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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.
Excavations and Trenching
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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.
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Sliding or sluffing may occur as a result of
tension cracks.
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Toppling occurs when the trench's vertical face
shears along the tension crack line and topples
into the excavation.
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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.
Soil Classifications
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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!