ExcavationsExcavations1926 Subpart PEXCAVATION

◦ Any man-made cut, cavity, trench, or depression in an earth surface that is formed by earth removal.

◦ Cave-ins are much more likely to result in worker fatalities than other excavation-related accidents.

TRENCH◦ A narrow excavation (in relation to its length)

made below the surface of the ground. ◦ The depth of a trench is greater than its width,

and the width (measured at the bottom) is not greater than 15 ft (4.6 m).

Soil MechanicsSoil MechanicsA number of stresses and

deformations can occur in an open cut or trench.

For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation.

The following diagrams show some of the more frequently identified causes of trench failure.

ExcavationsExcavations1926.651(b)

◦Locate underground utilities and installations

Means of Access/EgressMeans of Access/Egress1926.651(c)

◦Required in excavations 4 feet or deeper

◦Provide means of egress◦Maximum lateral travel distance in

25 feet

Hazardous AtmospheresHazardous Atmospheres1926.651(g)

◦Excavations 4 feet or deeper where hazardous atmospheres can reasonably exist

◦Oxygen deficiency, oxygen enriched◦Toxic gases, flammable vapors◦Provide ventilation and/or PPE

Emergency Rescue Emergency Rescue EquipmentEquipmentEmergency rescue equipment is required

when a hazardous atmosphere exists or can reasonably be expected to exist.

Requirements are as follows:◦ Respirators must be of the type suitable for

the exposure. Attended (at all times) lifelines must be

provided when employees enter bell-bottom pier holes, deep confined spaces, or other similar hazards.

Employees who enter confined spaces must be trained.

Control of WaterControl of Water1926.651(h)

◦PROTECT EMPLOYEES FROM THE ACCUMULATION OF WATER

◦MONITOR WATER REMOVAL EQUIPMENT

Protection of EmployeesProtection of Employees1926.652(a)

◦Protect employees in the excavation from cave-ins in excavations 5 feet and deeper Protection could be required at depths

less than 5 feet if there is an indication of a potential cave-in, work activities warrant protection, etc.

Not required for excavations made entirely in stable rock

Protection SystemsProtection Systems1926.652Sloping and benchingShielding, support, or other

protective system

Stable RockStable RockStable rock is natural solid

mineral matter that can be excavated with vertical sides and remain intact while exposed.

It is usually identified by a rock name such as granite or sandstone.

TYPE A SOILSTYPE A SOILSExamples of Type A cohesive

soils are often: clay, silty clay, sandy clay,clay loam.

Compressive forces of 1.5 tsf or greater

TYPE B SOILSTYPE B SOILSType B soils are: angular gravel;

silt; silt loam; previously disturbed soils unless otherwise classified as Type C

Compressive forces greater than .5 tsf but less than 1.5 tsf

TYPE C SOILSTYPE C SOILSType C soils include granular

soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable.

Compressive forces .5 tsf or less

Soil Field TestsSoil Field TestsThumb testPlasticity Pocket

penatrometerTorvane shear Sedimentation

Thumb Penetration TestThumb Penetration TestASTM test designation D 2488Retrieve a large clump of undisturbed

spoilAttempt to penetrate the soil with the

tip of the thumbType A soil can be penetrated only with

great forceType B will penetrate to the full length

of the thumb nailType C will penetrate easily several

inches and can be molded by light finger pressur

Plasticity TestPlasticity TestRoll a moist sample

of spoil into a ballRoll the ball out into

a 1/8" by 2 " threadIf this can be done,

hold it on endIf it remains

suspended without tearing the soil is cohesive

Pocket PenetrometerPocket Penetrometer POCKET PENETROMETER.

Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils.

Once pushed into the soil, an indicator sleeve displays the reading.

The instrument is calibrated in either tons per square foot (tsf) or kilograms per square centimeter (kPa).

However, Penetrometers have error rates in the range of ± 20-40%.

Torvane ShearTorvane ShearSelect fresh clod or

block of undisturbed soil from spoil pile

Cut a smooth surface on the clod

Insert vanes of device into the soil

Retract vanes to show foot imprint

Set indicator at zeroHold device firmly

against soil and twist in clockwise manner until soil fails in shear

Consistency Term

Shear Strength,psf

Unconfined CompressiveStrength, psf

Soil Type

Very Soft

Soft

Medium

StiffStiff

Very StiffHart

<250

250-500

500-1000

1000-15001500-2000

2000-4000>4000

<500

500-1000

1000-2000

2000-30003000-4000

4000-8000>8000

TYPE “C”

TYPE “B”

TYPE “A”

Sedimentation TestSedimentation Test Flat bottom container

at least 7 inches high Fill glass jar 5 inches of water on

top of soil 1 1/2 inches of soil

• Place lid on jar and shake – Set jar down– Rotate slightly– Larger particles settle out

immediately– Wait 30 seconds – Mark jar– Silt after several minutes– Fine clays in an hour– Make second mark

Sloping OptionsSloping OptionsNO STEEPER THAN 1 1/2

HORIZONTAL TO 1 VERTICAL (34 DEGREES)

OSHA TABULATED DATA (SOIL CLASSIFICATIONS)

OTHER TABULATED DATAREGISTERED PROFESSIONAL

ENGINEER DESIGN

Slopes for Types of SoilSlopes for Types of SoilStable Rock (Vertical) 90 degrees TYPE A - (3/4 Horiz. TO 1 Deep)

(53 degrees)TYPE B - (1 Horiz. TO 1 Vertical)

(45 degrees)TYPE C - (1 1/2 Horiz. TO 1 Deep)

(34 degrees)

ShoringShoringProvision of a support system for

trench faces used to prevent movement of soil, underground utilities, roadways, and foundations.

Shoring or shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical.

Types of ShoringTypes of ShoringThere are two

basic types of shoring, timber and aluminum hydraulic

Shoring systems consist of posts, wales, struts, and sheeting.

Hydraulic ShoringHydraulic ShoringHydraulic shoring

provides a critical safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring.

Hydraulic ShoringHydraulic ShoringHydraulic shoring provides a critical

safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring.

Other advantages of most hydraulic systems are that they: ◦are light enough to be installed by one

worker;◦are gauge-regulated to ensure even

distribution of pressure along the trench line;◦can be adapted easily to various trench

depths and widths.

Installing/Removing Installing/Removing ShoringShoringAll shoring should be installed

from the top down and removed from the bottom up

Hydraulic shoring should be checked at least once per shift for leaking hoses and/or cylinders, broken connections, cracked nipples, bent bases, and any other damaged or defective parts

ShieldingShieldingTrench boxes are different from

shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents.

The space between the trench boxes and the excavation side are backfilled to prevent lateral movement of the box.

Trench BoxesTrench BoxesThe box should

extend at least 18 in above the surrounding area if there is sloping toward excavation.

This can be accomplished by providing a benched area adjacent to the box.

Earth excavation to a depth of 2 ft below the shield is permitted, but only if the shield is designed to resist the forces

Trench BoxesTrench BoxesThe box should extend at least 18 in

above the surrounding area if there is sloping toward excavation.

This can be accomplished by providing a benched area adjacent to the box.

Earth excavation to a depth of 2 ft below the shield is permitted, but only if the shield is designed to resist the forces

Spoil PileSpoil PileThere is also a hazard from

excavated material and equipment outside of the excavation falling or rolling down into the trench.

Temporary spoil must be placed no closer than 2 ft from the surface edge of the excavation

Materials and EquipmentMaterials and EquipmentEmployees must also be

protected from excavated materials or equipment that could pose a hazard by falling or rolling into the excavation

This is achieved by placing and keeping materials and equipment at least 2 feet back from the edge of the excavation.

Competent PersonCompetent PersonEmployers are

required to have a competent person on site when trenching and excavation work is in progress.

This person should be knowledgeable of the cave-in hazards and have sufficient authority to act when necessary.

The designated competent person should have and be able to demonstrate the following:◦ Training, experience,

and knowledge of: - soil analysis, - use of protective systems

◦ Ability to detect: - conditions that could result in cave-ins, - failures in protective systems, -hazardous atmospheres, and - other hazard

◦ Authority to take prompt corrective measures

InspectionsInspectionsDaily inspections of the

excavation, adjacent areas and protective systems should be made by a competent person.

The inspections should be made at the start of the work shift and as deemed necessary throughout the shift.

Inspection FrequencyInspection FrequencyThe following guide specifies the

frequency and conditions requiring inspections:◦ Daily and before the start of each shift.◦ As dictated by the work being done in the trench.◦ After every rain storm.◦ After other events that could increase hazards, e.g., snowstorm,

windstorm, thaw,earthquake, etc.

◦ When fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom, or other similar conditions occur.

◦ When there is a change in the size, location, or placement of the spoil pile.

◦ When there is any indication of change or movement in adjacent structures.