Environmental Engineering Reference
In-Depth Information
to ensure that supervisors and workers know the hazards. They should know that the
major hazard is cave-ins, which can crush or suffocate them. They should know that
trenches and excavations can contain poisonous gases. There is also the very real
possibility of uncovering a pocket of combustible vapors or gases; for example, when
digging around interceptor lines it is not unusual to run into a pocket of methane,
leading to the danger of fire or explosion. Supervisors and workers also need to know
that OSHA requires excavations to be protected from cave-ins by an adequate pro-
tective system that is designed to resist without failure all loads that are intended or
could reasonably be expected to be applied or transmitted to the system.
Trenches and excavations can be full of additional obstacles. For example, care-
lessly placed tools and equipment or excavated material can cause injuries due to
slips, trips, or falls. After explaining the general hazards that are present with any
trenching or excavation job, the person performing the training needs to inform the
workers about the causes of cave-ins so the workers know what to look for. Workers
need to understand that cave-ins occur when an unsupported wall is weakened or
undermined by too much weight or pressure or by an unstable bottom.
One of the danger signs to look for in trenching or excavation work is surface crack-
ing. These cracks usually occur near the edge of the trench or excavation. Overhangs
and bulges are other signs of danger. An overhang at the top—or a bulge in a wall—
can cause soil to slide into the trench or excavation. Whenever cracks or overhangs are
discovered, work should be stopped and the problem reported immediately.
Weather and climate can have a serious impact on trenching and excavation activ-
ities. Rain, melting snow, groundwater, storm drains, nearby streams, and 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 recognition proce-
dures. Soils with high silt or sand content are very unstable, unless properly shored or
sloped. Wet or back-filled 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 shoring operations
is the presence of vibration. Vibrations can loosen soil and cause walls to collapse
unless proper shoring or sloping is used. There are several sources of vibration at the
work site: 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 excava-
tion. 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.
