excavations & trenching cfr 1926 subpart p

8/14/2019 Excavations & Trenching CFR 1926 Subpart P

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Excavations and Trenching

CFR 1926 Subpart P


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An Excavation is any man-made cut,

cavity, trench, or depression in an earth

surface that is formed by earth removal.



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A Trench is a narrow excavation (in

relation to its length) made below the

surface of the ground.In general, 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) theexcavation is also considered to be a trench.



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Working around and in excavations is one

of the most dangerous jobs in the

construction industry.



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1 The man in this photo could be digging his own grave. His yellow

hardhat is a pathetically inadequate safety device, given the danger

that surrounds him. He is alone at the bottom of an 22 ft. deep vertical

trench.2 Excavated soil has been stockpiled at the very edge of the trench,

adding to the pressure on the trench walls.

3 Vibrations from a backhoe operating at the perimeter of the excavation

increases the likelihood of a cave-in.4 Neither a ramp nor ladder has been provided for the worker to escape.

5 Finally there is nothing to protect him from a cave-in; noslopingback

of the trench walls, noshoringof the walls, and noshieldingof the

worker. Unfortunately, this is an all too common site in excavation

work.



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Excavation cave-ins are a major source of

fatalities within the construction industry.

Trenching accidents on U.S. construction sites

account for an estimated100 fatalities per year,

with at least 11 times as many workers injured.



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Whatarethe

hazardsof

trenchesand

excavations?



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Hazardous

Atmospheres

Acces

s

and

Egress

Fallsand

Equipmen

t

Cave-

ins

WaterAccumulation


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Cave-ins and the associated hazards are the

most serious of the hazards that are

associated with working in a trench.Cave-in means the separation of a mass of soil or

rock material from the side of an excavation, or the

loss of soil from under a trench shield or support

system, and its sudden movement into theexcavation, either by falling or sliding, in sufficient

quantity so that it could entrap, bury, or other wise

injure and immobilize a person.



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Soil isHeavy!

Soil Mechanics


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A cube of soil measuring 1 ft. on a side

weighs around 100 lb. A cubic yard of soil

contains 27 of these, or 2700 lbs. total. Thisweighs about as much as a mid-sized

automobile. A trench wall collapse might

contain 3 to 5 cubic yards of soil, weighingfrom 8,000 to 14,000 lbs.

Soil Mechanics


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Undisturbed soil is kept in place by the

horizontal and vertical forces of adjacent

soil. The soil removed to create a trench, is no longer

available to provide support for the soil left behind

in the trench wall. With no support, soil from the

trench wall will eventually move downward andinward into the excavation. This creates a serious

life-threatening hazard for workers in the trench.

Soil Mechanics


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From a soil mechanics point of view, one

can visualize the soil as a series of multiple

columns of soil blocks, with the blockspiled one on top of the other.

Soil Mechanics


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In the figure, each soil block measures one foot

square, weighs approximately 100 lbs, and supports

the weight of all of the blocks above. This means

that a block sitting at a five-foot depth supports itsown weight and the combined weight of the four

blocks resting on it. The combined weight of this

column is 500 lbs spread over a one-square-foot

area; 500 pounds per square foot (psf). This fiveblock column constitutes a 500 pound force exerted

vertically on whatever lies below.

Soil Mechanics


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When a trench is excavated, the stable

relationship described in the previous section

no longer exists. The horizontal pressure on thesoil blocks along the trench wall is no longer in

equilibrium, and a block may not be able to

support its weight and the weight of any blocks

above. At the point where the soil can no longerwithstand the pressure, the wall will shear and

break away from its stable position.

Soil Mechanics


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Tension cracks usually form at a horizontal

distance of 0.5 to 0.75 times the depth of the

trench, measured from the top of the verticalface of the trench.

Soil Mechanics


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Sliding or sluffing may occur as a result of

tension cracks.

Soil Mechanics


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Toppling occurs when the trench's vertical face

shears along the tension crack line and topples

into the excavation.

Soil Mechanics


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An unsupported excavation can create an

unbalanced stress in the soil, which, in turn,

causes subsidence at the surface and bulging ofthe vertical face of the trench.

Soil Mechanics


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Dirt is dirt - right? Not necessarily so.The extent of soil contamination and its

relative risk to biological receptors atRMA has been determined andclassified in the EPA Record of Decision(ROD) for RMA. The three classifications

of relative risk are identified as HumanHealth Exceedance/Principal Threat Soil,Biota Soil, and Priority Soil. The threeclassifications can be defined as follows:

Soil Classifications


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Priority 1 Soil: Native surficial soil from0 to 12 inches below the existing groundsurface with contaminant concentrations

considered to pose a low-level risk toanimal populations after remediation ofall ROD-specified areas.



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Biota Soil: Biota soils contain acontaminant or group of contaminantsthat may pose a risk to biota and animal

populations. The primary risk to biotaincludes bioaccumulation andbiomagnification of contaminants in thefood chain. The biota soils do not exceed

EPAs Human Health Risk range.



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Human Health Exceedance/PrincipalThreat Soil: Human healthexceedance/principal threat soil contains

a contaminant or group of contaminantsthat, through various exposure pathwayssuch as ingestion, inhalation or skincontact, could result in a health effect to

humans in excess of EPA allowable risks.



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Classifying soils according to therelative risk to workers and wildlife at

RMA determines the type of remediationto be performed and its final outcome.Ensuring that access to areas withcontaminated soil is restricted to

qualified personnel, that thecontaminated soil is contained in itsintended location, and that theprocedures for working in areas with

contaminated soils are followed, is



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Prior to conducting any excavation

activities, an Intrusive Activity Permit must

be completed. It must be reviewed by RMA Infrastructure.

If it is in a potential UXO area, it must be

reviewed by an ordnance specialist.

It must remain on file at the job site during

the activity

Excavation Requirements


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No one can predict accurately if an excavation is

safe to enter without a proper support structure

being provided. A worker does not have to be

completely buried in soil to be seriously injured orkilled. Workers who have been only buried up to

their waist have died as a result of the pressures

exerted by the soil on their bodies.

Excavations in or near "back-filled" or previouslyexcavated ground are especially dangerous since the

soil is "loose" and does not support itself well.

Remember!


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Water increases the possibility of a cave in. The

increased water pressure exerted on the soil can be

the final factor in causing the walls to collapse.

Clay can be extremely treacherous if dried by the

sun. Large chunks of material can break off a trench

wall after having been stable and solid for a long

period of time.

It is not safe to assume that because the walls of anexcavation are frozen that it is safe to enter. Frozen

ground is not an alternative to proper shoring.

Remember!


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An excavation should be considered a confined

space and appropriate evaluation and controls

undertaken to ensure workers are not exposed to

contaminated atmospheres.

Shoring must be adequate to overcome additional

pressures from piles of excavated material,

adjoining structures, and vehicular traffic.

Remember!

excavations & trenching cfr 1926 subpart p

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