Excavation
Any man-made cut, trench, or depression in an earth surface, formed by earth removal.
Benching:
A method of protecting employees from
cave-ins by excavating the sides of
an excavation to form one or series of horizontal levels
or steps, usually with
vertical or near vertical surfaces between levels.
Cave-In
The separation of
a mass of soil or rock materials from the side of an excavation, or the loss of
soil from under a trench shield or support system, and its sudden movement into
the excavation, either by failing or sliding, in sufficient quantity so that it
could entrap, bury, or otherwise injure and immobilize a person.
Competent Person
One who is capable
of identifying existing and predictable hazards in the surroundings or working
conditions, which are unsanitary, hazardous, or dangerous to employees, and who
has authorization to take prompt corrective measures to eliminate them.
Trench
A narrow excavation below the surface of
the ground, less than 15
feet wide, with a depth no greater than the width.
Protective
System A
method of protecting employees from cave-ins, from material that could fall or
roll from an excavation, or from the collapse of adjacent structure. Protective
systems include support systems, sloping and benching systems, shield systems,
and other systems that provide necessary protection.
Shield
A structure that is capable of
with standing the forces imposed on it by a cave-in and
There by protects employees within the
structure. Shields can be permanent structures or can be designed to be
portable and moved along as work progresses.
Sloping
A method of protecting workers from
cave-ins by excavating to form sides of an excavation that are inclined away
from the excavation to prevent cave-ins. The angle of incline required to
prevent a cave-in varies with differences such as soil type, length of
exposure, and application of surcharge loads.
GENERAL REQUIREMENTS:
1-The following specific site conditions should be taken into accounts
for safe excavations:
a- Traffic
b- Nearness of structure and their
conditions
c- Soil
d- Surface and ground water - the
water table
e- Overhead and underground utilities
f- Weather
2- Before any excavation actually
begins, the standard requires the employer to determine the estimated location
of utility installations: Sewer, telephone, fuel, electric, water lines or any
other under ground installations that may be encountered during digging.
3- No employee should operate a piece
of equipment without first being properly trained to handle it and fully
alerted to its potential hazards.
4- The standard requires that a
competent person inspect, on a daily basis, excavations and the adjacent areas
for possible cave-ins, failures of protective systems and equipment, hazardous
atmospheres, or other hazardous conditions.
5- Adequate protective systems will be
utilized to protect employees. This can be accomplished through sloping,
shoring, or shielding.
6- Workers must be supplied with and
wear any personal protective equipment deemed necessary to assure their
protection.
7- All spoil piles will be stored a
minimum of two (2) feet from the sides of the excavation. The spoil pile must
not block the safe means of egress.
8- If a trench or excavation is 4 feet or deeper, stairways,
ramps, or ladders will be used as a safe means of access and egress. For
trenches, the employee must not have to travel any more than 25 feet of lateral travel
to reach the stairway, ramp, or ladder.
9- No employee will work in an
excavation where water is accumulating unless adequate measures are used to
protect the employees.
10-A competent person
will inspect all excavations and trenches daily, prior to employee exposure or
entry, and after any rainfall, soil change, or any other time needed during the
shift. The competent person must take prompt measures to eliminate any and all
hazards.
11-Excavations and
trenches 4 feet
or deeper that have the potential for toxic substances or hazardous atmospheres
will be tested at least daily. If the atmosphere is inadequate, protective
systems will be utilized.
12-If work is in or
around traffic, employees must be supplied with and wear orange reflective
vests. Signs and barricades must be utilized to ensure the safety of employees,
vehicular traffic, and pedestrians.
SOIL CLASSIFICATION AND IDENTIFICATION
The OSHA standards define soil classifications within the simplified
soil classification systems, which consist of four categories:
a-
Stable Rock
b-
Type A Soil
c-
Type B Soil
d-
Type C Soil
Stability is greatest in stable rock and decreases through type A and B
to type C, which is the least stable.
Stable Rock: is defined as natural solid mineral matter
that can be excavated with vertical sides and remain intact while exposed.
(Example: granite or sandstone).
Type A Soil: are cohesive soils with unconfined
compressive strength of 1.5 tons per square foot or greater. (Example: clay,
silty clay, sandy clay, clay loam)
Type B Soil: are cohesive soils with an unconfined
compressive strength greater than 0.5 tons per square foot but less than 1.5
(tsf)
(Example: angular gravel, silt, silt loam)
Type C Soil: are cohesive soils with an unconfined
compressive strength of 0.5 tsf or less.(Example: gravel, sand and loamy sand,
submerged soil, soil from which water is freely seeping.
SOIL TEST & IDENTIFICATIONS:
Many kinds of equipment and methods are used to
determine the type of soil prevailing in an area, as described below:
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.
Visual Test: If the excavated soil is in clumps, it is
cohesive. If it breaks up easily, not staying in clumps, it is granular.
Thumb Penetration Test: The thumb penetration procedure
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.
Dry Strength Test: Try to crumble the sample in your hands
with your fingers. If it crumbles into grains, it is granular. Clay will not
crumble into grains, only into smaller chunks.
Wet Manual Test: Wet your fingers and work the soil
between them. Clay is a slick paste when wet, meaning it is cohesive. If the
clump falls a part in grains, it is granular.
EXCAVATION PROTECTION SYSTEMS:
There are three basic protective systems for an excavation and
trenches:
a-
Sloping and Benching Systems
b-
Shoring Systems
c-
Shields Systems
The protective systems shall have the capacity to resist without
failure all loads that are intended or could reasonably be expected to be
applied to or transmitted to the system.
SLOPING & BENCHING SYSTEMS:
Sloping
Systems:
Maximum allowable slopes for excavations less than 20 feet (6.09 m ) based on soil type
and angle to the horizontal are as follows:
Soil Type
|
Height/Depth ratio
|
Slope Angle
|
Stable Rock
|
Vertical
|
90 deg.
|
Type A
|
¾ : 1
|
53 deg.
|
Type B
|
1 : 1
|
45 deg.
|
Type C
|
1½ : 1
|
34 deg.
|
Benching Systems:
There are two types of benching, simple and multiple. The type of soil
determines the horizontal to vertical ratio of the benched side.
As a general rule, the bottom vertical height of the trench must not
exceed 4 feet
(1.2 m )
for the first bench. Subsequent benches may be up to a maximum of 5 feet (1.5 m ) vertical in Type A
soil and 4 feet
(1.2 m )
in Type B soil to a total trench depth of 20 feet (6.0 m ).
Shoring
Systems:
Shoring is the provision of a support system for trench faces used to
prevent movement of soil, underground utilities, roadways, and foundations.
Shoring is used when the location or depth of the cut makes sloping back to the
maximum allowable slope impractical. Shoring systems consist of posts, wales, struts,
and sheeting. There are two basic types of sharing, timber and aluminum
hydraulic.
Hydraulic Shoring:
Hydraulic Shoring is a prefabricated strut and/or wale system
manufactured of aluminum or steel. Hydraulic shoring provide a critical safety
advantage over timber shoring because workers do not have to enter the trench
to install or remove hydraulic shoring.
All shoring should be installed from top down and removed from bottom
up.
Pneumatic Shoring:
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.
Shielding
Systems:
A- Trench 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 outside of the trench box
and the face of the trench should be as small as possible. The space between
the trench boxes the excavation side are backfilled to prevent lateral movement
of the box.
Combined Use:
Trench boxes are generally used in open areas, but they also may be
used in combination with sloping and benching.
The box should extend at least 18 inch (0.45 m ) above the surrounding area if there is
sloping toward excavation. This can be accomplished by providing a benched
areas adjacent to the box.
Spoil:
Temporary spoil must be placed no closer than 2 ft . (0.61 m ) from the surface
edge of the excavation
