Excavation is one of the most hazardous construction operations. Refer OSHA Subpart P, Excavations, 29
CFR 1926.650,651 and 652.
CONFINED SPACE is a space that is limited openings for entry and exit, unfavorable natural ventilation,
may contain or produce hazardous substances, and is not intended for continuous employee occupancy.
EXCAVATION is any man-made cut, cavity, trench or depression in an earth surface that is formed by earth
removal. A TRENCH is a narrow excavation 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 15ft (4.6 m). if a form or other structure installed
or constructed in an excavation reduces the distance between the form and the side of the excavation to 15ft or less (measured
at the bottom of the excavation), the excavation is also considered to be a trench.
UNCONFINED COMPRESSIVE STRENGTH is the load per unit area at which soil will fail in compression. This
measure can be determined by laboratory testing, or it can be estimated in the field using using a pocket penetrometer, by
thumb penetration test, or by other methods.
of Trench Failures
Tension cracks: Tension cracks usually form at a horizontal distance form at a horizontal distance of
0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench.
Sliding: It is also called as sluffing may occur as a result of tension cracks.
Toppling: In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s
vertical face shears along the tension crack line and topples in to the excavation.
Subsidence and Bulging: An un supported excavation can create an unbalanced stress in the soil, which
in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can
cause failure and entrapment of workers in the trench.
Heaving or Squeezing: Bottom heaving or squeezing is caused by the downward pressure created by the downward
pressure created by the weight of the soil. This pressure causes a bulge in the bottom of the cut.
Boiling: It causes by the upward water flow into the bottom of the cut. A high water table is one of
the causes of boiling
of Soil type.
Stable rock: Natural solid mineral matter that can be excavated with vertical sides and remain intact
Type A soils: It’s a cohesive soils with an unconfined compressive strength of 1.5 tons per sq.ft
(144 kPa) or greater. Clay, silty clay, sandy clay, clay loam. No soil is Type A if is fissured, is subject to vibration of
any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into the excavation on a
slope of 4 horizontal to 1 vertical (4H:1V) or greater, or has seeping water.
Type B soils: It’s a cohesive soil with an unconfined compressive strength greater than 0.5 tsf
(48 kPa) but less than 1.5 tsf (144 kPa). E.g. Angular gravel, slit loam, previously disturbed soils unless otherwise classified
as Type C, soils that meet the unconfined compressive strength or cementation requirements of Type A soils but are subject
to vibration, dry unstable rock and layered systems sloping into the trench at a slope less than 4H:1V (only if the material
would be classified as a Type B soil).
Type C soils: It’s a cohesive soil with an UCCS of 0.5 tsf (48kPa) or less. Other Type 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. Also included in the classification is material in a slopped, layered system where the layers dip
into the excavation or have a slope of four horizontal to one vertical (4H:1V) or greater.
Layered geological strata: where soils are configured in layers, i.e., where a layered geological structure
exists, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each layer may be classified
individually if a more stable layer lies below a less stable layer, i.e., where a type C soil rests on tope of stable rock.
Equipment and Methods for Evaluating Soil type:
Pocket Penetrometer: Penetrometer are direct-reading, spring operated instruments used to determine the
unconfined compressive strength of saturated cohesive soils.
Shearvane (Torvane): To determine the UCCS of the soil with a Shearvane, the blades are pressed into
a level section of undisturbed soil and the torsional knob is slowly turned until soil failure occurs. The reading must be
multiplied by 2 to get result in tons/sqft.
Thumb Penetration Test: This involves an attempt to press the thumb firmly into the soil in question.
If the thumb makes an indentation in the soil only with great difficulty, the soil is probably Type A. if the thumb penetrates
no further than the length of the thumb nail, it is probably Type B soil, and if the thumb penetrates the full length of the
thumb, it is Type C soil. This test is less accurate of the three methods.
Dry Strength Test: Dry soil that crumbles freely or with moderate pressure into individual grains is
granular. Dry soil that falls into clumps that subsequently break into smaller clumps (smaller clumps can be broken only with
difficulty) is probably clay in combination with gravel, sand or slit.
Plasticity or Wet Thread Test: This test is conducted by molding a moist sample of the soil into a ball
and attempting to roll it into a thin thread approximately 1/8 inch (3mm) in dia (thick) by 2 inches (50 mm) length. The soil
sample is held by one end. If the sample does not break or tear, the soil is considered cohesive.
Visual Test: Vibrations, crack-line openings along the failure zone that would indicate tension cracks,
look for existing soil disturbance, signs of bulging, boiling or sluffing as well as for surface water seeping.
is the provision of a support system for trench used to prevent movement of soil, or any structures near by. Shoring or shielding
is used when the location or depth makes sloping back to the maximum allowable slope impracticable. Shoring systems consist
of posts, wale’s, struts and sheeting. Timber, aluminum, pneumatic and hydraulic are the major type of shoring.
Hydraulic shoring: Used often today. All shoring should be installed from the top down and removed for
the bottom up. Hydraulic shoring should be checked at least once per shit fro leaking hose and or cylinders, broken connections,
cracked nipples, bent bases and any other damaged or defective parts.
Pneumatic shoring: It works in a manner similar to hydraulic shoring. The primary difference is that
pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneumatic shoring is that
an air compressor must be on site.
Trench Boxes: These are primarily to prevent protect workers from cave-ins and similar incidents. The
excavated area between the outside of the trench box and the face of the trench should be as small as possible. The space
between the trench boxes and the excavation side are back filled to prevent lateral movement of the box.
Sloping: Maximum allowable slopes for excavations less than 20 ft (6.09 n) based on soil type and angle
to the horizontal are as follows:
slopes – for stable rock the Height/Depth ratio is vertical and slope angle is 90 deg.
– for Type A
the Height/Depth ratio is ¾:1 and slope angle is 53 deg.
for Type B the Height/Depth ratio is 1:1 and slope angle is 45 deg.
for Type C the Height/Depth ratio is 1.5/1 and slope angle is 34 deg.
for Type A (short term) the Height/Depth ratio is 0.5:1 and slope angle is 63
note that for a maximum excavation depth of 12 ft.
Benching: There are two basic types of benching, simple and multiple. The type of soil determines the
horizontal to vertical ratio of the benched side.
a general rule, the bottom vertical height of the trench must not exceed 4 ft(1.2 m) for the first bench. Subsequent benches
may be up to a maximum of 5ft (1.5 m) vertical in Type A soil and 4 ft (1.2 m) in Type B to a total trench depth of 20 ft
(6.0 m). All subsequent benches must be below the maximum allowable slope of that soil type. For Type B soil the trench excavation
is permitted in cohesive soil only.
Temporary Spoil: Not to be placed closer than 2ft (0.61 m) from the surface edge of the excavation, measured
from the nearest base of the spoil to the cut.
Permanent Spoil: Should be placed at some distance away from excavation.
the cut, cavity, or depression a trench or an excavation?
the cut, cavity, or depression more than 4ft (1.2 m) in depth?
there water in the cut, cavity or depression?
there adequate means of egress?
there any surface encumbrances?
there exposure to vehicular traffic?
adjacent structures stabilized?
mobile equipment have a warning system?
a competent person in charge of the operation?
equipment operating in or around the cut, cavity, or depression?
procedures required to monitor, test, and control hazardous atmosphere?
a competent person determine soil type?
a soil testing device used to determine the soil?
the spoil placed 2ft or more from the edge of the cut, cavity or depression?
the depth 20 ft (6.1 m) or more for the cut, cavity, or depression?
the procedure require benching or multiple benching? Shoring? Shielding?
provided, do shields extend at least 18 in (0.5 m) above the surrounding area if it is sloped toward the excavation?
shields are used, is the depth of the cut more than 2 ft (0.6m) below the bottom of the shield?
any required surface crossing of the cut, cavity, or depression the proper width and fitted with hand rails?
means of egress from the cut, cavity, or depression no more than 25 ft (7.6m) from the work?
emergency rescue equipment required?
there documentation of the minimum daily excavation inspection?