NBSIR 79-1936

AN ANALYSIS OF THE RESPONSES FROM AN

ASSOCIATED GENERAL CONTRACTORS OF

AMERICA (AGO SURVEY OF TRENCHING AND

SHORING PRACTICES

of

ubbartt

HOV 2 4 1980

Hc>l dec

r m^ ^

79 - /936

/9 20

Lawrence A. Salomone

Felix Y. Yokel

Geotechnical Engineering Group

Structures and Materials Division

Center for Building Technology

National Bureau of Standards

Washington, D.C. 20234

July 1979

Issued January 1980

Prepared for Occupational Safety and Health Administration

Department of Labor

Washington, D.C. 20210

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

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
' s « . J

«»

k
PREFACE

In June 1976, The Occupational Safety and Health Administration (OSHA)

engaged the National Bureau of Standards, (NBS), to a) study the com- patibility of the technical provisions in the OSHA regulations for

excavation, trenching and shoring with actual construction practice

and with the state of knowledge in geotechnical and structural

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

their effectiveness.

As part of this effort, a field study of present practice in excavation,

trenching and shoring and of the impact of the OSHA regulations as perceived by contractors was conducted. Some of the information was provided by the Associated General Contractors of America (AGC) who

sent a questionnaire to its members. In this report the responses to

this questionnaire are summarized and discussed.

Mr. Arthur Schmuhl, Director of Safety and Health Services, and Mr. Joseph

P. Ashooh, Director of the Municipal - Utilities Division of AGC, conducted

the survey and provided many useful suggestions. The data were reviewed

and analyzed by Dr. Richard L. Tucker and Dr. Lymon C. Reese of the

University of Texas at Austin and by Dr. Jimmie Hinze of the University

of Missouri at Columbia.

Ill
ABSTRACT

Results of an Associated General Contractors of America (AGC) survey of

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

of the OSHA Regulations for Excavation, Trenching and Shoring as perceived by a selected number of the membership are presented. The survey

consisted of forty-seven (47 questions. A response of about fifty percent resulted in twenty-three (23) questionnaires being completed and

returned to AGC. Although the twenty-three responses did not merit a rigorous statistical analysis, the data are useful in making some general

statements about trenching and excavation operations.

Keywords: Construction practices; Construction Safety; Excavation; Shoring; Trenching.

IV
TABLE OF CONTENTS

Page

PREFACE ill

ABSTRACT iv

1.0 INTRODUCTION 1

1.1 GENERAL 1 1.2 PURPOSE AND SCOPE 1

2.0 DISCUSSION OF RESULTS 2

2.1 INTRODUCTION 2

2.2 FINDINGS 2

3.0 SUMMARY 4

4.0 REFERENCES 5

APPENDIX A THE AGC QUESTIONNAIRE A-1

APPENDIX B RESPONSES TO THE AGC QUESTIONNAIRE B-1

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1.0 INTRODUCTION

1.1 GENERAL

The OSHA Regulations for Excavation, Trenching and Shoring were promulgated in 1974 [1]*. Experience accumulated since their promulgation indicates that the regulations are difficult to enforce because of technical

ambiguities. Consequently in June 1976, OSHA engaged the National Bureau

of Standards (NBS) to study construction practices and to recommend potential modifications that could improve the effectiveness of these regulations. The state of knowledge in geotechnical and structural engineering

and the experience accumulated since the promulgation of the regulations

were to be considered.

The NBS study consisted of: 1) a field survey of present practice in

excavation, trenching and shoring and of the impact of the OSHA regulations; 2) a study of the technical provisions in the regulations; 3) a study of the timber presently used for shoring and; 4) a federal workshop on excavation safety held on September 19 and 20, 1978.

As part of the field survey, data were collected from various geographical regions in the United States. NBS studied field conditions in nine

metropolitan areas [2]

.

Essentially all states were represented by more

than 300 responses (anonymous) to three letter surveys conducted by the

National Utility Contractors Association, the Associated General Contractors of America, (AGC), and the National Association of Plumbing, Heating

and Cooling Contractors. This report presents the responses to the questionnaires sent out by the AGC, while the responses from the other letter

surveys are presented in Reference 2.

1.2 PURPOSE AND SCOPE

The purpose of the (AGC) survey was: 1) to study field practices in

excavation bracing, particularly empirical practices that seem to be adequate and generally accepted in the industry, and 2) to obtain information on the problems and difficulties encountered by contractors with

existing OSHA regulations.

The information presented herein was obtained through a 17-page questionnaire sent by AGC to its members in the fall of 1977. Forty-seven questions were included in the questionnaire, which covered such topics as trench bracing systems, soil types encountered, OSHA decision-making processes on field work and field investigations. Respondents were free to

elaborate in their answers. A response of about fifty percent resulted

in twenty-three (23) questionnaires being completed and returned to AGC.

It should be pointed out that due to the amount of time required to

complete this 47-question survey only a small number of questionnaires

was sent out. When interpreting the responses, how representative they

* Numbers in brackets Indicate reference given on page 5.

1
are must be considered. This question involves factors that differentiate those that responded from those that did not. For example, the

respondents might be those contractors most anxious to comment on the

OSHA provisions. This and other possibilities must be considered when

interpreting the responses.

With the small sample size obtained, it was determined that the most

meaningful analysis of the data could be made by tabulating the results.

Although the twenty-three responses did not warrant a rigorous statistical

analysis, the data are useful in making some general statements about

trenching and excavation operations. A discussion of the responses to

questions follows in Section 2.0.

2.0 DISCUSSION OF RESULTS

2 . 1 INTRODUCTION

A copy of the questionnaire is provided in Appendix A and the results of

the survey are presented in tabular form in Appendix B. All but two

of the questions have been tabulated. The responses to Question 3 have

not been presented. This question concerned the location of the area in

which each company did most of its work. It was thought that the identity

of some of the firms might be revealed if the responses to this question

were presented. Consequently, this information is presented in

a generic manner because anonymity of the companies responding to the

survey had been agreed to. The data from Question 5 are also omitted.

This question involved the number of injuries incurred by the workers in

each company during the previous year. This information was not relevant

to this report because normalized injury frequencies would not be tabulated as the corresponding manhours of exposure was not obtained. The

data which are presented in the next section show similarities between

some contractor practices concerning trenching and excavation operations

and in some instances the data demonstrate how widely some of the seem- ingly similar contractors differ in their daily operations. General con- clusions will be drawn. However, these conclusions are largely conjecture

as no statistical analysis was made. Further study may reveal that other

relationships exist.

2.2

FINDINGS

The annual volume of business conducted by each firm was related to the

characteristics of the firm's operations. Sixty percent of the firms

participating in the survey had an annual volume of business less than

or equal to five million dollars. The service area of the larger firms were over a wider geographic area and trenching comprised only a small

fraction of their total volume. On the other hand the smaller firms

had trenching as a major portion of their annual volume. About 70 percent of the trenching work was reported for sewerage, with another 23

percent for water systems. Fifty percent of their work utilized sloped

sides, and bracing systems were used an average of 43 percent of the time.

2
Soil type had a strong influence on field practices. Typical soil types

identified by the respondents were:

In addition, written responses were given regarding the soil type causing

the most concern for safety against collapse. Although they varied somewhat, the most frequent responses were "clay likely to crack" and "loose

silt or sand". Bracing systems are used more frequently in soft to

medium clays, loose sand, muck and loess. Trench boxes are the most

frequently used bracing system for trenches, while hydraulic shores and

screw jacks are the second most common form of trench bracing. Sloped

trench walls are most often used in stiff clays or in soils comprised

of sand and gravel.

Most trenches deeper than 15 feet (4.6m)* are braced. Braced trenches are generally 4 feet (1.2m) to 8 feet (2.4m) wide. The majority of the

contractors stated they left their trenches open less than one day. In

fact, almost half of the trenches are left open less than 2 hours. The

equipment used in opening a trench is usually a backhoe with the typical

length of an unbraced trench section averaging 40 feet (12m) and the

typical length of a braced section averaging approximately 50 feet (15m).

Although the greater percentage of the contractors stated that collapses

of trenches more than five feet (1.5m) deep were rare, six contractors

indicated that this happened occasionally.

Most contractors indicated that disturbed soil was a major contributing

factor to trench hazards and that this type of condition was met occasionally. Soils changing radically in character such as granular to co- hesive and unanticipated utilities are the most frequently encountered

worse conditions. Rainfall, vibration, subsurface water and drying were

the most common causes of soil instability.

While most firms have at least one registered engineer on the staff, the

majority of the field decisions are not made by engineers but by field

personnel. The responses to the questionnaire showed a preponderance

of decision-making authority belonging to job site supervisors. General

superintendents and foremen are heavily involved with decisions regarding

bracing or sloping. The job foreman or superintendent is almost universally considered to be the person most qualified to describe the physical

characteristics of a soil.

Stiff Clay

Medium Clay

Soft Clay

Sand & Gravel

Loose Sand

Muck

Loess

29%

18%

16%

15%

10%

6 %

6 %

* 1 ft = .305 meters

3
In familiar areas many contractors will use the soil borings provided

by the owner. A large number of contractors also will drill additional

soil borings and/or dig test holes. When work, is to be performed in a new area, the contractors surveyed generally indicated that they explored

the subsurface conditions in slightly greater detail.

No further generalization can be made. Many of the remaining questions

of the survey drew specific responses. Any attempt to generalize these

responses could be misleading. However, the responses to these questions are presented in Appendix B so that they may be studied individually. Specifically, the reader should refer to the responses to Questions 44 and 45 for comments on OSHA standards.

3.0 SUMMARY

The smaller firms participating in the survey had trenching as a major

portion of their annual volume. Of the 23 responses the greater percentage of their trenching work was for sewerage. Fifty percent of

their work utilize sloped sides with bracing systems used an average of

43 percent of the time.

Contractors encounter a wide variety of soils which have a strong

influence on field practice. Soils often vary with depth in a trench.

Unexpected conditions frequently occur and soils may lose stability

with time due to weather or environmental factors.

Most trenches are opened for less than a day using a backhoe. Trenches

deeper than 15 (4.6m) feet are braced. Braced trenches are generally

4 feet (1.2m) to 8 feet (2.4m) wide with a typical length of a braced

section averaging approximately 50 feet (15 m) . Most decisions regarding

bracing or sloping are made by job site supervisors.

In general, the contractors were dissatisfied with the OSUA standards.

OSHA Tables P-1 and P-2 were considered inappropriate, impractical and

needed revision. The experience of field personnel and the use of trench

boxes should be considered and be made a part of the OSHA standards.

4
4.0 REFERENCES

1. Department of Labor, Occupational Safety and Health Administration,

Construction Safety and Health Regulations, Part II, Subpart P,

Washington, D.C., June 1974.

2. Hinze, J., "A Study of Work Practices Employed to Protect Workers

in Trenches," NBSIR No. 79-1942, National Bureau of Standards,

December, 1979.

5
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APPENDIX A

THE ASSOCIATED GENERAL

CONTRACTORS OF AMERICA (AGC)

QUESTIONNAIRE

A-1
QUESTIONNAIRE FOR CONTRACTORS

1.

Approximate average annual volume of work of your company: $_

2. Approximate percentage of this total volume of work in trenching and excavation:

Trenching % Excavation %

3. In what areas (or states) does your company do most of its work?

4.

Type of trenching work your company does (circle appropriate ones):’

storm sewer %

sanitary sewer

other:

telephone

electric

% water

gas

5.

How many recordable injuries occurred in the company iti 1975?

Of these injuries, how many were lost time injuries?

6. In terms of linear footage, what percent of your trenches are:

sloped % Braced % Unsupported Vertical

7. What soil types do you typically encounter?

Hard, solid, stiff clay %

Likely to crack or crumble,

medium clay 7o Loose silt, soft clay

Sand and gravel

Loose sand / Muck 7« Loess X

Which of these soils causes you the most concern for safety against collapse?
8.

Bracing systems you use in trenching ;

Trench box %

Steel sheet piles %

Tight timber sheeting with

timber wales and struts %

Skeleton (skip) bracing %

Hydraulic shores or skrew jacks %

Soldier piles and lagging %

Other % Specify

9.

Support systems in excavations ;

Backties %

Raker braces %

Cross lot braces %

Sheetpiles (cantilever) %

Other 7o Specify

10.

Depth of trenching work (give percent in terms of linear footage of trench):

Depth (feet) Braced Trenches Sloped Trenches

0-5

5-10

10 - 15

15 - 20

20 - 25 over 25

%

%

%

%

% . % %

11.

Width of braced trenches ;

less than 4 feet %

4 feet to 8 feet %

8 feet to 12 feet %

12 feet to 15 feet %

over 15 feet %

12.

Length of time braced trenches are left open before backfilling:

less than 1 hour %

1 to 2 hours %

less than 1 day %

1 to 2 days %

longer than 2 days %
13. Length of braced trench open at one time;

Unbraced Portion Braced Portion

typical ft. ft.

maximum ^ft. ft.

14. What horizontal clearance between cross braces in trenches do you need in order

to carry out your work? (explain reasons carefully) For excavation:

for insertion of pipe:

for other operations where clearance is important:

15.

Do you need several choices of horizontal spacings of cross braces to keep

member sizes down? (explain carefully)

16.

What vertical clearance betx/een the lowest brace and the bottom of the trench

do you need to carry out your work? (explain carefully)

17.

Do you ever need to do one of the following in a braced trench?

a) Temporarily remove a horizontal cross brace:

length of time reason

b) Impose a load on a horizontal cross brace:

load in pounds

reason
18.

2

Would you consider an allowance for a surcharge of 300 lb /ft (equivalent of

3 ft. of surcharge) to account for storage of excavated material and other

construction loads

to be excessive reasonable Inadequate

Explain :

! 19. Percentage of trenches (in terms of linear footage) where a high water table is

I a problem: %

f

j 20. What methods do you normally use in trenches to deal with a high water table?

Explain :

21. Slopes in temporary sloped excavations;

Soil type

vertical in

vertical in

vertical in

vertical in

vertical in

vertical in

horizontal

horizontal

horizontal

horizontal

horizontal

horizontal

22. Do you ever use compound (composite) slopes ? If YES, draw typical cross-section.

YES ’ NO

[I

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!

A-5
Timber

sizes

used

by

your

company

(note

If

you

use

rough

cut

or

dressed

timber,

softwood

or

hardwood,

and

If

cross-brace

size

Is

changed

for

wide

trenches).

Give

sizes

you

consider

best,

regardless

of

OSHA

provisions.

(a)

Timber

shoring

(wales

and

cross

braces) —

include

large

horizontal

spacings

(16-24

ft.)

if

used

by

your

company,

but

do

not

include

skip-bracing .

fO

fN

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A-6

(continued)
23.

(a)

Continued

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A-7

(continued)
23.

(a)

Continued

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A-8 Ii!i

(continued)
(a)

Continued

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60 CO a <u H c 4J ^0) u

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A-9

(continued)
(a)

Continued

C4
I 23. (b) Skeleton (skip or spot) bracing.

Soil Type Depth

(ft)

Timber Uprights Cross Braces

Size Spaclngs

(c to c)

Size or Type

Number

-

5-10

10-15

15-20

5-10

10-15

15-20

'

I

A-11

(continued)
23. (b) Continued

Soil Type Depth Timber Uprights Cross Braces

(ft) Size Spacings

(c to c)

Size or TypeNumber

5-10

10-15

15-20

-

(c) Hydraulic Shores (such as speedshores)

Soil Type Depth

(ft)

Horizontal Center-to-Cento.r Spacing

Between Uprights (ft)

5-10

10-15

15-20

20-25

A-12 (continued)
23. (c) Continued

Soil Type Depth

(ft)

Horizontal Center-to-Center Spacing

Between Uprights (ft)

'i

5-10

10-15

15-20

20-25

5-10

10-15

15-20

20-25

•

A-13
I 24.

Under what conditions do you use skeleton bracings and how do you decide on horizontal spacing of uprights?

25. On a normal project, who in your firm will be most familiar and qualified to

describe the physical characteristics of the soil?

Owner/Management General Superintendent Job Foreman

Backhoe Operator Other

26. If you have previously done trenching work in the vicinity of a project, what

type of soil investigation do you accomplish prior to beginning work?

Make and evaluate soil borings

Utilize information from available soil borings and

other related information

Dig test holes

Other (please explain)

None

27.

If you have not previously done trenching work in the vicinity of the project,

what type of soil investigation do you accomplish prior to beginning work?

Make and evaluate soil borings

Utilize information from available soil borings and

other related information

Dig test holes

Other (please explain)

None

28.

On most jobs, who decides that sloping or bracing of a given segment of a trench

will be required?

Management

Contractor's design engineer, or consultant

General superintendent

Job foreman

^Based on local codes or design by owner

A-14
29. On trenches where sloping is employed, who determines the angle of the slope?

Owner/management , based on General superintendent, based on

Foreman, based on Other (please explain)

30. What percentage of the time do you use the following items of equipment in

trenching operations?

Backhoe Z

Trenching machine %

Power shovel %

Drag line %

Clam shell %

Other (please specify)

Do not do trenching

31. Are the soils in which you noirmally work generally the same from the ground

surface to the bottom of the trenches?

YES

NO

If NO, please give typical characteristics

32. Do you ever add moisture to a dry, sandy soil to increase its stability long

enough for your trenching operation?

YES

NO

33. Which of the following site conditions do you consider the most hazardous from

the standpoint of safety?

Straight trench

Manhole

^Intersections of trenches

Vibrating areas

Disturbed soils

Narrow right of way

Other (please specify)

A-15
34.

Listed below are some factors which might be difficult to anticipate throughout

a trenching project. Please identify, with a symbol, the frequency with which

these or other surprises occur on your projects.

Symbols: r = rarely; o = occasionally; f = frequently.

Unexpected ground water

Radical change in soil type

Tree roots

i Utility lines not anticipated

Previous excavations or disturbed soil

Unusually heavy surcharge

Other (please specify)

35.

Do you frequently encounter soils which lose stability as a result of:

Rainfall

Vibration

^Increased subsurface water

Drying

Temperature changes

^Inclinded bedding

Other

If any of the above secondary factors cause significant concern, please explain

further:

36.

If there is an environmental change such as a heavy rainfull, or unanticipated

subsurface conditions which require a change in bracing, who makes the decision

on the required changes?

37.

How often has your firm experienced unexpected collapse of sides of trenchings

more than 5 feet deep (with or without persons in the trench)?

More than once per month

^Several times per year

Occasionally

Very rarely

38.

In what types of soils and circumstances do these collapses most often occur?
NO

39.

Did a braced excavation ever collapse on any of your projects? YES

If not, are you aware of any cases where a braced excavation did collapse?

YES NO Please explain reasons if known:

40.

What, in your experience, are identifyable signs of imminent distress in trenches

and excavations?

41.

What are the most likely causes of collapse of bracing systems (other than

carelessness) in trenches?

In excavations?

42.

On what occasion would the company consult the following: (state if never)

A. A soils engineer to explore subsurface conditions?

B. A professional engineer to design bracing systems?

43.

Do you have a registered professional engineer on your staff? YES NO

Do you have a trained engineer who designs bracings? YES NO

A-17
44.

In the OSHA standards, contractors are provided with guidelines as set forth

in OSHA Tables P-1 and P-2. Please comment on how these could be improved:

45.

What specific other changes would you recommend to be made in the OSHA standards

regarding "Excavation, Trenching, and Shoring"?

46.

From your experience with trenching, can you suggest simple field tests which will

Indicate the stability of the trench (for both braced and sloped excavations)?

If so, please explain.

47.

From your experience, please suggest any practices of any type which might be

commonly adopted and would improve trenching safety.

A-18
APPENDIX B

RESPONSES TO THE ASSOCIATED

GENERAL CONTRACTORS OF AMERICA (AGC)

QUESTIONNAIRE

B-1
RESPONSE

NUMBER

3:OC

t:

n:<

s:g

ccwa

C/j so

CuOD

o;

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SPECIFIC REPONSES TO THE AGC QUESTIONNAIRE

Question 14: What horizontal clearance between cross braces in trenches

do you need in order to carry out your work?

Response Number For Excavation For Insertion of Pipe

2 10 feet (minimum) Pipe length plus 2 feet

3 — Pipe length plus 2 feet

5 — 20 feet

8 12 feet (minimum) Depends on pipe length

9 — 3-5 feet

12 — 3-20 feet

13 8 feet (minimum) 8 feet

14 10-24 feet Depends on pipe length

15 4 feet 4 feet

16 25 feet (2 or 3 yd 25 feet (large pipe)

backhoe)

17 12 feet 8-40 feet

18 — 6-10 feet

19 6 feet 6 feet (use box or jacks)

20 — Pipe length plus 2 feet

21 — Depends on pipe length

23 6-8 feet (backhoe) 4-8 feet

Question 15: Do you need several choices of horizontal spacings of cross braces to keep member sizes down?

iponse Number Yes or No Explanation

2 Yes Size of the cross brace is dependent

upon horizontal and vertical spacing

and size of waler.

3 Yes Use different sizes of work braces and even steel braces.

5 No —

12 Yes Needs to fluctuate with the length of

pipe

13 Yes Spacing is dictated by size of items

installed.

9,14 N.A. Use trench box almost exclusively.

15 Yes —

16 Yes Largely dependent on size of pipe

18 Yes Rarely

19 Yes Different pipe lengths

21 Not necessarily — see tables in the

Wisconsin Admin. Code

22 No —

23 No One size used regardless. Probably

too big for some close spacing

B-10
Question 16; What vertical clearance between the lowest brace and the

bottom of the trench do you need to carry out your work?

Response

Number Clearance

1

1, Explanation

1 N.A.

1

1 Use trench box.

2 O.D, of pipe plus 1 foot

!

3 Clearance above the pipe —

5,12,21 — Varies with the pipe

8

“ Depends on pipe diameter or special foundation requirement or dewatering equipment

9 2 feet (minimum) To insert pipe and bedding

13 6 inches above top of pipe Varies with pipe size

14 12 inch (minimum) Depends on size of pipe

Clearance needed for trench box

in order to keep pipe joints

intact when moving the box

16 6-10 feet —

17 — Depends on pipe diameter

18 3-4 feet (typical) Depends on size of pipe

19 2-3 feet Depends on type and size of

pipe

20 O.D. of pipe plus 2 feet —

22 7 feet For moving boring machine in

pit

23 2-8 feet Depends on pipe size

B-11
t

Question 17A: Do you ever need to do one of the following in a braced

trench: Temporarily remove a horizontal cross brace?

Response Number Yes/No Length of Time Reason (or Comment)

2 Yes Would make other provisions ti

3 Yes Under one hour

carry load

5,9,19,20 No — —

8 Yes Varies Need for working room

12 Yes 5-15 minutes To install pipe or structure

13 No — Should never have to

15 Yes — To insert pipe

16 Yes 4-8 hours To allow access

17 Yes 10 minutes To lower long lengths of pipe

18,21 Yes 10-15 minutes To place a length of pipe

22 Yes 30 minutes To install longer casing

23 Yes 10-15 minutes Clearance for pipe or working

Question 17B: Do you (ever need to do one

space

of the following in a braced

Response Number

trench: Impose a load on a

Yes/No Load

horizontal cross brace?

(lbs) Reason (or. Comment)

3

5,8,13,16 ,19, No

Try to avoid it

20,22,23

9

(Oci

Yes

casionally) — Only minor loads

15

18

Yes

Yes

(Rarely)

Plat form

No working room

B-12

1
.

' 2 Question 18: Would you consider an allowance for a surcharge of 300 lbs/ ft

(14.4 kPa) (equivalent of 3 feet of surcharge) to account for

storage of excavated material and other loads to be excessive,

reasonable or inadequate?

Response Inadequate

Number Answer Explanation

2 Inadequate Force lines in the piles or the banks are rarely if ever vertical. Loads imposed on

the ground usually have an inverted conical

sphere of influence.

3 Reasonable Must assume some sort of surcharge

7 Inadequate

8 Inadequate Construction materials, equipment and methods

routinely account for surcharges of many times

300 psf without becoming unsafe practices.

9 Reasonable Usually there is no more room for excavated

material. Backfilling loads would depend

on equipment being used. ^

12 Each case must be designed on its own conditions.

13,16,18, Reasonable

19,20,21 —

21 Reasonable Loads greater than this should be kept from

open trench.

B-13
Question 19: Percentage of trenches (in terms of linear footage) where

a high water table is a problem.

Response Number Reply

17,18 2%

4,15 5%

7,11,12,22 10%

16 15%

1,2,8,13,19 20%

20 25%

14,21 35%

9 40%

5 70%

23 75%

3 100%

V »
Question 20: What methods do you normally use in trenches to deal with

a high watnr table?

Response Number Response

12

3

4

5

7

8

9

11

12

13

14

15

16

17

18

19

20

21

22

23

Pumps

Well points

Well points, drain water from pipes in stone bedding

Pumps, central wells, sand points (use trench box)

Well points or stone and pump depending on the volume

of water

Sump pumps in trenches, drain tile and gravel under

line in excavations

Dewatering, sheet piling, trench shield

Sump pumps

Slope the overburden for vertical-walled rock trenches

Pumps, well points, deep well

Sump pumps, well point (depends on amount of water)

Deep wells in clean sand in trenches over 12 feet

deep, well points if material is real fine. In

trenches under 12 feet deep we use well points

because it is faster and less expensive. | %'

Pumps and shores

Sheet pile - H pile and lagging (trenches open for

long periods)

Gravel bedding with pumps and in extreme cases - well

points

Sand casing well point and pumps

Pumping from sand pits

Sand box and pumping, well points, wells

Caissons with submergible pumps

Sump and pumps

Well points, wells, open sump and submergible pumps

(depend on level of water, permeability of soil,

drawdown area, depth of invert, money available)

B-15
Question 21; Slopes in temporary sloped excavations?

Response

Number

11

13

Soil Type; Vertical to Horizontal

Sandy silty clay; 2 to 1

Soil Type; Vertical to Horizontal

Good clay; 2 to 1 Soft, plastic; 1 to 1 1/2Loam; 1 to 1 Running; 1 to 2

Hard, solid, stiff clay; 1 to 1 1/2

Loose silt, soft clay; 1 to 1

Loose sand and gravel; 1 to 1 Hard solid stiff clay; 1/2 to 1

Sand; 1 1/2 to 1

Likely to crack clay; 1 to 1

Compact sand and gravel; 3/4 to 1 I

14 Hard clay; 1 to 1 - Medium; 1 to 1 Loose silt; 1 to 1 Sand and gravel; 1

16 Firm clay; 1 to 1 Medium clay; 1 to 2

Silt; 1 to 3 Sand; 1 to 3

Muck; 1 to 4

19 Solid clay; 4 to 1 Normal clay; 2 to 1 Sand; 1 to 1

20 Sand; 1 to 1 Medium clay; 1 to 1 Firm clay; 1 1/2 to 1 Loess; 1 to 1

21 Hard clay; 2 to 1 Gravel and sand; 1 Wisconsin code)

23 Sand; 1/2 ito 1 Silt and loam; 1/4 1 Water and Sand; 1 to 1

B-16
Question 22: Do you ever use compound (composite) slopes ?

Response Number

2

3

5

7

8

9

11

12

Response

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

Response Number

13

16

17

18

19

20

21

22

Response

No

Yes

Yes

Yes

Yes (benching)

Yes

No

Yes

Typical trench cross-sections using composite slopes

No. 3

No. 5, 20

No. 11

No. 12

B-17
Question 22 (continued)

No. 16
Question 23: Timber sizes used by your company. Give sizes you consider

best, regardless of OSHA provision.

Response No. 2

Response No. 3

Response No. 5

"We almost never use an all timber system for trenching anymore. We do use 2" rough green hardwood for

sheeting with prepared steel wales and bracing or with

aluminum shoring systems." Work is primarily in soil

which is hard, compact, and solid.

The OSHA tables are used in all soil types. In depths

exceeding 20 feet, steel sheeting would be employed

which is designed by a professional engineer.

Where solid sheeting is required, steel sheeting with

wide flange beams is used. In situations where skeleton

or skip shoring is appropriate, a steel trench box would

be used.

Response No. 9 Trench boxes are used.

Response No. 13 Use interlocking steel sheets in all given soil types

whether hard, compact, solid, likely to crack, s'and,

soft clay or hydro-muck. The same applied where skip

shoring might be used.

Response No. 15 Do not use hydraulic shores, but use wood for skeleton

shoring. Generally 2 x 10 oak members are used for

the timber uprights. Trench jacks (screw jacks) are used for cross braces. Spacing depends strictly on site

conditions.

Response No. 16 Hard, compact, solid material was encountered in which

timber shoring was used. Regardless of trench depth,

14" H steel members were used as wales. The wales were unsupported for 20 feet. The cross braces were

also 14" H Steel members with a 10 foot horizontal

spacing. Sheeting was generally 3" rough hardwood at

10 foot maximum vertical spacing.

Response No. 17 Use primarily hydraulic shores. In hard soil material

the spacing of the shores is 8 feet in depths of 5 to

10 feet and the spacing is reduced to 6 feet in depths

of 10 to 15 feet. In medium clay the spacing of these

shores is 6 feet in depths of 5 to 10 feet and 4 feet

in depths of 10 to 15 feet.

Response No. 18 The following spacing is used for hydraulic shores.

These spacings apply to loess and hard clay

B-19
Question 23 (continued)

Depth (ft.)

5-10

10-15

15-20

20-25

Spacing (ft.)

6-8

5-7

4-6

4-6

Response No. 19 In hard clay hydraulic shores are used at a 4 foot

spacing up to 15 foot depths. At greater depths, two rows of shores are installed with a 4 foot spacing.

Response No. 20 Work is done in hard, compact, solid soil, likely to

crack soil, sand, soft clay, and hydro-muck. Regardless of trench depth 6x8 wales and cross braces are used. The wales have a 20 ft. unsupported span in

depths up to 10 feet but this spacing is reduced to

10 feet in depths beyond 10 feet. Cross braces are spaced in the same manner. Sheeting thickness is

increased from 1** to 3" at depths beyond 10 feet.

The vertical spacing of wales is 5 feet in depths up

to 10 ft and 8 ft. at greater depths. In hydro-muck,

however, a 5 foot spacing is always maintained.

In skeleton shoring 2 x 10 uprights are used at an

8 ft spacing. At depths beyond 10 feet, the spacing

is reduced to 5 feet. Screw jacks are used for cross braces. Generally 2 screw jacks are used per upright

with 3 being employed at depths over 15 feet.

Response No. 23 No skip or skeleton shoring is used. Solid timber

shoring is used in hard, compact, solid soil, likely

to crack soil, sand and hydro-muck. In soil which is

likely to crack, this contractor would use multiple

hydraulic shores mounted on 3 x 12 's and backed with

3/4" or 1" plywood. In the remaining soils 6 x 8's

would be used for wales and cross braces. The wales

would be unsupported for 11 feet with wales stacked

4 foot apart in the trench. The skirting used would

be 2" rough cut, Douglas fir.

B-20
Question 24: Under what conditions do you use skeleton bracings and how

do you decide on horizontal spacing of uprights?

Response Number Reply

2 We only use trench boxes.

3 Used in 8-12 ft cuts with 2 braces minimum, somet ime s 3

5,9,19 We don't use skeleton bracing.

8 Soils engineering determines choice of bracing.

13 When existing utilities are in conflict, we use hydraulic shores as needed, determined by the fore- man.

16 As per OSHA requirements

20 Used in hard clay. Spacing is based on past suc- cessful experience and on the recommendation of the

shore manufacturer.

21 See Wisconsin code.

23 Generally don't exceed 4 or 5 feet. It is not that

difficult to add an extra hydraulic jack or two.

B-21
Question 38: In what types of soils and circumstances do these (trench

wall) collapses most often occur?

2 sand

2 clay

2 soft clay

2 joint clay

1 highly expansive clays

(exposed to the air)

1 sand (with water)

1 sandy clay

1 disturbed soil

1 ground water

1 silty sand and wet clay

1 medium clay

2 all types of soil

B-22
Question 40: What, in your experience, are identifiable signs of imminent

distress in trenches and excavations?

Response Number Signs of Distress

1,7,20,21,22 Tension cracks in the soil

1 3,16,23

l|

Cracking of timber, bowing or bending of braces or sheeting, cracks behind sheeting and "boils" in

bottom of excavation

1 ^ Crack in dirt with a lot of pressure on trench box

1

? 5 After heavy rains in sandy clay soils

6 Bottom sloughing away, embankment undercut, cracks

in embankment

s 8

i

t

Methods are the most identifiable warnings of

distress. There will never be a substitute for a qualified soils engineer and a knowledgeable,

experienced general superintendent.

9

1 Excess groundwater in fine silty sands carrying away

trench walls

10

1

Surface tension cracks, soil movement, moisture content, vibration of equipment, and a bulge in the

trench or excavation walls

11 Cracking in soil at edge of trench

12 Cracking soils or lateral movement

13 Water at flow line

15 Ground cracking, bending of cross braces or vertical

members

17 Undercutting of vertical wall by sloughing, parallel

cracks back from trench edge

18

1

A crumbling of small pieces of soil like the soil was working

1 19

1

1

Changes in soil conditions (moisture appearing)

(cracking) (movements) (changes in soil color)

! 21 Loose particles dropping from trench walls

i B-23

I
Question 41A:

Response Number

2,14

3

4,13

5

6

7

8,19

9

10

15

16

18

20

21

22

23

What are the most likely causes of collapse of bracing

systems (other than carelessness) in trenches?

Cause of Collapse

Inadequate support members

Not "squarely" placed braces, excavating operators

knocking out braces

Inexperienced personnel

Heavy rains

Bracing improperly or inadequately engineered use of

poor quality materials.

Underdesign, careless installation

Improper assessment of the loads the bracing was to

support

Excessive loads from surcharge, trucks too close to

trench, excessive rain

Improper position, wrong bracing system, improper

size and/or grade lumber used

Overload, material deficiency

Inadequate shoring — excessive surcharge — virbrations

! Water, loading, vibration

Not getting bracing installed quick enough

Underdesign and poor installation

Improper bracing

Underdesigned

B-24
Question 4lB: What are the most likely causes of collapse of bracing

systems (other than carelessness) in excavations?

Response Number Cause of Collapse

3 Superimposed loads, change in water table, operator

problem

5 Bottom blowout, sheeting too short, not enough toe or failure of dewatering system

8 Improper assessment of the loads the bracing was to

support

9 Excessive rain or ground water

10 Inadequate means of secondary bracing if applicable,

lumber used rather than steel piles, wrong type

bracing system

12,23 Underdesigned

13 Lack of experience

14

20

Too light material

20 Failure to do a first class job of waling
Question 42: On what occasion would the company consult the following

A. A soils engineer to explore subsurface conditions?

Response Number Reply

1 Never

2 Unusual or known treacherous soils

3 If found to be different than bid borings

4 Never

5 Never

7

> Documentation

8 In the event of any doubt whatsoever

9 We have in-house engineers

10 Depends on job specifications

11 Never

12 Seldom

13 Excessive leaks. When dispute arises over shoring

methods

14 We do our own

16 All projects of any size or concern

17 When doubt exists and economics justify

19 Under bad conditions

20 Never

21 Extremely bad soil conditions

22 Never '

23 Never

B-26
Question 42: (continued)

B. A Professional engineer to design bracing systems?

Response Number Reply

1 Never

2 VAiere soils engineer's analysis indicated

loadings

unusual

3 Over 20 ft, cuts

4 Never

'j Only if required by the consultant engineer

7 Never (use in-house engineers)

8 All bracing systems

9 We have in-house engineers

10 Confining areas, i.e. between two or more

structures

existing

11 Never

12 Seldom

13 As required by Cal/OSHA

14 Whenever we have a job where no one in the

had past experience

company

15 When design data is required

16 All projects of any size or concern

17 When doubt exists and economics justify

19 Never

20 When excavation is to be open for extended periods

21 Extremely deep and wide trench

22 Never

23 Never

B-27
Question

Response

2

3

8

10

12

15

17

19

20

21

23

44: In the OSHA standards, contractors are provided with quidelines as set forth in OSHA Tables P-1 and P-2, Please comment on how these could be improved.

Number Improvement Suggestion

These tables are worth next to nothing — no consideration is given to types of timber used.

Generally, they fit our area pretty well.

They are totally inappropriate and in fact conflict

with the text of the regulation, i,e,, slopes from

bottom of trench.

Table P-1 is good — keep it simple. Table P-2 is

also a good guideline to follow provided lumber is

used. However, hydraulic shores are less costly

and more efficient. Hence Table P-2 merely takes

up space.

Each trench must be designed on its own merits.

Instruct inspectors that alternate systems are suitable and equal.

There should be a provision for compound sloping

with a safety factor equivalent to that permitted 5'

vertical unshored. The safety factor of 5'-l" sloped

1/2 to 1 is far beyond 5' unshored vertical that’ is

safe and the additional trenching and backfilling is

not justified by increased safety.

Adjustment to soil conditions or a closer basis, not

a blanket design for entire areas.

More leeway should be given for experience of field

personnel.

These need complete revision. They are impractical

for use of today's equipment and are even unsafe in

a number of areas.

They try to use a blanket design to fit all situations.

This is not really practical and it is very expensive

for the owner in many cases. In many situations the

compliance officer is not knowledgeable in the excavation field and it is one continuous hassle to show

him (if possible) that you are working safely even

though you have deviated from the book directions

slightly.

B-28
Question

Response

2

5

8

11

13

17

19

21

23

45: What specific other changes would you recommend to be made

in the OSHA standards regarding "Excavation, Trenching and

Shoring"?

Number Other Changes

Excavation and trenching need to be more completely

separated. They are different operations. A con- tinuous trench, regardless of width, is different

from an excavation.

A more realistic interpretation of the OSHA standards

We emphasize that a soils engineer must determine the

suitability of the methods, as surely as a structural

engineer must design a building skeleton.

More realistic sloping of trenches

The inclusion of a specification on trench shields,

particularly concerning the danger of exposure to the

trench walls above the shield

Definite provisions for professional engineer-designed

bracing based on soils analysis as an alternate only.

Obviously many jobs are too small to justify this,

consequently sloping or shoring beyond the minimum

requirement would have to be done as it is now.

More liberal rules on fining contractors for first

offenses

Refer to the Wisconsin Code for practical standards

If some of us have to live by these poorly thought

out standards, make sure that there are enough

enforcers so that we all are in the same situation.

Don't let the cheaters get away with anything.

B-29
Questions 46: From your experience with trenching, can you suggest

simple field tests which will indicate the stability of

the trench (for both braced and sloped excavations)?

Response Number Field Test

2 Experience itself is the only proven field test.

There is simply no way any fool can determine this.

3 Be sure of the water table — quite often work is

done at different conditions than are shown by the

borings as the water table changes

5 Common sense

8 Consult: Soil Mechanics in Engineering Practice

by Terzaghi and Peck and "Fundamentals of Soil

Mechanics" by Taylor

13 Any person experienced in excavation can make a decision about the nature of the soil. Soil is

very unpredictable. Most times a test is no indication because soil can change each foot.

15 If it doesn't fail, it is good.

16 Braced trench: pressure gage to indicate loads on bracing. Sloped bank: Soil test performed

19 Experience by qualified sewer foreman and superintendent. Experience from years of working ditches

is your most valuable tool for analyzing the existing conditions.

21 There are no simple tests.

23 No simple test! Too many variables

B-30
Question 47: From your experience, please suggest any practices of any

type which might be commonly adopted and would improve

trenching safety.

Response Number Practices Suggested

1 Use trench boxes

2 The use of trench boxes is the best advance in

trench safety in 20 years. Lab tests to determine the coefficient of elasticity and cohesion if

coupled with the time element are helpful in

determining slope stability.

3 In all excavations over 5 feet deep, always use some protection for your personnel.

4 Stronger and lighter trench boxes

5 The use of trench boxes with a greater allowable

trench width

7 Since 80% of accidents are caused by unsafe acts

and only 20% by unsafe conditions, scrap OSHA and

start a safety education program.

8 The Engineers who prepare the Plans and Specifications, from which contractors build, must engineer

virtually every phase and certify thereto by the

fact of their seal. Yet soils engineering is not

addressed or liability expressly denied which should

be a violation of their responsibility. Legislate

the review and approval by soils engineers of soils

engineering projects, particularly trenching and

excavation.

10 Open cut all trenches provided there is enough room

to do so. However, this method is also the most

costly

.

11 Realistic pay items by the contract-letting body

(city, state, etc). The contractors is forced to

bid work not using shoring in order to be competitive. Shoring like solid rock should be paid for if

required. Engineers in their own job estimates

rarely provide enough allowance for shoring. If the

cost is included in the bid, the job is thrown out

for being over the estimate.

12 Experience

B-31
Question 47: (continued)

Response Number Practices Suggested

13

15

16

17

19

21

23

I feel the presence of a trench shield, trench jack

and steel sheets are the most up to date, economical

method to use.

Always have shoring supplies available.

Trenching contractors to be licensed by State and

required to maintain a qualified Engineer for design

of shoring

Expenditure of more time and effort on those contractors who do not shore their trenches, either through

ignorance or economy. Most accidents occur in unshored

trenches of depths exceeding 5 feet.

Work with designing engineers on location and depth

of work-many times the designers will place the lines

in areas of high risk when some common sense would

save problems by relocation, etc. Prior location of

utilities in relation to your work. Pre-planning by

all utilities would be an aid.

Bracings systems such as shields (trench boxes) and

hydraulic braces installed from trench top are the

only sure way to prevent accidents. Any bracing

systems which require men to enter an unbraced area

to install pipe are dangerous and will result in

accidents and deaths!

Shore trenches and keep the men behind the shoring

as much as possible.

B-32
NBS-I14A (REV. e- 78 )

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1936

Z, & Access(OR No.

4. TITLE AND SUBTITLE

An Analysis of the Responses from an Associated General

Contractors of America (AGC) Survey of Trenching and

Shoring Practices

5. Publication Daie July 1979

LlOtganiutiofl

7. AUTHOR(S)

Lawrence A. Salomone and Felix Y. Yokel

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 fsrreer, city, St^te, ZIP)

Occupational Safety and Health Administration

Department of Labor

Washington, D.C. 20210

13. Type of Report & Period Covered

-v'=-x

'

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-^word or teas factual aummary of moat aignificant information. If document includes a significant bibliography or

literature survey, mention it here,)

Results of an Associated General Contractors of America (AGC) survey of present

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

Regulations for Excavation, Trenching and Shoring as perceived by a selected

number of the membership are presented. The survey consisted of forty-seven

.(47) questions. A response of about fifty percent resulted in twenty-three

(23) questionnaires being completed and returned to AGC. Although the twentythree responses did not merit a rigorous statistical analysis, the data are useful in making some general statements about trenching and excavation

operations

.

17, KEY WORDS (six to twelve entriea; alphabetical order; capitalize only the first letter of the first key word unleaa a proper name;

separated by semicolons)

Construction practices; Construction safety; Excavation; Shoring; Trenching.

18. AVAILABILITY IITI Unlimited

I I For Official Distribution. Do Not Release to NTIS

QJ Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

rx1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCL/^SIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

22. Price

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