SAFETY TIPS: TRENCHING
AND EXCAVATING
by Jamie Knotts
We've all seen the headlines: "Worker Dies as
Ditch Collapses." "Trench Caves In, Killing One." "Unstable
Soil Buries Man Alive." Although many know the dangers of unstable
soils when digging holes, trenches, or ditches, some workers digging in those
unstable soils don’t take the safety precautions needed to prevent accidents.
The fact is, excavating is among the most hazardous construction operations.
Drinking water utility workers often perform trenching and excavating work on distribution systems. Workers install new lines, repair ruptured mains, or tap into existing lines well beneath the ground’s surface, and each time they dig a trench or excavate a ditch, they put themselves and their co-workers at risk for injury.
The National Institute for Occupational Safety and Health (NIOSH) analyzed
workers’ compensation claims from 1976 to 1981 and found that excavation
cave-ins caused approximately 1,000 work-related injuries each year. Of these,
about 140 resulted in permanent disability, and 75 resulted in death. Excavation
cave-ins account for nearly one percent of all annual work-related deaths in the
nation.
What Is A Trench?
The Occupational Safety and Health Administration (OSHA), the federal agency
that oversees workplace safety, defines a trench as "a narrow excavation
(in relation to its length) made below the surface of the ground. In general,
the depth is greater than the width, but the width of a trench (measured at the
bottom) is not greater than 15 feet (4.6 meters). If forms or other structures
are installed or constructed in an excavation so as to reduce the dimension
measured from the forms or structure to the side of the excavation to 15 feet
(4.6 m) or less (measured at the bottom of the excavation), the excavation is
also considered to be a trench."
OSHA defines an excavation as "any man-made cut, cavity, trench, or
depression in an earth surface formed by earth removal." Even a
three-foot-deep by two-foot-wide trench dug for a new water line is a trench.
And make no mistake--it also can be dangerous.
Workers sometimes believe these shallow trenches are safe to work in with no
sloping or safety devices to stop cave-ins. The fact is, workers can be injured
in these trenches, especially if nearby machinery vibrates the soil or the soil
itself is water-laden and unstable.
What Causes Trench Walls to Fall?
Soil is an extremely heavy material, sometimes weighing more than 100 pounds per
cubic foot. A cubic yard of soil (3 ft. x 3 ft. x 3 ft.) which contains 27 cubic
feet of material may weigh nore than 2,700 pounds. That is nearly one and a half
tons (the equivalent weight of a car) in a space less than the size of the
average office desk. Wet soil, rocky soil, or rock is usually even heavier. The
human body cannot support such heavy loads without being injured.
As a worker removes soil forming a trench, the walls may become unstable.
Horizontal pressure on the soil along the trench wall is no longer in
equilibrium, and a portion of the wall may not be able to support its weight and
the weight of any adjoining soil. At the point where the soil can no longer
withstand the pressure, the wall will shear and break way from its stable
position.
Soil failure often occurs first at the bottom of the wall as the soil moves into
the trench, but whole chunks can topple down from the sides, and portions can
heave up from the floor of the trench, also. This movement creates an undercut
area at the base of the trench as soil falls into it. In many cases, more of the
wall collapses. Finally, the erosion at the base of the trench leaves the upper
part of the wall supported only by cohesion to the soils around it, and
more soil from the wall will fall into the excavation. Many rescue attempts are
unsuccessful because rescuers try to save victims before the second and third
failures take place, often trapping the would-be rescuers along with the
original victims.
Inspect Soils Before and During Excavating
While a worker is digging, he or she should identify the soil characteristics.
This could help predict the chance of a cave-in. When using a visual test of the
excavation site, look at adjacent soils in the area. See if the soil clumps
together or seems coarse-grained like sand or gravel.
During a visual test, check for crack-line openings along the site that indicate
tension cracks, potential starting points for a cave-in. Look for signs that the
soil was previously disturbed. Also look for evidence of bulging or sliding
material as well as signs of surface water seeping from the sides of the
excavation or from the water table.
Workers Should Use Protective Devices
Workers should stabilize the area they are working in to minimize the risk of
cave-ins. Four methods include:
Benching -
protecting employees from cave-ins by excavating the sides of an excavation to
form one or a series of horizontal levels or steps, with vertical or
near-vertical surfaces between levels.
Shields -
or "trench boxes" designed to withstand
forces that may be imposed on them if a cave-in occurs.
Shoring -
metal hydraulic, timber, or other types of mechanical systems that support an excavation’s sides.Short Tips to Consider:
•
Do not go into unsupported excavations.•
Prevent the sides and ends from collapsing by battering them to a safe angle or supporting them with timber, sheeting, or other support systems.•
No one can accurately predict if an excavation is safe to enter without a proper support structure.•
Remember that even work in shallow trenches can be dangerous. You may need to provide support if the work involves bending or kneeling in the trench.•
Never work ahead of a support.•
Do not store spoil (excavated soils) or other materials close to the sides of excavations. The spoil may fall into the excavation and the extra weight will make the sides more prone to collapse.•
Make sure the edges of the excavation are protected against failing materials.•
Wear a hard hat when working in excavations.•
A worker does not have to be completely buried in soil to be seriously injured or killed. Workers who have been only buried up to their waist have died due to soil pressures exerted on their bodies.•
Excavations in or near "back-filled" or previously excavated ground are especially dangerous because the soil is loose and does not support itself well.•
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 from a trench wall after having been stable and solid for a long time.•
Do not assume that frozen excavation walls are safe. They are not. Frozen ground is not an alternative to proper shoring.•
Shoring must be adequate to overcome additional pressures from piles of excavated material, adjoining structures, vehicular traffic, and nearby equipment.•
Fence off all excavations in public places to prevent pedestrians and vehicles from falling into them.•
Where children or others might get onto a site after hours, take precautions, such as backfilling or securely covering excavations to reduce the chance of injury.• Provide good ladder access or other safe ways of getting in and out of the excavation.
References
"Request for Assistance in...Preventing Deaths and Injuries From
Excavation Cave-Ins," NIOSH Alert: July 1995, Publication No. 85-100
"Excavations: Hazard Recognition in Trenching and Shoring," OSHA
Technical Manual: TED 1-0 15 A
Brown, Larry C. et al. "Trenching and Excavation: Safety Principles,"
Ohio State University.
"Guidelines for Excavation Work," Government of Manitoba, Workplace
Safety and Health Division.
"Safety in Excavations," Health and Safety Executive, United Kingdom.
Jamie Knotts is Assistant
Editor with the National Drinking Water Clearinghouse. Reprinted with permission
from On Tap,
Spring 2001, and the National Drinking Water Clearinghouse.