Environmental Engineering Reference
In-Depth Information
As mentioned earlier, weather and climate can have a serious impact
on trenching and excavation activities. Rain, melting snow, groundwater,
storm drains, nearby streams, or damaged water lines can loosen soil
and increase pressure on walls. At the opposite extreme, extremely dry
weather can also be dangerous because it tends to loosen soil. Frozen
ground presents another problem. When the frozen ground thaws, walls
of a trench or excavation can be weakened. When the excavation is a long-
term job, there may be a need for extra weather and climate protection.
Sides and faces of the dig should be covered with tarps to reduce danger.
Supervisors and workers also need to be trained in soil-type recog-
nition procedures. Soils with high silt or sand content are very unstable
unless properly shored or sloped. Wet or backfilled soil is also unstable
and requires wall support. Even hard rock can present a problem unless
it is properly supported. Hard rock that cracks or splits through a fault
can break away and fall into the excavation.
One area of danger that is often overlooked in trenching and shor-
ing operations is the presence of vibration. Vibrations can loosen soil
and cause walls to collapse unless proper shoring or sloping is used.
Sources of vibration at the worksite include vehicles, moving machinery,
blasting operations, and machines that might be used nearby such as
punch presses and forging hammers. Excavated material can also pose
a hazard to the excavators. Excavated material (or spoil) should always
be stored at least 2 feet from the edge of a trench or excavation. Never
let excavated material accumulate near wall sides. Additionally, moving
excavated soil can also pose a hazard to excavators. Heavy equipment
operating near the trench or excavation can exert tremendous pressure
on walls.
To protect excavators against accidents, proper techniques and
equipment must be used in the trenching and shoring operations. Trench
shoring material should consist of sheeting, bracing, and jacks. Never
use shoring materials that have not been certified for use by a licensed
professional engineer. After installation of the correct shoring materi-
als, these materials should be inspected daily before anyone enters the
trench or excavation.
When the decision is made to use ground sloping techniques to
prevent cave-ins, the sides of a trench or excavation must be sloped cor-
rectly so the soil will not slide. Determining the
angle of repose
is criti-
cal in shaping the proper slope. The angle of repose is the steepest angle
at which trench or excavation walls will lie without sliding. The more
unstable the soil, the flatter the angle should be. For example, the angle
of repose for solid rock should be set at 90°. For average soil, an angle of
45° is recommended. For loose sand, the proper angle of repose should
be set at about 25°.
Several other safety considerations are necessary whenever a
trenching or excavation project is undertaken. Workers must understand
that it is important to provide site protection. Site protection protects
workers from rocks or other objects kicked or thrown into the trench as
well as pedestrians who might inadvertently fall into an open trench or
