.

4.7 CYCLIC TESTS

4.7.1 Cyclic Tests on the Silt Site

Typical test results are shown in figure 4.30. Specimen STlll was loaded to

what was estimated to be 0.75 Q^. Note that most of the displacement took

place in the first load cycle and no further residual displacement occurred

after 100 load cycles. This phenomenon is the result of gradual compaction

causing the displacement to be entirely elastic after 100 load cycles. Similar

load-displacement curves resulted at 50 and 25 percent of the estimated pullout

load (tests ST116 and 118). Specimen ST123 was preloaded to 0.84 and subsequent load cycles were applied at 0.67 Q^. Note that was overestimated for

his specimen, resulting in a preload which was^ close or equal to the ultimate

load (the pullout load after cyclic loading, Q^, was less than the preload).

Nevertheless, the total displacement after 200 cycles was only 1.2 in. However,

unlike in the other tests plotted, the preloaded specimen had small residual

displacements for each load cycle up to 200 cycles. The tests on the silt site

also included two tests on preloaded 4-inch double helix anchors. These tests

had no further residual displacements after 100 load cycles.

It is of interest to consider whether the load capacity of the anchors was diminished as a result of the cyclic loading. The five 6-inch single helix

anchors which were subjected to high cyclic load (0.75 vs. 0.67 Q^) had an averged pullout strength = 5240 lb. with a coefficient of variation

V = 0.09. This compares with an average pullout strength of 5090 lb and v = .11

for the anchors which were not subjected to cyclic load. Thus loading of up to

300 cycles of 0.75 apparently had no significant effect on the pullout

strength of the anchors. Indeed some of the anchors were subjected to cyclic

loads as high as 0.9 Q^, since the actual pullout strength was not known when

the cyclic load was applied (specimen ST123 was probably preloaded to ultimate

and thereby weakened).

The total cumulative displacement of the preloaded anchors was well within

limits acceptable in present standards (see section 4.8).

4.7.2 Cyclic Tests on the Sandy Site

Typical test results from the sandy site are shown in figure 4.31. Tests SD34

and SD35 are unpreloaded and preloaded axial tests, and tests SD38 and SD40 are

82
1

Pc = 4500lb

1 /"^^^ ffu^SOOOIb

6" Single helix anchors

SUty soy

Pc = 3000 lb

/ Pp = 4737 lb /Sn23

Pc = 3778 H) Q'u = 4550lb

Pc = 1500lb

1 1 100 2W 300

NUMBER OF CYCLES

Figure 4.30 Cyclic load tests in silty soils

83
T

-J L.

100 200

NUMBER OF CYCLES

Figure 4.31 Cyclic load tests in sandy soils

84
unpreloaded and preloaded tests on vertical anchors pulled at 40° to the

horizontal. Note that on the sandy site there was also some compaction effect,

but there were small residual displacements in each load cycle up to the 200

load cycles applied in the test. The total cumulative displacement of -the preloaded specimens after 200 load cycles of 2/3 the ultimate load was approximately

2 in. If the assumption is made that 2/3 of ultimate would be the maximum

design load that can be reasonably permitted, and when the effect of these 200

cycles is compared with the hurricane history described in section 3.3.2(4),

and the design load is compared with the "maximum" wind load, it is conservatively estimated that a similar hurricane would have resulted in a cumulative

anchor head displacement for preloaded anchors of not more than 1 inch (see

also section 4.7.3).

The effect of cyclic loading on anchor-load capacity is somewhat difficult to assess from the test data. For the vertical tests, four 6-inch single helix

anchors had an average failure load of 4675 lb. with v = 0.07. This com- pares with = 5290 with v = 0.08 for the sand site if submerged tests are excluded. For the 4-inch double helix anchors, the average was 2325 lb.

This compares with 2390 lb. with v = 0.4 for all the tests in sand, but only

1610 lb. with V = 0.08 if tests if the area of the dense crust are excluded.

Thus no conclusive trend emerges from these tests.

Many of the inclined tests, when pulled out after cyclic loading failed by

hardware failure rather than pullout (helixes broke off). The loads resisted

before hardware failure tended to exceed the average static load capacity under

this type of loading. Only in test SD13 was there a pullout as a result of

strength deterioration by cyclic loading. The overall conclusion that can be

drawn is that 200 cycles of 2/3 of the ultimate load are not likely to cause progressive anchor failure in either of the two loading modes used or to substantially weaken load capacity. However the anchor hardware will be weakened

by the cyclic load in the inclined loading mode, and progressive soil failure

could occur if the applied cyclic load approaches the load capacity of the

anchors

.

4.7.3 Cyclic Tests on the Clay Site

Typical test results are shown in figure 4.32. Specimens C24 and C25 are vertical anchors coaxially loaded to what was thought to be 0.75 Q^. Both

anchors experienced progressive failure. Specimen C26 is a preloaded specimen

which performed well. However, its companion specimen, C27 (not shown) which

was similarly preloaded, experienced progressive failure. An examination of

the preloading curves of specimens C26 and C27 , shown in figure 4.33, indicates

that C27 experienced yielding during preloading. Thus the preload was very

close to the ultimate load.

It is interesting' to note, when comparing tests C25 and C26, that on the clay

site the preloading effect did not occur in the initial load cycle, but rather

tended to be gradual. This is attributed to the fast rate at which the cyclic

load was applied. This loading rate did not permit enough time for the full

85

Figure 4.33 Comparison of the preloading curves of tests C26 and C27

87
displacement to occur in the first few load cycles. The load, rather than being

resisted by the soil skeleton, induced porewater pressure gradients. The displacement occured gradually over many load cycles as these porewater pressure

gradients dissipated.

Curve C41 in figure 4.32 is for a preloaded vertical anchor subjected to

inclined pull. Note that, even though this specimen was preloaded, it experienced a movement of 4 inches during the 200 applied load cycles. Some of the

anchors in clay failed before reaching 200 load cycles. Of eight 6-in single

helix anchors, four failed before reaching 200 load cycles. If we divide these

into preloaded and unpreloaded anchors, 3 of 4 unpreloaded, and 1 of 4 preloaded

anchors failed. Of the eight 4-in double helix anhors tested, two of the four

unpreloaded anchors and none of the four preloaded anchors failed.

Anchor C33 , a 4-inch double helix, was loaded to 450 cycles in order to ascertain

whether failure could be induced in anchors which perform satisfactorily for

200 load cycles. The results of test C33 are plotted in figure 4.34. Note

that in the 10 to 200 cycle range the creep increment per cycle was about constant. After 200 cycles, the specimen deteriorated and failure occurred at

450 cycles. From the preloading curve, it appears that this specimen was loaded to 80 percent rather than 67 percent of ultimate. It is reasonable to

assume that at these high cyclic loads all specimens would fail if enough load

cycles are applied.

It is difficult to determine whether the load capacities of the specimens which

did not fail were impaired by the application of 200 load cycles, since all the

weaker specimens failed. Perhaps the best information can be derived from the

coaxially-tested 4-in double helix anchors, which all survived the cyclic

test (except that the test on anchor C33 was continued for 45(j) cycles until

failure occurred). The three anchors tested had an average 0^ of 2067 lb

This compares with an average load capacity of 1930 lb for the anchors tested

statically. Since the variability of these test results is very small

(v = 0.07 for the cyclic tests, and 0.03 for the static tests), this is taken

as an indication that the load capacity of the anchors was not significantly

affected by the cyclic tests.

The question should be asked whether cumulative displacement caused by windload

effects would be within tolerable limits. Looking at the wind data in section

3.3.3(4) and assuming that increments of displacement would be approximately

proportional to increments of load and that the cyclic load applied is equal

to the design load, and thus the maximum windload experienced in the storm,

the storm would cause a cumulative displacement of less than 100 cycles of the

design load (this is a very conservative estimate). This would result in a cumulative displacement of 3 1/2 inches for anchor C41 and of less than 1 inch

for anchor C26. Even though these displacements are considerable, they are not

considered excessive for the extreme conditions assumed.

It is of interest to compare the anchor performance under cyclic load at the

clay site with the results of other studies of the cyclic shear strength of

88
^ 4

2

-

4" Double helix anchor

Clay

Pp = 1230 lb

Pc = 980 lb

200 300

NUMBER OF CYCLES

400

Figure 4.34 Results of the cyclic loading test of specimen C33 on

the clay site

89
clays. Seed and Chan [16] studied three different clay types and found that,

for loading conditions similar to those of the anchor tests (no shear stress

reversals) the shear strength under 100 load cycles varied from 70 percent of

the static shear strength for a soft sensitive clay to 80 percent of the static

shear strength for compacted sandy clays. These findings are compatible with

the results of this study and give further corroborations to the fiding that

anchors in clays can survive 100 cycles of 67 percent of their failure load

without failure.

4.8 COMPARISON OF ANCHOR PERFORMANCE WITH PRESENT STANDARD REQUIREMENTS

ANSI Standard A119.3 [2] requires that anchors resist a load of 4725 lb without

failure, where failure is defined as a 2-inch displacement of the anchor head

in the vertical direction or a 4-inch displacement in the horizontal direction.

The HUD Mobile Itome Construction and Safety Standard [9] sets even more conservative requirements by stipulating that loads be increased by 50 percent for

the design of foundations. Hereafter these requirements are compared with the

test results.

On the Silt Site (refers to table B.l), P2v for the vertical full-depth 6-inch

single helix anchors ranged from 3000 lb to 5750 lb and averaged at 4270 lb

with V = 0.19. The average, as well as the lowest strength, are lower if the

submerged tests are considered. The resistance of the diagonally loaded vertical anchors at 4-inch horizontal displacement and 45° pull ranged from 1100 to

1300 lb and averaged 1260 lb with v = 0.16. The values for the 4-inch double

helix and the swivel anchors are not listed here since they fall far short of

required capacities.

On the Sandy Site (refers to table B.3), P2v for the vertical full-depth 6-inch

single helix anchors ranged from 3900 lb to 6000 lb and averaged 4800 lb with

V = 0.13. The resistance of the diagonally loaded vertical anchors at 4-inch

horizontal displacement and 40° load inclination ranged from 2800 to 4000 lb

and averaged 3200 lb.

On the Clay Site (refers to table B.5), P2v for the vertical full-depth 6-inch

single helix anchors ranged from 2300 lb to 3500 lb and averaged 3100 lb. The

resistance of the diagonally loaded anchors at 4-inch horizontal displacement

was negligible.

Thus, even though the sites selected were competent sites, anchor capacity fell

far short of present standard requirements. Even on the sandy site, where

anchors were relatively stiff and the average performance of the vertical

anchors met standard requirements, many individual anchors did not meet the

requirements. Not a single diagonally loaded specimen met the standard

requirements.

It can be concluded from the test results that presently used anchor technology

with present installation procedures did not deliver the performance required

by ANSI Standard A119.3. '

90
4.9 PERFORMANCE OF ANCHOR HARDWARE

In coaxial pullout, there were relatively few anchor hardware failures at loads

lower than the stipulated 4725 lb capacity. In four instances, anchors failed

below the 4725 load level, always by a break in the weld which connects the

hel ix to the shaft. Two of the four failures occurred in cyclic tests. Many

of the noncoaxially tested specimens failed because of anchor hardware failure.

But these failures occurred at very high load levels and were in part caused

by the fact that the soil resistance was extremely high.

Most anchors withdrawn had bent helixes (mushroom shaped). While the bending

of the helix did not cause anchor failure, it may well have reduced the load

capacity of the anchors and increased displacements. Almost all the anchors

withdrawn had their paint stripped off. The paint stripping probably occurred

during insertion. Thus it is concluded that painting does not provide effective

corrosion protection. In the anchors which were subjected to non-coaxial pull,

the anchor shaft was severely bent (see figure 4.7). Figure 4.35 shows typical

anchor hardware failures.

The conclusions that can be drawn from the anchor hardware performance are that

anchors should be galvanized or otherwise effectively protected against corrosion. The corrosion protection should not be damaged by Installation and remain

effective where yielding occurs during anchor installation or loading, i.e., on the anchor shaft and the helix. Another conclusion that can be drawn is that

the load capacity of anchors could probably be improved by using a thicker helix

plate that does not bend during withdrawal, and that during fabrication care should be exercised to insure the integrity of the weld between the anchor

shaft and the helix plate.

91
FACTWG PAGE: Tfie "Soil Tut PAobe," u^^d to pn.^dlct anchortoad capacAXy aj> aJii>o a poto^nttatly iii>^(<^uil dzvtcQ. {jOH. thd -ln-6tta m(iai>uJtmzYit o{^ thd

6h2,aA 6tn.mgtk o^ 6ott6 cut a i>kaZJioM d(ipth.

Figure 4.35 Typical anchor hardware failures

92
5. SUMMARY OF CONCLUSIONS

5.1 GENERAL

The findings presented herein are based on tests conducted in this project.

Since soil is not a man-made material, the anchor behavior observed is not

necessarily characteristic for all the sites that will be encountered in

practice.

93
5.2 VIRGIN LOAD-DISPLACEMENT CURVES

All anchors tested had a unique virgin load-displacement curve which depended

on the characteristics, installation depth and loading mode of the anchor and

the soil conditions. The virgin load-displacement curve is a strength envelope

which can not be changed by a limited number of intermediate unloading and

reloading cycles, except that each unloading and reloading cycle will result in

a small residual displacement. However, if a great number of intermediate load

cycles was applied at a load level close to the pullout strength of the anchor

they did in some instances cause incremental failure on the sand and the clay

site. The virgin load-displacement curves observed were a unique function of

the installation depth of the anchor. For instance, if an anchor is installed

at the depth of 3 ft and withdravra to the depth of 2.5 ft, unloaded and subsequently withdrawn, its load-displacement curve during withdrawal will differ

from that of an anchor which is initially installed to a 2.5 ft depth and then

withdrawn.

5.3 RELOADING CHARACTERISTICS

When an anchor is loaded to a certain load level and then unloaded and reloaded,

the secant reloading modulus (reload/displacement caused by reload) will be

several times larger (stiffer) than the secant modulus of the virgin loading

curve. Thus, a preloaded anchor will have much more favorable load-displacement

characteristics than an anchor which is loaded for the first time.

5.4 EFFECTS OF LOADING CONFIGURATION

Helix anchors installed vertically and pulled at an angle had a virgin loaddisplacement curve which exhibited much less stiffness than that of coaxially

loaded anchors. However, the reloading characteristics of these anchors were

superior to those required in present standards, even though they were adversely

affected by elastic rebound of the anchor shaft. Helix anchors installed at an

angle and withdrawn coaxially developed less load capacity than anchors

installed vertically and pulled coaxially. The load capacity of coaxially

loaded inclined anchors was roughly equal to that of coaxially loaded vertical

anchors installed at the same helix depth below the ground surface (the difference in load capacity reflected the difference in embedment depth).

Vertically-installed helix anchors pulled at an angle to the vertical developed

higher load capacities than coaxially loaded vertical anchors. Their load

capacity increased as the angle of withdrawal with the horizontal was decreased.

However, their resistance to displacement in the initial loading stages was very low.

Inclined helix anchors loaded at a 90° angle to the shaft had very low load

capacities and low resistance to displacement.

Swivel anchors pulled at an angle to the vertical had either the same load

capacity as vertically-pulled anchors (triangular anchors) or their load

94
capacities decreased with a decrease of the angle of pull with the horizontal

(pipe anchors).

5.5 EFFECTS OF SOIL TYPE

Anchors installed in the silt site had considerable ductility. (They could be

withdrawn for a relatively large distance without a reduction in load capacity.)

Anchors installed in clay had also considerable ductility, but their load capacity reached a peak and gradually decreased as they were further withdrawn.

Anchors installed on the sand site lost their load capacity abruptly upon

further withdrawal after their peak load capacity was reached.

5.6 PREDICTION OF ANCHOR LOAD CAPACITY BY IN-SITU TESTS

(1) Soil Test Probe

Soil Test Probe readings did correlate with anchor capacities, except that on the clay site some of the Test-Probe readings were abnormally low, apparently

because of porewater pressure buildup. The capacity of the 6-inch single helix

anchors can be reasonably correlated with Test Probe readings where the tip of

the test probe was near the helix. The capacity of the 4-inch double helix

anchors can be reasonably correlated with the average test probe reading over

the depth of the anchor. There appears to be a good correlation between the

in-situ shear strength of the soil and the test probe reading on the sand and

the silt site. The results from the clay site were erratic.

(2) Installation Torque

It appears that it is possible to determine a lower bound for anchor pullout

capacity from measurements of the installation torque at maximum penetration.

However, the scatter of the data is considerable and the lower-bound prediction

is too conservative to be of practical value.

(3) Standard Penetration Test

It does not appear that the Standard Penetration Test is a useful tool for

predicting the strength of shallow anchors, mainly because of the short drill

stem length used in shallow depths.

5.7 THEORETICAL PREDICTION OF ANCHOR-LOAD CAPACITY

Theoretical prediction of anchor-load capacity can only be as good as the

estimate of the in-situ shear strength of the soil. Since in-situ shear

strength at shallow depths is difficult to determine, the practical

applicability of theoretical models is limited.

Correlation of the test results on the silt and clay sites with presently used

theoretical pullout capacity models was poor. It was observed that anchor

capacity per unit area of anchor plate increases as the anchor-plate area

95
decreases. It was also observed that the swivel anchors have a higher load

capacity per anchor-plate area than the helix anchors. In general, anchor load

capacities were much higher than those that would be predicted on the basis of

existing theoretical models and available data on soil-strength characteristics.

Correlation of the test results on the sandy site with presently used

theoretical models was poor because the sand was overconsolidated and possibly

cemented. Effects of depth and anchor plate size were similar to those predicted by theoretical models, but the anchor capacities were much higher than

those that would be calculated on the basis of available data on soil-strength

characteristics

5.8 PREDICTION OF ANCHOR LOAD CAPACITIES ON THE BASIS OF TEST ON SIMILAR

SITES

The coefficient of variation of anchor strength on the sites ranged from 0.18

to 0.40. There probably would be more variation if anchor test results from

one site are used to predict anchor strength at another site.

5.9 ANCHORS SUBJECTED TO CYCLIC LOAD

On the silt site, anchors tended to stabilize after 100 load cycles and additional

cycles caused no further creep. On the sand and clay site, creep displacement

continued indefinitely and failure could be induced if enough load cycles are applied. Failure actually occurred in some specimen where the applied cyclic

load was close to the anchor load capacity.

The cyclic-load performance of anchors, preloaded to a load higher than the

applied cyclic load was superior to the performance of unpreloaded anchors.

There was no evidence that the pullout strength of anchors was reduced by

applying 200 cycles of about 2/3 of their pullout strength.

It appears that on all three sites anchors could survive the effects of a major

hurricane with displacements smaller than those permitted in the present ANSI

Standard (2-inch vertical and 4-inch horizontal), provided that the maximum

wind load effect does not exceed 2/3 of the pullout strength of the anchor and

the anchors are preloaded.

5.10 COMPARISON OF ANCHOR PERFORMANCE WITH PRESENT STANDARD REQUIREMENTS

Presently-used anchoring technology did not provide the anchor performance

required by ANSI Standard A119.3, neither in terms of load capacity, nor in

terms of load-displacement characteristics.

5.11 PERFORMANCE OF ANCHOR HARDWARE

Anchor hardware generally developed the required load resistance. After

installation, painted anchors do not seem to have effective corrosion protection because of paint stripping. Most anchor helixes were bent after anchor

96
withdrawal, Indicating that anchor performance could probably be Improved by

thicker helix plates.

The shafts of anchors Installed vertically and withdrawn at an angle were severely bent before the anchors reached their maximum load capacity.

Most anchor hardware failures occurred by a failure of the weld between the

shaft and the helix plate. Some of these failures were Induced by cyclic

loading.

5.12 USE OF SOIL ANCHORS IN PERMANENT MOBILE HOME FOUNDATIONS

The possibility of using anchors In permanent mobile home foundations has

recently received some consideration.

It is evident from the test results, that if anchors are to be included as part of a permanent foundation they must have adequate corrosion protection

to retain their structural integrity throughout the service life of the

mobile home and they should be preloaded to insure adequate performance

under anticipated extreme loads.

Such anchors would also have to be adequately protected against potential

effects of frost heave.

97
FACIWG PAGE: VuIZoat t£At o^^ ^ubmoAQnd anckon. lYiAtatldd

98
6. RECOMMENDATIONS

6.1 REQUIRED LOAD CAPACITY

Recommendation ;

It Is recommended that the requirement for a 4725 lb load capacity for anchors

be abandoned. Instead, It Is recommended to stipulate the required total working

load that the anchoring system must resist, and then determine the number of

anchors required to achieve this performance on the basis of anchor capacity

99
that can be achieved at particular sites. This requirement will have to be

coupled with a maximum allowable spacing requirement to avoid unreasonably wide

spacing in dense soils. The implementation of this approach will require an initial estimate of the anchor capactiy at an installation site either by a load test or by previous experience. To avoid unnecessary costs the allowable

working load for the anchors could be established during the pre-loading of

the first anchor installed on the site.

Commentary ;

As a result of this test program, it was determined that on the sites selected,

which had competent soils, existing anchor technology did not provide the

required 4725 lb load capacity required by ANSI A119.3.

6.2 INSTALLATION REQUIREMENTS

Recommendations ;

The following procedure is recommended:

1. Each anchor installed must be preloaded to 1.25 its working load.

2. One anchor per mobile home, or three anchors per site where the soil

conditions are uniform, must be preloaded to 1.5 the working load.

3. The working load (P^^) is defined as the anchor load induced by the design

wind pressure (without the 50 percent increase required by HUD) [9].

A suggested preloading procedure for diagonally loaded anchors is shown

schematically in figure 6.2. Loading devices for vertically loaded anchors are commercially available.

Commentary ;

Figure 6.1 illustrates the intent of the recommended procedure. The dashed

curve is the loading curve of specimen C7. The load capacity Ou ~ 3.8k, preload Pp = 3.17k and working load P^ = 2.53k. If the specimen is preloaded in

accordance with the recommended procedure, the reloading modulus will be high

and the anchor performance, accordingly, excellent. Had the specimen been preloaded to , it is conceivable that the load capacity upon reloading would be

less than Q^. The 1.5 safety margin is intended to provide sufficiently high

probability that the anchor will resist the working load. The 1.25 P^ preload

will insure good anchor performance. Note that the reloading modulus would be

much lower if the preload were only 1 P^^.

The provision that some of the anchors should be loaded to 1.5 P^ is to provide

some assurance that the preload will not approach the anchor-load capacity since

this could weaken the anchors. Another way in which this could be accomplished

100
1

6^ Smgle helix anchors

Clay

Pp = ai7k I

Qu=a8k

I

I1/

Pw = 253k 1/

IIn

II Reloading modulus ||

i|

lii

I L_

I /

/

/

DISPIACEMENT, in

Figure 6.1 Illustration of the recommended preloading requirement and

the resulting anchor performance

101
Figure 6.2 Suggested preloading procedures for diagonally loaded anchors

102
would be to stipulate that the preload be held for five minutes without measurable creep (say not more than 1/4 inch). However such a procedure is considered

too cumbersome.

It is conceivable on sites where the variability of anchor capacity is high

that in some instances Pp will be as high as Q^. A good installer would probably decide in such a case that he needs more anchors and would reduce his

preload. However, if he keeps pulling in order to try to reach the preload, he

may pull out his anchors too much during the preloading process. This could be

forestalled by stipulating a maximum allowable displacement during the preloading. However, it is difficult to come up with a displacement magnitude which

could apply to all anchor types and soil conditions. A stipulation that preloading should be discontinued if an anchor moves more than two inches without an increase in applied load would probably provide adequate protection against

overloading.

The test results also indicate that anchors preloaded by the recommended

procedure would perform adequately under wind-induced cyclic loading.

6.3 CORROSION PROTECTION

Recommendat ion

Anchors should have adequate corrosion protection for the service life of the

mobile home. The corrosion protection should remain effective after the anchor

is subjected to inelastic deformations similar to those anticipated during preloading. Painting or any other coating that could be damaged by installation

and preloading is not acceptable.

Commentary ;

Most anchors tested lost much of their coat of paint during installation.

Galvanized anchors would probably perform adequately, but there is concern that

the anchor may become vulnerable to corrosion because of deformations induced

by preloading or service load. All diagonally loaded anchors experienced large

deformations in the shaft, and most anchors tested experienced deformations in

the helix.

6.4 ANCHOR HARDWARE CAPACITY

Recommendations ;

Anchor hardware should resist two times the service load without rupture.

Inelastic deformations are permitted if it can be demonstrated that the

durability of the anchor will not be impaired.

103
Commentary ;

It is unrealistic to expect that anchor hardware should not experience inelastic

deformations as the anchor adjusts to the applied load. The safety margin of

two is to insure that welds will not fail during anticipated cyclic loads.

lOA
7 . REFERENCES

[ 1] Adams, J. I. and Radhakrishna, H. S., Uplift of Auger Footings in Fissured

Clay, Ontario Hydro Research Quarterly , Vol. 22, No. 1, 1970, pp. 10-16.

[ 2] ANSI Standard 119.3 (NFPA No. 501), Standards for the Installation of

Mobile Homes, National Fire Protection Association, Boston, MA, 1975.

[ 3] Andreadis, A. et al., "Embedded Anchor Response to Uplift Loading, J . of

Geotechnical Engineering Division of ASCE , Vol. 107, No. GTl, Jan. 1981.

[ 4] ASTM Designation D 1586-67 (Reapproved 1974), Standard Method for

Penetraion Test and Split-Barrell Sampling of Soil, American Society for

Testing and Material, Philadelphia, PA, 1967.

[ 5] ASTM Designation D 2487-69, (Reapproved 1975), Standard Method for

Classification of Soils for Engineering Purposes, ASTM Standards, Part 19,

American Society of Testing Materials, Philadelphia, PA, 1975.

[ 6] Balla, A., The Resistance to Breaking Out of Mushroom Foundations for

Plyons, Proceedings, 5th International Conference of Soil Mechanics and

Foundation Engineering, Vol. 1, pp. 567-577, Paris, France, 1961.

[ 7] A. B. Chance Co., Encyclopedia of Anchoring, Bulletin 424, Centralia,

MO, 1969.

[ 8] Davie, R. J. and Sutherland, H. B., Uplift Resistance of Cohesive Soils,

J. Geotechnical Engineering Division , ASCE, Vol. 103, No. GT9, Paper

113223, Sept. 1977.

[ 9] Department of Housing and Urban Development, Mobile Home Construction and

Safety Standard, Federal Register, Vol. 70, No. 244, Title 24, Chapter II,

Part 280, Subpart D, November 1979.

[10] Harvey R. C. and Burley, E., Behavior of Shallow Inclined Anchorage in

Cohesionless Sand, Ground Enginerring , Vol. 6, No. 5, 1973, pp. 48-55.

[11] Kananyan, A. S., Experimental Investigation of the Stability of Bases of

Anchor Foundations, Soil Mechanics and Foundation Engineering , No. 6,

1966, Moscow, U.S.S.R., June 1966.

[12] Kovacs, W. D. and Yokel, F. Y., Soil and Rock Anchors for Mobile Homes,

A State-of-the-Art Report, NBS Building Science Series 107, National

Bureau of Standards, Washington, D.C., October 1979.

[13] Marshall, R. D., The Measurement of Wind Loads on a Full-Scale Mobile

Home, NBSIR 77-1289, National Bureau of Standards, Washington, D.C.

September 1977.

105
[14] Meyerhof, G. G., Uplift Resistance of Inclined Anchors and Piles,

Proceedings, VIII International Conference of Soil Mechanics and

Foundation Engineering, Vol. 21, pp. 167-172, Moscow, U.S.S.R.,

August 1973.

[15] Schmertmann, J. H. and Palacios, A., Energy Dynamics of SPT, Journal

,

Geotechnical Engineering Division , ASCE Vol. 105, No. GTS, August

1979.

[16] Seed, H. B. and Chan, C. K., Clay Strength Under Earthquake Loading

Condition, Journal, Geotechnical Engineering Division , ASCE, Vol. 92,

No. SM2, March 1966.

[17] Texas Department of Labor and Standards, Mobile Home Division, Standards

and Requirements 0.63.55.07, Texas Register, Vol 3., No. 78, Austin, TX,

October 1978.

[18] Tsangarides, S. N. , The Behavior of Ground Anchors in Sand, Doctoral

Thesis, University of Londong, Queen Mary College, January 1978.

[19] Yokel, F. Y., Chung, R. M., Yancey, C. W., NBS Studies of Mobile Home

Foundations, NBSIR 81-2238, National Bureau of Standards, Washington,

D.C., March 1981.

[20] Yokel, F. Y, Yancey, C. W., Mullen, C. L., A Study of Reaction Forces on Mobile Home Foundations Caused by Wind and Flood Loads, NBS Building

Science Series 132, National Bureau of Standards, Washington, D.C.,

March 1981.

106
8 . ACKNOWLEDGMENTS

The contribution of the following persons is gratefully acknowledged:

Mr. James McCollom of HUD monitored the NBS contract and made many constructive

suggestions; Dr. William D. Kovacs of NBS conducted the state-of-the-art study

which recommended this work and contributed to the planning and interpretation

of the tests; Mr. Christopher L. Mullen participated in the design of the test

rig; Mr. J. T. Odom of A. B. Chance Co. reviewed the design of the test rig;

Mr. Erik D. Anderson and Mr. Mike P. Glover participated in the testing work in

the field; MHA corporation, Minuteman Corporation, A. B. Chance, Tiedown Engineering and Abema, Inc., supplied anchor hardware and installation devices,

gave advice, and installed some of the anchors. Some of this assistance was rendered at no cost to the government. Schnabel Engineering Corporation identified two of the test sites and conducted the soil exploration on all the test

sites

.

107

APPENDIX A

Test Sites

A.l. Introduction

Appendix A Contains information on anchor location and subsurface

information on three test sites. Site A is in silty soil; Site B is in

sandy soil; and Site C is in clay soil.

Figures Al through A4 show the location of anchor tests, test borings and

test pits. The anchor-location coordinates are given for each test identified in Appendix B. Tests Wl through W17 on Site A are located in a

drainage swail where the soil is permanently submerged.

There are two separate test areas on Site B. Area B2 was chosen because part

of Area Bl was overlain by a dense crust and thus some of the test locations

were not utilized. A test pit was dug on Site B2 to explore soil conditions.

The appended soil exploration reports contain the logs of the borings shown

in the location maps, as well as laboratory test results. Information on

Soil Test Probe readings is included in the tables in Appendix B.

109
Figure A.l Anchor Test and Boring Locations on Site A

110
9

8

7

6

5

4

3

2

1

B2

15'

B4

B3'

A B C D E F G H J

-j- B1 thru B4, TEST BORINGS

SITEl

N

Figure A. 2 Anchor Test and Boring Locations on Site B.l

111
N

11

10

9

8

7

6

5

4

3

2

1 K L

TEST PIT TP1

rpi

10-

.= 20-

I

30-

a

40

50 H

M N P Q

SITE B2

WHITE SAND

BROWN SAND

WHITE SAND —Mmu SAND

WHITE SAND

^SAnT~COURSE CLAYEY

SAND

SANDY CLAY. SOME GRAVEL

CLAY

Figure A.

Anchor Test and Test Pit Locations on Site B.2

112
CO

H

G

C

B

^C1

C2

C4

2 3 4 5 6 7 1I 9 1 D 11

.C3

10(a)5'=50'

45' to edge of pavement

-f- CI thru C4, TEST BORINGS

Figure A. 4 Anchor Test and Boring Locations on Site C

113
A.2 Soil Exploration Reports

114
M

O

O

•S EVa 53

llflS i-H U3 f-t O -rt U m •a > uV U 4i

ft) tM » (0 g g O £

00 kT)

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tri *-»

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V

u 4 4-1 U < o 1

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4.1 10 •a . s SI 8

115
biclonr* (2) Coactmct to. M79M3

SU>tlARY or SOIL LABQgATORT TESTS

orlni

No. Dapth

Saapla Typ«

Saacrlptloa

of Sou

Spaclaan

Percant Paaalng

No. 200 Slave

At

I,

carl lal >arg f

Natural Molatura

Natural

Danaltjr (pcf) Soil LL PL PI (X) Vat Br, Paraaatara taurka

A-S l.S' Jw CLATR SILT, soaa

flna aaad, browi (HL) 39 27 12 17.6 Bottos of Saapla

A-7« 0.S-2.S 3" TubaCL&TET SILT, aow

flna aand with quart!

fraisanta, bronB 00.)

61.9 19.

Saa Gradation Cum

4- 7a i-i 3" CLMm SILT, aow

flaa aaad with quarti

fiacaaDta, brovn (ML)

S3.0 21.6 lie. 7 97.« • - 30<» c • 200 paf qu- 4029 paf

« a • 4. OX

Saa Gradatloo, OncoBfinad Coapraaaloci A Mract Shaar Curraa

otaa)

1. (oil taat la aecerdnca with appllcabla 3. Vlaual Idantlflcatloo of aaaplaa la Am ttandarda.

toll claaalf Icatlse ayabola ara ill aecordanca with Unlflad SoU Claaalflcatlon Sjrataa, baaad on taatlnf Indlcacad

and *laual Idaatiflcatloa.

In aecordanca altb tba ayataa uaad

hy Chla flra.

Kay to abbrarlatlonai LL*Llquld Ualt;

Pl^Plaatlc Unit; Pl-PIaatlcity Indax;

q„-Unconfli>adCoapraaalva Stransth; c*

Apparent Cobaaloo; ^Soll Frlctlaa

Ansla; a-Strala.

Natural aolatura contant dataratnatlona »ara run on

Jar aavlaa froa all borloca. laaulta ara abovn on Taat lorlng laport.

Soil teata vara cenductad by

I. Trullo. K. Santlll, E. Soonanbarg AT. Charlton.

116
A-7»

A-7«.

DtlCWI»TtOW M ton. t*M>Lt TeSTCO griAfEf 6ILT V3M£ FINE <^NC> WITH

CX«?f6Y Sl*T. "••WC . p"«M"g WITH

ATlOMCVaiVft

_ Wt A 6MTM«e««»UR<3 , MP

OAT« MOV 5 > IW5 eoMTK MO MTaVB

117
Li -rt (0 II W

V C iJ 4J

*-> V V c

V U Vt C o

3 «M at rt M-l £ « own.

(0 in o c 3

4J U U C V" • ^ u o 0) >. > o h ^ It 01 1-13

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2P at Li -o w C 'J O O 3 u ~- n u X

O I > 0) « c

,o f-1 -o a e--H

01 01 o

0) ft) M >

C U

00 -o 01

118
BORING NUMBER

GROUND SURFACE ELEVATION

0.2'

3.5'

Topsoll —

3+5+7 andr allcFILL, with

quarti fTagaenta.molsC, 4+7+9

broui

tine sandy SILT(ML), 2+*+5

traca quartz fragments.

•-l noist .Drovm ^

bottoa of boring 9 S.O'

18. 9Z

18. SZ

BORING NUMBER "

GROUND SURFACE ELEVATION

Topaoll^ ^ 0.2'

2.5*

5.0'

oandy allt.FlLL. »rtth quartz f ragnenta.Bolst, ^

brown

ClAYET SlLT(Mt), aome fln^

aand ,molat .brown

7+7+8 21. 7X

22. 6Z

bottoBi of boring 9 5.0'

8 Q] p n m

1 o r- n Z

2 in n

I H 2 o -

S z z

i z

\ R >

>

-\m

BORING COrPLETED 7-19-79

WATER LEVSL READINGS

ENCOUNTERED nona UPON C0HPI.ET10N none HATER non,. CAVED AT - AFTER 4-5 hra

BORING COMPLETED 7-19-79

WATER LEVEL READINGS

ENCOUNTERED none UPON COMPLETION none WATER none CAVED AT - AFTER 3

1^-

BORINO NUMBER A3 GROUND SUR^4CE ELEVATION

0.2'

1.5'

3.0'

5.0'

topsoll ^^-^ sandy all.t,FILI,,<i>olst, • 6+9+9 brown

CLAYEY SILT, sone fine . 4+5+6

sand wltn quartz frag- nenta , moist , brown /~~

fine saniy SILT, nolst 3+3+4 brown

23. 3X

20. 5Z

bottOB boring t 5.0'

BORING COMPLETED 7-19-79 WATER LEVEL i^EADINGS

ENCOUNTEREIi none UPON COMPLETION none WATER n6ne CAVED AT - AFTER 2.5 hra

BORING N 'MBER GROUND SURFACE ELEVATION

0.2'

1.5'

3.5'

5.0*

topsoll '—

1

sandy alir, ILL,with 5+8+8

quarts trbgients,moist. "\brown / 4+7+9 CLAWSILT(^a),some fine

aand .moist , brown

fine aandy SILT(HL), 4+5+S

nolat,brown

18. OZ

17. 7Z

betCni of boring ( 5.0'

BORING COMPLETED 7-19-79 WATER LEVEL READINGS

ENCOUNTERED none UPON COMPLETION none WATER nona AFTER IH. hra CAVED AT -

inn

X

z

\ 0] r o m

5 O r- n Z Own

1 c z

J [; o

S z z

> o E! 2 ^

0 5 a 3i|

1 S >

5 u) tn z O

" nI

U)

o o

5 ?

119
BOPm NUMBER a$ GROUND SURFACE ELEVATION

topaoll0.2*

3.0*

5.0'

r

. 1 —

1

CLAYEY SILT(ML).aoiiie 2+4+4

0.2' flm Mnd,sols t,brown r 4+5+6 17. 6Z

2.5'

(Ini! aandjr SILT (ML).

Bol.1t,brown r 5+7+9 16. IZ

5.0'

SORING NUMBER a6 GROUND SURFACE ELEVATION

tOplOll Mndy silt.FILL,Bolst J+5+5 brown ' 6+7+7

CLAYEY SILT(ML),sc»a

fine (and .aoUc .brown

22.(Z

i IS.K

Z

S o

III

b^tcoa of boring f 5.0' bottoa of boring 9 5.0*

BORING COMPLETED 7-19-79 WATER l :vel readings

ENCOUNT''RED none UPON COMPLETION non. WATER none to 4.9'. AFTER 4 hra CAVED AT *.9'«><1 dry

BORING CO^PLETED 7-19-79 WATER LEVE . READINGS

ENCOUNTERtO "one UPON COMPLETION non* WATER non AFTER 5 hra CAVED AT - Bp If*

BOP NG NUMBER a7 and A7a GROUND Si'RFACE ELEVATION

top' oil

3.5

S.O

ClAYFI SILT (ML) .soma

fln«t aand.molflt .brown

tine aandy SILTCML) .with quarlz fragventa.ioolat

,

'Xbro.'n x

be.tob of boring C 5.0'

BORINO CO«>LETED 1-19-79

WATEP '-EVEL READINGS

ENCO.NTEaED none UPON COMPLETION none WATER noaa AFTER 1.5 hr CAVED AT - Alternate boring A7a drilled to recover taidlaturbed aaaplea

BORING NUMBER

GROUND SURFACE ELEVATION

• '+8+9rn P-24"

0.2".

4+4+8^ B-24''

19. 8Z 2.8<

4+<+8^ P-24" R-22" ni.6Z 5.0*

topsail —i| 2+4+5

aandy (llt.rn.1..wltb quartz fragraenta 6 organ ' 4+8+9

Ic natrtcr.Bolst .brown

fine aandy SILT(ML). olat.brown " 3+4+5

16. 9Z

26.2Z

bottoa of boring I 5.0'

BORING COMPLETED 7-19-79

WATER LEVEL READINGS

ENCOUNTERED none UPON COMPLETION none WATER none CAVED AT - AFTER 3 »>»•

^8

u8 z Sg

5 10 tn

IM3

i a

RIO }

120
B0RJN6 NUMBER a9 GROUND SURFACE ELEVATION

topsoll ^ 0.2*

3.0'- 4.0'-

5.0'-

Miidy llC.PUL.wlth

rock f ragMBta.Bolat,

btown

topaell fine aandy CLAYEY SILT

*—\(tlL),iiolat, brown / bettoB of boring C S.O'

BORING COMPLETED 7-19-79 WATER LEVEL READINGS

ENCOUNTERED nono UPON OJMPUTIOM nona WATER »o»» AFTER 3 h'* CAVED AT -

BORING NUMBER aid GROUND SURFACE ELEVATION

topaoll 2+*+5 O.J'-

+4+4 24. 3X 1.5'-

8/6"

4+5+7 30. 8X

5.0'-

aandy allc .FILL.boIsC. 4+5+«

brown

. 7+9+10

flna aamljr SILT(HL) ,vtth rock fragmcnta.aotat.

brown r 4+S+8

18. qx

12. ^X

13. jX

bottoa of borlnt 9 S.O'

BORING COMPLETED 7-19-79 WATER LEVEL READINGS

ENCOUNTERED none UPON COMPLETION non« WATER - AFTER

CAVED AT -

backfill on coaplatlon

Z

lis

i 2 "

° m >

i

3

BORING NUMBER ah

GROUND SURFACE ELEVATION

topaoll

^ 0.2'

3.0'-

flne'alUy SA)ID(SM),

18. IX Boiat,brown • 4+5+5 14. 2X

• 3+5+« 14. IX

bottoa of boring % S.O' BORING COMPLETED 7-19-79 WATER LEVEL READINGS ENCOUNTERED •><»« UPON COMPLETION <">»• •'ATER - AFTER

CAVED AT - backfill en eeaplatloo

8

Z

S a> p o m

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n w n l\l

5 VI u) « 8

1(0

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and

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test

result;

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tests

and1

>ve

analyses

w<

triaxial

cotapi

specimens

to< %

indicate

tl

le

to

coarse

si

lifications

ac(

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described

le

sand

are

all

ire

contents

rj

about

92

and 1

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with I

testing.

Tr:

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moisture

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cube

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Sic e

gradation,

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on

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and

2< 0

psi

back

pressure

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Enclosure (2) Contract No. H79043 SWtWtT OF SOIL LABORATORY TESTS

Percent Natural Renolded Remolded orlnt Snple

Oaacrlptlon

of Soil PassingNo. 200 Moisture

Density

(PCF) Moisture

OenRlty (PCD

Soil ParaNo. Oapch Type Speelaen Sieve «) Wet Dry «) Wet Ury neters Remarks

Bl 6' Jar Fine to coarse SAND

(SP), trace silt t gravel, broun t white

3.7 See: Gradation Curve

B2 I'to 3" Fine to coarae SAND 26.

S

3.3 loe.i lO^i.? See: Gradation Curve

3' Tube (SM),S0K slit, light brown

«.o 122.5 117.8

3' to

5' 3" Tub* Fine to aedluB SAND 1.8 2.0 94.0 92.2 1.5 96.4 95.0 See: Gradation and Trl-

(SP), trace allt. 2.2 96.1 94.0 3.1 95.6 92.7 axial Conpresslon Curve

light brown t white 2.1 95.5 93.6 4.7 '96;7 92.3" c-0.6 kef Notes 6 & 7 do do do do 1.9

3.1 ~r4-

94.3

96.7

"95.2"

92.5

93.7 93.0

6-29°

c-0

See: Direct Shear Curves

Note 6 13 *' Jar Fine to coarae SAND

(SP). trace allt 4 gravel, brown

2.2 See: Gradation Curve

M 2' Jar Fin* to aadlua SAND

(SP), trace allt, light brown

4.6 See: Gradation Curve

etes!

1. Soil tast In accerdane* ultfa appllcabla ASTM Standards.

2. toil claaslf Icatloa ayabols are In accordance with Unified

Soil Claaalf Icaclon Syataa, based on testing Indicated and vlaual identification.

3. Vlaual Identification of aaaiplaa la In accordance with

the ayatcB used by thla firs.

«, Key to abbreviations: do - dltto^ c • apparent cohesion

• i- soil friction anKlc , 5. Soil tests were conducted by L. Trullo, T, chnrlton i

E.' Sonnenberg,

6. Speclnens retoolded to spproxlnate natural density

and Bolaturc for strength testing.

7. A 50 pal back pressure was spplJcd to trlaxlal

apec laens

.

124
U • BTANOAKO •live ftllC

C"«IN IN MILLIurTCNS

O

"A

DEPTI

3'

63

DrSCIHIT ION or ',01 L iiAMPi . T£GTl D

BNE, TO .MEDIUM .'E'AND/^'n^ I'l'-T^ LIOaMT BROv^N WHITE

nrJE TO coap'je. 'pAND.ti ace .FIME.. C3l?AVEL e^l-T^ BROWN

SCHNUEL INCINECRIWO ASSOCIATES

GRADATION CURVES

rnoifci MO&ILE. lii.Vvll

DATE S'-?^. t-.J [cONTH NO U 2X^'\

W < ITANDAHO ElCVt »IIC

U( senior ION OF SOIL SAMPLE TESIt I) ZW^-.JO e^R'^a ^AND. "some e^iLT.

FINE. TO MEp!(JW._t»'<ND, TB^E «>ILT^

Ll Assir •i2> SCIINUCL ENOINtiniNO USOCI*T(S

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128
)ws

required

t<

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three

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30

inch.

in

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irentheStfs

are

ii

inspection.

es

the

number

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ii

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is

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SjTabols

in

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Symbols

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a

s

by

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ication

sys

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col

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,

1-3

as

Indicate

ASTM

D-1586

on

of

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1

soil

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Soil

C

an:

B.

Sple

•H C 0 U -H 0 W

B <M 0 00

cati ied d

Un

orem

(9 « -4

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the

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re

It

the

particular

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130
b *-> n a

Remarks

See;

Gradation

and

Unconflned

Compression

Curves

See:

Gradation

and

Trlaxlal

Compression

Curves 1

See:

Gradation

Curve

Soil Parameters

01k* —i

(A ^« (0

r> 4) m .H

3 k. CP©* to

O r- 0> •

1 1 « u

1

Speciric

Gravity

2.56 'J

1 1

Natural Density

(pcf)

81.1

100.3 1 1

113.

6

123.5

1 1

Natural

Moisture o~a

23.1 27.6

aCM

Atterberp, Limits

PI Ov

PL o o o

o\ CX> m rg

Percent Passing

No.

200

Sieve 89.1 72.2 82.6

Description of

Soil

Specimen

SILTY

CLAY

(CL)

,

trace

fine

sand

with

organic

matter,

gray

SILTY

CLAY

(CL)

,

some

fine

to

coarse

sand

with

organic

matter,

gray

SILTY

CLAY

(CL),

some

fine

sand

with

organic

matter,

gray

Fine

sanoy

d-niZ't

(CL-ML),

with

organic

matter,

gray

Sample Type

t> =m 3H

01 : M

<n 3H

Jar Jar

Sample Depth ST

1 **t rx

3.5'

Boring No. U 2C 30 u

.2

Ou O O BUM U R

Cl< •OV

c —

" -H C

•r* O

e c <H •HOW

m3 U 01 ex* -o o o e

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•H 0) 0)

CD -H 4J

(D tM • CD <H e C

»M e o o

o a

>v e B

f* e

I 4J « p

131
1

GAAIH t)ZC IN MILLIMETCHS

IC 2-4-

zc

OESCRIPTION Of SOIL SAMPLE TESTED ^IIXY CUJi^ fgACa FINE. 'gAND WITH

t^lLTY ^LA^Y^ "sOMZ. FINE TO COAE^r"

J&AND VVlT.H_CjRWEl- l(,OI?iiANIC MATTER^ _

<SjRAV

CL, 23.

1

2r5

2^ 15.

SCHNMEL tNeiNEERINQ ASSOCIATit

ORAOATION CURVU

pnojECT isi MOBILE HOME

ANCHQR^<3E PROJECT

D«TE^Ep,f:22,l'?ec coNTB.w. M7gp4.5

U S •laNOAKO «ICVC *IK

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1 —

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1

1

1 \\ 11 ,.

.

1. i ,i II 1 OWAIN Size IN MILLIMCTCWS TSe"

3.5

DESCRIPTION OF SOIL SAMPLE TESTED FINE -jAMPr- CLAYEY SI LT WITH Ol?^NlC-. MATreB^CiCAl

CL-ML SCHNAaCl f NGINECNING AtSOCIATIC

OnWATION CUKVM

pnojccT MOBILE HOME

132
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136
APPENDIX B

Test Results

B.l. Introduction

The test results are presented in a series of six tables. The data

in the tables were taken from x-y plots produced electronically in the field,

except that in tests STl to ST12 the data were recorded manually. Anchors of

several makes were used in the study. Since it was not the intent of this

study to compare the performance of different products, anchors of different

makes are identified by letters only. The swivel anchors were inserted by a

percussion tool. The time it took to install the anchors is identified in the

footnotes

.

B.2 Symbols Used in the Tables

Static test results:

Test Number Designations ;

ST = Silty Site

SD = Sandy Site

C = Clay Site

Test Location ;

Coordinates in location maps in Appendix A

Anchor Type ;

H-6 = 6-inch single helix anchor

D-4 = 4-inch double helix anchor

H-3 = 3-inch single helix anchor

P-10 = 10 X 1 3/4 in. pi,pe anchor

P-6 = 6 1/2 X 1 1/4 in. pipe anchor

AH-6 = 6-inch arrowhead anchor
Anchor Inclination:

Loading :

SM = Static monotonic

SUR = Static monotonic with several unloading and reloading cycled

CP = Creep test

Soil Condition :

M = Moist

W = Wet

S = Submerged

Other Symbols :

STP = Soil Test Probe reading, in-lb

= Installation torque, ft-lb

P^^ = Load at 2-inch vertical displacement, lb

P^jj = Load at 4-inch horizontal displacement, lb

= Ultimate load capacity, lb

= Anchor head displacement in the direction of pull at Q^, in

Rg^ = Reloading modulus at 85 percent of Q^, lb/in

P = Cyclic load, lb

c n = Number of cycles

A^, A^Q, A^QQ, A = total anchor head displacement in 1st, 10th, 100th and last

cycle

138
Reloading modulus in 10th cycle, lb/in

Ultimate load capacity determined by anchor pullout after completion

of cyclic tests

139
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NBS»n4A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR : REPORT NO, -,f- NBS BSS 142 S

2. Performing Organ. Report No. 3. PubU<:at1ori Date May 1982

4. TITLE AND SUBTITLE

Load—Displaceinent Characteristics of Shallow Soil Anchors

5. AUTHOR(S)

Felix Y. Yokel, Riley ¥.. Chung, Frank A. Rankin and Charles W. C. Yancey

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions;

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. Stole. ZIP)

Office of Policy Development and Research

U.S. Department of Housing and Urban Developinent

Washington, DC 20410

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 82-600509

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information,

bibliography or literature survey, mention it here^

If document includes a si gnificant

Tests on shallow soil anchors, commonly used by the mobile home industry,

including 6-in single helix and 4-in double helix anchors as well as three

types of swivel anchors, were conducted on three sites: a silty site, a sandy

site, and a clay site. Test variables included direction of anchor installation; direction of loading; anchor depth; size of anchor plate; and cyclic

load effects. The effect of these test variables on load-displacement characteristics, measured at the anchor head, is investigated. It is concluded that on most sites the anchor types tested, when installed in accordance with present

industry practice for mobile home tiedown systems, did not deliver the anchor

performance required in present standards. It is recommended that minimum load

capacity requirements for anchors be waived; that all anchors be preloaded to

1.25 times the design load; and that one anchor per mobile home, or three

anchors per site if soil conditions are uniform, be preloaded to 1.5 times the

design load.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

anchors; cyclic loading; field testing; flood forces; foundations; load capacity;

mobile homes; soil anchors; soil mechanics; stiffness; wind forces

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

Pr~| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D C

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

163

15. Price

$6.50

i>u . s . GOVERNMENT PRINTING OFFICE 1982-360-997/2098

USCOMM-DC 6043-P80
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