NATL INST OF STAND & TECH

A11107 E1D7S3

NBS BUILDING SCIENCE SERIES 127

Recommended Technical Provisions for

Construction Practice in Shoring and

Sloping of Trenches and Excavations

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

The National Bureau of Standards' was established by an act ot Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

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KATJONAl BUREAU OF STANDARDS

LIBRARY JUN 5 1980

NBS BUILDING SCIENCE SERIES 127 tl%

Recommended Technical Provisions for

Construction Practice in Shoring and

Sloping of Trenches and Excavations

Felix Y. Yokel

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for the Occupational Safety and

Health Administration Department of Labor Washington, D.C. 20210

and

National Institute for Occupational Safety

and Health Department of Health, Education and Welfare Washington, D.C. 20201

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1980
Library of Congress Catalog Card Number: 80-600068

National Bureau of Standards Building Science Series 127

Nat. Bur. Stand. (U.S.), BIdg. Sci. Ser. 127, 84 pages (June 1980)

CODEN; BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20234

Stock No. 003-003- Price (Add 25 percent for other than U.S. mailing)
ERRATA TO ACCOMPANY NATIONAL BUREAU OF STANDARDS

BUILDING SCIENCE SERIES 127

Recommended Technical Provisions for

Construction Practice in Shoring and

Sloping of Trenches and Excavations

Felix Y. Yokel

Item Change

Page XIV - Skeleton Sheating Skeleton Sheat]hing

Page 10 - Second line from bottom = equivalent weight effect

of page in . . . k, (£=0)

Page 38 - Third line from bottom R^ = — s is corrected as k. (£=0)

Page 56 - Second line from bottom and guidance throughout

should read

Page 68 - Table A. 9 should read

as follows:

Tabic A. 9 Tlaber Shoring Wale Sit** la Aecordaccc vtth Standard Practice

Tranch

Dtpth

(ft)

Soil trv* Horizontal Strut Spacing (ft)

10 12 14 16 20

Sheeciog

TTllckoe**

(In)

5-10 BC

6 8

8 s 10

810X 10

X 12 1012 X 10X 12*

10 z 10 10 X 12 12 X 12 12 X 12

2

3

10-15 B 8x8 10 X 10 10 X 12* 10 X 12 12 X 12 2 C 10 z 12 12 X 12* X 12 3

15-20 B 8 X 10 10 X 12 12 X 12* 12 z 12 3 C 12 X 12 12 X 12 IHTDUCDIATE STUtTrS TO TBJ! RIGHT OF LIKE

3

(1) All liabcr •Ite* arc aet<lal (not noalnal) • Isc* In inchaa.

(2) Terdcal apaclnf not to axced 5 ft eentcr-tv-cencer.

(3) All horizontal apaelng 1* center-to-center.

(*) LoB« *lde of cro**-*«ctloa of rectangular acaber* to be horizontal.

(5) • Indicate* *llght OTer*tre**.

(6) Wale tlxca to the right of dividing line require Inaertlon of Interaedlatc

atrut before vorker* enter the trench.

(7) If vertical dlatance froa the center of the lowect wale to the bottoa of the trench exceed* 2 1/2 ft, aheetlng (hall be flralv eabeddcd below ^he bottoa of the trench or auddll ahall be uaed. The vertical dlatance

froa the center of the loweat vale to the bottoa of the trench ahall not •zceed 3 ft, or 3 1/2 ft If aulalll 1* uaed.

ABSTRACT

On the basis of studies conducted by the National Bureau of Standards

technical provisions for the sloping and shoring of the banks of trenches

and excavations are recommended. Included are a recommended standard

practice for trenching which can be used by construction supervisors

and compliance officers of the Occupational Safety and Health Administration, and proposed engineering guidelines for the design of shoring

systems and other means to prevent mass movement of soil or rock in

excavations

Key words: Braced excavation; construction; retaining structures;

shoring; slope stability; soil classification; soil pressure;

standards; trenching.

COVER: Onu the, pn,oblm6 mcoayitoA^d In many tAmdx-

-ing opoAoutioYii) Li, a noAAOM mn.k ^pac2. (couAte^y,

HoutiovmJi UtAJbity ContAOicXofU ki,i>ocLijoution,

Mcuhington, V.C.)

iii
PREFACE

The Occupational Safety and Health Administration (OSHA) Regulations for

Excavation, Trenching and Shoring [13]-' were promulgated in April 1971.

In June 1976, OSHA engaged the National Bureau of Standards to study the

compatability of the technical provisions in these regulations with actual

construction practice and with the state of knowledge in geotechnical and

structural engineering, review the experience accumulated since their promulgation and recommend potential modifications that could improve their

effectiveness. The NBS study consisted of three parts: A field study of

present practice in excavation, trenching and shoring and the impact of

the OSHA regulations as perceived by contractors, labor unions and State

and Federal enforcement agencies; a technical study consisting of the

assessment of the technical provisions in the present regulations and of

available options for improving the soil classification method and the

technical provisions for sloping and shoring; and a study of timber presently used for shoring in order to reasonably assess the load carrying

capacity of timber shoring systems. Initial guidance was also provided by

a trenching hazards identification task force appoinment by the Building

and Construction Trades Department of AFL-CIO who organized a workshop to

identify excavation safety problems. The findings of this work are presented in separate reports on the field study, [10, 17], the soil classification study [21] and the shoring timber study [11], respectively. This

report contains recommendations which are based on those findings. A

draft of initial recommendations was discussed at a workshop which was

held in September 1978 [16]. As a result of this workshop some of the

original recommendations were re-studied and subsequently modified.

The study was conducted by the Geotechnical Engineering Group of the

Center for Building Technology. The preparation of this report was funded

in part by the National Institute for Occupational Health and Safety

(NIOSH).

Numbers in brackets refer to literature references in Section 6.

iv
TABLE OF CONTENTS

Page

List of Figures vii

List of Tables viii

List of Symbols ix

SI Conversion Units xii

Definitions xiii

1. INTRODUCTION 1 2. ACCEPTABLE PRACTICE 3

3. RECOMMENDED STANDARD PRACTICE 7

3.1 Scope 7 3.2 Sloped Excavations 8

3.3 Shored Excavations 8

3.3.1 Strength of Shoring Systems 8

3.3.2 Loads Acting on Shoring Systems 10

3.3.3 Rating of Shoring Systems 11

3.3.4 Determination of Load Capacity by Test 14

3.4 Soil Classification 15

3.4.1 General 15

3.4.2 Simplified Soil Classification System 15

3.4.3 Matrix Soil Classification System 15

3.5 Spaced Sheeting 15

3.5.1 General 15

3.5.2 Strength of Spaced Sheeting 19

3.5.3 Maximum Spacing of Spaced Sheeting 19

3.6 Recommended Special Provisions 20

4. ENGINEERING GUIDELINES FOR THE DESIGN OF SHORING SYSTEMS

AND OTHER MEANS TO PREVENT MASS MOVEMENT OF SOIL AND ROCK .. 23

4.1 General 23

4.2 Scope 24

4.3 Design Loads 24

4.3.1 General 24

4.3.2 Soil and Water Loads 24

4.3.3 Surcharge Loads 25

4.3.4 Operational Loads 25

4.3.5 Dynamic Loads 25

4.3.6 Restraint Loads 25

V
Page

4.4 Design Criteria 26

4.4.1 General 26

4.4.2 Sloped Excavations 26

4.4.3 Braced Excavations 27

4.5 Information on Accepted Engineering Practice 30

4.5.1 General 30

4.5.2 References 30

4.5.3 Summary of Information 30

5. COMMENTARY ON RECOMMENDED STANDARD PRACTICE 35

5.1 Sloped Excavations (Sec. 3.2) 35

5.2 Strength of Shoring Systems (Sec. 3.3.1) 36

5.3 Loads Acting on Shoring Systems (Sec. 3.3.2) 36

5.4 Soil Classification (Sec. 3.4) 51

5.5 Spaced Sheeting (Sec. 3.5) 51

5.6 Recommended Special Provisions (Sec. 3.6) 51

6. REFERENCES 53

7. ACKNOWLEDGMENT 55

APPENDIX A - Timber Shoring 57

vl
LIST OF FIGURES

Page

Figure 1 Explanation of Symbols xi

Figure 2 Components of Shoring Systems xv

Figure 2.1 Effect of Foundation Loads on Shoring 5

Figure 3.1 Allowable Configurations of Sloped Excavations 9

Figure 3.2 Loads Acting on Shoring Systems 12

Figure 3.3 Loads Tributary to Members of the Shoring System 13

Figure 5.1 Symbols Used in Derivation of the Equation

for the Sloped Backfill Effects 39

Figure 5.2 Simplified Expressions for Rg 41

Figure 5.3 Relationship Between Rg, (!> , 6 and B 42

Figure 5.4 Comparison of Proposed Equation for R^ with NAVFAC

Criteria 43

Figure 5.5 Effects of Heavy Construction Equipment 45

Figure 5.6 Dimensions Used In the Derivation of Simplified

Expressions for Heavy Equipment Effects 47

Figure 5.7 Effects of m on 48

vli
LIST OF TABLES

Page

Table 3.1 Simplified Classification System 16

Table 3.2 Soil Classes in Matrix Classification System 17

Table 3.3 Minimum Acceptable Stability Requirements for the

Matrix Classification System 18

Table 3.4 Maximum Center to Center Spacing (in ft) of Spaced

Sheeting for the Simplified Soil Classification System . 19

Table 3.5 Maximum Center to Center Spacing (in ft) of Spaced

Sheeting for the Matrix Classification System 20

Table 4.1 Typical Values of Unit Weight of Soils [15] 32

Table 4.2 Relationship Between Properties of Cohesionless Soil and

Standard Penetration Test Results [19] 32

Table 4.3 Properties of Cohesive Soil and Standard Penetration

Test Results (Goldberg, et al. , 1976) [9] 33

Table A.l Allowable Unit Stresses in psi for Hardwood

Trenching Lumber 58

Table A. 2 Typical Minimum Sizes of Timber Skip Shoring (inches) ... 60

Table A. 3 Typical Minimum Sizes of Timber Shoring of Trenches 61

Table A. 4 Adjustment of Strut Size for Trench Width 62

Table A. 5 Analysis of Table A. 2 64

Table A. 6 Analysis of Table A. 3 64

Table A. 7 Timber Skip Shoring Sizes in Accordance with Standard

Practice 66

Table A. 8 Timber Shoring Strut Sizes in Accordance with Standard

Practice 67

Table A. 9 Timber Shoring Wale Sizes in Accordance with Standard

Practice 68

viii
LIST OF SYMBOLS

B - Width of excavation

c - Cohesion (undrained shear strength) of material in bank, lb/ft

- Undrained shear strength of material below bottom of excavation, Ib/ft^.

- relative density of soil

h/v - Slope (horizontal over vertical)

H - Depth of Excavation, in feet.

- Equivalent depth for sloping backfill in ft

- Surcharge converted to equivalent soil depth in ft

kg - Coefficient of active earth pressure, as defined by pertinent

equations listed.

m - Coefficient in lateral force equation as defined by Peck

(1969).

N = yH/c - Stability number, based on shear strength of material in the

bank

= yR/c-^ - Stability number, based on shear strength of material below

bottom of excavation and weight of material in bank.

N - Blowcount in standard penetration test using traditional U.S.

methods (rope and cathead) in blows per foot.

p - Uniformly distributed lateral soil pressure in lb/ft .

Pq - Uniformly distributed lateral soil pressure caused by equipment load in lb/ft .

q - lineload parallel to trench in lb/ft.

S - Load capacity in lb

S - Average strength (average failure load corrected for load

duration, if applicable) in lb

V - Coefficient of strength variation, corrected if necessary for

sample size.

ix
W - total downward force exerted by the weight of heavy equipment

in lb.

Wg - Equivalent weight effect of soil in lb/ft .

X - distance from edge of excavation in ft S3. - ratio of for sloping backfill to k„ for horizontal backfill 3

a - Angle between back of retaining structure (facing the soil) and

the horizontal, in degrees (see figure 1).

g - Angle between surface of sloping backfill and the horizontal in

degrees (see figure 1).

Y - Unit weight of soil (in natural condition or as assumed for worst

case) in Ib/ft^.

o Yg^j. - Unit weight of saturated soil, in lb/ft .

^sub ~ Unit weight of submerged in lb/ft .

Y^ - Unit weight of water in lb/ft .

(5 - Angle of friction between retaining structure and retained backfill in degrees.

- horizontal pessure against rigid retaining wall caused by lineload parallel to trench in Ib/ft"^

- average horizontal pressure against rigid retaining wall caused

by lineload parallel to trench in Ib/ft^

(|) - Angle of shearing resistance (internal friction) of soil, in

degrees

X
Figure 1. Explanation of Symbols

xl
SI CONVERSION UNITS

In view of the present accepted practice for building technology in

this country, common U.S. units of measurement were used throughout the

report. In recognition of the position of the United States as a signa

tory to the General Conference on Weights and Measures, which gave offi

cial status to the International System of Units (SI) in 1960, the tabl

below is presented to facilitate conversion to SI units. Readers interested in making further use of the coherent system of SI units are referred to: NBS SP 330, 1972 Edition, The International System of

Units; and ASTM E380-76, Standard for Metric Practice.

Table of Conversion Factors to SI Units

Convert From To Multiply By

inch millimeter 25.4*

inch meter 2.54* . 10"^

foot meter 3.048* . 10"

lb (force) newt on 4.4482

lb (mass) kilogram 0.4536

Ib/in^ pascal 6.8947 X 10^

Ib/ft^ pascal 47.880

ton/ft^ pascal 95.760 X 10^

Ib/ft^ (mass) kg/m^ 16.018

o lb/ft (equivalent force) N/m^ 157.14

1

* Exact value, others are rounded to five digits.

xii
DEFINITIONS

Acceptable Practice is practice which meets the minimum requirements

recommended in this report.

Allowable Working Stresses are stresses which should not be exceeded

under the most critical combination of working loads.

Average Strength is the average failure load corrected for effects of

load duration.

Design Criteria are design rules which, if followed, would reduce the

risk of occurrence of design limit states to acceptable levels.

Design Limit States are failure modes which endanger workers in, or adjacent to, excavations.

Design Loads are loads used for the design of shoring systems or the

determination of slope stability. Design loads may be working loads

or ultimate loads.

Engineer is a registered professional engineer.

Fractured Rock is rock which could spall or crumble when excavated with

vertical slopes. In general, rock with a pattern of fractures or joints,

which are, on the average, closer than 1 ft apart should be considered

fractured. Rock is not considered fractured when rock slopes are secured

against mass movement and spalling by rock bolts, netting, and other means

approved by an engineer.

Load Capacity is a measure of strength defined in Section 3.3.1(2).

Long Term Excavations are excavations which are open for more than 24

hours.

Safety Factor is the ratio of load capacity to the effect of the most

critical combination of working loads. In the case of excavation stability, the safety factor is the ratio of resisting forces to driving

forces. For excavation slope stability the safety factor can be taken as the ratio of critical height to actual height.

Safety Margin is any measure of excess strength over that required to

resist the working loads.

Shoring Systems are structural systems supporting the bank of an excavation. The components of shoring systems are defined in Figure 2.

Short Term Excavations are excavations which are open for 24 hours or less

xiii
Short Term Strength Properties of Soils are the strength properties of

the soli adjacent to the excavation during a 24 hour period of exposure.

Some of this strength can be lost with the passage of time by such

effects as desiccation and lateral expansion. A typical short term

strength property Is apparent cohesion In moist sands. In some Instance

there may also be an Increase In strength with time (for Instance drained

vs. undralned strength).

Skeleton Sheatlng Is spaced sheeting In which the sheeting members

(upright or horizontal) are supported by wales (see figure 2).

Skip Shoring Is spaced sheeting In which the sheeting members are directly supported by struts.

Standard Practice Is the trenching and shoring practice recommended

In Section 3 of this report.

Ultimate Loads are working loads multiplied by the following factors:

1.7 for long term excavations

1.3 for short term excavations

Working Loads are loads which should reasonably be anticipated and which

must be resisted with appropriate safety margins. All loads defined In

the Standard Practice and the Engineering Guidelines are working loads,

unless otherwise noted.

xiv
Figure 2. Components of Shoring Systems

XV
FACTWG PAGE: Utilltij tAmck with hycUiaaLic 6ho^ng.

Typ-lddttij , woKk in ^uah tAznchd^ p^ocd^d^ rapidly and dici^ionM on i,hoKA.nQ

ajKL ma.d<L by the, coniiyuiction {^on.man.

{CouAXeySy, Ei^g Red Sa^^ety and VK.od{icXi-on,

Inc., NewoJik, Cati{^o^niji]

XV
1 . INTRODUCTION

One of the most important conclusions that can be drawn from an NBS field

study of present trenching practice [10, 17] as well as from recomtoendations made in an August 1978 Federal workshop on this topic [16] is that

it is, in many instances, not practical to require that a professional

engineer design the shoring or determine the steepest allowable slopes for

utility trenches or shallow excavations. Reasons for this conclusion are

the fast pace at which this work typically proceeds, the frequently rapidly changing soil conditions which sometimes require instantaneous decisions from the work supervisor, and the typical chain of command at the

job site which would not permit an engineer to intervene rapidly enough

to effect necessary changes in shoring or sideslopes. Thus, it is nec- essary to have standards and provisions which can be understood and

implemented by the supervisory personnel in the field.

1
The most important element of any such standard is the soil classification

systems which provides the means of relating site conditions to required

shoring or steepest permissible sideslopes. Because of the importance of

the soil classification systems, two potential approaches are recommended

for consideration. The approaches and their advantages and disadvantages

are discussed in detail in a report on soil classification [21]. One of

the two approaches, but not both, should be considered for inclusion in a future trenching standard. The choice should be made after careful study

and deliberation by all the parties involved (contractors, labor unions,

engineers and government officials). In accordance with the two alternate soil classification systems, two alternate sets of provisions are recommended for consideration wherever the provisions are tied to the

soil classification systems.

The recommended provisions provide two options: Use of a recommended

standard practice, or design by an engineer. The standard practice can

be used by supervisory personnel in the field to select shoring and

determine maximum allowable slopes. The shoring from which the selection

is made should be designed by an engineer for pre-determined conditions

of use.

The recommended soil classification systems are also correlated with

traditional timber shoring practice. Even though NBS analysis of timber

shoring as presently used leads to the conclusion that it could not

resist the design soil pressures recommended in this report with safety

margins consistent with present engineering practice, the NBS study could

find no field evidence that properly installed traditional timber shoring

is unsafe. There are two possible reasons for this paradox: (1) Allowable stresses presently used in timber design may incorporate safety margins against failure larger than those assumed in the analysis (2) In

absence of significant data on forces acting on shallow-trench bracing

the recommended design lateral forces are conservatively high. The timber

shoring practice is discussed in Appendix A. "Strength-equivalency" to

traditional timber shoring members should not be accepted as proof of

adequacy for other shoring systems which should be designed in accordance

with Section 4.

"Acceptable practice" is defined in Section 2; Section 3 contains the

recommended standard practice; recommended guidelines for the design of

shoring systems and sloped excavations by engineers are given in Section

4; Section 5 is a commentary on the recommended standard practice; timber

shoring practice is discussed in Appendix A.

The acceptable practice recommended in this report serves the purpose of

protecting workers avjainst trench cave-ins. It may not prevent damage to

adjacent properties by excessive settlements.

FACING PAGE: Typical. timboA. ^kofU-viQ. A hydAoutlc. i>ko^(i

AJi uAdd to -inioAt the. 6t/iu.ti>. 2
2. ACCEPTABLE PRACTICE

"Acceptable Practice" is defined as practice that meets the miniraum

requirements recommended herein. Compliance with these minimum requirements, whether by consulting an engineer or by following recommended

standard practices, insures in most instances adequate protection of

workers against cave-ins. However, soil is not a man-made material

and its characteristics are not as predictable as those of common building materials. Thus the contractor who is doing the work has the

responsibility to ascertain if there are local site conditions which

present special hazards.

For all excavations deeper than 20 feet (except as noted herein) the

adequacy of shoring systems and the stability of sideslopes should be

determined by an engineer in accordance with the guidelines in Section 4

of this report. An engineer should also be consulted whenever the bottom

of a building foundation adjacent to an excavation extends into the
zone between Imaginary sideslopes rising at a slope of 1 horizontal: 1 vertical from the outer edges of the bottom of the excavation (see Figure

2.1).

The shoring or sideslopes of excavations deeper than 5 ft and less than

20 ft deep (except as noted herein) should be deemed acceptable if they

meet either one of the following conditions: (1) they comply with the

standard practice recommended in Section 3 of this report; or (2) an engineer determines that they are adequate in accordance with the guidelines

in Section 4 of this report.

Excavations less than 5 ft deep and all excavations in unfractured rock

regardless of depth should be exempt from shoring and sloping requirements.

The shoring and sloping requirements should also not apply whenever it can

be demonstrated beyond reasonable doubt that the cave in of an excavation

would not pose a danger to workers, other persons, or adjacent equipment

and property. All other excavations should be sloped or shored in accordance with the recommended acceptable practice.

4
\

\ \

\ LIMIT OF CRITICAL ZONE

FOOTING "A": STANDARD PRACTICE CAN BE FOLLOWED

FOOTING B: AN ENGINEER SHOULD BE CONSULTED

Figure 2.1 Effect of Foundation Loads on Shoring

5
FACIWG PAGE: Ship i>liohA,nQ Ivi i>tll{^ clay. Fl66a/L2A aould
3. RECOMMENDED STANDARD PRACTICE

3.1 SCOPE

The recommended standard practice applies to all excavations deeper than

5 feet and less than 20 feet deep except those in unfractured rock.

Whenever a distinction is drawn between long term and short terra excavations, the definition of long term and short terra excavations given in

this report applies (24 hours is the division point). "Excavations"

include, but are not limited to, utility trenches.

I

7
3.2 SLOPED EXCAVATIONS

Sloped excavations should not have sideslopes steeper than those stipulated in Tables 3.1 or 3.3-'. If there is any indication of general or local instability slopes should be cut back to be at least 1/4 hor. :1

vert, flatter than the "stable" slopes (the "stable slope" is the slope

which will remain stable for the duration of the excavation). The slope

configuration of short term excavations can be modified as shown in

Figure 3.1.

3.3 SHORED EXCAVATIONS

3/ 3.3.1 Strength of Shoring Systems- '

Shoring systems should be designed to resist the working loads stipulated

in Section 3.3.2. The term "designed to resist" should be interpreted as follows [(1) or (2)]:

(1) The following stresses are not exceeded: 1.33 times the allowable

working stresses in short term excavations; 1 times the allowable working

stresses in long term excavations. "Allowable Working Stresses" are defined as the "allowable stresses" stipulated in applicable standards

[1, 2, 3, 12] in conjunction with traditional "working stress" design

(using unfactored loads and safety margins). For timber shoring which is

left in place (not re-used for other excavations) allowable working

stresses can be adjusted for load duration as follows: 2-day duration for

short term excavations; 6-month duration for long term excavations.

Allowable stresses for hardwood timber are recommended in Appendix A.

Allowable stresses for softwood timber should be in accordance with Ref.

[12]; or (2) The system has adequate load capacity to resist the following

factored loads: 1.3 times the working loads stipulated in Section 3.3.2

in short term excavations. 1.7 times the working loads stipulated in

Seciton 3.3.2 in long term excavations. "Load capacity" is defined as one of the following:

(a) "Required strength" as defined for reinforced concrete

members in ACl 318-77 [2];

or (b) "maximum strength" as defined for steel members

in Part 2 of the AISC Specifications [3];

or (c) S = S^ (1 - 2v), where:

Either one or the other applies, depending on the soil classification

system adopted (see Section 3.4).

Consideration could be given to exemption of conventional timber shoring systems from the minimum strength requirements (see Appendix A).

8
— = steepest allowable sideslope

Case I

Case II

Case III

Case I - Ordinary slope Case II - Compound slope with bench no more than 3 ft. high Case III - Configuration must meet following criteria:

1. No vertical bank to exceed 5 ft, the vertical bank adjacent

to the work area not to exceed 3 ft.

2. Imaginary slopes ah and cd not to exceed max. allowable.

If steps are used [left side of Case III] imaginary slope |abj not to exceed 1 :

1

3. Excavated area equal to or greater than area within abcda

Figure 3.1 Allowable Configurations of Sloped Excavations

9
S = load capacity

S = average strength (failure load corrected for load duration if

applicable)

V = coefficient of variation of strength, corrected if necessary for

sample size

.

3.3.2 Loads Acting on Shoring Systems

( 1 ) General

All loads given in this section are "working loads". They are loads which

should reasonably be anticipated and which must be resisted with appropriate safety margins.

(2) Operational Loads

The following minimum load should be used for the design of all struts

(cross braces): A gravity load of 240 pounds distributed over any 1 foot

long portion of the strut.

In addition, trench boxes and other shoring systems installed by methods

which do not assure that the sheeting bears tightly against the excavated

bank before workers enter the trench (there may be an open space between

the bank and the sheeting) should withstand without failure an impact

energy of 240 ft- lb applied at any point against the sheeting side facing

the bank (inward). In shoring systems whose struts are pre-loaded to

exert a force of 500 lb or more against the excavated bank, the sheeting

is considered to bear tightly against the bank.

(3) Lateral Soil Pressures

Lateral soil pressure per unit surface area of shoring should be calculated by eq (3.1), eq (3.2) or eq (3.3), whichever results in the largest

lateral pressure

P (H + 2) eq (3.1)

P eq (3.2)

P

w

e

P a uniformly distributed lateral soil pressure in lb/ft .

equivalent weight effect on lb/ft as stipulated in Tables 3.1 or 3.3.

H depth of excavation from top of supported bank to bottom of

e:xcavation in feet.

10
= equivalent height for sloping backfill in ft as defined in

Figure 3.2

Hq = height of surcharge (spoil or stored material converted to

equivalent soil depth)in ft.

Pq = lateral pressure caused by equipment or traffic loads in lb/ft

as defined in figure 3.2

Equation (3.1) applies to excavations in level ground (not steeper than 3

horizontal: 1 vertical) and has an allowance for surcharge which is adequate for most ordinary conditions (2 feet are added to the height H).

Equation (3.2) applies whenever the ground or the retained backfill slopes

upward at an angle steeper than 3 horizontal: 1 vertical (sloping backfill can be disregarded when the slope is less than 3 horizontal: 1 vertical)

.

Equation (3.3) applies when very heavy equipment is used (producing lateral pressures greater than those caused by the 2 ft surcharge in eq

(3.1)). The three cases in eq (3.1), eq (3.2), and eq (3.3) are illustrated in Figure 3.2.

(4) Loads Tributary to Members of the Shoring System

The following portion of the lateral loads caused by the uniform lateral

soil pressure p shall be assumed to act on members of the shoring system:

100 percent of the tributary load shall be assumed to act on all struts,

80 percent of the tributary load shall be assumed to act on wales (members

directly supported by struts shall be designed as wales), 67 percent of

the tributary load shall be assumed to act on sheeting. Tributary load

shall be calculated in accordance with Figure 3.3.

3.3.3 Rating of Shoring Systems

Components or subassemblies of shoring systems, or fully assembled selfcontained shoring systems, should be rated and subsequently used to resist

working loads equal to, or smaller than, those for which they are rated

to be adequate.

Rating may be accomplished as follows:

Struts may be rated for the compressive working loads they are allowed

to resist. If struts are extendable, the rating should consider

length effects on load capacity. Rating of struts should include con- sideration of the 240 pound vertical downward load stipulated in

Section 3.3.2 (2).

Wales supported at given length intervals could be rated for allowable

load per linear foot of wale. For strut-wale assemblies the wale

should be designed to resist moments and shears not less than 80 peril
TOP OF SUPPORTED

BANK\^

BOTTOM OF EXCAVATION^

SURCHARGE

[a| AVERAGE CONDITION, TERRAIN NOT

STEEPER THAN 3 hor : 1 vert

p = we|H+2) - eq (1)

Z

iJ L

|b| SLOPED BACKFILL

p

= We He - eq (2)

He^H(l+2j):^H(1+0.04/^|

or He=H, +2 WHICHEVER IS LESS

(C) HEAVY SURCHARGE

P

= We(H+Hql + pq W X 0.8W

Pq = H(J+x) I'-0-6hI - H(5+x)

Pq CAN BE DISREGARDED WHEN x>H

S= LENGTH OF EQUIPMENT OR

LINELOAD IN THE DIRECTION OF THE TRENCH

W = TOTAL FORCE EXERTED BY WEIGHT

OF EQUIPMENT OR LINE LOAD

NOTE: If wheel spacing Is wide, should be also checked for x = distance

from edge of excavation to closest wheel and W = weight supported by

closest wheel.

Figure 3.2 Loads Acting on Shoring Systems

12
Strut 1

Strut 2

Strut 3

di

da/a

d3/2

d4/2

d4/2

Sheeting

di Strut 1 ^ di

4

Strut 2

4

'

62/2

ds

d3/2

Strut 3 d3/2

d4

^Mudsill

Strut 4 ^ d4 2>

Strut 2

Strut 1

Strut 2

Strut 3

Strut 4

CASE 1. Sheeting is not embedded

CASE 2. Embedded sheeting

Note: Use mud sill or equivalent

support unless sheeting is firmly embedded

Figure 3.3 Loads Tributary to Members of the Shoring System

13
cent of those resulting from a load per linear length equal to the sum

of the tributary allowable strut loads divided by the length of the

wale.

Self-contained repetitively-used shoring systems such as trench boxes,

hydraulic shoring systems or pr e-f abr icated strut-wale assemblies

could be rated either for allowable working loads (in lb per square

foot of trench wall), or for pr e-deterrained conditions of use. Rating

for conditions of use would include designation of maximum allowable

depth (or equivalent depth if the backfill is sloped) for given soil

types. Thus a trench box could be rated for use in a 20 foot depth

in Type B (Class II) soil (see Tables 3.1 and 3.3) or, alternately,

for an allowable working load of 880 lb/ft .

Rating Procedures ;

The rating should be based on the professional opinion of an engineer and

marked on the component or assembly. It could be accomplished by engineering analysis or testing. In addition to the loads stipulated in Section 3.3.2 the engineer should consider loads resulting from installation

and construction procedures.

Repetitively used assemblies or components should be kept in good repair.

This could be accomplished by limiting the validity of the rating to a 1 year period, and renewing the rating after inspection by an engineer. In

this case the rating should have an effective date. Hydraulic shores

should be tested at least once a year to 1.25 times their allowable working load, and the load should be maintainable for at least 5 minutes

without a pressure drop.

3.3.4 Determination of Load Capacity by Test

If the load capacity of structural components of a shoring system is

determined by test, the following minimum requirements are recommended:

1. Strength variability should be considered in accordance with the

definition of load capacity.

2. Under no circumstances should the allowable working load of struts in

short term excavations exceed 67 percent of the mean failure load or in long term excavations 50 percent of the mean failure load.

3. For struts the test load should be applied with an eccentricity of not

less than 1/3 the thickness of the strut with respect to any one of

the principal axes (but not simultaneously with respect to both axes),

or with the eccentricity producing an end moment equal to the centerspan moment caused by a concentrated load of 240 lb times the applicable load factor (3.3.1(2)) applied at the center of the strut, whichever is greater. Load eccentricities should be of the same magnitude

and direction on both ends of the strut (single curvature).

14
4. For wood members the provision of ASTM D2915 [4] should serve as a guideline.

5. Impact load should be applied by a 60 lb sand filled leather bag

fabricated in accordance with Section 12.2 of ASTM E72-77 [5]. During

the impact test the sheeting should be supported as in actual working

conditions. Three successive impact tests should be applied. "Failure" under impact load is defined as any one of the following: rupture

of the sheeting or any of its structural supporting members; any

structural damage that would lower the load capacity of the shoring

below that required; excessive bending which could endanger workers

in the trench.

3.4 SOIL CLASSIFICATION

3.4.1 General

Two alternate soil classification systems are recommended: The "Simplified Classification System", and the "Matrix Classification System." One

of the two systems should be incorporated into the standard practice.

3.4.2 Simplified Soil Classification System

The Simplified Classification System is shown in Table 3.1. Soils are divided into three types: A, B, and C. For each soil type the "equivalent weight effect", w^, to be used in eq. (3.1), (3.2) or (3.3) for the"

calculation of lateral soil pressure, and the maximum permissible sideslope for sloped excavations are stipulated. The notes in the table provide guidance for the selection of the soil type. Identification

procedures and tests are further described in Chapter 5 of Reference [21].

3.4.3 Matrix Soil Classification System

The Matrix Classification System is shown in Table 3.2. Soils are divided

into four classes (I, II, III, and IV). "Equivalent weight effects" and

maximum allowable slope for each soil class are given in Table 3.3. Guid- ance for the selection of the Soil Class is given in the footnotes to

Table 3.2 and identification procedures and tests are further described in

Chapter 5 of Reference [21].

3.5 SPACED SHEETING

3.5.1 General

Spaced sheeting can be used only when there is no evidence of spalling or collapse of the unsheeted bank between sheeting members.

15
Table 3.1 Simplified Classification System

Soil Type Description w^lb/ft

Steepest Allowable Slope hor.:vert.-^

Depth 12 ft or less Depth 12-20 ft

A Intact Hard 20^^ 3/4:1 1:1

B Medium 40 3/4 : 11/2:1

C Submerged or Soft 80 11/2:1 2:1

Note:

1. Intact Hard Soils (Type A) include stiff clays and cohesive or cemented sands and gravels^^

(hardpan, till) above the ground water table which have no fissures, weak layers, or inclined layers that dip into the trench. Stiff clays included have cn unconfined com- pressive strength (pocket penetrometer reading)®' - 1.5 tsf or more. Intact hard soils

subject to vibrations by heavy traffic, pile driving or similar effects are Type B .

2. Medium Soils (Type B) are all soils which are not Type A or C.

3. Soft Soils (Type C) include cohesive soils*' with an unconfined compressive strength

(pocket penetrometer reading)®' of 0.5 tsf or less and soils that cannot stand on a slope of 3 hor. : 1 vert, without slumping (muck).

4. Submerged Soils (Type C) are assumed whenever water drains into the trench from the soil

forming the bank; or water is retained by tight sheeting; or there is a possibility that

the trench may be fully or partially flooded before workers leave it or may be entered by

workers within 5 hours after more than half of its depth was flooded and pumped out.

5. Fractured Rock shall be considered Type B when it is dry and Type C when it is submerged.

Unfractured rock is exempt from shoring and sloping requirements.

6. Layered Systems (two or more distinctly different soil or rock types or micaceous seams

in rock) which dip toward the trench wall with a slope of 4 hor.: 1 vert, or steeper

are considered Type C. Layered soils are classified in accordance with the weakest layer.

7. Spaced Shoring Systems (Skeleton sheathing or skip shoring) are permitted in Type A and

B cohesive soils with maximum center to center spacing in accordance with Table 3.4

In long term excavations use w^ - 40 lb/ft

If there is any indication of general or local instability slopes shall be cut back to a slope which is at least 1/4 hor.: 1 vert, flatter than the stable slope.

In long term excavation steepest allowable slope should be 1:1

Cohesive soils are clays (fine grained) or soils with a high clay content which have

cohesive strength. They do not crumble, can be excavated with vertical sideslopes, are

plastic (can be molded into various shapes and rolled into threads) when moist and are hard to break up when dry.

The pocket penetrometer is a small (vest pocket sized) commercially available device that measures In-situ shear strength of cohesive soils.

16
Table 3.2 Soil Classes In Matrix Classification System

Site

Condition

Soil

Water in Trench No Yes Fissures

No Yes Fissures

No Yes

Stiff Cohesive^ I II III

Medium Cohesive II III III IV

Granular^ II III

Soft IV IV

Notes

1. Water In Trench is assimed whenever water drains into the trench from the soil forming

the bank, or water Is retained by tight sheeting, or there is a pKissibility that the trench may become fully or partially flooded before workers leave It, or may be entered

by workers within 5 hours after more than half its depth was flooded and pumped out.

2. Vibrations ; Soils subject to vibrations by heavy traffic, pile driving or similar effects

shall always be assumed fissured .

3. Stiff Cohesive Solls^ include stiff clays and cohesive or cemented sands and gravels (till,

hardpan). Stiff clays Included have an unconfined compressive strength (pocket penetrometer

reading)^' of 1.5 tsf or larger.

A. Medium Cohesive of Solls^ have an unconfined compressive strength (pocket penetrometer reading)

between 0.5 and 1.5 tsf.

5. Granular Solls^^ are gravels, sands and silts that can stand on a slope steeper than 3 hor.:

1 vert, without spalling or slumping.

6. Fractured Rock shall be treated as granular soil. Dnfractured rock Is exempt from shoring

and sloping requirements.

7. Soft Soils are cohesive soils with an unconfined compressive strength (pocket penetrometer

reading)^ of 0.5 tsf or less and soils that can not stand on a slope of 3 hor.: 1 vert,

without slumping (muck)

.

8. Layered Systems (two or more distinctly different soil or rock types, micaceous seams in rock) which dip toward the trench wall with a slope of 4 hor.: 1 vert, or steeper are considered Class IV soils.

9. Disturbed Cohesive Soils (backfill) shall be treated as fissured medium cohesive or sof t cohesive soil.

10. Spaced Shoring Systems (skeleton sheathing or skip shoring) are permitted in stiff and

medium cohesive soil with maximum center to center spacing in accordance with Table 3.5.

Cohesive Soils are clays (fine grained) or soils with a high clay content which have

cohesive strength. They do not crumble, can be excavated with vertical sideslopes, are plastic (can be molded into various shapes and rolled into threads) when moist and are hard to break up when dry.

Granular Soils have no cohesive strength. They normally cannot be excavated with vertical

sideslopes (some moist granular soils will exhibit apparent cohesion and temporarily stand

17
Table 3.3 Minimum Acceptable Stability Requirements

for the Matrix Classification System

Soil Type . Wg Ib/ft^

Steepest

Depth 12

Allowable Slope

ft or less

hor . : vert

.

Depth 12 -20 ft

I 2oa/ 1/2 : it/ 1 : 1

II 40 3/4 : l£/ 1 1/4 : 1

III 60 1 : 1 1 1/2 : 1

IV 80 1 1/2 : 1 2 : 1

No t e s :

1. If there Is any Indication of general or local Instability, slopes

shall be cut back to a slope which Is at least 1/4 hor. : 1 vert,

flatter than the stable slope.

2. In layered soils stability requirements are governed by the weakest

layer

.

In long-term excavations use w^ = 40 lb/ft

In long-term excavation steepest allowable slope should be 3/4:1

In long-term excavation steepest allowable slope should be 1:1

18
3.5.2 Strength of Spaced Sheetlng^ ^

1. Struts and Wales ; Struts and wales supporting spaced sheeting should

be designed to resist the full tributary lateral load (the same load

that would be calculated for tight sheeting).

2. Sheeting ; Spaced sheeting members should be designed to resist the

lateral load tributary to an area equal to the length of the member

times the center to center spacing between the sheeting members (this

includes the unsheeted portion of the trench wall) as follows;

Sheeting members supported by wales (skeleton sheathings) should be

designed to resist 67 percent of the lateral soil pressure "p"

[3.3.2.(3)]; sheeting members directly supported by struts (skip

shoring) should be designed to resist 80 percent of p.

3.5.3 Maximum Spacing of Spaced Sheeting

The maximum center to center spacing of spaced sheeting for the Simplified

Soil Classification and the Matrix Soil Classification, respectively, is

given in Tables 3.4 and 3.5.

Table 3.4 Maximum Center to Center Spacing (in ft) of Spaced Sheeting

for the Simplified Soil Classification System

Depth, ft

Soil Type 5-10 10-15

A 8 (6)^/ 6 (4)*/

B 3 2

Numbers in parentheses are preferred spacings and maximum

spacing for long-term excavations.

Consideration should be given to exemption of conventional timber shoring

from minimum strength requirements.

19
Table 3.5 Maximum Center to Center Spacing (in ft) of Spaced

Sheeting for the Matrix Classification System

Depth, ft

Sol 1 CI 5-10 10-15

I 8 (6)3/ 6 (4)3/

II 3 2

III 3 2

a/- Numbers in parentheses are preferred spacing s and maximum

spacing for long-term excavations.

3.6 RECOMMENDED SPECIAL PROVISIONS

(1 ) Intersecting Trenches

When two trenches with vertical or steep (not sloped) sidewalls intersect

and one trench is shored, the intersecting trench should also be shored to

a distance of not less than its depth from the intersection of the two

trench walls.

(2) Sloping Backfill

If the slope of the backfill behind the trench shoring exceeds 3 hor . in 1 vert, workers in the trench must be protected against objects rolling or sliding from the sloped backfill. This can be accomplished by projecting

the sheeting at least 18 inches above the ground surface or by a specially

constructed protective sill.

(3) Excavation Below the Bottom of the Sheeting

Excavation up to 2 ft below the bottom of the sheeting or trenchbox could

be permitted in short-term excavations provided that:

1. No soil movement below the bottom of the sheeting is evident; and

2. The forces acting on the bracing system are calculated for the full

depth of the excavation, and the sheeting projecting below the lowest wale as well as the lowest wale are designed to resist the

forces that would result if the sheeting would be projecting to the

bottom of the excavation.

20
(4) Restriction on Placement of Equipment and Material

Unless special provisions are made to provide support for the resulting loads

and protect workers against rolling and sliding objects, construction equipment

and excavated and other material should not be placed closer than 2 ft from

the edge (top of the bank) of any excavation (spoil piles should preferably

be placed no closer than 3 ft from the top of the bank).

21
FACIWG PAGE: LoAgz zxcavatlon lUlng itdoJi 6oldi.QJi pULoJi

,

Moodm taggA^ng, baaktloJ) [ndOA top), H.ak2A

bfiacdi, [ndjoui bottom], and 6top<zd i>td<iii Mk^Ao.

^z/uibtz [background]. Tkd 6koKtng and

6loptng uii>zd wcl6 dd^tgndd by an mg-lmoA

a^^tix caAii{^uit 6oit zxptoK.atlon. [Cou/itQJ>y,

ScknoMoJt Foundation Company, E^he^da,

MoAyland )

.

22
4. ENGINEERING GUIDELINES FOR THE DESIGN OF SHORING SYSTEMS AND OTHER

MEANS TO PREVENT MASS MOVEMENT OF SOIL AND ROCK

4.1 GENERAL

These guidelines are for engineers who design shoring systems or determine

sideslopes in excavations. The guidelines are not meant to be a standard

from which an engineer cannot deviate. Rather, they recommend minimum

design loads and safety margins against mass soil and rock movement which

are considered appropriate and design limit states which should be considered by engineers. It is recognized that the design of shoring systems,

the stability analysis of slopes, and the assessment of soil conditions

are not an exact science which can be approached with a set of rigid

rules, but rather an art which requires judgment, experience and recognition of unique local conditions. Thus these guidelines can neither be

imposed as mandatory rules, nor can a professional engineer forego his

responsibility to determine in each instance whether the stated guidelines

are adequate. M 23
4.2 SCOPE

The guidelines contain recommended minimum requirements for the protection

of workers in excavations against death and Injury by mass movement of soil

and rock. They do not cover other important parameters which an engineer

must consider, such as protection of adjacent structures, utilities and

Improvements against damaging settlements, or effects of ground water

fluctuations on adjacent properties. They also do not cover other safety

requirements in excavations which are unrelated to soil and rock movement.

Three methods of preventing soil and rock movement are considered in the

guidelines: sloping of the banks of excavations; shoring; and shielding

of the work space by protective devices. Other methods could also be used

such as soil stabilization by freezing or grouting. The guidelines do not

apply to excavations whose collapse does not endanger workers.

4.3 DESIGN LOADS

4.3.1 General

All the design loads listed, but not necessarily only the listed loads,

should be considered. Unless specifically stated otherwise in the design

criteria, the most critical combination of design loads should be considered. The design loads quantified herein are "working loads" (see

definition)

.

4.3.2 Soil and Water Loads

Loads caused by soil and water pressures should be calculated in accordance with accepted engineering practice and these guidelines.

(1 ) Loads Caused by Water

Hydrostatic loads, hydrodynaralc loads .and seepage forces should be considered where applicable. Special attention should be given to the effects

of potential groundwater fluctuations, saturation of previously drained

deposits, and water penetration into fissures. The following conditions

are recommended as the basis for determining critical loads:

For long-term excavations: conditions caused by the 5-year flood.

For short-term excavations: conditions caused by the 1-year flood or alternatively the most severe condition that will not cause interruption of work and evacuation of the workers from the excavation.

(2) Soil Loads

Soil loads should be determined in accordance with the state of the art in

geotechnlcal engineering. Special attention should be given to fissures,

planes of weakness and previously excavated soils. The following conditions are recommended as a basis for determining critical loads.

24
For long-term excavation: Drained as well as undralned conditions

should be considered If applicable. Short-terra strength characteristics should not be assumed to contribute to stability. Effects of

exposure, lateral expansion, desiccation cracks, freezing, erosion,

and change In confining pressures should be taken Into account.

For short-term excavations: In most Instances only undralned conditions should be considered. Short terra strength characteristics

could be considered, provided that an adequate assessraent Is raade

of conditions that could lead to loss of strength.

Further Information is provided In Section 4.5

4.3.3 Surcharge Loads

Surcharge loads should be determined on the basis of actual anticipated

working conditions. Consideration should be given to: the amount and

location of accumulated spoil material; stored construction material;

construction equipment; vehicular and human traffic; and foundations

adjacent to the excavation.

In no case shall the surcharge load be assumed less than 200 lb/ft distributed over the entire ground surface or the equivalent of an additional

2 ft depth of material excavated on the site (using average unit weight

of soil deposits), whichever is more.

4.3.4 Operational Loads

All loads caused by the anticipated excavation work must be considered.

These Include excavated or construction material supported by portions of

the shoring system and workers climbing on the shoring systera. The following minimum load should be used for design: A gravity load of 240 lb

distributed over any 1 ft long portion of any strut.

4.3.5 Dynamic Loads

Dynamic loads which can reasonably be anticipated as a result of pile

driving, blasting, vehicular traffic and construction equipment should be

considered

.

4.3.6 Restraint Loads

Restraint loads caused by temperature, moisture, or other factors causing

dimensional changes in structural members of the shoring systera should be

considered when applicable. In general, it can be assumed that the empirically based lateral loads calculated in accordance with present engineering practice contain a reasonable allowance for temperature effects on struts

.

25
4.4 DESIGN CRITERIA

4.4.1 General

This section conveys design limit states and design criteria. "Design

limit states" are events which constitute a failure to meet safety

requirements. "Design criteria" are design rules such as factors of

safety to be used which, if followed, should prevent the occurrence of

the design limit states. It is conceivable that an engineer could deviate

from the design criteria if the occurrence of the design limit states can

be prevented by other means.

4.4.2 Sloped Excavations

(1 ) Design Limit States :

1. Slope stability failure (part or all of the embankment)

2 . Sloughing

(2) Design Criteria ;

1 . Long-term Excavations

(a) Granular soils (no cohesion):

Slope angle should not exceed angle of shearing resistance.

(b) Cohesive Soils:

The safety factor against stability failure should be

<^ greater than 1.5, unless the excavation is monitored by an engineer using instrumentation and other means. The safety

factor should always be greater than 1.3. Suitable surface

and subsurface drainage should be provided to prevent sta-

; :Ai bility failures or sloughing induced by seepage or erosion.

Maximum unbraced height of vertical bank:

5 ft for all soils or fractured rock. No limitation for

unfractured rock.-'

2 . Short-term Excavations

The safety factor against stability failure should exceed 1.3

except that for dry cohesionless soils a slope angle equal to the

angle of repose may be maintained. Short-term strength properties could be utilized, provided that there are adequate safeguards against conditions which could cause strength degradation.

A geotechnical engineer or engineering geologist should determine whether

the rock is unfractured.

26
Maximum unbraced vertical bank: For Intact hard clays the

unbraced height could exceed 5 ft provided that an engineer can document that there is substantial empirical evidence that the

risk is not excessive. For all other soils, including fractured

rock, the maximum unbraced height should not exceed 5 ft. There

are no limitations for unfractured rock.

. 3 Braced Excavations

Design Limit States

1. Stability failure of the bank

2. Base instability

3. Partial caving or sloughing of the bank between spaced vertical or horizontal supports.

4. Failure of the soil supporting struts, anchors, or soldier piles.

5. Failure of structural components of the shoring system.

Design Criteria

1. Stability of the Bank

A stability failure of the bank is the collapse of all or part of

the bank caused by sliding of a soil mass along a failure surface. The failure surface may lie outside the support points

of structural members of th« shering systems (supports of raker

braces, soil anchors, or the bottom of soldier piles or cantilever sheeting) and thus render the shoring Ineffective, or it

may be caused by the structural failure of members of the shoring

system.

The safety factor against any stability failure ©f the bank should

exceed 1.5.

2. Base Stability

Base Instability leads to heaving of the base of the excavation,

which in turn can cause dislocation and collapse of the shoring

system. The safety factor against base Instability should exceed

1.5. Potential effects of uplift resulting from artesian pressure in confined aquifers should be considered. Dewatering

should be adequate to prevent piping (quick condition) caused by

seepage of groundwater into the base of the excavation. In deep

clay deposits, base instability should be considered a problem

27
whenever Nl exceeds the following values: > 6 for trenches

where S > 3; > 5.14 for very wide excavations: intermediate

values for 0 < — < 3.

B -

where = y^/c^ = stability number for base failure.

o Y = unit weight of soil, lb/ft

H = depth of excavation, ft.

B = width of excavation, ft.

c^ = undrained shear strength below excavation base, lb/ft

3 . Soil Stability between Spaced Supports

There is no generally accepted theoretical approach by which the

ability of a soil to arch between successive supports can be

evaluated or correlated with strength properties of the soil.

There is empirical evidence that short-term supports can be

spaced up to 8 ft on center in hard clay, very stiff sandy clays

or glacial tills, and 2 to 3 ft on center in slightly fissured

clays

.

Guidance is given in Section 3.5 and should be compared with

empirical field evidence.

4 . Soil Support for Struts, Anchors or Soldier Piles

A minimum safety factor of 2 is recommended against bearing failures of members of the shoring systems such as raker braces. A

safety factor against shear failure of the supporting soil of not

less than 1.5 should be used when passive earth pressure is

relied upon to support embedded portions of soldier piles and

sheeting or deadmen.

All soil anchors should be proof load tested to 1.33 times their

working load. If the load capacity of soil anchors is determined

by tests, it should be not less than 1.5 times the working load

for anchor inclinations of 2 hor. : 1 vert, or flatter, and

increase to 2.0 times their working load for inclinations of 1 hor: 2 vert. When anchor capacity is determined by analysis the

safety factor should not be less than 3. Soil anchors subjected

to the working load should not show creep when the load is

sustained for 15 minutes.

28
5. Design of Structural Components of the Shoring System

(1) Applicable Standards

Structural members should be designed In accordance with the

following standards:

Steel: Specifications for the Design, Fabrication and Erection of

Structural Steel Construction, American Institute of Steel

Construction, New York, N.Y. , Feb. 1969 [3].

Concrete: Building Code Requirements for Reinforced Concrete, (ACI

318-77), American Concrete Institute, Detroit, Michigan,

November 1977 [2]

.

Aluminum: Specifications for Aluminum Structures, The Aluminum

Association, New York, N.Y., November 1967 [1].

Wood: National Design Specifications for Wood Construction,

National Forest Products Association, June 1977, for softwood lumber stresses [12].

Because formally approved allowable working stresses do not

exist for most hardwood species, applicable ASTM Standards

may be followed in conformance with procedures recognized

under the American Lumber Standard, PS70/70. (For allowable

stresses refer to Appendix A.)

(2) Allowable Stresses

Allowable stresses should be determined in accordance with the applicable standards. In long-term excavations allowable stresses should

not be exceeded under any applicable combination of working loads.

In short-term excavations allowable stresses in structural members

may be exceeded by up to 33 percent, however, allowable stresses

should not be exceeded in connections between structural members.

(3) Ultimate Strength Design

Ultimate strength, rather than working stress design may be used

whenever such a procedure is stipulated in the applicable standard

or load capacity is determined by test. Ultimate loads should be

taken as 1.7 times the working load for long-term excavations and

1.3 times the working load for short-term excavations, and should

not exceed the load capacity as defined in Section 3.3.1.

(4) Determination of Load Capacity by Test

Determination of the load capacity of structural components of the shoring

system by tests should be in accordance with Section 3.3.4.

29
4.5 INFORMATION ON ACCEPTED ENGINEERING PRACTICE

4.5.1 General

This section contains a brief summary of information on commonly used

engineering practice which is considered to provide adequate protection

against the mass movement of soil and rock. The choice of the referenced

design approaches should not be interpreted as an endorsement of these

approaches over other approaches which are consistent with the present

state of the art.

4.5.2 References

The following references provide guidance in the calculation of lateral

loads on excavation bracing. Loads calculated in accordance with these

references are considered to be working loads: Department of the Navy,

1971 [7]; Goldberg et.al., 1976 [9]; Peck, 1969, [14]; Peck et. al., 1974

[15]; U.S. Steel Co., 1975, [20].

The preceding references contain design approaches which are not neces- sarily identical. However, all these approaches are widely used and are considered adequately conservative.

4.5.3 Summary of Information

Hereafter is a summary of information derived from references in Section

4.5.2. The suggested pressure envelopes are not intended as an endorsement

of one single approach to the problem, but rather as a summary of commonly

used approaches.

1. Lateral Pressures

(1) Sands (Peck, 1969)

k^ = tan^ (45 - <t)/2) H

30
(2) Soft to Medium Clays, when N > 6 (Peck, 1969) (if pressures

calculated under (3) using 0.4 yH are larger, use (3)).

0.25H

N = yH/C

0.75H

1.0 kn y H

= 1 - ra -

a N

When cut is underlain by deep, soft,

normally consolidated clays: m = 0.4

All other cases: m = 1.0

(3) Stiff Clays, whenever N < 4. (Peck, 1969) (if 4 < N < 6 use (2) or

(3), whichever gives larger pressures)

RANGE

0.25H

0.50H

0.25H

0.2 yH

^0.4yH —

Relatively Uniform

0.80H

0.20H

Upper Third of Cut

Dominated by Cohesionless Sands

-QI5yH- — 0.25yH

31
2. Soil Properties

TABLE A.l. Typical Values of Unit Weight of Soils [15]

Silty or clayey sands & gravel Moist U.W. above Saturated U.W. Below

Soil Type W.T. , ^(Ib/ft^) W.T., Ysat^^^/^'^^

Poorly graded sand 105-115 115-125

Clean well graded sands 115-125 125-130

Sllty or clayey sands 120-130 125-135

Silty or clayey sands & gravel 125-135 130-145

Soft to medium clay 100-115 100-115

Stiff to very stiff clay 110-125 110-125

Organic silt or clay 90-100 90-100

^sub " l^sat ~

Y„ - 62.4 Ib/ft^

TABLE 4.2. Relationship Between Properties of Coheslonless

Standard Penetration Test Results [19]

Soil and

SPT, N Soil Type blows/ft.

Relative

Density ^

Dj.% (after Peck)

Very loose sand <4 0-15 29° >0.35

Loose sand 4-10 15-35 29°-30'' 0 .35-0.33

Medium dense sand 10-30 35-65 30''-36° 0 .33-0.25

Dense sand 30-50 65-85 36°-41'' 0 .25-0.21

Very dense sand >50 85-100 >41'' <0.21

32
TABLE 4.3 Properties of Cohesive Soil and St andard Penetration

Test Results [9]

Clay SPT, N Shear Str. Comp. Str. Consistency Identification blows/ft Ib/ft^ Ib/ft^

Very soft Easily penetrated several

Inches by fist. Extudes

between fingers when

squeezed In hand.

Soft Easily penetrated several

Inches by thumb. Molded

by light finger pressure.

Medium Can be penetrated several

inches by thumb with moderate effort. Molded by

strong finger pressure.

Stiff Readily indented by thumb

but penetrated only with

great effort. Indented

by thumb.

Very stiff Readily indented by

thumbnail.

Hard Indented with difficulty.

<2 250 <500

2-A 250-500 500-1000

A-8 500-1000 1000-2000

8-15 1000-2000 2000-4000

15-30 2000-4000 4000-8000

>30 >4000 >8000

The correlation between N values and soil properties for clays can be regarded as no more than a crude approximation, but for sands it is often

reliable enough to permit the use of N values in design. Unconfined com- pression tests or triaxlal tests_are more reliable for clays. It should

also be noted that the value of N can be influenced by numerous factors

such as: the depth at which the test is made; the location of the water

table; presence of boulders in the deposits; irregularities in performing

the test; etc. In general, N values used here are representative of those

obtained by the traditional U.S. (rope and cathead) methods. If other

methods are used, a correction for delivered energy is desirable.

33
FACING PAGE: A it^ol. tAmch box U> a^zd to pn.ote,cX mtkoM.

In ptoJidnt pfiacXi-Cd tkd dxcavatlon o^^tm nx- tmdU, bol-ow th<i bottom o(^ tko, t/Lmch box.

(CouAtdiy , NcutionciZ UtiUJiij CowtAactou

A^^ocAjCution, {iJaikington, P.C.).

34
5. COMMENTARY ON RECOMMENDED STANDARD PRACTICE

5.1 SLOPED EXCAVATIONS (Sec. 3.2)

A commentary on the recommendations In this section Is provided In Section

6.2 of Reference [21]. The sloped excavation configurations In Figure 3.1 are suggested In order to give more flexibility to contractors. Vertical

sides are frequently required in specifications for pipe bedding. The

configuration in Case II would permit contractors to meet such specifications. In Case III any configuration is permitted that would provide

overall stability (not necessarily local stability) equivalent to that of

the sloped excavation. The 3-ft limitation of the height of the vertical

unsupported bank in the vicinity of the work area is provided to minimize

the effects of a potential localized collapse and is based on the recom- mendations of an AFL-CIO Trenching Hazards Identification Task Force

35
report dated April 25, 1977 which is presented as an appendix to Ref [10].

The 5 ft limitation on the height of any vertical unsupported bank in

Case III is consistent with present OSHA regulations.

Steepest allowable slopes are given in Tables 3.1 and 3.3. Steeper slopes are allowed for trenches with depths up to 12 ft. There are two reasons

for this recommendation: (1) in cohesive soils a slope which is stable in

a shallow excavation may not be stable in a deep excavation; (2) the time

of exposure and thus the risk of collapse is less for shallower trenches.

For the steeper slopes a distinction is also made between short term and

long term excavations, since a steep slope is considered risky when the

exposure time is long enough to result in a change in the short-term

strength properties of the soil.

5.2 STRENGTH OF SHORING SYSTEMS (Sec. 3.3.1)

A distinction is made between long-term and short-term excavations. For

working-stress design allowable stresses in short-term excavations can be

increased by 33 percent. For ultimate-load design the load factor for

short-term excavations is 1.3 and that for long-term excavations is 1.7.

This approximately corresponds to the 33 percent increase allowed for

short-term excavations. There are several reasons for decreasing the

design loads (or increasing allowable stresses) for short-term

excavations

:

1. The lateral loads exerted by the retained soil tend to be less

for short-term excavation bracing (see Ref. [21]).

2. The time of exposure in short-terra excavations is less. This

decreases the risk of extreme loading conditions and also the

accident risk.

3. Structural members of the shoring system of short-term excavations are stressed for a shorter time.

The definition of "load capacity" is designed to produce safety margins

similar to those required in present standards (ACI, AISC, etc.).

5.3 LOADS ACTING ON SHORING SYSTEMS (Sec. 3.3.2)

(1) Operational Loads : [Section 3.3.2(2)]

The 240 lb load on struts is intended to insure that struts would not fail

when workers occasionally climb on them. In a study of loads on guardrails [8] it has been determined that about 95 percent of male workers

will have body weights of less than 220 lb (including shoes and safety

helmet but not clothing) . This weight has been conservatively increased

to 240 lb. Larger gravity loads have been proposed by others (Appendix

to [10], AFL-CIO report). However, it is suggested that a design for any

load other than that caused by the occasional support of workers may lead

to misuse and overloading. It is more desirable to prohibit the loading

of struts unless they are specifically designed for the loading. The 240

36
ft-lb impact load specified for sheeting that does not bear tightly Cor

exert any thrust) against the trench wall is a precaution against the case where an airspace is left between the trench wall and the trench box (or

sheeting). Impact load could be generated when the trench wall suddenly

collapses and fills the empty gap. It is assumed that in all instances

where struts are not pre-loaded to exert a pressure against the trench

wall there will be occasional gaps.

(2) Lateral Soil Pressures [Sec. 3.3.2 (3)]

The lateral soil pressure diagrams associated with equations (3.1), (3.2),

and (3.3) are for simple soil classification systems (Sec. 3.4) which

do not provide enough information to justify the selection of more complex

pressure diagrams such as those in Section 4.5.3.

Derivation of Simplified Equations for Pressure Increase Caused by Sloped

Backfill.

Equation (3.2)

Eq. (3.2) and the equations for equivalent weight in Figure 3.2 (b):

P = ^e«e

= H(l + 2v/H) = H(l + 0.04 3)

are derived hereafter:

The lateral support provided by excavation bracing is not rigid, nor are most soils purely cohesionless . However, the complexity of the case of a soil which has cohesion as well as an angle of shearing resistance, bearing against a flexible support is so great, and there are so many variables that a generalization cannot be made. The assumption made in this

derivation, which tends to be conservative and is consistent with present

engineering practice, is that the effect of sloping surcharge of a cohesionless soil against a rigid retaining wall can be assumed to give a reasonable indication of the magnitude of the effect of sloping surcharge

on the lateral soil pressures on excavation bracing.

The coefficient of active soil pressure for cohesionless soils acting

against a rigid retaining wall has been analytically calculated by Coulomb

[6] as follows:

sin (a + (})) cos 6 k =

a

. . .eq(l)

sin a sin (a - 6) 1 +

sin (<t> + 6) sin ((j) - 8)

sin (a - 6) sin (a + 6)

37
where: k„ =

a coefficient of active earth pressure

6 = angle of friction between backfill and the wall (is positive

if the top of the wall rotates away from the retained soil

and negative if the bottom rotate out)

(|) = angle of shearing resistance of the soil

the other sjonbols (a, 3) are explained in Figure 5.1.

For a vertical wall (a = 90°) this reduces to:

2 cos

k =

a

1 +

sin (()) + 6) sin ((}) - 3) cos 6 cos 6

. . .eq(2

)

if e =0 (level backfill)

cos (f) k = a

1 +

sin ((}) + 6) sin

cos 6

if 6 =0 and the backfill is level eq (3) reduces to:

k, = tan^ (45 - <d/2)

. . .eq(3

)

it is assumed that the ratio

(6?^0)

Rg = is a reasonable measure of the pressure increase for

(8=0)

sloping backfill for any given value of 6. If a = 90°:

2

R =

s

^ _l_

/ sin ((}) -f 6) sin (j) \ cos 6

1 + J sin (<t> + sin (<t> - B)

1 cos 6 cos 3

. . .eq(4)

38
Figure 5.1 Symbols Used in Derivation of the

Equation for the Sloped Backfill Effects

39
if 6 = 0 this reduces to:

1 + sin

R =

s sin 4) . sin - B) cos B

For the limiting case where B = (j), eq. (5) reduces to;

,eq(5)

Rg = (1 + Sin ,eq(6)

this can be approximated by the following expression in terms of the backslope angle 6 in degrees:

Rg = 1 + 0.04 B (if 3 = <t>)

or in terms of backslope gradient (h/v):

Rg = 1 + 2v/h

and Hg H(l + 2v/h)

. . .eq(7

)

. . .eq(8)

Eqs. (6), (7), and (8) are compared in Figure. 5.2

The case of B = <{) is an extreme but not unusual case, since frequently the

bottom of a trench is shored while sideslopes are maintained for the re- mainder of the depth. These sideslopes are likely to approach the steepest stable slope. A similar situation arises on steep sidehills.

In Figure 5.3 a range of R^ values for various values of ^ and 3 is shown

for the two typical cases of 6 = -^/2 and 6 = +<t)/2. In an actual field

situation it is difficult to predict the probable effects of friction

between the wall and the soil. Coulomb's assumption on a negative or positive 6 is predicated on a rigid-body rotation by the wall relative to

the retained soil. In a braced excavation relative movement would mostly

occur as a result of wall deformation in flexure. In this case, the dir

ection of relative slippage between wall and soil is not likely to be the same over the full height of the wall. Thus the assumption that 6=0

seems reasonable. The curve for <}> = 3 and 6 = 0 is therefore a conservative upper bound for the effects of sloping backfill in cohesionless

soils

.

In Figure 5.4, R^ calculated by eq. (6) is compared with curves calculated

from Figure 10-9 in NAVFAC Manual DM-7 [7] which is representative of

present practice. Note that the curves include clays and that eq (6)

40
Figure 5.2 Simplified Expressions for R

hi
0 10 20 30 40 50

DEGREES

Figure 5.3 Relationship Between R^, ,j) ,6 , and 6

42
Rs2.o|-

— Rs ()8=</), 8=0]

CLEAN SAND & GRAVEL

DIRTY SAND & GRAVEL

STIFF RESIDUAL SILTS & CLAYS

MIDIUM TO STIFF CLAYS DEPOSITED IN CHUNKS

& PROTECTED FROM INFILTRATION

VERY SOFE-SOFT CLAY, SILTY CLAY

ORGANIC SILT & CLAY

40 50

DEGREES

Figure 5.4 Comparison of Proposed Equation for with NAVFAC Criteria

43
appears to be a conservative upper bound. Since the NAVFAC curves are not

carried to the point where g is the steepest possible slope, eq (6) is not

quite as conservative as it appears to be in this figure.

It is also of interest to examine the equation for k„ if g = * and 6=0:

^a(g=(t), 6=0) = ...eq(9)

This can be rewritten in terms of slope (h/v)

2 h2

k = = = ,..eq(10)

^ ^ v2 + h2

This equation can be converted into an expression for p that could be subby multiplying by the weight of soil; thus

-ng Y

stituted for equation (3.2)

assuming y - 100 lb/ft

p (4. = B, 6 = 0)= ^QQ^

^

• H ...eq(ll)

V + h

Equation 11 is simple and compact and could be included in the "standard

practice"

.

Equation (3.3)

Equation (3.3) and the equation for heavy equipment load effects in Figure

3.2 (c):

p = Wg(H + Hq) + p^;

W X 0 . 8W

p = _ ___ n - 0.6 —) < —:——-—- are derived hereafter:

q H()l + x) ^ H - H(jl + x)

Figure 5.5 shows some heavy construction equipment (the wheel positions

are shown by black squares) of total weight W acting through its center

of gravity.

44
Figure 5.5 Effects of Heavy Construction Equipment

45
It is assumed that the effect caused by weight of the heavy equipment can

be reasonably represented by an equivalent line load "q" parallel to the

side of the trench.

Using Boussinesq's equations, the distribution of horizontal pressures

(a^^) against a vertical plane resulting from a line load can be calculated

as

:

2

g = 2 q ^ m n ...eq(l)

^ ^ (m^ + n2)2

where: H = depth of excavation

m = x/H

. n = z /H

q = lineload (force/length)

X = distance from lineload to side of trench

z = depth below surface.

The dimensions used in eq (1) are shown in Figure 5.6

Terzaghi [18] recommended values of approximately twice the Boussinesque

pressures. Thus the following equations are commonly used to calculate

pressures against rigid retaining walls:

a = -Ai. • , ifm>0.4 ...eq(2)

h ttH , 2, 2.2

(m -l-n )

a

• ^^-T^ ifm<0.4 ...eq(3)

Figure 5.7 shows a plot of the nondimensionalized measure of horizontal

pressures ay^ • H/q for three values of m. The same plot shows also a

46
q

Figure 5.6 Dimensions Used in the Derivation of Simplified

Expressions for Heavy Equipment Effects

47
48
uniformly distributed lateral pressure (o^^) which would produce a total

resultant force on the bracing system equal to that produced by the lineload. The equivalent uniform lateral pressure can be calculated by

integrating eqs. (2) and (3) as follows:

for m < 0.4; a, = "'^^^^ — ha H

1

1 ndn

J

(0.16+n^)

0.203q

H

- 2-0.16

4x0. 16(0. 16+n)

= 0.548 . . .eq (4)

H

the maximum ratio of — occurs at the depth of n = 0.231 and is '^-^^ = 1 ^ha 0-548

for m > 0.4; a 4m

ha TT ndn

(m +n )

4m

TT H 2 2

2(m +n )

n = 1

n = 0

71 (m +1)

_3_

H

in the practical range of values for m, 0.4 < m < 1 this is closely

approximated by:

= - (.7074 - .3973 m) = — (0.7 - 0.4 m) ...eq(5)

H H

Thus it has been shown that a^^ can be calculated by rather simple expressions. Eq (4) would be for the case where the surcharge would be at a distance of 40 percent of the depth or less from the edge of the trench.

Eq (5) for distances between 0.4H and IH. Any surcharge load located

further than IH from the trench wall can be disregarded. For moving

equipment which could be positioned close to the edge of the trench,

= 0.5 q/h is a reasonable worst-case approximation. For fixed lineloads such as lanes of heavy traffic eq (5) may be appropriate.

49
As is apparent from Figure 5.7 and also from the ratio calculated for

m < 0.4, ^^/^Yia ~ '-•^^» average value a^^over the

entire surface of the shoring is not conservative, particularly for a large H where the upper struts may be subjected to excessive loads. On

the other hand, one should realize that as the distance of a piece of

equipment of limited length from the edge of the trench increases, its

representation by an infinite line load becomes more conservative.

Another factor that must be recognized is that equipment loads are dynamic

and an impact factor should be used in their representation. Recognizing

all these variables, the recommended design (working) load was derived

as follows:

W . . .eq(6)

«- + X

where: W = total load

I = length of actual load parallel to the trench

X = distance from edge of trench

by this equation the assumed lineload will decrease as the distance from

the trench increases. To account for the shape of the pressure diagram

and impact effects, a lateral pressure (Pq) of 1.5 o^^ is recommended

thus

for X < 0.4 H (see Figure 1)

p_ = _0jl8W__ .eq(7)

: H(il + x)

^

and for 0.4 H < x < IH

Pa = ^ (1-0.6 2i) ..,eq(8)

H(jl + x) H ' ^H(jl + x)

p„ should be assumed 0 if — > 1 q H

(3) Tributary Loads [Sec. 3.3.2(4)]

The recommended loads tributary to members of the shoring system are in

accordance with accepted engineering practice. The load reduction for

wales and sheeting is to account for "arching effects", which will cause members to experience bending moments and shear forces smaller than those

produced by a uniformly distributed load.

50
5.4 SOIL CLASSIFICATION (Section 3.4)

A detailed commentary on the recommended soil classification systems is

provided in Ref [21], which also describes field identification and testing procedures associated with the recommended systems.

5.5 SPACED SHEETING (Sec. 3.5)

It is assumed that even though spaced sheeting does not cover the entire

exposed trench wall, the shoring system must still resist the same resultant force that would be resisted by a shoring system with tight sheeting.

There are presently no data by which soil properties such as cohesive

strength can be correlated with the ability of the soil to stand in the

interval between the spaced supports without collapse or spalling. The

recommended provisions are based entirely on empirical practice and on field observations reported by experienced contractors and foremen. In essence NBS could find no evidence that the present OSHA requirements with

respect to spaced sheeting are unsatisfactory. In Ref [21] there is some additional information by which the ability of unbraced vertical cuts to

stand in the field without collapse or spalling is correlated with

recommended provisions for spaced sheeting.

5.6 RECOMMENDED SPECIAL PROVISIONS (Sec. 3.6)

The recommended requirement to shore both trenches near a trench intersection is based on data obtained in the field study [10, 17], and on the

fact that a pre-loaded strut near a trench intersection could shear off

the corner unless both trenches are shored.

The recommendation to permit some excavation below the bottom of the

sheeting is based on the fact that this practice is presently used succesfully. However, it is not recommended to permit this practice if there is

any evidence that perceptible soil movement occurs below the bottom of the

sheeting or trench box. As an added protection it is required to provide

added strength to the bottom wale and projecting sheeting and to calculate

the lateral pressure on the basis of the full trench depth.

The recommended restriction on placing objects and construction equipment

near the edge of excavations extends a similar restriction in present OSHA

provisions to construction equipment. However, it permits the contractor

to make special provisions in tight situations.

51

6. REFERENCES

[I] Aluminum Assocation, Specification for Aluminum Structures,

New York, N.Y., November 1967.

[2] American Concrete Institute, Building Code Requirements for Reinforced Concrete (ACI 318-77), Detroit, Mich., November 1977.

[3] American Institute for Steel Construction (AISC), Specifications for

the Design, Fabrication and Erection of Steel Construction, New

York, N.Y. , Feb. 1969.

[A] ASTM, Standard Methods for Evaluating Allowable Properties for

Grades of Structural Lumber, ASTM 2915-74, ASTM Standards Part 22,

pp 830, American Society for Testing and Materials, Philadephia, PA,

1979.

[5] ASTM, Conducting Strength Tests of Panels for Building Construction,

ASTM E72-77, ASTM Standards Part 18, American Society for Testing

and Materials, Philadelphia, PA, 1979.

[6] Coulomb, C.A., Essai sur une Application des Regies des Maximis et

Minimis a quelques Problemes de Statique Relatifs a 1' Architecture

,

Mem. acad. roy. pres. divers savants. Vol. 7, Paris, France, 1776.

[7] Department of the Navy, Design Manual, Soil Mechanics, Foundations

and Earth Structures, NAVFAC DM-7, Chapte 10, U.S. Naval Publications and Forms Center, Philadelphia, PA, September 1971.

[8] Fattal, S.G. and Cattaneo, L.E., Investigation of Guard Rails for

the Protection of Employees From Occupational Hazards, NBSIR76-1139,

National Bureau of Standards, July 1976.

[9] Goldberg, D.T. , Jaworski, W.E. , and Gordon, M.D. , Lateral Support

Systems and Underpinning, Report No. FHWA-RD-7 5-129 , Vol. II, Federal Highway Administration, Washington, D.C., April 1976.

[10] Hinze, J., and Carino, N.J., A Study of Work Practices Employed to

Protect Workers in Trenches, NBSIR No. 79-1942 National Bureau of

Standards, March 1980.

[II] Knab, L.I. , Yokel, F.Y., Galligan, W.L. , and Senft, J.F., A Study of

Lumber Used for Bracing Trenches in the United States, BSS 122

National Bureau of Standards, Washington, D.C., March 1980.

[12] National Forest Products Association, National Design Specifications

for Wood Construction, Washington, D.C., June 1977.

[13] Occupational Safety and Health Administration, Safety and Health

Regulations for Construction, Subpart P, Excavation, Trenching and

Shoring, Federal Register, Vol. 39, No. 122, June 24 1974.

53
[14] Peck, R.B., Deep Excavations in Soft Ground, Proceedings of the

International Conference on Soil Mechanics and Foundation Engineering, State-of-the-Art Volume, pp. 225-250, Mexico City, 1969.

[15] Peck, R.B., Hanson, W.E., and Thurnburn, T.H., Foundation

Engineering , Chapter 27, John Wiley and Sons, New York, N.Y., Jan.

1974.

[16] Salomone, L.A., and Yokel, F.Y. , Proceedings, Federal Workshop on Excavation Safety, September 19 and 20, 1978, NBSIR 79-1935,

December 1979.

[17] Salomone, L.A., and Yokel, F.Y. , An Analysis of the Response from an Associated General Contractors of America Survey of Trenching and

Shoring Practices, NBSIR 79-1936 National Bureau of Standards,

Washington, D.C., January 1980.

[18] Terzaghi, K., Anchored Bulkheads, Transactions ASCE, Vol. 119, p.

1243, New York, N.Y., 1954.

[19] Tschebotariof f , T. P., Foundation, Retaining and Earth Structures , McGraw-Hill, New York, NY, 1973

[20] United States Steel Company, Sheet Piling Manual, U.S. Steel,

Pittsburgh, Pa., July 1975.

[21] Yokel, F.Y., Tucker, R.L. , and Reese, L.C., Soil Classification for

Construction Practice in Shallow Trenches, BSS 121 National Bureau

of Standards, Washington, D.C., March 1980.

54
7. ACKNOWLEDGMENT

The assistance of the following persons collectively contributed to the

development of the recommendations presented in this report:

Mr. James Bryson from NBS provided direction and valuable assistance as chief of the construction safety section.

Professor Jay B. Kim of Bucknell University carried out a major phase of

this study which consisted of a summary of the state-of-the-art and an analytical study which was part of the assessment of present trenching

practices and the technical provisions in the present OSHA regulations.

Professor William D. Kovacs of Purdue University prepared the data

for sloping backfill shown in Figure 5.3

Professors Richard L. Tucker and Lymon C. Reese from the University of

Texas conducted the soil classification study which provided the background information for Tables 3.1, 3.2 and 3.3.

Dr. Lawrence I. Knab from NBS, Mr. William L. Galllgan and Mr. B. Allen

Bentsen from the Forest Product Laboratory, and Professor John F. Senft

from Purdue University, conducted the timber study on which Table A.l is

based.

Professor Jimmie Hinze from the University of Missouri at Columbia con- ducted the field study.

Mr. Lawrence A. Salomone from NBS prepared the report on the National

Workshop and analyzed the results of an Associated General Contractor's

Survey on Trenching and shoring practices.

Dr. Nicholas J. Carino of NBS reviewed and revised the field study report.

Messrs. William S. Zoino, James J. Schnabel, James H. Kleinfelder and

John P. Guaedinger, Consulting Engineers and members of the Association of

Soil and Foundation Engineers (ASFE) helped define the role of the

professional engineer, and provided many valuable comments which are reflected in the recommendations made in Tables 3.1, 3.2 and 3.3 and in

Section 4 of this report. They also helped formulate the overall approach

in the Recommended Technical Provisions.

Messrs. Arthur L. Schmuhl and John P. Ashooh of the Associated General

Contractors (AGC), John H. Panullo and Robert G. Griffith of the National

Utility Contractor's Association (NUCA) and Patrick J. Higgins of the

National Association of Plumbing Heating and Cooling Contractors (NAPHCC)

surveyed field practices and received assessment of current OSHA regulations among members of their associations and gave valuable advice and

assistance

.

55
Mr. James L. Lapping, Professor Jack 0. Mlckle and Messrs. M. Ahlquist,

Carl F. Gill and Robert C. Mack, Sr., of the AFL-CIO provided invaluable

guidance in a workshop and Task-Force report on Trenching Hazards Identification.

Messrs. Peter J. Ellis, George Bradberry, W.M. "Red" Cass and David 0.

Plank of the excavation and shoring industry provided invaluable guidance

and advice and helped develop the approach taken in this report.

Messrs. Thomas H. Seymour, David 0. Hadden and Allen E. "Ike" Martin of

OSHA provided much needed assistance and guidance and throughout this

project

.

56
APPENDIX A. TIMBER SHORING

A.l General

There are two reasons for the separate consideration of timber shoring in

this appendix:

(1) Even though hardwood is widely used throughout the U.S. in timber

shoring practice, there are presently no hardwood standards and grading rules and procedures similar to those for softwood. Thus engi- neers have no guidance on allowable working stresses and there is no effective procedure to assure quality control in the field.

(2) NBS could not demonstrate that traditional U.S. timber shoring

practice complies with the recommended standard practice.

Section A. 2 of this appendix contains a recommendation on allowable

working stresses in hardwood trenching timber. Proposed grading rules

were presented in Ref. [11]. Sections A. 3 and A. 4 contain information

on, and an assessment of, traditional timber shoring practice. Timber

sizes which comply with the recommended standard practice are shown in

tables A. 7, A. 8 and A. 9. Appendix A supplements the recommended standard

practice.

A. 2 Recommended Working Stresses in Hardwood Timber Shoring

Recommended allowable working stresses for hardwood trenching lumber are shown in Table A.l. The recommendation was developed in the NBS trenching

lumber study and is taken from Ref [11]. Note that various species and

combinations of species are considered. "Grade No 2" was used as the

basis for the recommendation. This is based on the findings of the NBS

field study, and will require that some of the members currently used in

trenching will have to be discarded or downgraded.

Note that certain limitations on "wane" have to be observed for No. 2 grades in accordance with the 1977 Grading rules of the Southern Pine

Inspection Bureau.

It is strongly recommended that industry extend present grading practice to

hardwood trenching lumber and consider developing special provisions for wane. In the meantime, contractors and suppliers should comply with the

1977 SPIB softwood grading rules for No. 2 grade when selecting and supplying trenching lumber (struts, wales and spaced sheeting). These rules

should be followed, for all trenching lumber , even that used for timber

shoring in conjunction with traditional practice.

The following symbols are used in Table A.l.

= allowable bending stress (flexure)

F. = allowable tensile stress (axial tension)

57
Table A.l Allowable Unit Stresses in psl for Hardwood Trenching Lumber^

Hardwood group'' F^^ F^. F^ F^ F^j. E

2 to 4 in. thick, 2 to 14 in. wide

White 875 575 550 105 355 800,000

Mixed oak<^ 850 550 500 80 355 800,000

Mixed hardwoods 725 475 375 65 165 800,000

Mixed hardwoods 600 400 350 50 115 800,000

5 in. and thicker, 5 to 20 in. wide

White oak^ 975 650 525 120 355 800,000

Mixed oak"^ 925 625 475 90 355 800,000

Mixed hardwoods 800 550 350 75 165 800,000

Mixed hardwoods Ilf 675 450 325 60 115 800,000

b

Ref. Southern Pine Inspection Bureau Grading Rules, 1977 edition, for general grade description as follows:

Grade Paragraph Size

No. 2 313 2 to 4 in. thick, 2 to 4 in. wide

No. 2 343 2 to 4 in. thick, 5 to 14 in. wide

No. 2 SR 406 5 in. and thicker, 5 to 20 in. wide

Assumes 10-yr. load duration basis. For new (first use) lumber, adjustments

for load duration may be made: for 1-yr. duration multiply by 1.1; for 1 wk.,

multiply by 1.25; for 2 days, multiply by 1.30. Load duration adjustments

for used trenching lumber are not recommended. For hardwood trenching

lumber, requirements are waived for manufacture, compression wood, firm knots,

skips, stain and warp. Holes limited as knots; wane limited as given for

No. 2 grade in SPIB, 1977 edition.

Hardwood species defined per ASTM D 1165.

^ White oak: The follwing white oaks—bur, chestnut, live, overcup, post, swamp chestnut, swamp white, white.

^ Mixed oak: Red oak (black, cherry bark, laurel, northern red, pin, scarlet,

southern red, water, willow); white oak (footnote c).

Ash (black, blue, green, Oregon); beech; birch (sweet,

yellow); cherry; elm (American, rock, slippery); hackberry; hickory

(mockernut, pignut, shagbark, shellbark); locust (black, honeylocust)

;

magnolia (cucumber, southern, sweetbay); maple (bigleaf, black, red,

silver, sugar); mixed oak (footnote d); pecan (bitternut hickory, nutmeg

hickory, pecan, water hickory); red alder; sassafrass; sugarberry; sweetgum;

sycamore; tanoak; tupelo (black, water); yellow poplar. Excludes all

Cottonwood, all aspen, basswood, and balsam poplar.

f Mixed hardwoods II: All hardwoods in Mixed hardwoods I (footnote e) plus

black and eastern cottonwood; quaking and bigtooth aspen; basswood.

Excludes balsam poplar.

58
= allowable compressive stress parallel to grain

F^ = allowable shear stress

F^j^ = allowable compressive stress perpendicular to grain

E = modulus of elasticity (Young's Modulus)

F^ and F^ should be considered for flexural stresses in wales, and com- pressive stresses in struts, respectively.

A. 3 Traditional Timber Shoring Practice

Traditional timber shoring practice varies widely from location to location and frequently depends on such variables as sizes and characteristics

of available timber, soil conditions, and local work practices. In some locations these practices have been used for many years and appear to be

satisfactory to all the parties concerned. Three such locations are the

State of Wisconsin, New York City, and the State of California (where

mainly softwood is used).

In order to assess traditional lumber practice, an attempt has been made

to compile tables of typical sizes by combining common elements of available codes and local practices. Table A. 2 is for skip shoring (spaced

sheeting). Table A. 3 for tight sheeting and Table A. 4 shows the increase

in strut sizes with trench width for trenches wider than 4 ft. Table A.

2

is for Type A and B soils since skip shoring would not be used in Type C

soils. Table A. 3 is for Type B and Type C soils since tight sheeting is

not likely to be used in Type A soils. The following correlation should

be used in conjunction with the matrix soil classification system.

Table Soil Type Soil Class

A. 2 A I

A. 2 B II, (III)*

A. 3 B II

A. 3 C III, IV

* Class III soil falling in this category would be fissured medium clay.

A. 4 Assessment of Traditional Timber Shoring Practice

Traditional timber practice can be compared with the proposed standard

practice. This is done hereafter, making the following assumptions:

1. Allowable working stresses are those for "Mixed Hardwood I,"

No. 2 grade.

59
Table A. 2 Typical Minimum Sizes of Timber Skip Shoring (inches)

Trench Soil Type Minimum Strut Size Depth (ft) Minimum Size of Upright Minimum

Wale Size

Horizontal Center-to Center Strut Spacing

4 ft 6ft 8 ft

Horizontal Center-to-Center Spacing

of Uprights

2 ft 3 ft 4 ft 6 ft 8 ft

5-10

10-15

5-10

10-15

10-15

4x4

4x4

4x4

4x4

4x4

4x6

4x4 2x6 2 X 2x6 3x8

2x6

2x6

2x6

None Required

None Required

4x6

4x6

4x6

(a) All lumber sizes are actual sizes in inches

(b) Strut sizes are for 4 ft trench width.

For wider trenches, see Table A-4 rows 1 and 2

(c) Vertical center-to-center spacing of struts and wales should not exceed 4 ft.

(d) 3x6 struts can be substituted for 4x4 struts

in trenches of up to 4-ft width.

60
Table A. 3 Typical Minimum Sizes of Timber Shoring of Trenches

Minimum Strut and Wale Size

Trench Soil Depth Type

(ft)

Horizontal Center^to-Center Strut Spacing (ft)

8 10 12 lA 16 20

Maximum Minimum Center-to-Center Sheeting Vertical Wale Thlclcness Spacing (ft) (In)

5-10

5-10

10-15

10-15

15-20

15-20

Type B

Type C

Type B

Type C

Type B

Type C

4 X A 6x6

6x6 8x8

6x6 6x8

6x8 8x8

6x8 8x8

6x6

8x8

8x8

10 X 10

8 X 10 8x8 10 X 10 10 X 12

8x8 8x8 10 X 10 10 X 10

8 X 10 10 X 10 10 X 12 12 X 12

8 X 10 10 X 10 10 X 12 12 X 12

10 X 10 10 X 12 12 X 12

10 X 10 10 X 12 12 X 12

12 X 12 12 X 12

(a) All lumber sizes are actual sizes In Inches.

(b) Strut sizes are for 4 ft. trench width. For wider trenches, also see Table A. 4.

(c) For strut spacing greater than 10 ft., Insert Intermediate struts before workers

enter trench.

(d) If vertical distance from the center of the lowest strut to the bottom of the trench exceeds 2 1/2 ft., sheeting shall be firmly embedded or mudsill shall

be used. The vertical distance from the center of the lowest strut to the bottom of the trench shall not exceed 36 in, or 42 in if mudsill is used.

61
Table A-4 Adjustment of Strut Size for Trench Width

Trench

Width Strut Sizes in Inches (Read Down for Equivalent Size)

(ft)

4 4 X 4 4 X 6 6 X 6 6 X 8 8 X 8 8 X 10

6 4 X 4 4 X 6 6 X 6 6 X 8 8 X 8 8 X 10

9 4 X 6 6 X 6 6 X 6 6 X 8 8 X 8 8 X 10

12 6 X 6 6 X 6 6 X 8 8 X 8 8 X 8 8 X 10

15 6 X 6 6 X 6 6 X 8 8 X 8 8 X 10 10 X 10

Note: 10 x 10 or larger sizes need no adjustment.

62
2. A 33 percent increase is allowed for working stresses since the

shoring is for short term excavations.

3. 80 percent of the calculated moment is used for wales, and 100

percent of the calculated compressive load for struts (in accordance with recommended standard practice).

4. Strut load effects include the effect of a 240 lb downward load

in the center of the strut.

5. It was conservatively assumed that rectangular wales may be

installed with the larger cross sectional dimension vertically,

and that wales are not continuous over several struts (simply

supported beams with no end restraint.

Tables A. 5 and A. 6 contain an analysis of Tables A. 2 and A. 3, respectively. They have the same format as Tables A. 2 and A. 3, but instead of

the member size they give a "stress ratio": allowable working stress/

actual working stress. If the actual working stress is greater than the

allowable stress this ratio is smaller than one, and the implicit safety

margin is less than that used in standard design practice.

In accordance with Table A.l, the allowable stress could be 1.3 times the

stresses shown in the Table in a short-term excavation where the timber is

not re-used. Thus if the "stress ratio" is 0.78 the member would still

meet the requirements of the standard practice, provided it has not been

used before. In every case where the stress ratio is below 0.78, the

member does not meet the requirements of the standard practice.

An analysis of Table A. 2 for skip shoring is presented in Table A. 5.

Stress ratios for struts are calculated for a 4 ft wide trench.

The stress ratios for the struts in Type A soils indicate that the strut

sizes comply with the standard practice. In type B soil there is a 20 percent overstress in new struts and a 50 percent overstress for re- used struts. The wales used are in all instances substantially

overstressed

.

Table A. 6 shows an analysis of Table A. 3 for tight sheeting. Two types

of stress ratios are shown for spacing larger than 10 ft. The left number

shows the stress ratio without intermediate struts and the right number

with intermediate struts (see footnote c to Table A. 3). The anlaysls

indicates that while struts are reasonably in compliance with the standard

practice, again wales are much undersized, particularly for spacings up to

10 ft where no intermediate struts are required.

The question needs to be asked why timber shoring does not fail more frequently if the wales are as undersized as Table A. 5 would indicate. It is

assumed that several factors combine to produce this phenomenon:

(1) Most timber members can probably be stressed to 2 to 3 times

their allowable stress or more before failure actually occurs.

63
Table A. 5 Analysis of Table A.

2

Trench Depth (ft) Soil Type Struts Wales

Horizontal Spacing (ft)

4 6 8

5-10 A 1.13 0.92

1.15^' 0.96

10-15 A 1,13 0.85

1.15 0.88

5-10 B 0.67 .29 0.71

10-15 B 0.72 .18

10-15 B 0.67 .37 0.71

a/- Bottom numbers are for the case where 3x6 struts are substituted for 4x4 struts.

Table A. 6 Analysis of Table A.

3

Horizontal Center-to-Center Spacing (ft)

Trench Soil Struts

12a/ I4S/ a/ 2Qa/ Depth Type or 6 8 10 16 (ft) Wales

S .80 .97 .79 1.19 2.30 1.03 1.99 1.42 2.78 1.14 2.24

5-10 B W .20 .25 .19 .26 1.05 .19 .77 .29 1.15 .18 .74

S .66 .90 .73 .76 1.49 .82 1.62 .86 1.71 .83 1.65

5-10 C W .22 .30 .19 .16 .66 .19 .75 .17 .69 .16 .64

S .87 .88 .96 1.01 1.97 1.09 2.14 1.15 2.26 1.11 2.19

10-15 B W .30 .22 .25 .22 .88 .25 1.01 .23 .92 .21 .85

S .60 .61 .77 .64 1.27 .66 1.31 .69 1.38

10-15 C W .20 .20 .25 .17 .68 .15 .60 .17 .67

S .88 .90 .91 .95 1.88 .98 1.94 1.04 2.05

15-20 B W .30 .30 .27 .26 1.03 .23 .90 .25 1.0

S .61 .72 .69 .69 1.38 .60 1.18

15-20 C W .26 .29 .22 .22 .89 .16 .65

left number in column is for full unsupported wale length, right number with

intermediate strut supports.

64
(2) Wales are very flexible members and lateral soil pressures will

tend to decrease as they bend away from the trenchwall. The 20 percent stress reduction allowed in the standard practice may only

partially account for this effect.

(3) Lateral design soil pressures recommended in the standard practice are probably much larger than those actually experienced in

most (but not all) instances (this is discussed in Reference [21]).

The following conclusions can be drawn from the analysis of traditional

timber practice:

1. The strut sizes used are reasonably in compliance with standard

practice. Even in instances where they are not in compliance, there

is probably still a safety margin left.

2. Wale sizes are in all instances deficient and it can not be

analytically demonstrated that there is an adequate safety margin

against failure.

Since, in spite of the results of this analysis, NBS could find no evidence that traditional timber practice, if properly executed, is unsafe,

consideration could perhaps be given to temporarily exempting conventional

timber shoring from the lateral load requirements until lateral load

effects can be further studied by actual measurements in the field. If

such an approach is adopted, it may be more reasonable to endorse proven

local shoring practices on a regional basis, only where such shoring is

widely used. It is not recommended to use a single scheme such as Tables

A. 2, and A. 3 nationwide, since local practice evolved on the basis of

local workmanship, material supplies and soil conditions.

It has been previously noted that the struts used in conventional timber

shoring reasonably comply with the requirements of the standard practice.

Thus, with relatively minor adjustments (mostly upgrading) all struts could

be brought into compliance. It should also be noted that the adjustments

to bring skip shoring into compliance with the standard practice would only

require substantial upgrading of the wales, which are used rather infrequently. Tables for timber shoring, which would be in compliance with the

recommended standard practice are given below. Table A. 7 is for skip

shoring. Table A. 8 is for struts and Table A. 9 for wales. In Table A.

9

it was assumed that wales would be used with the long cross-sectional

dimension horizontal to give maximum moment resistance, and that the overstress in large spans would not exceed 100 percent before the intermediate

struts are installed. It is recommended to retain traditional practice

for uprights in skip shoring and sheeting thickness in tight sheeting,

since this practice is widely used in all types of excavations and there

is no evidence that it is inadequate. Tables A. 7 through A. 9 are for

short-term excavations.

65
Table A. 7 Timber Skip Shoring Sizes in Accordance with Standard Practice

Trench Soil Strut Sizes (in) Sizes of Upright (in) Wale

Depth Type Sizes

(ft) (in)

Horizontal Spacing (ft)

4 6 8 2 Horizontal Spacing3 4 (ft)

6 8

5-10 A 4x4 4x4 2x6 2x8

10-15 A 4x4 4x4 2x6 3x8

5-10 B 4x6 2x6 6 z 8

10-15 B 6x6 2x6 8 z 8

10-15 B 4x6 2x6 6 X 6

Notes

:

(1) All lumber sizes are actual (not nominal) sizes in inches.

(2) 3x6 struts can be substituted for 4x4 struts in trenches

up to 4 ft width. For trenches wider than 4 ft use Table A. 4 for strut size adjustment.

(3) All horizontal spacing is center-to-center.

(4) Vertical center-to-center spacing of struts or wales not

to exceed 4 ft.

(5) Longer side of wale cross section to be horizontal.

66
Table A. 8 Timber Shoring Strut Sizes In Accordance with Standard Practice

Trench Soli Horizontal Strut Spacing (ft) Depth Type

(ft) 6 8 10 12 U 16 20

5-10 B 4 X 6 6 X 6 6x6 8x8 8x8 8 X 10 10 X 10

C 6 X 8 8 X 8 8 X 10 8 X 10 10 X 10 10 X 12 12 X 12

10-15 B 6 X 6 6 X 8 8x8 8 X 10 10 X 10 10 X 12 12 X 12

C 8 X 8 8 X 10 10 X 10 10 X 12 12 X 12

15-20 B 6 X 8 8 X 8 8 X 10 10 X 10 10 X 12 12 X 12 C 8 X 10 10 X 12 12 X 12

(1) All lumber sizes are actual (not nominal) sizes in inches.

(2) For trenches wider than 4 ft adjust strut sizes by Table A. 4.

(3) Vertical spacing not to exceed 5 ft center-to-center.

(4) All horizontal spacing is center-to-center.

67
Table A. 9 Timber Shoring Wale Sizes in Accordance with Standard Practice

Trench Soil Horizontal Strut Spacing (ft) Sheeting

Depth Type Thickness

(ft) (in)

6 8 10 12 14 16 20

5-10 B 6 z 8 8 X 10 10 X 10 10 X 10 10 X 12 12 X 12 2 C 8 X 10 10 X 12 12 X 12* 12 X 12 3

10-15 B 8 X 8 10 X 10 10 X 12* 10 X 12 12 X 12 2 C 10 X 12 12 X 12* 12 X 12 3

15-20 B 8 X 10 10 X 12 12 X 12* 12 X 12 3 C 12 X 12 12 X 12 3 USE INTERMEDIATE STRUTS TO THE RIGHT OF LINE

(1) All lumber sizes are actual (not nominal) sizes in inches.

(2) Vertical spacing not to exced 5 ft center^to-center

.

(3) All horizontal spacing is center-to-center.

(4) Long side of cross-section of rectangular members to be horizontal.

(5) * indicates slight overstress.

(6) Wale sizes to the right of dividing line require insertion of intermediate

strut before workers enter the trench.

(7) If vertical distance from the center of the lowest wale to the bottom of the trench exceeds 2 1/2 ft, sheeting shall be firmly embedded below Jhe bottom of the trench or mudsill shall be used. The vertical distance

from the center of the lowest wale to the bottom of the trench shall not exceed 3 ft, or 3 1/2 ft if mudsill is used.

68
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS BSS 127

2. Gov't Aecesslon No. 5. ft«£ip>ent'$ Acc^ssiort No. :|

4. TITLE AND SUBTITLE Recommended Technical Provisions for Construction Practice

in Shoring and Sloping of Trenches and Excavations

5. Publication Date June 1980

C. Performing Organization Code

7. AUTHOR(S)

Felix Y. Yokel

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Pro}ect/Ta8i</WorK IJnit No.

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12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rs<ree(. City. st<,te. ziP) Occupational Safety and Health Adm. Nat. Inst, for OccupationDepartment of Labor al Safety and Health

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Washington, D.C. 20201

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16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here,)

On the basis of studies conducted by the National Bureau of Standards technical

provisions for the sloping and shoring of the banks of trenches and excavations are recommended. Included are a recommended standard practice for trenching which can

be used by construction supervisors and compliance officers of the Occupational

Safety and Health Administration, and proposed engineering guidelines for the design

of shoring systems and other means to prevent mass movement of soil or rock in

excavat ions

.

17. KEY WORDS (six to twelve entriea; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) Braced excavation; construction; retaining structures; shoring; slope stability;

soil classification; soil pressure; standards; trenching

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