FftTCOW

^ ’^9 NOT REMOVE

NBSIR 79-1945/^

Soil Classification for Construction

Practice in Shallow Trenching

Felix Y. Yokel

Geotechnical Engineering Group

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Richard L. Tucker

Lymon C. Reese

University of Texas

Austin, TX 78712

December 1979

Prepared for Occupational Safety and Health Administration

Department of Labor

Washington, D.C. 20210

NBSIR 79-1945

SOIL CLASSIFICATION FOR CONSTRUCTION

PRACTICE IN SHALLOW TRENCHING

Felix Y. Yokel

Geotechnical Engineering Group

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Richard L. Tucker

Lymon C. Reese

University of Texas

Austin, TX 78712

December 1979

Prepared for Occupational Safety and Health Administration

Department of Labor

Washington, D.C. 20210

U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technoiogy

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Soil Classification for Construction Practice in Shallow Trenching

by

F. Y. Yokel, R. L. Tucker, and L. C. Reese

ABSTRACT

Construction practices in trenching and data on potential causes of

trenching accidents are reviewed. A study is made of the soil propertie

and site conditions that must be identified in order to determine the

stability of shored and sloped excavations against cave-ins. Two possible alternate soil classification methods are recommended. The

methods are simple enough to be used by construction foremen and at the same time use parameters which can be measured or identified without

ambiguity. The classification methods are supplemented by appropriate

field tests and correlated with allowable side slopes and lateral soil

pressures on shoring.

Keywords: Braced excavations; construction; excavation; geotechnical

engineering; retaining structures; shoring; slope stability;

soil classification; soil pressure; soil testing; trenching.

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TABLE OF CONTENTS

Page

ABSTRACT iii

PREFACE viii

EXECUTIVE SUMMARY ix

1. INTRODUCTION 1 2. BACKGROUND INFORMATION 3

2.1 GENERAL 3 2.2 TRENCHING CHARACTERISTICS AND PRACTICES 3

2.2.1 General 3

2.2.2 Contractor Characteristics 4

2.2.3 Characteristics of Trenching Operations 4

2.2.4 Sloping 4

2.2.5 Shoring 3

2.2.6 Excavation Equipment 3

2.3 TRENCH CAVE-INS 5

3. SOIL CLASSIFICATION AND CHARACTERIZATION 7

3.1 GENERAL 7 3.2 SOIL PROPERTIES 8

3.3 SOIL CHARACTERIZATION IN CURRENT ENGINEERING PRACTICE

IN TRENCHING AND EXCAVATION 14

3.3.1 General 14 ^ 3.3.2 Soils Information Required in Current

Engineering Practice 14

4. PARAMETERS AFFECTING TRENCH STABILITY 19

4.1 THE NATURE OF SHALLOW TRENCHES 19

4.2 PARAMETERS AFFECTING THE STABILITY OF TRENCHES 22

4.2.1 General 22

4.2.2 Secondary Soil Structure 22

4.2.3 Site and Working Conditions 25

5. RECOMMENDED SOIL CLASSIFICATION SYSTEMS 31

5.1 GENERAL 31

5.1.1 Purpose 31

5.1.2 Criteria 32

5.1.3 Existing Classification Systems for Trenching ... 33

IV
TABLE OF CONTENTS (Continued)

Page

5 .

2

RECOMMENDED SYSTEMS ...» 34

3 • 2 o i Ce ne^ai 34

5®2o2 Definition of Lateral Pressures and Allowable

Sideslopes Associated with the Classification

Systems 33

5.2.3 Simplified Classification System 35

5.2.4 Matrix Classification System 38

5.3

FIELD IDENTIFICATION 38

0.3.1 Cene ral 9.099«9999099999099999999999e9999999e99.. 38

5.3.2 Visual-Manual Examination 43

5.3.3 Strength Tests 46

5.3.4 Drying Tests 46

5.3.5 Suitability for Spaced Sheeting 47

5.4

CLASSIFICATION PROCEDURES 47

5.4.1 Simplified Classification System 47

5.4.2 Matrix Classification System 49

6. COMMENTARY ON RECOMMENDED SOIL CLASSIFICATION SYSTEMS 51

6.1 GENERAL 51

6.2 ALLOWABLE SIDESLOPES (Figure 5.1) .... 51

6.3 LATERAL PRESSURE DIAGRAMS (Figure 5.2) 52

6.4 SIMPLIFIED CLASSIFICATION SYSTEM (Table 5.1) 52

6.4.1 Soil Types (Table 5. 1) .......... ..............o. 52

6.4.2 Lateral Soil Pressures (Table 5.1) 52

6.4.3 Allowable Slopes (Table 5.1) 53

6.4.4 Spacing of Spaced Supports (Table 5.2) .......... 54

6.5 MATRIX CLASSIFICATION SYSTEM 55

6.5.1 Soil Classes (Table 5.3) 55

6.5.2 Lateral Soil Pressures (Table 5.4) 55

6.5.3 Allowable Slope (Table 5.7) 56

6.5.4 Spacing of Spaced Supports (Table 5.5) .......... 56

6.6 FIELD IDENTIFICATION 56

6.6.1 Resolution of Conflicts ......................... 56

6.6.2 Drying Test (Section 5.3.4) 57

7. REFERENCES 59

V
TABLE OF CONTENTS (Continued)

Page

8. ACKNOWLEDGMENTS 61

APPENDIX: BACKGROUND INFORMATION ON TRENCHING PRACTICE AND

ACCIDENTS 63

A. 1 GENERAL 64

A. 2 BACKGROUND INFORMATION 64

A. 2.1 Sources of Information 64

A. 2. 2 The Thompson and Tannenbaum Data 64

A. 2. 3 NBS Field Study 64

A. 2.4 Interviews with Selected Contractors 65

A. 2. 5 The Associated General Contractors (AGC)

Questionnaire 66

A. 2. 6 OSHA Compliance Officer Reports for 1973-1974 68

A. 2. 7 Summary of Background Information 69

A. 3 TRENCHING CHARACTERISTICS AND PRACTICES 69

A. 3.1 General 69

A. 3. 2 Right-of-Way 70

A. 3. 3 Depth 70

A. 3.4 Time and Site Conditions 70

A. 3. 5 Codes and Specifications 71

A. 3. 6 Sloping 71

A. 3. 7 Shoring 71

A. 3. 7.1 General 71

A. 3. 7. 2 Types of Sheathing 71

A. 3. 7.3 Wale Systems 72

A. 3. 7.4 Bracing Systems 72

A. 3. 7.5 Skip Shoring 73

A. 3. 8 Trench Boxes 73

A. 3. 9 Equipment 74

A. 3. 10 Contractor Characteristics 75

VI
LIST OF TABLES Page

Table 3.1 Soil Descriptions for Trenching 9

Table 3.2 Data Required for Soil Identification ................. 10

Table 3.3 Properties of Cohesive Soil Correlated with Standard

Penetration Test Results [6] 12

Table 3.4 Relationship between Properties of Cohesionless

Soil and Standard Penetration Test Results [20] 13

Table 3.5 Typical Values of Soil Unit Weight 13

Table 3.6 Soil Properties that must be Quantified to Calculate

Lateral Pressures 16

Table 5.1 Simplified Soil Classification System 40

Table 5.2 Maximum Center to Center Spacing (in feet) of Spaced

Sheeting for Simplified Soil Classification System .... 41

Table 5.3 Soil Classes in Matrix Classification System 42

Table 5.4 Minimum Acceptable Stability Requirements for Matrix Soil

Classification Systems 43

Table 5.5 Maximum Center to Center Spacing in ft. of Spaced Sheeting

for Matrix Soil Classification System ................. 43

LIST OF FIGURES

Figure 3.1 Lateral Soil Pressure Diagrams 15

Figure 4.1 Profiles of Cohesive Strength vs. Depth in Normally Consolated and Overconsolidated Clays 24

Figure 4.2 Desiccation Effects Observed in Cuts in Clays .o...... 27

Figure 4.3 Effect of Trench Intersection on Sideslope Stability . 29

Figure 5.1 Recommended Allowable Configurations of Sloped

Excavations 37

Figure 5.2 Assumed Diagram of Lateral Soil Pressure Acting on Shoring 38
PREFACE

In June 1976 the Occupational Health and Safety Administration (OSHA)

engaged the National Bureau of Standards (NBS) to study the provisions

for Excavation, Trenching and Shoring of the Safety and Health Regulations for Construction [9]^ and to recommend potential modifications

that could improve their effectiveness. As part of this study, a team

from The University of Texas at Austin studied the problem of soil

classification under an NBS Contract and submitted a study report in

January 1979 [22]. In a National workshop at the Department of Labor

in September 1978 [11a] the preliminary findings of the NBS study,

including preliminary recommendations of the Texas study were reviewed

and discussed by working groups consisting of contractors, union representatives, consulting engineers, and government officials.

The recommendations presented in this report are based on analytical and

field studies by NBS, the University of Texas report, and recommendations

and comments made in the workshop.

The report was prepared by the geotechnical Engineering Group of the

Center for Building Technology.

1 Numbers in brackets identify literature referenced in Section 7.

viii
EXECUTIVE SUMMARY

As a result of a National Bureau of Standards' study of the problems

associated with excavation safety, it has been concluded that there is

a need for a simple soil classification system that can be used by field

supervisors to make rapid decisions on sloping or shoring requirements.

The soil classification system should meet the following criteria: it

should be comprehensive (cover essentially all the conditions that could

be encountered); it should consider (at least implicitly) all the critical conditions that can cause instability; it should not be ambiguous

(two persons classifying a site should arrive at the same conclusion);

it should be usable by construction supervisors and OSHA compliance

officers in the field without the assistance of an engineer.

Two alternative systems are proposed for consideration; The "Simplified

Classification System" and the "Matrix Classification System." The simplified system requires fewer decisions by construction supervisors, but

it also somewhat narrows the number of choices available to contractors.

The matrix system is more sophisticated and would require better training

of construction personnel and OSHA inspectors. The systems are summarized

in Tables 1, 2, and 3. The tables contain definitions of soil types as well as stability requirements associated with the various soil types

identified. Lateral loads that shoring systems should be designed to

resist are defined by the "lateral weight effect" w^ and further ex- plained in Figure 1. Recommended allowable configuration of sloped ex- cavations associated with the steepest allowable side slopes defined in

Tables 1 and 3 are shown in Figure 2.

The soil classification systems are also correlated with provisions

for spaced sheeting (skip shoring). Simple field identification methods

are recommended which can be used to determine the soil type (or class)

in the field. These consist of a visual-manual identification method,

supplemented when necessary by in-situ strength tests using a pocket

penetrometer^ or a hand operated shearvane and by a drying test which

is performed on an undisturbed sample and used to determine whether

the soil is fissured and whether it is cohesive or granular. In case

of doubt or dispute these simple field tests can be further corroborated

by traditional in-situ and laboratory tests.

2 The Pocket Penetrometer is a small (vest-pocket sized) commercially

available device that measures in-situ shear strength of cohesive soils.

ix
Table 1. Simplified Soil Classification System

Soil Type Description Ib/ft^i/

Steepest Slope

hor. : vert

.

A Intact Hard 20 3/4:1

B Medium 40 3/4:1

C Submerged or Soft 80 1 1/2:1

1. Intact Hard Soils (Type A) include stiff clay and cohesive or cemented sands and gravels (hardpan, till)-' above the groundwater

table which have no fissures, weak layers, or inclined layers that

dip into the trench. Stiff clays included have an 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 com- pressive strength (pocket penetrometer reading) of 0.5 tsf-' or less

and soils that cannot stand on a slope of 3 hor. in 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 6 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. Intact rock is exempt from shoring and

sloping requirements.

6. 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 Type C. Layered

soils are classified in accordance with the weakest layer.

^ Ib/ft^ = 1.63 N/m^

^ 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 side slopes, are plastic (can be molded into

various shapes and rolled into threads) when moist and are hard to

break up when dry.

1 tsf = 96 kPa

X
Table 2. Matrix Classification System

'

^ Site —--^^,^_^^^^^Co^ition

Soil

Water in Trench

No Yes

Fiss

No

ures Yes

Fissures

No 1 Yes

Stiff Cohesive^*'^ I II III

Medium Cohesive II III III IV

Granular^/ II III

Soft IV IV

Notes

1. Water in Trench is 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 become fully

or partially flooded before workers leave it or may be entered by

workers within 6 hours after more than half its depth was flooded

and pumped out.

2. Vibrations : soils subjected to vibrations by heavy traffic, pile

driving or similar effects shall always be assumed fissured.

3. Stiff Cohesive Soils-*^ include stiff clays and cohesive or cemented

sands and gravels (till, hardpan). Stiff clays included have an unconfined compressive strength (pocket penetrometer reading)

= 1.5 tsf-' or larger.

4. Medium Cohesive Soils^^ have an unconfined compressive strength

(pocket penetrometer reading) between 0.5 and 1.5 tsf-'.

5. Granular Soils^ '*^ are gravels, sands and silts that can stand on a slope steeper than 3 hor. : i vert, without spalling or slumping.

6. Fractured Rock shall be treated as granular soil. Intact 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

silts that can not stand on a slope of 3 hor.: 1 vert, without

spalling or 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

xi
9. Disturbed C ohesive Soils (backfill) shall be treated as fissured

medium cohesive or soft cohesive soils

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 (although some moist granular

soils will exhibit apparent cohesion and temporarily stand on a vertical slope), they cannot be molded when moist and crumble easily

when dry.

1 tsf = 96 kPa

Table 3. Minimum Acceptable Stability Requirements for the

Matrix Classification System

Soil Type Wg Ibf/ft^

^ Steepest Slope

hor : vert

I 20 1/2:1

II 40 3/4:1

III 60 1:1

IV 80 1 1/2:1

Notes

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.

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

layer.

1 Ibf/ft^ = 1.63 N/m^

xii
P =We(H+2)

P = p xH

H = Height of supported bank, ft.

(2 ft. are added to allow for surcharge)

p

= Distributed horizontal earth pressure, Ib/ft^

P = Resultant horizontal force per unit length lb /ft.

We = Lateral weight effect, Ib/ft^

Note: 1m = 0.30 ft., 1 lb - 4.5 N, 1 Ib/ft^ = 48 Pa,

1 Ib/ft3 - 157 N/m3

f-lgu/ie 1 : LcitO-Xat Loads on ShoH^ing Syi,te.m^

xili
h

V

= Steepest allowable sideslope

Case I

Case II

a d

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 bank adjacent to the

work area not to exceed 3 ft

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

If steps are used (left side of III) imaginary slope (ab)

not to exceed 1 :

1

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

VIq{xa.<i 2 : Altomble, SZop^ Con^^gu/LCuU-ovU

xiv
1. INTRODUCTION

Each year more than 100 lives are lost in the U.S. and many more injuries occur as a result of excavation cave-ins. Of these fatalities over 87

percent occur in trenches less than 20 ft. (6 m) deep [18]. The implementation of the Occupational Safety and Health Administration (OSHA)

regulations for Excavation, Trenching and Shoring [9] so far has had no observable effect on the number of trench fatalities [19]. Part of

this lack of success is attributable to the difficulties encountered in

the interpretation and implementation of these regulations.

A field study conducted by the National Bureau of Standards [7] indicates

that one of the major complaints received from contractors is that the

soil classification in subpart P of the OSHA Regulations for Excavation,

Trenching and Shoring [9] is too vague and that numerous disagreements

between contractors and compliance officers arise over identification of

1
soil types. Most contractors expressed willingness to cooperate with OSHA

but were confused about the applicability of the present regulations,

particularly those in Table P-1 of the regulations, to their particular

construction sites [22].

Data on the correlation of various soil types with accident rates are difficult to obtain. A review of selected Occupational Safety and

Health Review Commission cases [22] indicated that data concerning soil

types were generally not recovered in the reports on fatal accidents.

Thompson and Tannenbaum [18] conducted a survey of 86 newspapers in order

to determine the soil types associated with trench and excavation caveins. The data indicate the following correlation between soil type and

frequency of accidents.

Soil Type No. of Accidents

Clay and/or Mud 32

Sand 21

Wet dirt (probably silty sand) 10

Sand, gravel and clay 8

Rock 7 Gravel 4

Sand and gravel 2

The source of these data raises some questions since soils were generally

not identified by qualified persons ; however, the report does provide an indication that there is a strong correlation between the soil type

encountered and the risk of a fatal accident.

Utility contractors tend to be familiar with the soil conditions in

their particular areas and frequently use generic names such as "Del Rio

Clay," "Taylor Marl," etc. These generic descriptions cannot always be

easily correlated with the soil categories used in the OSHA regulations.

Thus, there is frequently no common language between the utility contractors and OSHA compliance officers.

The purpose of this report is to recommend soil classification systems

which could adequately relate shoring and sloping requirements to the

soil type encountered in the field, and which could provide a common language between contractors, engineers, and compliance officers.

General background information is given in Section 2. Methods of soil

classification and characterization and parameters affecting the safety

of trenching operations are discussed in Sections 3 and 4. In Section 5 two alternate soil classification systems are recommended, and Section 6

is a commentary on the recommended soil classification systems. Detailed

background information on trenching practices and accidents which was assembled in the Texas study [22] is provided in the Appendix.

FACIWG PAGE: Saakho^ uu> 2.d to zxc.avat^ 6topo,d t/imah

2
2. BACKGROUND INFORMATION

2.1 GENERAL

A detailed summary of background information and its sources is given in

Appendix A to this report to enable the reader to familiarize himself

with the trench stability problem. Information pertinent to soil classification is summarized in this section. The information has been derived

from references 7, 12, 18, 19, 20 and 22.

2.2 TRENCHING CHARACTERISTIGS AND PRACTICES

2.2.1 General

As noted in the introduction, most fatalities from cave-ins occur in

excavations less than 20 ft (6 m) deep. The majority of these accidents

i
occur in utility trenches which are less than 12 ft (4 ni) deep (mostly sewer trenches). The information provided herein is therefore confined

to utility-trench excavations.

2.2.2 Contractor Characteristics

Utility contractors typically bid on projects within a 50 to 75 mile (80-

-120 km) radius from their home base. Their companies tend to be relatively small and stable, and their crews, and particularly their foremen,

have typically many years of experience in trenching within their area.

The contractors tend to specialize in one type of trenching work and

their machinery and the experience of their crews are key factors in

their decision on the methods to be employed to obtain trench stability.

2.2.3 Characteristics of Trenching Operations

For all but very large diameter lines, pipe laying is a dynamic operation, moving at a rate of several hundred feet per day. Soil structure,

moisture content, and related factors therefore tend to change frequently

requiring rapid decisions on methods of sloping or shoring to cope with

these changing conditions. As a consequence most firms rely on their

job foremen and equipment operators for on-the-spot decisions on sloping

or shoring. Foremen normally have a thorough working knowledge of the

physical characteristics of soils in their area. Shoring methods tend

to be developed on the basis of this experience and shoring systems are employed without detailed design and analysis.

Pipe laying and backfilling operations tend to follow closely behind the

trenching. As a result less than 100 ft (30 m) of trench are normally

open at any one time. The time elapsed between opening of a trench and

backfilling after completion of the pipe laying operation is on the average less than 2 hours and seldom exceeds one day. However, smaller

trenches, such as those for sewer connections to individual houses, are sometimes open for longer periods, if the excavation and the pipe laying

are carried out by different contractors. A similar situation occurs where two trenches intersect. In such locations backfilling may be

delayed until the lateral pipeline is connected.

2.2.4 Sloping

Sloping is the contractor's preferred method of trench stabilization,

since it reduces contruction material handling and provides the most

reliable personnel protection. Sloping is normally performed by

hydraulic backhoes. The choice of the angle at which banks are sloped

is usually at the discretion of the backhoe operator, who is rarely

aware of the exact slope angle required by the OSHA regulations [9],

but instead relies on his own experience and knowledge of the soil

conditions

4
2.2.5 Shoring

Shoring is employed where sloping is not feasible or economical because

of right of way restrictions, adjacent structures or trench depth (sloping is difficult and uneconomical for deep trenches). Sometimes the

bottom portion of a trench is shored and the upper part sloped. The

width of shored trenches seldom exceeds 6 ft (2m). Details on shoring

systems presently used are provided in Appendix A.

2.2.6 Excavation Equipment

Backhoes are by far the most widely used and most versatile excavating

machines. They are suitable for vertical-side as well as sloped trenches

and permit the operator to deal conveniently with changing conditions.

In very regular trenches where there are no abrupt changes in sideslopes

or depth, wheel or ladder type trenching machines are used. These machines

are capable of relatively fast digging with accurate controls for the depth

and width of the excavation. In some instances, where the trench is deep

or the spoil pile is far from the trench, draglines are used.

2.3 Trench Cave-Ins

Correlations between soil conditions and trench cave-ins have been discussed in Section 1. The greatest safety hazards identified by contractors can be rank-ordered as follows: Disturbed soils (by previous

excavations, usually unknown to the contractor); intersecting trenches;

narrow right of way; vibrations (usually by construction equipment);

increased seepage of subsurface water; rainfall; drying of exposed

trench walls; and inclined layers of soil dipping into the trench.

Records also indicate that many accidents occur during installation and

removal of shoring. Sometimes cohesive soils which are disturbed or fissured stand up long enough to permit workers to enter the trench but

subsequently collapse.

It is significant that the records indicate that most injured workers

were mature individuals. Thus, the number of accidents involving older,

and presumably more experienced, personnel tends to refute the argument

that experience is an adequate factor in judging trench wall stability.

5
FACIWG PAGE: Spot b/iactng tn den^e cia^ai/ 6M

6
3. SOIL CLASSIFICATION AND CHARACTERIZATION

3 . 1 GENERAL

A great number of soil classification systems have been introduced over

the years to serve a variety of purposes. Many of these systems are still in use today. The type of system used in any particular instance

usually depends on the soil properties that need to be characterized.

The soil properties that are presently most commonly described by classification systems are grain size distribution, plasticity, and organic

content. These can be obtained from analysis of disturbed soil samples.

Pedological classification systems used for agriculture generically

describe the stratification and drainage characteristics of the upper

soil horizons. Soil characterization for most engineering purposes

7
needs to describe the undisturbed in-situ soil conditions such as density

of granular soils or consistency of clays and is frequently provided as an addition to the soil classification by one of the conventional

systems

The Unified Soil Classification System ("Standard Method for Classification of Soils for Engineering Purposes," ASTM D2487-69) [2] has gained

wide acceptance in the engineering community. For engineering purposes

this classification is supplemented by information on density or consistency, or by direct measurements of the shear strength of the soil. In

the Unified Classification System the soil classification is based on grain size distribution, plasticity (Atterberg Limits) and organic content. Soils are generally classified as "coarse-grained (sands and

gravels)," "fine grained," or "organic". Fine grained soils are further

classified into different types of silt and clay on the basis for their

plasticity.

A less precise field identification system which is widely used for engineering purposes is the "Recommended Practice for a Visual Manual Procedure for Description of Soils," (ASTM D2488-69) fl]. The recommended

practice is based on field identification by appearance and by simple

tests which, although approximate in nature, provide a quantitative measure of the soil strength. Thus, this system provides a simple and

approximate method of identification of those properties which need to

be characterized for engineering purposes.

Table 3.1 compares various classification methods presently used in published standards for trenching. No quantitative values are given in

the table. The coexistence of these classification methods causes con- siderable confusion and the terminology used is difficult to correlate

with conventional classification techniques or accepted engineering

practice

3.2 SOIL PROPERTIES

Terzaghi and Peck [17] list 18 properties which should be determined to

identify soils. These properties are shown in Table 3.2, together with

checkmarks indicating which of the properties would be used to identify

any one of 15 different soil types. However, not all the properties

listed in Table 3.2 are identified in presently used classification

systems

Most classification systems, including the Unified Classification

System [2], describe soils in terms of their particulate properties. The

information gained from the tests associated with this type of classification is from disturbed samples and therefore can not completely

describe the in-situ behavior of the undisturbed soil. The information

does however provide a basis for determining whether a soil is granular

(derives its strength primarily from intergranular friction, has little

strength when soil is unloaded, strength increases rapidly when soil is

8
Table 3.1 Soil Descriptions for Trenching

OSHA [9] CAL OSHA [4]

Table P-1 Table P-2 Sect 1541 Appendix C WisconsinOSHA [24]

Solid rock, shale

or cemented Hard, Hard, All soils except

Rock

Hard, Solid

sand and gravel compact compact wet sands, satuCompacted angular

gravel

Average soil Likely

rated soil and

soft plastic clays

Soil that

splits easily

Gravelly or

Compacted

to crack

Unstable Wet sands

filled in gravel

Sand or very wet sharp sand

Loose rounded

sand

Soft,

sandy,

filled

Hydrostatic

pressure

1

Running Saturated soil

soft and plastic

clay

soil

9
Table 3.2

Data Required for Soil Identification fl7]

Type

of

Soil

General

Information

Results

Classification Tests

Intact Samples

Disturbed

Samples

o

c;

Odor

Texture

Dilatancy

Grain

properties

Dry

strength

Natural

void

ratio

e

Natural

water

content

w

Unit

weight,

natural

state

Y

Unit

weight,

oven-dried

Yj

Unconf.

compressive

strength

qy

Sensitivity

S|.

X

E

0) o

L. "O>

X

s:

Min.

void

ratio

6^,^^ X

o

3

C“

Plastic

limit

P„

I

Mechanical

analysis

Carbonate

content

Hardpan X X X X Sand, gravel X X X X X X Inorganic silt X X X X X X X X X X X X X Organic silt X X X X X X X X X X X X X X Clay X X X X X X X X X X X Organic clay X X X X X X X X X X X X Peat X X X X X X Till X X X X X X X X Tuff, fine-grained X X X X X X X X X X Loess X X X X X X X X X X X X X X X Modified loess X X X X X X X X X X X X X X X Adobe X X X X X X X X X X X X X X X Marl X X X X X X X X X X X X X Lake marl X X X X X X X X X X X X X Gumbo X X X X X X X X X X X X

10
loaded) or cohesive (derives its strength primarily from interparticle

cohesion, has strength when unloaded, its strength changes slowly when

it is loaded). The soil properties most often used in engineering design

are those associated with strength, in-place unit weight, compressibility

and permeability. Most of these properties can be correlated with the

relative density of sands and the consistency of clays, and all the

properties are related to the undisturbed (in-situ) condition of the

soil. Their measurement is either performed by in-situ tests or by

laboratory tests on undisturbed samples.

Since most conventional classification systems do not convey information

on these properties, they have to be supplemented by additional information. It may be noteworthy to mention that when soil is used as a structural or borrow material (engineered fills) information on the

particulate properties tends to be more important than information on

in situ properties. In this case conventional classification systems are extremely useful. However this case is not germain to the excavation

problem.

Typical values of strength properties and in-situ unit weight and their

correlation with the relative density of sand or the consistency of clays

and with widely used in-situ tests are given in Tables 3.3, 3.4 and 3.5.

The following symbols are used in the tables:

= relative density, percent d niilL- . j[qq

d ''^dmax - ”*^dmax

o <1) = coefficient of active earth pressure = tan (45 “^)

N

Y^d

dmax

dmin

sat

^sub

Y

blowcount (per foot) in Standard Penetration Test

angle of shearing resistance, degrees

in place unit weight, Ib/ff^

dry unit weight, Ib/ft^

maximum dry unit weight Ib/ff^

minimum dry unit weight, Ib/ft"^

unit weight of undisturbed soil in saturated state

in place bouyant unit weight, Ib/ft^ “ unit weight of water, Ib/ff^

[3]

, Ib/ft^

Table 3.3 shows approximate strength parameters for clays and their cor- relation with consistency and with two widely-used in situ tests: the

thumb penetration test (ASTM D2488-69) [1] and the Standard Penetration

Test (SPT) (ASTM Dl 586-67 ) [3 ] . The correlation between the R-values of

the Standard Penetration Test and the properties of clay should be

regarded as no more than a crude approximation which is not always reliable. However the values for sands shown in table 3.4 are much more reliable and are frequently used for engineering design. This is particularly true since undisturbed samples of sand are difficult to take

and the relative density (Dj.) is a parameter which can not be easily

11
Table 3.3. Properties of Cohesive Soil Correlated with Standard Penetration

Test Kesults [6]

Clay

Consistency Identification (ASTM D2488-69) SPT, M blows /ft

Undrained

Shear Str. Ibf/ft^

Unconfined

Compressive

Strength

Ibf/ft^

Very soft Easily penetrated several

inches by fist. Exudes

between fingers when

squeezed in hand.

<2 250 500

Soft Easily penetrated several

inches by thumb. Molded

by lisht finger pressure.

2-4 250-500 500-1000

Medium Can be penetrated several

inches by thumb with moderate effort. Molded by

strong finger pressure.

4-8 500-1000 1000-2000

Stiff Readily indented by thumb

but penetrated only with

great effort.

8-15 1000-2000 2000-4000

Very stiff Readily indented by thumb nail 15-30 2000-4000 4000-8000

Hard Indented with difficulty by

thumbnail >30 >4000 1 >8000

Mote: 1 ft “ 0.30 m

1 Ibf/ft^ - 48 Pa

12
Table 3«4 RelaCionahip between Properties of Cobesionless Soil and Standard

Penetration Test Results [21]

Soil Type

SPT, N blows/ft

Relative

Density

®r* (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* 0.33-0.26

Dense sand 30-50 65-85 36-41* 0.26-0.21

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

Approximate soil weights are given in Table 4.5.

Note: 1 ft = 0.30 m

Table 3.5 Typical Values of Soil Unit Weight [11]

Soil Type

Moist Unit Weight

Above Water Table,

'^(Ib/ft^)

Saturated Unit Weight

Below Water Table

^satdb/ft )

Poorly graded sand 105-115 115-125

Clean well graded sands 115-125 125-130

Silty or clayey sand & 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

Y * Y “ Y sub sat w

- 62.4 Ib/ft^

Note: 1 Ib/ft^ - 16 kg/ia^

13
measured in the laboratory. In the case of clay unconfined compression

tests [ASTM D2166-66 (1972)] or other laboratory strength tests on un- disturbed samples give reasonably good results, and simple in-situ tests

such as cone penetration (ASTM D3441-75T) and vane shear tests (ASTM

D2573-72) have been successfully used. It should be noted that the

N values given in Tables 3.3 and 3.4 are representative of those obtained

in traditional U.S. practice (rope and cathead). Adjustments should be

made if other methods (such as trigger release hammers) are used. In

Table 3.5 typical values for soil unit weight are given and correlated

with various soil types.

3.3 SOIL CHARACTERIZATION IN CURRENT ENGINEERING PRACTICE IN TRENCHING

AND EXCAVATION

3.3.1 General

The purpose of this discussion is not to provide a comprehensive review

of current engineering practice and of available data, but rather to

identify those soil properties which need to be quantified or defined

if lateral soil pressures are to be calculated or slope stability determined in accordance with current engineering practice.

3.3.2 Soils Information Required in Current Engineering Practice

Figure 3.1 shows typical pressure diagrams extensively used in present

U.S. engineering practice to calculate lateral loads on excavation bracing. Diagram (1) is for sands; Diagram (2) is for soft to medium clays;

Diagram (3) is for stiff clays; and Diagram (4) is for dense cohesive

(cemented) sands or for very stiff sandy clays for two different conditions which are specified in the figure. The symbols used in the figure

are those previously used in tables 3. 3-3. 5. Additionally:

c = cohesion (cohesive strength)

H = depth of excavation

m = lateral force coefficient

N = "Stability number" = YH/c

The diagrams in Figure 3.1 are widely accepted in current engineering

practice; however, they are not the only pressure diagrams used. Other

types of pressure distribution, such as those proposed by Tschebotariof

f

[21] also have gained wide acceptance. The following discussion of the

soil properties that need to be defined in order to use the pressure

diagrams in Figure 3.1 equally applies to these other approaches.

It is important to identify those soil properties which need to be quantified or defined in order to apply the pressure diagrams in Figure 3.1

to any specific case. These properties are shown in Table 3.6. The

bracketed checkmarks are for properties which may have to be defined in

order to identify the "soil type."

14
1. Sands Ref [10]

II

'' 0.65ka7H|^

ka = tan^(45 - 0/2)

2. Soft to medium clays, when N > 6

(if pressures caSclated under 3. using

0.4 7H are larger, use 3.)

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. (if 4 -« N < 6 use 2. or 3., whichever gives larger pressures)

4.

Dense cohesive sands, very stiff sandy clays

Upper third of cut

Relatively uniform dominated by cohesionless sands

VUg/WLiM, /, 2, and 3 aitta P^ak (I9M1U0], Vlagaxin, 4 af,tvi Goldbeag U.al

(I976)[6)

F^gcuie 3.1: LcuteAoI SoU Vuigmrni

15
Table 3.6. Soil Properties That Must Be Quantified

to Calculate Lateral Pressures

Property

Soil Type Y <t> c Sand /

Soft to Medium Clay / /

Stiff Clay / “TTT

Dense Cohesive Sand / (/)

Very Stiff Sandy Clays / (/)

Another engineering problem associated with trenching and excavations is

slope stability. The soil properties that need to be quantified in

order to determine slopes that will be stable without shoring are the

angle of shearing resistance, 41 , for granular soils, and the cohesion, c, the angle of shearing resistance, 4), and the in-place unit weight, y,

for cohesive soils. Another important parameter in cohesive soils which

is not strictly a "soil property" is the depth to which tension cracks

(fissures) extend from the surface.

Certain soil properties must be defined or quantified in order to determine the "soil type" in Table 3.6. A determination must be made whether

the material is a "clay" or a "sand", or rather whether it should be

considered a cohesive material where the lateral forces are calculated

on the basis of the cohesion c, and 4

> is assumed to be small or negligible,

a non-cohesive material where c is assumed to be small or negligible and

the forces are calculated on the basis of 4

> alone, or a "cohesive sand" or "sandy clay" where both, c and 4

> should be considered. If the

material is classified a "clay" a decision must be made whether it is

"soft or medium," "stiff", or "very stiff". This determination is based

on the value of the undrained shear strength (c) as shown in Table 3.3.

There are several methods by which the soil properties discussed above can be determined. The value of 4

> in sands correlates with the "relative

density", , a property which is not easily measurable in the laboratory.

It does however correlate reasonably well with a number of in situ measurements such as the Standard Penetration Tests [3], cone penetration

tests and pressure meter tests. The angle of shearing resistance, 4,, could also be deduced from the observed angle of repose, (however, if

sands are moist, the angle of repose could be steeper because of apparent

cohesion), or it could be determined by laboratory tests, though it is

difficult to secure undisturbed samples of sands or otherwise reproduce

the in-situ density. Sowers and Sowers [15] proposed a manual procedure

to determine the density of sands.

Values of c for clays can be determined by a variety of methods. Two

methods, the thumb penetration test and the Standard Penetration Test

(SPT) are shown in Table 3.3; however the SPT is generally not recom- mended for this purpose. Other in-situ methods include vane shear tests, cone penetration tests and pocket penetrometer tests. Determination of

16
c by laboratory tests on undisturbed samples such as the unconfined

compression test tend to be quite reliable.

In conclusion it should be noted that the important parameters associated with present engineering practice related to excavation are all

dependent on density or consistency. Neither of these parameters is

considered in the commonly used soil classification methods, with the

exception of the manual and visual procedures specified in ASTM D2488-69.
FACIWG PAGE: SkoZoXon iheaXhZng ivZth pA.&^abAXccU&d 6tA.at-Mat2, cui^mbly

18
4. PARAMETERS AFFECTING TRENCH STABILITY

4.1 THE NATURE OF SHALLOW TRENCHES

In section 3.2 current engineering practice in calculating lateral soil

pressures and slope stability for excavations was discussed, together

with the soil properties that need to be determined in order to apply

this practice. The so-called "pressure envelopes" shown in Figure 3.1 were developed on the basis of measured data such as those in Reference

[5] which originated from deep excavations (deeper than 20 ft). Because

of the time element usually associated with such excavations, the data

are from excavations which were open for weeks or even several months.

There are fundamental differences between such excavations and typical

shallow utility trenches which are subsequently discussed.

19
(1 ) Depth

One of the effects of depth on the nature of the lateral-pressure envelope is recognized in Figure 3.1. A dimensionless parameter, defined as the "stability number" N = YH/c is introduced for cohesive soils. A

vertical cut can stand without external support when N ^ 4 or the depth

H ^ 4c/Y. Thus whenever the depth of the excavation is less than this

"critical" depth an arbitrary decision has to be made with respect to

the lateral forces that the bracing should be designed to resist, since

theoretically these forces are zero. This problem is recognized in

Figure 3.1 and guidance is provided in Figure 3.1 (3). For a stiff clay

c is from 1000 to 2000 psf (48-96 kPa). If we assume that the in-place

unit weight Y is between 100 and 150 Ib/ft ( 1600-3200kg/m^) the corre- sponding critical depth is 40-50 ft (12-15 m) . For a very soft clay of

c = 250 psf (12 kPa) the critical depth would be about 10 ft (3m). Since

most utility trenches that need to be braced are sewers which tend to be on the average 10 ft (3 m) deep, and most clays tend to have cohesive

strengths far in excess of 250 psf (12 kPa) it is evident that in the

majority of cases shallow trenches in cohesive soils can stand at least

temporarily without support. One conclusion that can be drawn from this

observation is that it will be extremely difficult to make meaningful

measurements of forces acting on shallow-excavation bracing in cohesive

soils. This is probably one of the reason for the absence of data. The

observation also casts some doubts on the relevance of present engineering practice in excavations to the special case of shallow trenches.

It is also important to recognize the effect of depth on soil characteristics. Even in areas where soil conditions are relatively uniform, a condition that is typical for many clay areas, the upper soil horizons

differ in character from deeper layers. There is the zone of active

plant life where organic materials accumulate and root systems break up

the soil structure. In addition, soil layers near the surface are subject

to oxidation, leaching, desiccation, frequent volume changes by alternate

wetting and drying and frost action, and to many other disturbances.

Thus cohesive soils near the surface tend to be disturbed by fissures

and tension cracks. Desiccated and fissured stiff or medium clays may

not have any cohesion left and therefore could act like a granular soil.

In some instances surface water penetrates into tension cracks in otherwise intact clays and causes sudden increases in lateral pressures. The

effect of this disturbed zone may not be very significant in a deep

excavation where it' constitutes a relatively small portion of the overall

depth, but it may dominate the behavior of a shallow trench. Frequently,

when engineering soil properties are determined by in situ tests or laboratory tests on volumetric samples, the test results will not reflect

the effects of structural disturbances such as tension cracks which have

a dominant influence on the magnitude of lateral forces.

( 2 ) Time

It has already been noted that the measurements on which Figure 3.1 is

based were taken in excavations which were open for a long period of

20
time. On the other hand the typical utility trench is open for less

than two hours. In many instances, even if a condition exists that would

lead to failure in a long term excavation, the failure in a short term

excavation will not develop before the backfill is placed. This is true

for clays as well as sands. Moist sands frequently have cohesive

strength (apparent cohesion). Under these conditions excavations will

be temporarily stable with sideslopes that would slump with a change

in moisture content (either dryer or wetter conditions) and the forces

exerted on excavation bracing are also smaller in the short term. This

transient condition can not be relied on in engineering practice, but

it plays an important role in construction.

(3) Excavation Methods

It has been shown [16] that the distribution of soil pressures on retaining structures is influenced by the deformations and displacements that

the retaining structure will undergo after the load is applied and by

the restraints on the deformation of the retained soil mass. Thus the

pressure distribution against a rigid retaining wall tends to be triangular when the top of the wall is allowed to rotate away from the

retained soil and parabolic if the bottom rotates out. The magnitude

of the pressures corresponds to the active pressure if the retained soil

mass is allowed to expand out sufficiently, the passive pressure if the

wall is pushed toward the soil mass to the verge of failure and the

prevailing in-situ pressure if no movement is allowed to occur. Thus

the pressure diagrams shown in figure 3.1 are not independent of the

displacements that occurred in the soil mass during excavation and the

magnitude of the preloading forces on the struts.

In present deep-excavation practice the struts (cross braces) are placed

and preloaded while the excavation is in progress. This effectively

restrains the displacements near the top of the excavation. As the excavation progresses downward significant inward displacements do occur near

and below the bottom of the excavation [8] but they are restrained as each successive tier of cross braces is placed and preloaded. On the

other hand shallow trenches are typically excavated to their full depth

before cross braces are placed. This may cause lateral pressures to

decrease in many instances; however in some other instances it may promote the development of fissures. In some types of shallow trench bracing, such as trench boxes, the protective sheeting may actually exert no thrust on the side of the excavation, and in many cases a void remains

between the side of the excavated trench and the sheeting. In this

instance pressure will only be exerted if the sides of the excavation

move in as a result of deformation, a general shear failure, or the

localized separation of some volume of soil.

Thus, it is reasonable to assume that the magnitude and distribution of

soil pressure on shallow-trench bracing may significantly differ from

that shown in Figure 3.1.

21
4.2 PARAMETERS AFFECTING THE STABILITY OF SHALLOWS TRENCHES

4.2.1 General

In Section 3 the soil properties which must be identified in present

engineering practice are discussed. The subsequent discussion in 4.1

draws a distinction between deep excavations and shallow trenches, and

identifies some other parameters which affect the behavior of shallow

trenches. It is convenient to divide the parameters which affect the

behavior of shallow trenches into three categories.

1. The primary soil properties which generally characterize the

soils as seen at a point within the soil mass or within a small volumetric element that is tested in the laboratory.

2. The secondary soil structure which characterizes the soil or rock mass adjacent to the excavation or rather that part of

the soil mass which could affect the stability of the trench.

3. Site and working conditions which affect trench stability.

Primary soil properties are discussed in Section 3. Some of the important effects of secondary soil structure are discussed in Section 4.1.

Other effects of the secondary soil structure as well as site and working

conditions are discussed hereafter.

4.2.2 Secondary Soil Structure

( 1 ) Discontinuities

Discontinuities are sometimes of a magnitude which escapes visual identification and may cause planes of weakness in a soil mass which are not detected in laboratory sized samples. This is perhaps best illustrated by an example from Reference [22]. A severely jointed clay with

slickensides in Houston, Texas was classified as a highly plastic clay

(CH) in accordance with the Unified Soil Classification System (ASTM

D2487-69). Undisturbed samples taken at their natural water content

in the in-situ condition before soil disturbance by excavation had an unconfined compressive strength of 4 tons/ft^ (383 kPa). Observations

indicate that when trenches are excavated this soil becomes unstable

and fissured. Laboratory and field investigation of the soil in its

fissured condition showed that the cohesion approaches zero. This

clay would eventually behave like a granular soil [14]. The development

of fissures is particularly pronounced when the soil is allowed to expand

laterally and when changes in water content weaken its shear strength.

Another type of discontinuity causes problems in rock excavations.

The bonding between bedding planes or across other discontinuities is

sometimes very weak or non-existent due to erosion or presence of weak

materials such as micaceous layers or clay seams. When these planes

slope toward the side of an excavation the shear stresses in the direc22
r

tion of the discontinuity may exceed the shear resistance provided by

friction and bond. The situation is sometimes aggravated by excess pore

water pressure and by the lubricating effects of seepage, micaceous

materials and clay seams. The danger is particularly great when the

excavation is classified as a rock excavation and therefore not braced.

(2) Overconsolidation

Soils which have been subjected to loads greater than those presently

exerted by the overburden are overconsolidated. Overconsolidation

can be caused by glaciation (weight of ice), tectonic movements, erosion,

desiccation and other effects. Most surface deposits of clay are overconsolidated by desiccation [13]. The potential effect of this

overconsolidation on the relationship between shear strength and depth

is shown in Figures 4.1(a) and (b) which show frequently observed

profiles of cohesive strength versus depth for overconsolidated and

normally consolidated clays, respectively (clays are normally consolidated when present vertical pressures were not exceeded in the past).

Note that in Figure 4.1(b) the cohesive strength decreases with depth

up to a certain depth. If a trench is excavated under these conditions

the bottom of the trench, where the soil is subjected to the greatest

shear stresses, may be located in the region of least shear strength.

The soil conditions shown in Figure 4.1(b) may also lead to erroneous

conclusions with respect to the critical height at which an unsupported

vertical slope will stand without collapsing.

Overconsolidated clays frequently exhibit high in-situ permeability. An

example of this condition was encountered in an area about 45 miles northeast of Austin, Texas [22]. An undisturbed sample resembled canned solid

white tuna in that it broke and crumbled in a similar manner. Water

placed on the sample quickly passed through the clay. An unconfined

compression test at the natural moisture content showed an unconfined

compressive strength in excess of 3 tons/ft^ (287 kPa). However when

additional water was added the sample began to disintegrate. The sample

taken in this instance was highly fissured. However other available

evidence indicates that water may rapidly flow through apparently un- fissured samples as well, and an increase in water content will result

in a significant decrease in shear strength.

Since overconsolidation of surface deposits of clays is a common phenomenon, an allowance for this possibility should be made when soils are classified for the purpose of trenching. The limitation of unbraced cuts

to a five foot (1.5 m) depth is reasonable for most conditions; however

in some instances precautions against sudden collapse caused by water

penetration may be necessary even for very shallow depths.

23
Depth

Depth

Cohesion

|b| Cohesion vs. depth of an overconsoiidated clay

Cohesion

|a| Cohesion vs. depth of a normally consolidated clay

f^goAd 4.1: o{^ Stfirngth. v6. Vdpth in MoAmalZij

Con^olaXdd and OveAcon^olidatdd Clay^

24
4.2.3 Site and Working Conditions

(1) Water

The effect of a change in the water content on the strength of soils was discussed in previous sections of this report. If a trench fills

with water, or if heavy rain and flooding occur, the water content of

the surrounding soil will in most instances increase. As previously

noted, such an increase in water content will weaken most soils. However

in medium to fine poorly graded sands, an increase in moisture content

may increase the shear strength due to tensile forces induced by capillary suction (apparent cohesion). This strength increase disappears

if the moisture content is either decreased or increased from a certain

optimum. One contractor interviewed [22| claimed to have deliberately

flooded sandy areas so he could excavate with steeper sideslopes.

Most major problems associated with water are caused by occasional flooding by rain, water line breaks and snow or ice melts. While a trench

is filled with water the stability of the sideslopes may actually in- crease in most instances since the soil is submerged and its weight

decreases and excess pore pressures tend to equalize. Since flooding

of trenches inhibits the construction work, contractors pump the water

and drain the trench as rapidly as possible. As the pumping proceeds,

seepage forces develop and a significant amount of finer particles drains

from the trench walls into the trench and is pumped out with the water.

This will cause an increase in the void ratio of the soil adjacent to

the trench and thus reduce the shear strength [26]. However rapid pumping may have other more serious effects. As a result of the rapid drawdown the trench could be almost empty while the surrounding soil is fully

saturated. This increases the unit weight of the soil and thus the

driving forces which can cause a stability failure while at the same time the shear strength is reduced by saturation and excess pore water

pressures. As a consequence the trench may collapse during or shortly

after the pumping operation. The severity of these effects depends to

a large degree on the drainage and strength characteristics of the sur- rounding soil. Not much data are available on this phenomenon [23].

Current OSHA regulations specify that trenches must be inspected after

rains. This provision may not be adequate. If the soil adjacent to a trench has undergone radical changes in moisture content the stability

of the trench may be in question. Additional shoring or flatter slopes

should be considered if the construction is in a location or season where flooding or severe rain could be anticipated.

(2) Desiccation

Desiccation (drying) of the trench wall or bottom could cause difficult

problems; however these problems are more significant in excavations

that are open for long periods of time. As previously noted desiccation

causes tension cracks and overconsolidation in the soil layers near the

surface. Desiccation occurs as a result of exposure of soils on the

25
side and bottom of an excavation to air. When the moisture content of

clays decreases the volume of the clays decreases. The shrinkage causes tensile stresses in intact clays which result in tensile cracks whenever

the tensile strength is exceeded and in fissured clays it causes existing

fissures to open up. Eventually the tensile strength can completely

dissappear and the material acts like a granular soil. However, unlike

sands, these desiccated clays may lose much of their shear strength when

water penetrates the fissures. Typical desiccation effects which were observed in Houston [22] are shown in Figure 4.2. The failure wedge

abc in Figure 4.2(a) is probably attributable to desiccation near the

surface which existed prior to opening of the trench. The spalling of

column a, a', b, c is attributable to desiccation on the side of the

excavation after opening of the trench.

Drying of moist sands may have adverse effects in sloped excavation where

the angle of slope depended on apparent cohesion.

(3) Previous Disturbances

Previous disturbances are most frequently observed at utility line

crossings. There is evidence from field studies by NBS [7] that contractors are frequently unaware of the presence of previously disturbed areas and accidents result as a consequence. The most dangerous situations occur

in excavations in cohesive (hard, compact) soils where skip bracing rather

than tight sheeting is used. The decrease in cohesive strength is not

always noticed during the excavation stage and subsequently the trench may

collapse during or after insertion of the bracing. It should be noted

that in most instances backfill in utility trenches is not compacted and

thus previously disturbed material tends to be much weaker than the sur- rounding soil.

(4) Vibrations

Vibrations are caused by excavating equipment, driving of piles and sheet

piles, traffic, and operations in the trench itself. In saturated sands or silts vibrations could cause substantial or complete loss of shear

strength (liquefaction), causing the soil to act like a liquid and substantially increasing lateral pressures. Loss of shear strength could

also occur in sensitive clays. In addition, the dynamic forces caused

by vibrations could temporarily increase lateral pressures. The most

dangerous potential consequence of vibrations on braced excavations is

their possible effect on the bracing itself. Struts are wedged against

the walls or uprights and exert lateral pressures against the trench

wall. Frequently it is this lateral pressure which keeps the struts

in place and no tie is provided. Vibrations can cause relative movement

between bracing elements which may result in a reduction or release of

these pressures causing the unloaded struts to separate and fall off.

This loss of support can trigger a collapse. Another consequence of

loss of, or reduction in, the preload of struts could be that the

retained soil may become disturbed.

26
a

/

/

/

"7

/

(a) Effect of initially desiccated zone

c a

(b] Desiccated pillar mode of failure

flguAz 4.2: Ve^lcdatlon 0b62Ave.d In CaZ{> in Clayi>

27
(5) Surcharge

Surcharge imposes additional lateral forces on trench bracing and

increases the driving forces which could cause a stability failure of

a sloped excavation. Surcharge loads are exerted by spoil piles, excavation equipment and stored building material, and sometimes by building

foundations which are close to the trench. As a rule of thumb, any load

which is located within the area where imaginary 45 degree sideslopes

intersect the ground surface has an effect on the stability of the exca- vation. Tliis rule of thumb, however, may not be conservative enough for

very soft or loose soils. A sideslope ascending from the edge of a vertical braced excavation also acts like a surcharge.

Present OSHA regulations require that spoil piles be kept two feet from

the edge of the excavation slope. While this provision alleviates sur- charge effects it does not eliminate these effects and it is primarily

intended to reduce the danger of spoil material spilling, or objects

rolling into a braced excavation, or sloughing of the upper corner of

sloped excavations. There should be similar provisions for excavating

and construction equipment and machinery.

(5) Trench Discontinuities

Field studies conducted by NBS [7] indicate that many accidents occur at

trench discontinuities, particularly at intersections between two

trenches which occur whenever lateral pipes are connected to sewer trunklines. Discontinuities also occur at manhole locations or when

a trench changes direction. One potential effect is shown in a simplified way in Figure 4.3. The shear strength of the soil required to

resist a slide along planes outlined by the dotted lines in Figure

4.3(a) is less than that required to prevent the failure at the corner shown in 4.3(b). Note that frequently at trench intersections only

the main trench is braced while the lateral trench is still unbraced,

particularly since installation of trunklines and lateral lines is often

done by different contractors and at different times. If this situation

arises, the thrust exerted by preloaded struts can cause a failure

of the corner at the trench intersection. There are presently no OSHA

provision to prevent this dangerous situation.

(6) Time

Effects of time were discussed in Section 4.1. Time effects include:

desiccation of clays; loss of shear strength in overconsolidated clays

after lateral expansion; loss of apparent cohesion in sands; weakening

of bedding planes of rock by seepage; and erosion. Some failures in

a soil mass do not occur suddenly but rather manifest themselves by

slow movements over considerable periods of time. This includes stability failures in cohesive soils. In many instances the time element

required for a failure to develop is much longer than the time typical

utility trenches are left open (two hours or less).

28
I

(a) Potential failure section of straight trench

|b) Potential failure section at corner

FXgo/ie 4.3: I action on StabdUXy

29
There is also a complex relationship between drainage conditions and

time. The "undrained" condition is usually more critical since the

effective shear strength is less under undrained conditions when effective normal stresses are reduced by excess pore water pressure. In

utility trenches the undrained condition usually governs. In larger

excavations both drained and undrained conditions may have to be considered, except when the excavation proceeds at a slow enough pace to permit

drainage while the work is in progress. In cohesive soils it is customary to rely only on the part of the shear strength attributable to cohesion when undrained conditions are anticipated. Cohesion, in turn, can

be determined in an unconfined compression test.

Time effects can be critical in overconsolidated clays. The following

observation illustrates one instance where reduced confining pressures

caused a failure [22]. A vertical 8-ft (2.4-m) deep trench was opened

in a fissured clay in Houston, Texas to observe its behavior. Initially

the soil appeared stable with no visual indication of fissures. Skeleton braces were installed 8 ft (2.4 m) on center. After 45 minutes sev- eral sections between the uprights suddenly collapsed. This is a typical

situation which can lead to fatal accidents, since there is sufficient

time for workers to enter the trench, but instability occurs before the

work is completed.

Unfortunately time effects on trench stability are difficult to quantify

and no recorded data are available. To the extent that accident reports

are available they seldom describe the events adequately or contain an accurate chronology of events. However there are indications that

trenches left open for a few hours behave differently from those open

for a few minutes, and that long term excavations generally require

stronger bracing or flatter sideslopes than short term utility trenches.

FACTWG PAGE: Compound 6-tope uAed -in loAge iJViiQaXion ph.ojeet

30
5. RECOMMENDED SOIL CLASSIFICATION SYSTEMS

5 . 1 GENERAL

5.1.1 Purpose

Any recommended soil classification system must be designed to serve the

purpose of its intended use. Thus, it is important to determine who will

use it and how it is to be used.

Engineers generally follow the practice outlined in Chapter 3, which

relies on the determination of in situ soil properties. Additionally,

engineers consider many other parameters such as ground and surface

water, drainage characteristics, tension cracks, and static and dynamic

surcharge loads. Since each location has its own problems, and engineers

are trained and qu'alified to deal with these problems in accordance with

31
the state of the art, a formal soil classification system to supplement

engineering practice in excavations would not necessarily serve a useful

purpose and could even be harmful by narrowing the range of options

available to engineers. Thus, the recommended systems are not intended

to supplement present engineering practice.

A different situation arises when pre-fabricated systems and devices

such as trench boxes or hydraulic shores are used. In this instance it

is necessary to determine in advance under which field conditions the

systems can be used. This is best accomplished by a classification system which categorizes anticipated field conditions and gives adequate

consideration to the critical parameters. The role of the engineer

would then be to determine whether the shoring system is adequate for

its intended use.

In shallow utility trenches where work proceeds at a fast pace and

soil conditions tend to change rapidly it is generally not realistic

for an engineer to design shoring systems or determine slope stability

while the work is in progress. For this case a classification system

is needed which allows the rapid identification of field conditions

by both construction supervisory personnel and OSHA inspectors.

Thus, the classification systems recommended herein are intended to serve

two purposes

1. to predetermine conditions for the use of pre-fabricated or predesigned shoring systems

2. to allow rapid determination of shoring and sloping requirements

for various field conditions by construction supervisors and OSHA

ins pectors

5.1.2 Criteria

On the basis of the discussion in Sections 3 and 4 and the results of

the field studies discussed in this report, the following principles

were derived for creating a soil classification system:

1. The system must be comprehensive (not exclude any condition that

could be encountered, except for extreme or unusual cases which

require supervision by an engineer).

2. All the critical parameters must be considered, either implicitly

or explicitly.

3. Ambiguity must be avoided (any two persons classifying a site should

arrive at the same conclusion) and in case of a dispute it should be

possible to make objective determinations.

4. The system must be simple enough to be useable by construction supervisory personnel and by OSHA inspectors.

32
Note that Criterion (3) is difficult to meet. Soil is a natural material

and no two locations are exactly alike. Thus, a precise definition that

does not require subjective judgment can only be based on the most

obvious and easy to recognize soil properties. Such an approach is by

necessity conservative, since it must satisfy the worst condition out of

the range of possible conditions within a given soil class.

Criterion (4) also poses problems since supervisory personnel and inspectors are neither trained in geotechnical engineering, nor do they have

the means to perform any but the most primitive tests. More accurate

laboratory tests will in most instances only be used to resolve disputes.

5.1.3 Existing Classification Systems for Trenching

Some of the present soil classification systems devised for trenching

are shown in Table 3.1. An attempt is made in the table to correlate the

soil descriptions by their position on a vertical scale which goes from

"strong" (rock) to "weak" (soft clay). Note that OSHA as well as CAL

OSHA have dual systems (Table P-1 vs. Table P-2, Section 1541 vs. Appendix C) which cannot be easily correlated with each other. In accordance with the NBS field study [7] the systems in CAL OSHA Appendix C and

Wisconsin OSHA are both successfully used and generally acceptable to

their users.

It is useful to examine the compliance of the systems in Table 3.1 with

the criteria in 5.1.2;

OSHA Table P-1 does not meet Criterion (1) except if everything

that is not defined is considered an "average soil" (this would

include everything from hard to soft clay). It does not meet

Criterion (2) very well, though it may be argued that many parameters are considered implicitly. It does not meet Criterion (3)

since even a qualified soils laboratory could not determine when

a sand is "sharp and compacted" and a gravel is "angular". For

the same reason the system can also not be considered simple.

OSHA Table P-2 meets Criterion (1) since any conditions could be

placed into one of the categories (this pre-supposes that the comma

in one instance stands for "and", in the other for "or"); it meets

Criterion (2) implicitly, even though hydrostatic pressure inadequately describes potential water effects; it does not adequately

meet Criterion (3), even though it could meet the criterion if cer- tain definitions were added for "hard", "likely to crack," and

"soft", and it reasonably meets Criterion (4).

CAL OSHA, Section 1541 could meet Criterion (1) with additional

definitions, it implicitly meets Criterion (2); it would need some

added definitions to meet Criterion (3); and it meets Criterion (4).

33
CAL OSHA, Appendix C meets Criteria (1), (3), and (4) but does not

completely meet Criterion (2) since it does not consider surcharge

effects.

Wisconsin OSHA does not meet Criterion (1) since it would be difficult to know where to fit soft clay; it probably meets Criterion (2) implicitly; it would need added definitions to meet

Criterion (3); and it meets Criterion (4).

Two questions arise in conjunction with these existing systems:

1. Is compliance with all four criteria proof that a system is sound?

The answer to this question is that compliance with the criteria is a necessary (or at least highly desirable) but not a sufficient

condition. In additon to compliance with the criteria, a system

should lead to technically sound and safe decisions (though this

could be the case if Criterion 2 is adequately satisfied) and should

provide enough options to the contractor. Thus, even if soil categories are clearly defined, the choice of these categories and the

lateral forces or slopes assigned to them may not be sound. This

could lead to a situation where it would be easy for a contractor

to determine a soil type and there would not be many disputes

between inspectors and contractors, but the accident rate would be

high because of inadequate shoring, or shoring cost would be exor- bitant because of excessive safety margins.

2. If a system does not fully meet all four criteria, can it still be

used? Experience indicates that the answer to this question is yes.

The two cases in point are the CAL OSHA, Appendix C system and the

Wisconsin system.

The CAL OSHA, Appendix C system is slightly deficient by not considering surcharge. It also assigns rather small lateral forces to soils

which are not wet sands, saturated, or soft clays. One reason for

the success of the system may be that its use is confined to a limited geographical area. If the system were to be adopted nationwide, extensive use of very small lateral pressures may have adverse

consequences. Similarly, if the Wisconsin sytem were used nationwide, it may lead to difficulties in soft clay areas.

5.2 RECOMMENDED SYSTEMS

5.2.1 General

Before a classification system is adopted, a decision must be made about

its degree of sophistication. The adoption of a system which is very

simple and uses few easily recognizable soil categories is likely to

reduce judgment errors by construction personnel, simplify enforcement

and prevent costly litigation. On the other hand, it is likely to

decrease the number of the contractor's options and thereby in some instances increase construction costs. A more sophisticated system

34
would require better training of construction personnel and inspectors

and may increase the chances for errors, but if skillfully used it could

result in more economical and in some instances safer construction.

Accordingly, two alternate systems are recommended. A ’’simplified"

classification system, and a more sophisticated "matrix" classification

system. Before a decision is made which of the two systems should be

adopted, the advantages of each system must be weighed against its disadvantages. It is not recommended that both systems be adopted.

5.2.2 Definition of Lateral Pressures and Allowable Sideslopes

Associated with the Classification Systems

The choice of a soil class or category determines allowable sideslopes

in sloped excavations and lateral pressures that shoring must be designed

to resist. These are explained hereafter.

Figure 5.1 shows allowable sideslopes* The steepest allowable sideslope

is specified as the ratio of horizontal over vertical (h/v) for each

soil category. Cases I, II and III in the figure show sloped-excavation

configurations that should be permitted to provide some flexibility to

contractors

.

Figure 5.2 shows the lateral pressure diagram assumed to act on the exca- vation bracing. The bracing should be designed to resist these lateral

pressures with suitable safety margins.^ The equivalent uniform pressure

Wg is specified for each soil category. Note that unlike some of the

pressure diagrams used in present engineering practice (figure 3.1) a simple rectangular pressure diagram is used for all soil types. A more

complex diagram would not be justified because of the approximate nature

of this approach. An additional 2 ft (0.6 m) are added to the depth

[H] for the purpose of calculating pressure in order to allow for sur- charge loads. It is recommended to use 100 percent of the calculated

load for the design of struts (cross braces), 80 percent for the design

of wales, and 67 percent for the design of sheeting.

5.2.3 Simplified Classification System

This recommended system is similar to the one used in CAL OSHA

Appendix C; however, it differs in one important aspect: the category

3 . . . . , . . Specification of safety margins is not within the scope of this report,

but as a general philosophy it is envisioned that allowable working

stresses permitted in present engineering practice should not be

exceeded in long-term excavations and could be exceeded by up to 33

percent in excavations open for less than 24 hours.

35
h

V = Steepest allowable sideslope

Case I

a d

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 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 III) imaginary slope |ab]

not to exceed 1 :

1

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

VlQuJiQ, 5.1: Recomrmnd&d AHomblz ConfilguAatloi^ Sloped

Excavation^

36
p

= We |H+2)

P = pxH

H = Height of supported bank, ft.

|2 ft. are added to allow for surcharge)

p

= Distributed horizontai earth pressure, Ib/ft^

P = Resuitant horizontal force per unit length lb /ft.

We = Lateral weight effect, Ib/ft^

Note: 1m = 0.30 ft., 1 ib = 4.5 N, 1 Ib/ft^ = 48 Pa,

1 ib/ft3 = 157 N/ms

F^guA.e 5.2: A66iumd diagfiam o(\ IcLtoAdl $o-iZ ActA.yiq on SkoA^ng

37
for the strongest soils (A) is more restrictive. This slightly more conservative approach was taken because even though there is evidence

that lateral pressures in shallow trenches are less than those predicted

by present engineering practice, the evidence has not been corroborated

by adequate data on actual strut loads in shallow trenches. Even though

this system is more conservative than CAL OSHA Appendix C, the lateral

pressures for Type A soils are still significantly lower than those

used in present engineering practice.

The simplified system is shown in Tables 5.1 and 5.2

5.2.4 Matrix Classification System

The matrix classification system was originally proposed in Reference [22].

A modified version is presented below. It is a more sophisticated approach that permits explicit consideration of several parameters which

are implicit in the simplified system.

The matrix classification system is summarized in Tables 5.3, 5.4 and 5.5.

5.3 FIELD IDENTIFICATION

5.3.1 General

It has been noted in section 5.1.2 that a field identification method for

soil should be free of ambiguity so that any two persons classifying a site will always arrive at the same conclusion and yet simple enough to

be useable by construction supervisory personnel. This is an ideal that

may never be fully achieved. Thus, the field identification methods proposed herein are not entirely free of possible ambiguities. The authors

believe that the system can only be gradually perfected as a result of

experience gained by its application.

The following field identification methods are recommended because they are considered suitable for rapid field decisions by construction personnel. Although relatively crude, they will permit identification of soil

categories and thereby provide the necessary information for bracing and

sloping decisions. The tests are not intended for engineering design.

Should an engineering analysis be desired, it is recommended that presently accepted methods of soil sampling and testing be used.

Three identification tests are proposed to permit rapid soil identification as the work progresses. A visual -manual examination, which in most

instances will provide adequate information; strength tests to provide

improved quantitative estimates of the unconfined compressive strength

of cohesive soils in case of doubt; and drying tests to identify fissured

^NBS study of widely used conventional timber shoring indicated that it

could not resist the pressures calculated by present engineering practice. [Reference 25, App. A]

38
Table 5*1 fiiaplified Soil Claaaificatioo Syatem

Wg Ib/ft^S^

Steepest Slope

Soil Type Description Hor. : vert.

A Intact Hard 20 3/4 : 1^/

3/4 : 1^/

1 1/2 : ife' B Medium 40

C Submerged or Soft 80

Notes:

!• 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 an unconfined com- pressive strength (pocket penetrometer reading) • 1.5 tsf*' or more. Intact hard soils

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

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

3. ^ft Soils (Type C) include 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. in 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 6 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.

Intact (unfissured) and unfractured rock is exempt from shoring and sloping

requirements.

6. 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 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 5.2.

1 Ib/ft^ - 1.63 N/m^

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.

Cohesive soils are clays (fine grained) or aoils with e 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.

1 tsf “ 96 kPa

39
Table 5.2 Maximum Center to Center Spacing (in feet) of Spaced Sheeting

for Simplified Soil Classification System

Depth, ft§/

Soil Type 5-10 10-15

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

B 3 2

1 ft. = 0.3 m.

Numbers in parentheses are preferred spacings.

40
Table 5*3 Soil Claaaes in Matrix Claaaification System

^

^ Site

Condition

Water ic Mo Trench

Yes

Soil

Fiss

Mo

ures

Yes Fissures

Mo 1 Yes

Stiff Cohesive*^ I II III

Medium Cohesive-^ II III III IV

Granular-^ II III

Soft IV IV

Motes

1« Water in Trench is 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 become fully or partially flooded before workers leave it, or may be

entered by workers within 6 hours after more thap 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 *

. . a/ . 3* Stiff Cohesive Soils-' include stiff clays and cohesive or cemented sands and gravels

(till, hardpan)* sFiff clays included have an unconfined compressive strength (pocket

penetrometer reading) * 1*5 tsf^' or larger*

4* Medium Cohesive Soils-^ have an unconfined compressive strength (pocket penetrometer

reading) between 0*3 and 1*5 tsf^'*

5* Granular Soils-^ 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* Intact rock is exempt from shoring

and sloping requirements*

7* Soft Soils are cohesive soils with an unconfined compressive strength (pocket penetro- meter reading) of 0*5 tsf-' or leas and granular soils thpt 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* Distrubed Cohesive Soils (backfill) shall be treated as fissured medium cohesive or soft 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 5*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 can not be excavated with vertical

sideslopes (some moist granular soils will exhibit apparent cohesion and temporarily stand on a vertical slope), they can not be molded when moist and curmble easily when dry*

1 tsf -= 96 kPa

41
Table 5.4 Minimum Acceptable Stability Requirements

for Matrix Soil Classfication System

Soil Type Wg Ibf/ft^^'^ Steepest Slope

hor :vert.

I 20 1/2:1

II 40 3/4:1

III 60 1:1

IV 80 1 1/2:1

Notes

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 set by the weakest layer.

a/ 1 Ibf/ft^ = 1.63 N/m^

Table 5.5 Maximum Center to Center Spacing in ft. of Spaced Sheeting

for Matrix Soil Classfication System

Depth in ft.^^

Soil Class 5-10 10-15

I 8 (6)^/ 6 (4)^/

II 3 2

III 3 2

1 ft = 0.3 m

Numbers in parentheses are preferred spacing.

42.
soils that can be readily identified visually and to differentiate

between cohesive and granular material in case of doubt.

5.3.2 Visual-Manual Examination

(1) General

The recommended soil classification systems are designed for rapid field

application. Normally all that is needed is a visual and manual examination of the soil and site condition as work progresses. In case of doubt

or dispute the other field tests recommended herein, or more accurate

laboratory or in situ tests can be used. While the methods and interpretations given herein provide guidance, they can not be substituted for

common sense and judgment which may dictate attention to events and

phenomena which are not discussed herein.

(2) Visual Examination

Visual examination includes examination of the site before excavation,

examination of the trench and the extracted material during excavation

and examination of the soil surface next to the excavated trench.

Examination of the soil surface before excavation should be for cracks

(fissures), previous disturbance, surface water and vegetation (if there

are known correlations between vegetation and subsurface conditions).

If there are cracks (usually tension cracks from drying of clays) the

soil should be considered fissured, unless the fissures are very shallow

(25 percent of the excavation depth or less). Also, if previous disturbance

is detected (adjacent or crossing utility lines should be considered

evidence of previous disturbance) the soil should be considered fissured.

Surface water may be an indication that the soil is saturated, but this

is not necessarily true for impervious soils (clays), where surface water

may be perched.

Examination of the soil during excavation should include several observations .

1. Can the soil stand with vertical walls? If yes, can it stand longer

than 10 minutes? If not, at what slope will it stand without further

slumping or spalling? If the soil can stand longer than 10 minutes

it may be a hard intact (Type A) soil, or, by the matrix system, a Class I soil. A Type A (Class I soil) should stand vertically without collapse or spalling for at least 2 hours. A soil that cannot

stand vertically for at least 10 minutes should not be shored by

spaced sheeting. A soil that can not stand on a slope steeper than

3 hor. : 1 vert, is a "soft soil" (Type C or Class IV).

2. Is the soil dry, moist or wet? A wet soil should be considered

"submerged" (water in trench) unless the ground water is lowered by

dewatering of the soil forming the bank. A "moist" soil shiOuld not

be considered "submerged" unless water drains from the banks into the

trench.

43
3 . Is the soil fissured or disturbed? If not, do fissures develop a short time after excavation? If fissures either are present, or develop a short time after excavation the soil is fissured

(Type B in simplified system).

4. Does the soil crumble or come out in clumps? If it comes out in

clumps, can the clumps be broken up? If the clumps are moist or wet

do they tend to slump under their own weight?

Soils that come out in clumps are probably cohesive (a more reliable

determination is discussed under manual examination). If moist or wet clumps tend to slump when dumped out of the bucket of the backhoe the soil is probably a "soft soil" (Type C or Class IV).

5. Do chunks of soil spall off the vertically excavated trench wall?

If yes, this is an indication that the soil is fissured (Type B).

6. Is the soil layered? If yes, do the layers dip into the trench?

This is a determination which is normally made entirely on the basis

of the visual examination, by looking at the two sides of the trench.

Additionally, visual examination during excavation will reveal changed

conditions such as color or moisture changes or changes in the previously

discussed conditions 1. to 5.

Examination of the soil surface next to the excavated trench should be

mainly for fissures parallel to the direction of the trench. If new

fissures appear before shoring is installed it is usually an indication

that the soil is fissured even if it was previously assumed that the

soil is intact. This situation frequently arises in overconsolidated

clays. In soft cohesive soils fissures next to an unshored vertical

trench are an indication that the cohesive strength of the soil can not Support the vertical cut. In either case shoring of appropriate

strength should be selected and great caution should be exercised during

its installation.

If new fissures appear next to a shored trench, the fissures should be

taken as an indication of inadequate shoring and possibly imminent collapse. Likewise, development of new fissures next to a sloped trench are an indication that the sideslope is too steep and a slide may be imminent.

(3) Manual Examination

Several tests are included in the manual examination:

1 . Dry Strength

If the soil is dry and crumbles on its own or with moderate pressure into

individual grains or fine powder it is granular (sand or silt). If the soil

is dry and falls into clumps which break up into smaller clumps, but the

smaller clumps can only be broken up with difficulty it is a fissured clay.

44
If the dry soil breaks into clumps which do not break up into smaller

clumps and which can only be broken with difficulty it may be an intact

clay, provided the visual examination of the trench and its vicinity did

not give any indicaton of fissures.

2. Plasticity

Moist or wet cohesive soils can be molded into various shapes and rolled

into threads as thin as 1/8 in. (3 mm) without crumbling. If at least a

2 in (50 mm) length of an 1/8 in thread can be held on one end without

tearing the soil is cohesive (though it may be fissured).

3. Thumb Penetration

The thumb penetration test (see Table 3.3) can determine the strength

of cohesive soils. Type A (Class I) soils can be indented by the thumb,

but can only be penetrated with very great difficulty. Type C (Class IV)

soils can be easily penetrated several inches by the thumb.

(4) Visual-Manual Examination by ASTM 02488-69 [1]

Some of the methods described in this section are compatible with ASTM

D2488, however the approach is simplified and lends itself to a more

rapid application. In case of doubt or dispute ASTM 2488 could be used

with the following interpretation:

Cohesive soils are clayey silt, sandy clay; silty clay, clay, and

organic clay chat have dilatancies ranging from rapid-slow to none and

"toughness of plastic thread" ranging from medium stiff to very stiff.

Consistency : Soft cohesive soils are defined as in ASTM, Table 1. Hard

Intact Soils (stiff cohesive in matrix classification) cover the upper

range of "very stiff" and "hard" as defined in ASTM Table 1 (1.5 ton/ft^

and up).

(5) Visual Identification of Distress

"Distress" is defined herein as a situation where a shored or sloped

trench is at the verge of total or partial collapse. Not only construction superintendents, but all workers entering or working next to a trench should be trained and instructed to identify and watch for signs

of distress. There are two phenomena which are often associated with

distress

1. Development of new fissures parallel to n shored or sloped trench.

2. Sudden separation of even minor amounts of soil from the slope, or

the vertical bank in spaced sheeting (pebbles or little clumps rolling off the bank or sands beginning to trickle down). The term

"ravelling" is often used to describe this phenomenon.

45
5.3.3 Strength Tests

A crude approximation of the unconfined compressive strength of cohesive

soils can be obtained by use of a pocket penetrometer. The pocket penetrometer should be utilized in place, preferably in the sides of a freshly-opened trench (workers performing the test must be protected by

shoring). Tests should be made at locations no further apart than 10 ft

(3m) horizontally along the trench or wherever the soil type changes.

At each location, several tests should be made on each side of the

trench preferably at 1/3 and 2/3 the depth and near the bottom if moisture content increases with depth. If it is dangerous to enter the

trench, tests could be performed on soil clumps removed from the trench.

As an alternative to the pocket penetrometer, a hand operated shearvane

may be used. In this case, readings should be doubled to obtain unconfined

compressive strength. There may be a tendency to consider the results

of shearvane tests as more reliable than is justified.

5.3.4 Drying Tests

The basic purpose of the drying test is to differentiate between granular materials, cohesive materials with fissures, and intact cohesive

materials. The test must be conducted on undisturbed samples, perhaps

taken with an ordinary shovel, since they will identify fractures, fis- sures and slickensides in cohesive soils as well as the coarse-grained

component of other materials.

The procedure for the drying test involves drying a sample of soil

approximately 1/2 to 1 inch (12 to 25 mm) thick and 6-inches (150 mm) in

diameter. The sample can be dried in an oven at 212®F (100°C) for 24

hours. Other field methods could be used, such as placing the sample on

a hot engine exhaust manifold or baking the sample in the sun during

hot weather. If significant fissures exist, they will become apparent

upon drying as the sample will crack along the fissures. If the sample

dries to a hardened state intact it should be broken by hand as described

in the following paragraph.

After drying, the sample is grasped between the thumb and forefinger of

each hand and broken in flexure. If considerable force is necessary to

break the sample, the soil has a significant clay content and should be

classified as a soft, medium, or stiff clay without significant fissures

or joints (intact). If the sample can be easily broken, the soil is

either a fissured clay or a granular material. Distinction between a fissured clay or a granular material can be made by attempting to pulverize the resulting dried clumps of material. A simple means would be to

squeeze them together in one hand. (Even stepping on them is all right.)

If they pulverize into very small fragments, the material is granular.

If the clumps do not pulverize easily, the material is a fissured clay.

46
5.3.5 Suitability for Spaced Sheeting

When the material is granular, spaced sheeting is not feasible since the

material would collapse. This is in most instances evident and no tests are needed. A dangerous situation may however arise when the material

stands up long enough to make installation of supports feasible but subsequently collapses between supports. This situation may arise in fissured clays as well as in moist soils which are predominantly granular.

In the latter case, failure may occur with a change in moisture content.

The following conditions are necessary, but not always sufficient for

the feasibility and safety of spaced sheeting.

1. The trench, when fully excavated, should stand up with vertical

walls for at least 10 minutes if spaced sheeting is to be used

at all. It should be able to stand 2 hours without any sign of

failure if the center to center spacing allowed for Type A

(Class I) soils is to be used.

2. For spaced sheeting to be permitted at all it should be possible

to pick up the dried drying test specimen on one end without

breakage

5.4 CLASSIFICATION PROCEDURES

5.4.1 Simplified Classification System

The classification procedure is outlined in the following flowchart.

Note that in most instances all the steps can be accomplished in the

visual-manual examination.

Additional decisions are needed when spaced sheeting is used according

to Table 5.2. Two decisions are necessary:

1. Can spaced sheeting be used? If the soil crumbles and a portion of

the trench will not stand for 10 minutes without support, spaced

sheeting cannot be used. If the trench does stand up but there

is a question whether adequate cohesive strength is available, an additional determination can be made by the drying test.

2. Is the soil Type A or B? This decision was previously described

and can be in most instances easily resolved by thumb penetration

or penetrometer tests. Trenches in Type A soil should also be capable of standing without support for two hours.
FLOW CHART FOR SOIL CLASSIFICATION PROCEnilRE,

SIMPLIFIED CLASSIFICATION SYSTEM

TAMi 5.1. NOTE 4 SUBMERGED?

YES

NO

TABLE 5.1. NOTE 6 INCLINED LAYERS\^ YES

DIPPING TO TRENCH? -—TYPE C

NO

TABLE 5.1. NOTE 3

IF COHESIVE: THUMB IMPRINT TEST < cr PENETROMETER TEST

•r TORVANE TEST ^ SOFT?

po

TABLE 5.1. NOTE 1 WRD?

YES

VISUAL EXAMINATION

tr THUMB IMPRINT TEST

'

•r PENETROMETER TEST Qu 2: 1*5 tsf

Tyes

NO

TYPE B

YISUAL EXAMINATION r MANUAL EXAMINATION

•r DRYING TEST FISSURED?

YES

48

TNO

nPE A
5.h.2 Matrix Classification System

In this system, more parameters are considered and thus, more decisions

must be made. The procedure is outlined in the following flow chart.

FLOW CHART FOR SOIL CLASSIFICATION PROCEDURE,

MATRIX CLASSIFICATION SYSTEM

(1) Table 5.3, Note 8

(2) Table 5.3, Note 3

Visual Examination

Thumb Penetration

Penetrometer or vane

(3) Table 5.3, Note 5 Visual Inspection

Drying Test

(4) Table 5.3, Note 1 (5) Table 5.5, Note 4

Thumb Penetration

Pocket Penetrometer or vane

Visual Examination

Manual Examination

Drying Test

CLASS III

49
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6 . 1 GENERAL

Two alternate systems are recommended, each of which has advantages and

disadvantages which should be carefully scrutinized by the intended users before a final choice is made. The success of any system will ultimately

depend on its acceptance by all the parties involved and on the ability

of the intended users to apply it. One of the two systems should be

adopted.

6.2 ALLOWABLE SIDESLOPES (Figure 5.1)

The sloped excavation configurations shown in Figure 5.1 are suggested in

order to give more flexibility to contractors. Vertical sides are frequently required in specifications for pipe bedding. The configuration

?n.Q,{^abHA,c.0Lt2,d ^t/uit-waZo. (U^mbly

6. COMMENTARY ON RECOMMENDED SOIL CLASSIFICATION SYSTEMS

51
in Case II would permit contractors to meet these 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 ( 1 m) 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. The 5 ft (1.5 m) limitation on the height of any vertical unsupported bank in Case III is con- sistent with present OSHA regulations.

6.3 LATERAL PRESSURE DIAGRAMS (Figure 5.2)

The simple soil classification systems recommended do not provide the

information necessary for the selection of one of the design pressure

diagrams shown in Figure 3.1. For instance, it is not always necessary

to distinguish between sands and clays in order to use the Simplified

System. Moreover , surface deposits are frequently heterogeneous and can

not be readily placed into one of the categories in Figure 3.1. The

uniform pressure diagram was chosen for all soil conditions.

A 2-ft (0.6 m) uniform surcharge is used to allow for accumulation

of spoil, stored construction material, machinery, traffic or any other

surcharge effects (normally construction equipment operates from one side and the spoil pile is on the other side).

6.4 SIMPLIFIED CLASSIFICATION SYSTEM (Table 5.1)

6.4.1 Soil Types (Table 5.1)

The intent of this system is to provide the simplest and least ambiguous

scheme possible while still providing a reasonable range of choices to

the contractor. It is felt that in most practical situations, three

strength categories of bracing systems is all a contractor would want to use. However, should intermediate strengths be desired, an engineer

could document the adequacy of the shoring using accepted engineering

practice.

The decisions that must be made in order to classify a site are shown in

the flow chart in Section 5.4.1.

6.4.2 Lateral Soil Pressures (Table 5.1)

In Table 6.1 th§ lateral soil pressures (weight effects) recommended for

the Simplified System are compared with those calculated by accepted

engineering practice in accordance with Figure 3.1. It can be seen that

the Simplified Method is equal to, or more conservative than, accepted

engineering practice for sands and soft clays, roughly equal for stiff

clays, dense cohesive sands or stiff sandy clays and on the low side

for medium clays. [Note that the upper limit for medium clays was calculated for the borderline case of 500 psf (24 kPa) cohesion and 20

ft (6 m) depth. This is an extreme case which in reality would be

Type C (the stability number for medium clays, and thus the pressure

52
coefficient increases with depth)]. The reason for setting these values

lower is based on several observations: (1) The CAL OSHA experience

which extends over two decades produced no evidence of unsafe conditions;

(2.) Analysis of traditional timber shoring leads to the conclusion that

actual field conditions for shallow trenches produce pressures which

are much lower than those predicted by engineering analysis; (3) The

nature of shallow trenches, which are open for short time periods and are generally excavated to their full depth before shoring is installed

(particularly in clays) and where the stability number (N) tends to

be low. These conditions will produce lateral forces in shallow trenches

which are much smaller than those measured in deep excavations (see

discussion Section 4.1). Nevertheless, before the system is actually

adopted, consideration could be given to changing the definition of

the "soft soil" category for clays and including clays with an unconfined

compression strength up to 1500 psf (72 kPa).

Table 6.1 does not include submerged sands and organic soil; however, in

both of these instances the Simplified System tends to predict lateral

pressures which exceed those calculated by engineering analysis.

Table 6.1 Comparison of Lateral Weight Effects Calculated by Various

Methods

Soil Type

Wg, Ibf/ft^

Eng. Practice Simplified System Matrix System

Loose Sand 25-26 40 40

Med. Sand 20-25 40 40

Dense Sand 17-20 40 40

Soft Clay 40-80^ 80 80

Med. Clay 44-70^ 40 40-60®

Stiff Clay (N<4) 22-33 20-40® 20-40®

Dense Cohesive Sand 20-30 20-40® 20-40®

Stiff Sandy Clay 20-30 20-40® 20-40®

a • • • Value that would be used xf clay is fissured.

^ Value changes with depth of trench.

6.4.3 Allowable Slopes (Table 5.1)

The philosophy used for allowable slopes substantially differs from that

used in the present OSHA regulations. Table P-1 [9]. Instead of stipulating "stable" slopes for many different conditions, the "steepest

allowable" slope of 3/4 horizontal to 1 vertical is stipulated for

"intact hard" and "medium" soils. It is reduced to 1-1/2 horizontal to

1 vertical for "submerged and soft" soils. It is realized that the

stipulated maximum allowable slope will not be stable in many instances.

53
For example, dry sands will not stand on an angle steeper than their

angle of shearing resistance. However, in those instances the contractor

will be unable to excavate at a steeper angle, and thus, the resulting

excavation will be safe. On the other hand, the NBS field study indicates that in many instances construction slopes in short-term excavations are steeper than the maximum stable slopes that would be predicted

by engineering calculations. This is particularly true for moist sands

and silts where apparent cohesion stabilizes slopes steeper than the

angle of shearing resistance. This and similar conditions are very

difficult to quantify and rigid limitations, which by necessity are based on the worst case, would be unrealistically conservative and thus

difficult to enforce. The difficulty of enforcing multiple choices of

sideslopes was confirmed in the NBS field study [7] where many comments

expressed unhappiness with existing provisions for sloping.

In recognition of these difficulties the approach recommended herein is

similar to the one used when speed limits are enforced for vehicles on highways. The maximum allowable speed is not reasonable under all conditions (i.e., ice, rain, etc.) but it is the only speed that can be

realistically enforced, since the "reasonable" speed is too difficult

to quantify.

As an added protection, footnote "b" in Table 5.1 stipulates that if

there is any sign of general or local instability the slope shall be

cut to an angle which is flatter than the stable slope as observed in

the field.

There are several reasons for the stipulated steepest allowable slopes.

In stiff and medium clays these slopes will be stable for depths up to

20 ft. It can also be shown [16] that as the shear strength of a clay

decreases and the excavation depth becomes critical, a base failure will occur even if the slope is flattened; for sands and silts as indicated

before, this slope should be viewed as a maximum that in many instances

cannot be attained. Data from the NBS field study, as well as experience with a similar provision in California indicate that the proposed

maximum slope provision should work well in practice, as long as it is

understood by contractors that this is a maximum allowable slope which

is not necessarily appropriate for all cases. The flatter 1-1/2 horizontal: 1 vertical slope was proposed for "submerged and soft soil"

to allow for potential instability caused by water seeping into the

trench. In many of the "submerged" and "soft soil" conditions, sloping

is not appropriate and would not be used by contractors.

6.4.4 Spacing of Spaced Supports (Table 5.2)

The maximum spacings correspond to existing practice and present OSHA

provisions. Spaced supports are not always feasible for Type B soils.

A procedure for field determination of whether spaced sheeting can be

used is outlined in Section 5.4.1.

54
6.5 MATRIX CLASSIFICATION SYSTEM

6.5.1 Soil Classes (Table 5.3)

In this classification system the two important parameters of water and

fissures are interfaced with four different soil types. This allows con- sideration of 16 possible situations. The advantage gained by this

approach is that the important parameters are explicitly considered.

This is accomplished at the expense of added complexity since it

requires more decisions which require knowledge of soil properties and

better training of the personnel that must make these decisions.

6.5.2 Lateral Soil Pressures (Table 5.7)

Comparison of the lateral soil pressures obtained by the Matrix System

with those obtained by the Simplified System (see Table 6.1) indicates

that for the cases tabulated (except for medium clay) the pressures are identical. However, there are differences for other cases. These are shown in Table 6.2 together with average pressures calculated by accepted

engineering practice (actual pressures would change with depth). It can be seen that for submerged stiff clays and sands the pressures used in

the Matrix System are closer to those used in engineering practice. In

any particular case, the pressures computed by engineering practice may

be even lower, since the "submerged" case includes every instance where

water enters the trench, which includes cases where only part of the

depth of the retained soil is submerged. If the Simplified System was

to be used, the contractor would have the choice between using the con- servative 80 Ibf/ft^ (130 N/m^) pressure or retaining an engineer. In

view of the fact that shallow excavation work is only rarely conducted

below the groundwater table (in preference of dewatering) the involvement

of an engineer in this situation seems reasonable and even desirable.

Table 6.2 Cases Where Lateral Soil Pressures Obtained by the Matrix

System Differ from those Obtained by the Simplified System

Wg, Ibf/ft^

Soil Type Eng. Practice

Simplified

System

Matrix

System

Submerged Med. Sand 43^ 80 60

Submerged Stiff Clay 44-45^ 80 60

Submerged Med. Clay 55-70^ 80 60

Submerged Fractured

Rock

52-80® 80 60

^Since by hydrodstatic pressure increases linearly with depth, 1/2 the

effect of hydrostatic pressure was added to make a comparison possible.

^Pressures were taken from Reference [10].

55
6.5.3 Allowable Slope (Table 5.7)

While the slopes in the Matrix System, like those in the Simplified System, are maximum slopes they permit a distinction between soil types and

also provide more guidance to the contractor. Note that a 1/2 horizontal

to 1 vertical slope is permitted for Class I soils (Category A). This

slope is not permitted in the Simplified System, where the emphasis is on elimination of complexity. Experience indicates that slopes are difficult to determine in the field and generally look steeper than they

are. Thus, a 1/2:1 slope will have the appearance of a vertical cut.

With even slight irregularities, a 1/2 horizontal in 1 vertical slope

may actually become a vertial cut over large portions of an excavation.

Thus work would have to be performed very carefully if 1/2 horizontal

in 1 vertical cuts were to be used.

6.5.4 Spacing of Spaced Supports (Table 5.5)

The commentary in Section 6.4.4 also applies to Table 5.5.

6.6 FIELD IDENTIFICATION

6.6.1 Resolution of Conflicts

For both proposed systems visual-manual examination is all that is nor- mally needed. However, decisions must be made on the basis of this

field examination, and the question arises how conflicting opinions

about the validity of these decisions are resolved. For the Simplified

System there are two decisions where conflicting interpretations could

arise: (1) is the soil "intact hard"?; (2) is the soil soft?

For intact hard soil there are three criteria:

a. There shall be no fissures.

b. The shear strength exceeds 1500 psf (72 Pa).

c. The excavated trench can stand without support (there is no sign of

any distress after two hours).

For (a) there are three methods: visual examination, manual examination,

and the drying test. All of these procedures will not always give re- peatable results, because soil is a heterogeneous material, tension

cracks come and go with the seasons and the drying test is not a very exact tool. Thus, there are some inherent difficulties in this

instance.

The ultimate test would be that the trench should be able to stand

unsupported for at least two hours without spalling or collapse.

For (b) there is a "hierarchy" of tests in terms of level of accuracy.

At the bottom is the "thumb penetration test" (Table 4.3). At the next

higher level are the "penetrometer" or "vane" tests for cohesive soils.

At a still higher level are in situ tests: the Standard Penetration Test

56
[6], Cone Penetration Tests (ASTM D3441-75T) and Vane Shear Tests (ASTM

D2573-72). At the top of the hierarchy, provided that an undisturbed

sample can be taken, would be laboratory tests such as the Unconfined

Compression Test [ASTM D2166-66 (1972)].

For (c) the test is not ambiguous in itself, but it is quite possible

that two sections of trench, excavated in the same general area and

condition, would behave differently. To determine whether a soil is

"soft” the same heirarchy of steps as described under (b) above applies

if the soil is cohesive. For cohesionless soils compliance with the

provision of a 3 horizontal: 1 vertical stable slope can be determined

visually. At a higher level an engineer could determine the angle of

shearing resistance by in situ or laboratory tests.

For the Matrix Classification System, one more potentially ambiguous

decision is whether a soil is "granular". Again, visual-manual examination will in most instances provide an answer. After this, the procedure

described under the drying test (Section 5.5.4) could be followed; the

next more accurate step would be conventional soil classification

(D-2487-69) [4]. The ultimate judgment could be made by determining

the cohesive strength (c) and the angle of shearing resistance (<f)

in a triaxial test. <t> should be more than 18®, otherwise the scil

would be a "soft soil".

6.6.2 Drying Test (Section 5.3.4)

This is a new test, even though it has some similarity with procedures

described in ASTM D2488-69 [5]. However, the ASTM procedure involves

remolding which would obliterate the in-si tu fissures.

The Drying Test is to be conducted on an undisturbed sample. The test

should be relatively insensitive to technique and to sample size, as long as the sample is large enough to include likely fissures and as long as the sample is adequately dried. After some experience is gained

in the field, the drying test should be quantified with standardized

sample sizes and testing procedures.

It should probably be mentioned that temporary shoring should be placed

in a trench while taking any test samples .

57
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REFERENCES

1. ASTM, Recommended Practice for a Visual Manual Procedure for

Description of Soils, Designation D2488-69, American Society for

Testing and Materials, Philadelphia, PA, 1978.

2. ASTM, Standard Method for Classification of Soils for Engineering

Purposes, Designation D2487-69, American Society for Testing and

Materials, Philadelphia, PA, 1978.

3. ASTM, Standard Method for Penetration Test and Split Barrel Sampling of Soils, Designation D1586-67, American Society for Testing

and Materials, Philadelphia, PA, 1978.

4. CAL/OSHA, State of California Construction Safety Orders,

Article 6, Excavation, Trenches, and Earthwork, and Appendix C,

Helpful Construction Methods, Sacramento, CA, January 10, 1976.

5. Flaate, K. S., Stresses and Movements in Connection with Braced

Cuts in Sand and Clay, Ph.D Thesis, University of Illinois, Urbana,

IL, 1966. '

6. Goldberg, D. T. , Jaworski, W. E. , and Gordon, M. D. , Lateral Support Systems and Underpinning, Vol. II, Design Fundamental, DOT

Report No. FHDA-RD-75-129, Washington, D.C., April 1976.

7. Hinze, J., A Sfudy of Work Practices Employed to Protect Workers in

Trenches NBSIR No. 79-1942, National Bureau of Standards, Dec 1979.

8. O'Rourke, T. D. , and Cording, E. J., The Observed Performance of

Subway Excavations, University of Illinois, NTIS Report PB 240905,

September 1974.

9. 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.

10. Peck, R. B., Deep Excavations and Tunneling in Soft Ground, Proceedings, 7th International Conference on Soil Mechanics and

Foundation Engineering, State-of-the-Art Volume, pp. 225-250,

Mexico City, 1969.

11. Peck, R. B., Hanson, W. E. , and Thornburn, T. H. , Foundation Engineering; John Wiley, New York, NY, 1974.

11a. Salomone, L. A. and Yokel, F. Y. Proceedings, Federal Workshop on Excavation Safety, September 19 and 20, 1978, NBSIR No. 79-1935

National Bureau of Standards, Washington, D.C., December 1979.

12. Salomone, L. A. and Yokel, F. Y. , An analysis of the Response

from an Associated General Contractors of America Survey of

59
Trenching and Shoring Practice, NBSIR 79-1936 National Bureau

of Standards, Washington, D.C., December 1979.

13. Scott, R. F. and Shoustra, J. J., Soil Mechanics and Engineering , McGraw-Hill, New York, NY, 1968.

14. Skempton, A. W., Long-term Stability of Clay Slopes, The Rankine

Lecture, Geotechnique, Vol. 14, No. 2, 1964.

15. Sowers, G. B., Sowers, G. F., Introductory Soil Mechanics and

Foundations , Macmillan Publishing Co., Inc., New York, NY, 1970.

16. Terzaghi, K. , Theoretical Soil Mechanics , John Wiley & Sons,

Inc., New York, NY, 1956.

17. Terzaghi, K. , and Peck, R. B., Soil Mechanics in Engineering

Practice , John Wiley, New York, NY, 1968.

18. Thompson, L. J. and Tanenbaum, R. J., Excavation, Trenching and

Shoring: The Responsibility for Design and Safety, Texas A&M

Research Foundation Report, College Station, TX, September 1975.

19. Thompson, L. J. and Tamenbaum, F. J., Responsibility for Trenching

and Excavation Design, J. of the Geotechnical Engineering Division,

Proceeding of ASCE, Vol. 103 No. GT4, April 1977.

20. Thompson, L. J. and Tanenbaum., R. J., Survey of Construction

Related Trench Cave-Ins, J. of the Construction Division Proceedings of ASCE, Vol. 103, No. C03, September 1977.

21. Tschebotarioff , T. P., Foundations, Retaining and Earth Structures , McGraw-Hill, New York, NY, 1973.

22. Tucker, L. R. and Reese, L. L. , Soil Classification for Shallow

Trenches, The University of Texas, Austin, TX, January 1979.

23. Winterkorn, H. F. and Fang, H. Y. , Foundation Engineering Handbook , Van Nostrand Reinhold Co., New York, NY, 1975.

24. Wisconsin Administrative Code, Chapter Ind 6, Trench, Excavation

and Tunnel Construction, Wisconsin Administrative Register No. 188,

Madison, WI, August 1971.

25. Yokel, F. Y., Recommended Technical Provisions for Construction

Practice in Shoring and Sloping of Trenches and Excavations,

Technical Note No. 1111 National Bureau of Standards, Washington,

D.C., 1980.

26. Youd, T. L. and Craven, T. M. , Lateral Stress in Sands During

Cyclic Loading, Proc. Paper 11093 (Technical Note), J., Geotechnical Division, ASCE, Vol. 101 No. GT2, February 1975.

60
8. ACKNOWLEDGMENTS

The contribution of the following persons is gratefully acknowledged.

Jim Be Ike, Speed Shore Corp.

Luther Bell, Hustler Construction Co.

George Breadberry, Speed Shore Corp.

Harry Cagel, Cagel Construction Co.

Red Cass, Big Red Construction Co.

Del Talley, Austin AGC

Pete Ellis, Inter County Construciton Co.

Tom Ervin, Boh Brothers Construction COf

R. G. Griffith, Griffith Construction Co.

Cliff Farmer, CAL-OSHA

William Hazel Jr., Hazel Construction Co.

Tom Gerry, FALCO Construction Co.

Mikey Landry, Boh Brothers Construction Co.

Jim Lapping, AFL-CIO

Steve O'Reilly, State Construction Co.

Henry Pate, Grad. Student, University of Texas

Dave Plank, Speed Shore Corp.

Larry Salomone, NBS

Roy C. Schmidt, Schmidt Construction

Art Schmuhl , A. G.C.

Tom Seymour, OSHA

Frank Speer, FALCO Construction Co.

Roger L. White, City of Austim

Jim Williams, Houston Contractors Assoc.

61
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APPENDIX

BACKGROUND INFORMATION ON

TRENCHING PRACTICE AND ACCIDENTS

63
A.l GENERAL

The background information presented herein was compiled in

Reference [22].

A. 2 BACKGROUND INFORMATION

A. 2.1 SOURCES OF INFORMATION

Very little published information is available which provides quantitative data regarding trenching failures or lateral earth pressures

against shoring in braced shallow trenches and moreover trenching practices vary widely in different geographic regions of the nation.

Thompson and Tannenbaum [18, 19, 20] presented results of extensive sur- veys of accident reports and contractor opinions. The National Bureau

of Standards [7, 22] obtained data regarding trenching practices among

members of the National Utility Contractors Association and interviewed

selected utility contractors to determine their practices with regard

to soil identification and trench bracing. The Associated General Contractors conducted a questionnaire survey in 1977 [12] which yielded

further information. In addition, reports of fatal accidents of 1973-

74, given by OSHA Compliance Officers, have been reviewed in an attempt

to determine common causes or characteristics of trench cave-ins.

Results of some of these sources are discussed below.

A. 2. 2 THE THOMPSON AND TANNENBAUM DATA

A report was written in 1975 [18] from data accumulated in a study

funded by the Associated General Contractors of America. Basic sources

of information came from 86 major newspapers and 120 questionnaires

returned by contractors. The newspaper clippings identified both a variety of soil types, mentioned in section 1, and many others as possible contributors to trenching collapses. Of 120 contractors

responding, 52 reported a total of 85 cave-ins during the history of

their companies. Information of soil types in the contractor reports were sketchy, but cave-ins in all types of conventionally identified

soils were reported. Probable factors other than soil conditions which were mentioned as contributing to cave-ins were excessive rainfall, seepage, vibrations, and equipment near trenches. The majority of the

failures reported by Thompson and Tannebaum were in trenches less than

20 ft (6 m) deep [18].

A. 2. 3 NBS FIELD STUDY

As a portion of the overall NBS study, Hinze [7] interviewed contractors

and their staff and analyzed results from a survey conducted by the

National Utility Contractors Association (NUCA). The NUCA survey had

223 responses. Ninety-three percent of the trenching work of responding

contractors involved depths of 15 ft (4.5 m) or less. About 75 percent

reported average depths to the water table less than 15 feet. Trench

64
widths were seldom mojre than 6 ft (2 ta). All types of soils were encountered, and most contractors mentioned both wet and dry conditions.

Over 75 percent of the contraetots reported le^ss than 100 ft (30 m)

of trench open at any one time. Contractors seldom employ soils engineers. A considerable amount of additional data related to safety practices and firm characteristics was assembled in an NBS field study [7].

A. 2.4 INTERVIEWS WITH SELgCTEB CONTRACTORS

Tucker and Reese [22] held interviews with 15 selected contractors,

city officials, manufacturers of shoring equipment, and safety officials. In general, the interview results were consistent with the

data obtained from Reference [7].

Most contractors work within a radius of 50 miles of their offices and

in soils with which they are generally familiar. They seldom have soil

boring results available prior to bidding or starting a project. Even

if such data are available, they are considered unreliable by the con- tractors. If contractors have concern about a site, they take a backhoe out and dig a test section to determine site characteristics. In

such cases, contractors are normally looking for depths to rock and

water, rather than soil characteristics.

All contractors interviewed prefer to slope trench sides for stability,

as compared to any other safety system. Some will slope even if the

trench walls could stand vertically, because of backfill convenience.

There are instances, however, where right-of-way restrictions require

vertical sides. All contractors use hydraulic backhoes extensively.

Decisions regarding trench bracing are normally made by job-site supervisors. Some contractors state they wopld always have a supply of

shoring material onsite to minimize the inconvenience factor in a shoring decision. Although shoring slows work slightly, it is not

normally considered a major factor for decision. The unanimous attitude of those interviewed was that safety takes the highest priority

and that job foreman or superintendents are encouraged to take safety

precautions if they have any doubts regarding stability.

Job site supervisors have major responsibilities for decisions. Superintendents and foremem normally have several years experience. Even

contractors with registered engineers on their staffs consider the job

supervisors as more capable of identifying soils and determining shoring

needs. Most soils are defined by local generic terms rather than geological or geotechnical terms.

A variety of crude tests and indicators are used for soil types and

stability indicators. The necessary slope angle is normally determined

by the backhoe operator who may strike a trial slope several times with

the backhoe bucket. If he can induce sloughing in this matter, he will

flatten the slope. Many contractors said they looked for backhoe teeth

marks in the excavation. If these are presented and make clear indenta65
'; ( tions, it is an indication of significant "clay content. Other contractors watch surface cifack movements, kic-king loose soil into them and

watching for progressive opening.' • Marly mentioned signs of "ravelling"

in clay soils and surface spalling on the trench sides indicating movement or embankment distress.

Most contractors expressed fears of "stable" soils that collapsed suddenly. They also felt that most accidents are caused by carelessness

of field personnel, who fail to exercise proper precautions in known

unstable environments . ‘Many mentioned such things as "layered" soils,

"fissured" clays, trenches left open too long, rainfall or seepage

water, trench discontinuities, vibrations, and disturbed soils as hazardous situations^..- >

A. 2. 5 THE ASSOCIATED GENERAL CONTRACTORS (AGC) QUESTIONNAIRE [12]

The Safety Division- and Municipal Utilities Division of the Associated

General Contractors distributed a lengthy survey questionnaire in late

1977, which received 23 responses from a wide geographic area. Of the

work performed by the 23 respondents, about 70 percent was for seepage,

and 23 percent for water systems. Fifty percent of the work utilized

sloped sides, while bracing systems

of the time.

Typical soil conditions encountered

Stiff clay

Medium clay

Soft clay ,1. . Sand and gravel >: , Loose sand . .

, . - Muck '

;

' Loess

were used an average of 43 percent

by the respondents were:

I 29 percent

18 percent

16 percent

15 percent

10 percent

6 percent

6 percent

Written responses were given regarding the soil type causing the most

concern for safety against collapse. Although these varied somewhat,

the most frequent responses were "clay likely to crack" and loose silt or sand."

Most contractors (79 percent) stated that they normally left their

trenches open less than. one day. Almost half of the trenches are left

open less than 2 hours. The typical length of an unbraced trench sec- tion averaged 40 feet (12 m) and the typical length of a braced section

averaged about 50 feet (15 m)

As indicated by the interviews with contractors, the questionnaire showed

a preponderance of decisionmaking authority with job site supervisors.

General superintendents and foremen are heavily involved with all decisions regarding .bracing and sloping. The -job foreman or superintendent

is almost universally considered to be the person most familiar and qualified to describe the physical characteristics of a soil.

66
One question related to the uniformity of soils from the ground surface

to the bottom of the trench. Of those fosponding^ 75 percent stated

that soils in their region sre norm^^lly not uniform. Responses indicated many combinations, but both soils incressing in strength and soils

decreasing in strength with depth were mentioned. Soils changing radically in character, such as granular to cohesive were frequently

mentioned.

Responses to the question regarding

safety hazards were as follows:

Straight trench

Manholes

Intersections of trenches

Vibrating areas Disturbed soils

Narrow right-of-way

site conditions causing the greatest

6 percent

4 percent

23 percent

14 percent

38 percent

14 percent

Another question was posed regarding "surprises” during a trenching project. Answers were given in

Responses were as follows:

terms of frequency of occurrence.

Frequently Occasionally Rarely

Unexpected Ground Water 3 10 9

Radical Change in Soil Type 9 8 5

Tree Roots 3 11 8

Unanticipated Utility Lines

Previous Excavations or

8 11 3

Disturbed Soil 3 15 4

Unusually Heavy Surcharge 0 7 10

The contractors were asked if they frequently encountered soils which

lose stability due to a variety of causes. Responses are tabulated

below.

Rainfall 12

Vibration 13

Increased

Subsurface

Water 13

Drying 11

Temperature

Changes 2

Inclined Bedding 3

Survey responses suggest that moisture changes and vibration have significant effects on soil stability.

67
In summary, a wide variety of soils is encountered by trenching contractors. Soils often vary^with depth in a trench. Unexpected conditions

often occur and soils may lose stability with time due to weather or environmental factors.

' ** * A. 2. 6 OSHA COMPLIANCE OFFICER REPORTS FOR 1973-1974

As a means of gathering information on actual failure circumstances and

situations, 64 reports from OSHA Compliance Officer investigations during 1973-74 were furnished by OSHA in September 1977. These have been

reviewed and the results summarized [22]. Although the data are sketchy,

the pertinent results are discussed below.

Collapses normally occurred in rather shallow trenches. Of the 46 cases

in which trench depths were given, 40 were less than 12 ft (3.7 m)

deep. Soil identification was not normally made in an extensive fashion.

However, of the 29 instances in which soil was identified, 17 cases occurred in soil classified as "disturbed or fill." Free water was noted

in seven instances, although no mention of water was made in 56 cases.

In at least 2 cases, vibrations were assumed to be contributing to the

accidents. Another 23 locations were near streets or railroads which

are possible sources of vibrations. Fourteen of the locations were not near possible vibration sources. No information was given in 23 other

ins tances

Of the 54 instances in which information was available, 42 of the

failures occurred in straight sections. Discontinuities of the trenches

were possibly involved in 12 failures.

Secondary collapses of the opposite walls of the trenches were noted

in 5 instances.

No information was given regarding sloping or shoring in 20 of the 64 cases. In those where information was available, 4 failures were in

sloped trenches, 6 failures had skip shoring, 7 tight sheeting and

2 trench boxes. Twenty-two occurred in trenches with no shoring. Six

accidents occurred during installation or removal of shoring.

In accordance with this data sample failures generally occurred during

good weather. The injured workmen were normally mature individuals.

Of the oldest persons injured, 40 were over age 30 and 18 were over

age 50. This information is consistent with that from the contractor

interviews [21]. Job site personnel are often experienced and tend

to stay with a contractor for "many ‘years . Thus, the number of accidents

involving older, and presumably more experienced, personnel tends to

refute arguments that experience is an adequate factor in judging trench

wall stability.

It is unfortunate that more data are not contained in the Accident

Reports. These could be the most valuable source of quantitative

68
information available regarding failure conditions. Such items as trench dimensions, soil properties and identification, and details

of shoring systems would be invaluable.

A . 2 . 7 SUMMARY OF BACKGROUND INFORMATION

Most of the background data were obtained from contractor questionnaires

or interviews, or from case histories of accidents. Very little data

of a quantitative nature are available, that could be used to quantify

numerical approaches to problems of trench stability. Soil properties

often vary widely both from the top to the bottom of a trench and

linearly along a trench. Job site personnel are considered by contractors the most qualified persons to judge trench stability and to determine necessary support measures. Such supervisory personnel normally

have several years experience in the area of work. However, the number

of fatalities and accidents involving experienced personnel indicates

that decisions on sloping and shoring should not be based solely on experience

.

It is apparent that many factors other than those identified in a con- ventional soil characterization influence trench stability. The more

important factors include soil cracks and fissures, free water, previous

disturbance, vibrations, and weather. Time is obviously a critical item,

as trenches will remain open long enough for some activities, but collapse suddenly after a period of time.

A. 3. TRENCHING CHARACTERISTICS AND PRACTICES

A. 3.1 GENERAL

In current practice in the United States, most utilitiy contractors prefer to stabilize the sides of trenches by sloping rather than by bracing.

This attitude is, in part, the result of the fairly recent development

and introduction of the hydraulic backhoe in the industry. These

machines enable contractors to dig and handle a wide range of soils at

rapid rates. A significant number of the utility contractors engaged

in new subdivision work have organized their entire operations around

the hydraulic backhoe and do not go into a detailed design of most of

the trenches they dig. Instead, they depend on past experience on jobs

in the local area in submitting a bid. They rely on their job foremen

and/or backhoe operators to slope the trench sides adequately to ensure stability during the relatively short period the trench will be left

open. In deep trenches or in highly non-cohesive soils, they often

resort to a combination of methods; sloping the sides of the upper

portion of the trench and bracing the lower portion. Many of the contractors will not bid on a project when conditions dictate extensive

bracing of the entire trench.

For water lines and other relatively small trenches, trenching machines

are sometimes employed to cut vertical sides which are seldom braced

because workmen can easily get out of the trench if the sides fail.

69
There are, however, some jobs involving deeper trenches and vertical

sides, particularly in urban areas. A small number of contractors

employ large trenching machines on these projects and rely upon bracing

to ensure stability of the sides. A similarly small number of contractors concentrate on jobs involving rock, and employ drilling and blasting to fracture the material so that it can be excavated. These

contractors cut vertical trench sides and do not use bracing except

when inclined bedding makes it necessary. If a given project requires

several types of trench work, the successful bidder frequently subcontracts the portions his crews are not familiar with.

A. 3. 2 RIGHT-OF-WAY

Restrictions imposed by narrow right-of-ways often make some form of

bracing necessary. This is particularly true in urban areas, where

interference with traffic or other utilities cannot be tolerated. In

older cities, these conditions are often further complicated by

disturbed soil from earlier projects, which makes stabilization by sloping less dependable. Bracing, rather than sloping, is also necessary

when trenches pass close to structures which could be damaged by failure

of the trench or excessive settlements.

A. 3. 3 DEPTH

For any slope other than vertical, the additional volume of soil to be

excavated to slope the sides is quadrupled each time the depth is

doubled. As trenches become deeper, sloping becomes less practical

because of the large volume of earth involved and the increased distance

from the trench centerline to the spoil pile. The maximum practical

depth which can be stabilized by sloping is a function of the soil and,

equally important, of the size and characteristics of the available backhoes. In some relatively deep trenches, a trench box is used to guard

against collapse of the vertical-sided lower portion of the trench and

the upper sides of the trench are sloped for stability. Some form of

bracing is generally employed on the vertical sides cut by trenching

machines when the depth exceeds four to five feet.

A. 3.4 TIME AND SITE CONDITIONS

The amount of time a trench will be open, coupled with the influences

of vibration, water, and soil structure, significantly affects the

stability of trench sides and influences the decision between sloping

and bracing. Vibrations from a number of sources can seriously reduce

the stability of a soil mass. The longer the trench is open, the

greater is the chance of failure. When trenching operations do not

involve large pipe or special construction, trenches are seldom left

open more than an hour or two, and a wide range of soils will retain

adequate stability for safety with slopes as steep and 3/4 horizontal

to 1 vertical. If a segment of trench is to remain open for a relatively long period, gravity forces on the exposed sides, vibrations,

and other factors reduce the stability of the soil mass and either

70
flatter sloping or stronger bracing is required. This is particularly

true for trenches through clayey materials, where dessication and re- sulting cracks and progressive deformation can weaken the soil structure and cause failure of a trench side which was stable when it was initially cut. If the soil is layered and the bedding is inclined,

flatter sloping or stronger bracing of one side may be required even when the trench is to be open for only a very short time.

A. 3. 5 CODES AND SPECIFICATIONS

In many cases, the choice between sloping and bracing is dictated by

codes or project plans. Areas where the water table is high and shifting of material is a problem often have codes which specify that con- duits be protected by subsurface sheeting. Bracing, rather than sloping,

is then the normal approach. Similarly, bracing is generally employed

when plans for the facility call for installation of forms for concrete

or select fill.

A. 3. 6 SLOPING

When determined to be the most efficient method and when right-of-way

restrictions don't preclude its use, sloping is by far the contractor's

preferred method of trench stabilization. This method requires a minimum of materials and handling and provides the most reliable means of

personnel safety. Sloping is generally performed by hydraulic backhoes

unless other equipment is available. Normally the backhoe slopes the

bank back as the trench is being dug, providing a safe trench for the

workers at all times. The choice of angle to which the bank is sloped

is usually at the discretion of the backhoe operator; unless directed

by the foreman, the operator will remove soil until he is satisfied

that there is no danger of sloughing or slumping of the bank. The backhoe operator rarely is aware of the exact angle required by Table P-1

of the OSHA provisions [9], but instead relies on his own experience

and knowledge of the particular soil conditions.

A. 3. 7 SHORING '

A. 3. 7.1 General

Shoring for trenches or narrow excavations is usually accomplished by

bracing one bank against the other. The b«sic shoring system includes

a sheath of wood planks or plywood placed against both sides of the

trench, horizontal members or wales placed lengthwise to support the

sheathing and a system of shores to brace the wales against the sheathing

A. 3. 7. 2 Types of Sheathing '

Skeleton Sheathing - consists of a continuous wood frame supporting

sheathing planks, placed vertically at intervals of 4-8 ft (1.2-2. 4 m)

71
against the trench walls. Skeleton sheathing is used where the banks

consist of intact stable cohesive soils, primarily to prevent the development of initial failures, cracks or spalling.

Close Sheathing - consists of planks placed side by side along a continuous frame. Its use is to prevent local crumbling of soils. Since some space exists between the planks, it should not be used with silts or other fine grained cohesionless soils which could seep out through these

cracks

.

Tight Sheeting - consists of specially edged planks, generally tongue

and groove, eliminating the crevices existing in close sheathing. Tight

sheeting is either wood or steel and is used where water or fine wet

soils must be retained. The bracing used for tight sheeting is designed

for this use from the start and is generally stronger than that required

for other types of sheathing.

Timber Sheet Piles - when using this system, the contractor normally

leaves the piling permanently in the ground since removal usually

destroys the wood's usefulness. This system consists of planks which

are either square edged or tongued and grooved and which are driven into

the ground before it is excavated. The toe of the piles is usually cut on a diagonal with the point on the grooved side so that in driving,

each pile will be continuously wedged back against the previously driven

pile. The sections are driven with the tongue leading. After driving,

adjacent pieces may be spiked together across the joints to form a solid wall.

A. 3. 7. 3 Wale Systems

Normally most contractors use large timbers for wale systems, although

some steel shapes are used and recently lightweight aluminum wales have

been introduced. Timber members are usually 8 x 8 in (200 x 200 mm)

which are set into position at the quarter points of the sheathing and

supported by cross bracing.

A. 3.7.4 Bracing Systems

Timber shores have been used extensively in the past, with sizes varying from state to state. One rule, taken from the Construction Safety

Orders of the State of California pertaining to hard or compact soils,

requires that bracing between wales in trenches from 5-10 ft (1.5-3 m)

deep should be 4 x 4 in. (100 x 100 mm) timbers not over 5 ft (1.5 m)

apart, increasing to 6 x 6 in. (150 x 150 mm) for depths over 10 ft

(3 m). Other states have similar standards.

Trench jacks consist of two shoes with pronged surfaces that bear

against the wale system, or in the absence of wales, directly against

the sheathing. The shoes are held against the sheathing by an extendable connecting bar (usually a screw jack which can be extended). This

72
assembly is placed against the sheathing and extended or jacked outward,

providing the necessary lateral pressure to support the sides of the

trench.

Aluminum hydraulic shores - have been developed in recent years. The

shore is a hydraulic cylinder either connected to aluminum bearing plates

or aluminum shores. This system has the advantage of remote installation. The man installing these shores can stay outside of the trench

during installation.

A. 3. 7.5 Skip Shoring

This method utilizes the cohesive strength of the soil along with arching effects, thereby , allowing the contractor to support the wall by

individual spaced timber struts, trench jacks or hydraulic shores. The use of spaced vertical or horizontal planks supported at the quarter

points by two struts (cross braces) is the most common method of skip

shoring. The foreman makes the onsite decision as to whether the soil

has the strength required. Preliminary soils investigations give estimates of the strength, but not until the trench has been cut can an accurate estimate of the strength be made* When a soil has demonstrated

its capacity to stand vertically with only skip shoring, the foreman

will allow workers into the trench.

A. 3. 8 TRENCH BOXES . ,

Trench boxes are rigid boxes which are placed in the trench before the

entry of workers. The sidewall of the boxes are strong enough to resist

the lateral soil pressures. Thus, inside the box is a protected work

area. A relatively recent development in providing worker safety,

trench boxes permit rapid construction work as well as providing protection for workers. Some designs allow assembly in minutes at the job

site, and space saving flat storage for shipment from job to job. Contractors can use the boxes in several ways. The box can be placed in

the line of excavation digging the trench from inside the box, removing

the soil and subsequently pushing each side wall of the box down to the

pipe grade. This technique reduces the amount of soil that must be

removed, and lessens the damage to surrounding landscape. Another technique is to dig the trench in front of the box, then pull it into the

new section of trench with the backhoe. The technique uses the backhoe

for all phases of trenching, reducing the need for several pieces of

machinery. This technique requires that the trench be dug at least a foot wider than the box so that skin friction between the box and the

sidewall of the trench is reduced as much as possible. After the trench

box has been pulled through a section of trench, another machine, usually

a dozer, backfills the trench immediately to reduce the risk of excessive

soil movement.

73
A. 3. 9 EQUIPMENT

Backhoes are excavating machines which are primarily used to dig below

the natural surface of the ground on which the machine rests. They are adapted to excavating trenches, pits for basements, and general grading

work which requires precise control of depths. In some respects backhoes are more versatile than wheel or ladder type trenching machines,

especially in digging utility trenches whose banks are sloped and for

which trench shoring will not be used. Hoes can remove the earth as

it caves in to establish stable slopes, whereas trenching machines cannot do this easily. There are two basic types of backhoes. Track

mounted backhoes are heavy duty and are generally quite large [their

weights range from approximately 32,000 lb to 120,000 lb (14,500 kg

- 54,400 kg)]. Loaded combos are designed for more general work.

They are mounted on rubber tires and are equipped with a loader bucket

on one end and a backhoe bucket on the other.

Trenching machines are primarily used for digging utility trenches for

water, gas, and oil pipelines, telephone cables, drainage ditches, and sewers where the job and soil conditions are such that they may be used

They are capable of relatively fast digging with accurate controls of

depths and widths of trenches which reduces expensive hand finishing to

a minimum. Trenching machines are usually crawler-mounted to increase

their stability and to distribute their weight over a greater area.

There are three basic types of trenching machines. A wheel-type trench

ing machine consists of a power-driven wheel, on which are mounted a number of removable buckets, equipped with cutter teeth. The machine

is operated by lowering the rotating wheel to the desired depth, while

the unit moves forward slowly. The earth is picked up by the buckets

and deposited into an endless belt conveyer, which can be adjusted to

discharge the earth on either side of the trench.

Continuous-chain trenchers are designed for fairly shallow, narrow

trenches using digging scoops along a chain. The chain is supported on

a boom attached to the rear end of the prime moving tractor unit. The

major disadvantage of continuous-chain trenchers is that the lack of

rigidity in the chain prevents the buckets from digging into tough

material; they tend to twist over or move out of such earth. Only in

loose materials can the buckets be expected to get full loads.

The third variety is the ladder type trenching machine. It combines

the advantages of a wheel trencher and a continuous-chain trencher.

A ladder like projecting frame serves as the boom which can be raised

or lowered into the excavated trench. The excavation depth can thus

be adjusted while moving. The ladder type trencher can therefore operate on a more irregular surface than the wheel type. It has two continuous chains on both sides of the projecting frame with buckets like

those on the wheel trencher attached to and between the two chains.

In this way it can achieve the high production rates, lateral rigidity,

and power of the wheel trencher.

74
A. 3. 10 CONTRACTOR CHARACTERISTICS

In current practice in the United States, trenching is by and large a local operation. Utility contractors seldom bid on projects which are located more than 50 to 75 miles from their home base. Significant

exceptions to this arrangement are the general contractors who have a trenching division and often move their trenching crews to other

localities when their projects include trenches. Other exceptions are

the large pipeline projects, which attract bidders from all over the

country.

Since they normally concentrate their work in a relatively small area,

utility contractors have considerable knowledge of the physical characteristics of the soils in their area. They are not concerned with

classical identification procedures ; the fact that a given soil contains certain percentages of clay, mica, sand, and other materials is

of only incidental interest to them. Instead, they rely on their personal experience and the experiences of their fellow contractors in

the area for deciding on the extent of sloping or bracing required

for a trench in a given locality during a given season of the year.

Their experience and knowledge also enables them to predict, with fair

accuracy, the problems they will encounter with subsurface water and

rock formations.

Contractors have only limited confidence in soil boring results. When

there is some doubt about the conditions they will encounter, many contractors, either individually or in cooperation with other bidders, dig

several test pits with backhoes along the trace of the project to obtain

the additional information they need in order to submit intelligent

bids. Presence of subsurface water and rock are primary concerns,

but the contractors also obtain additional information on site stability

from the digging process and from leaving the pits open several hours.

As indicated previously, utility contractors tend to specialize in one type of trenching work. Their machinery and the experience of their

crews are a fundamental element in their decision to obtain stability

by sloping or by bracing.

In all but the very large diameter lines, pipe-laying is a dynamic operation, moving along at the rate of several hundred or more feet per day.

Soil structure, moisture content, and related factors change constantly

and the frequency of significant changes increases rapidly with

increases in the rate of production. Decisions on stability must not

only be made accurately, but also promptly, if the contractor is to

be successful. As a consequence, most firms rely on their job foremen

and equipment operators for day-to-day decisions on sloping and bracing.

Although these people are normally neither engineers nor soils technicians, confidence in their judgment appears to be well-deserved in most

cases. The foreman with 20 or more years of experience in trenching

is the rule; 10 years or less is the exception. Foremen normally know

as much, if not more, about the physical characteristics of the soil

75
in their local area as the knowledge that could be obtained through

conventional soil exploration and laboratory tests. Some contractors

solve their bracing problems by simply stockpiling standard sizes of

timber on site for use when and if the foreman decides that bracing

is necessary. Bracing methods developed by the firm from experience

with soils in the area are normally employed without detailed design

and analysis.

76
NO>ll4A (REV. S-7e>

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1945

1. Acc«$sfori

4. TITLE AND SUBTITLE

Soil Classlf icaation for Construction Practice in

Shallow Trenching

5. Publication Date December 1979

V .. 7. AUTHOR(S)

Felix Y. Yokel, Richarci L. Tucker and Lymon C. Reese

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreer, City, St^te, ZIP)

Occupational Safety and Health Administration

Department of Labor

Washington, D.C. 20210

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

1_J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual aummary of moat aignificant information. If document includes a significant bibliography or

literature survey, mention it here,) Construction practices in trenching and data on potential causes of trenching

accidents are reviewed. A study is made of the soil properties and site

-conditions that must be identified in order to determine the stability of

shored and sloped excavations against cave-ins. Two possible alternate soil

classification methods are recommended, which are simple enough to be used

by construction foremen and at the same time use parameters which can be

measured or identified without ambiguity; The classification methods are supplemented by appropriate field tests.

17. KEY WORDS (six to twelve entriee; alphabetical order; capitalize only the firat letter of the first key word unless a proper name;

separated by semicolons)

Braced excavations; construction; excavation; geotechnical engineering; retaining

structures; shoring; slope stability; soil classification; soil pressure; soil

testing; trenching.

18. AVAILABILITY f^Unlimited

For Official Distribution. Do Hot Release to NTIS

[

: Or(Jer From Sup. of Doc., U.S. Government Printing Office, Washington, DC

' 20402, SD Stock No. SN003-003-

i_j Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

IS. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

22. Price

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