(Call 1-800-DIG-SAFE before doing any digging)
OSHA requires that all excavations over five feet deep be sloped, shored, sheeted, braced or otherwise supported. When soil conditions are unstable, excavations shallower than five feet also must be sloped, supported or shored.
Elimination of trenching and excavation failures is actually a very simple matter. Failures will not happen if we learn what causes them, plan accordingly, proceed in keeping with the standards, and insist on safe work practices. Through a system of options you can select methods best suiting your needs and equipment. The new standard recognizes these methods of employee protection:
sloping and benching of soil
trench shields
aluminum hydraulic shoring
other alternatives to wood-shoring, such as pneumatic hydraulic shoring and trench jacks.
TYPE A is defined as cohesive soils with an unconfined compressive strength of 1.5 tons per square foot or greater. Examples of cohesive soils are: clay, silty clay, sandy clay, clay loam and, in some cases, silty and sandy clay loam. Cemented soils such as calcite and hardpan are also considered Type A. However, no soil is Type A if:
It is fissured.
The soil is subject to vibration from heavy traffic, pile driving or similar effects.
The soil has been previously disturbed.
The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical or greater.
The material is subject to other factors that would require it to be classified as a less stable material.
The exclusions for Type A eliminate its use in most typical construction situations. What is left, typically are Types B and C.
TYPE B is defined as:
Cohesive soil with an unconfined compressive strength greater than .5 TSF but less than 1.5 TSF.
Granular cohesionless soils including: angular gravel, silt, silt loam, sand loam and in some cases silty and sandy clay loam.
Previously disturbed soils except those that would be classified as Type C soil.
Soil that meets the unconfined compressive strength or cementation requirements of Type A, but is fissured or subject to vibration.
Dry rock that is not stable.
Material that is part of a sloped, layered system where the layers dip into the excavation on a slope less steep than four horizontal to one vertical, but only if the material would otherwise be classified as Type B.
Most soils fall into this category until they become predominantly granular or saturated, at which time they become Type C.
TYPE C is defined as:
Cohesive soil with an unconfined compressive strength of .5 TSF or less.
Granular soils including gravel, sand and loamy sand.
Submerged soil or soil from which water is freely seeping.
Submerged rock that is not stable.
Material in a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical or steeper.
A competent person shall classify the soil type on the basis of at least one visual and one manual analysis. These tests are designed to determine stability based on a number of criteria: cohesiveness, whether or not it is fissured, the presence and amount of water, the unconfined compressive strength, the presence of layering, prior excavation and vibration.
One method of ensuring the safety and health of workers in a trench or excavation is to slope the sides of the cut to the “angle of repose”, the angle closest to the horizontal at which the soil will remain stable. The angle of repose varies with different kinds of soil, and must be determined on each individual operation. When an excavation has water conditions, silty material, or loose boulders, or when it is being dug in areas where erosion, deep frost, or slide planes are apparent, the angle of repose must be flattened.
We may use a trench box, a prefabricated movable trench shield composed of steel plates welded to a heavy steel frame. OSHA standards permit the use of a trench box as long as the protection it provides is equal to or greater than the protection that would be provided by a standard shoring system.
Sloping is the oldest and still a popular method of preventing cave-ins. Sloping means that the sides of an excavation are laid back to a “maximum allowable slope” from which they will not collapse. In the original standard this was known as the angle of repose, a term that was borrowed from stockpiling. That term is not used in the new standard. Superintendents wanting to slope using the new standard have four options:
Slope the angle required by the standard for Type C, which is the most unstable soil type.
Use the tables provided in the standard to determine the maximum allowable angle (after determining the correct soil type).
Use tabulated data prepared by a registered professional engineer.
Have a registered professional engineer design a sloping plan specifically for that job.
It should be noted that any job over 20 feet in depth requires the services of a registered professional engineer if sloping is to be used. Spoil piles are to be at least two feet from the edge of the trench and should likewise be sloped to a safe angle.
Shoring is the second means of providing cave-in protection. In shoring, the sides of the excavations are braced up by structural components strong enough to prevent a cave-in. There are numerous ways to shore up excavations. Screw jacks, aluminum pneumatic, aluminum hydraulic, sheet pile and combinations of the methods are commonly used. As in sloping, the standard provides options. After determining the soil type, the excavator may use charts provided in the standard for shoring up to depths of 20 feet; use manufacturer’s tabulated data or have a plan designed by a registered professional engineer.
Whatever support system is used, workers should always apply shoring starting from the top of the trench or excavation and working down. In installing the shoring, care must be taken to place the cross beams or trench jacks in true horizontal position and to space them vertically at appropriate intervals. The braces also must be secured to prevent sliding, falling or kickouts.
All materials used for shoring must be in good condition, free of defects, and of the right size.
Installing the shoring should closely follow the excavation work. It is dangerous to allow trenches to remain unshored even if work is not being done in them: Dirt walls will slough off, causing dangerous overhangs. The longer a trench is left unsupported, the greater the chance of a cave-in.
Superimposed loads in the vicinity of a trench or excavation increase the pressure on excavation walls. Heavy equipment and materials such as pipes or timbers should be kept as far back from the excavation as possible. When heavy loads must be located near an excavation, the walls must be braced, sheet piled, or shored to safely support the extra weight. In some cases, it may be necessary to lessen the pressure of these loads. Pile driving or cranes, for example, would be mounted on wooden mats or heavy planking to spread the weight more evenly.
Buildings, curbs, trees, utility poles, and other structures adjoining the excavation area also can place more stress on a trench side than it can safely accommodate. In these instances, OSHA requires that shoring, bracing, or underpinning be provided as necessary not only to protect workers but also to prevent the dislocation of the soil beneath the structures in the vicinity.
In addition, spoil, (the excavated material) can exert great pressure on the excavation walls. Spoil must be stored two feet or more from the edge of the excavation, and be barricaded or retained in an effective manner.
Even when this is done, large heavy objects can roll or slide down the incline and into the trench. Tools and rocks should either be placed on the outer slope of the excavated materials, or else on the other side of the trench if the surface is flat.
When workers are working on hard surface roads where a flow of traffic is being maintained, it is important that small stones be removed from the road. Stones are sometimes thrown with great speed by the tires of passing cars and can cause serious accidents.
Injuries can result when workers fall into construction trenches; people fail to look where they are going, when they walk too close to the edge; or when they attempt to leap across the trench.
Rocks and tools thrown near the edge are not only a hazard to employees working in the trenches, but can cause falls into the trench by workers working on the surface.
Use extra care in venturing near the edges of trenches and other excavations when the weather is bad or there are icy, muddy conditions.
Wales or shoring timbers should not be used as supports for platforms to carry concrete mixers or other heavy equipment. If it is necessary for a mixer to be used over the trench, vertical supporting members should be cut in between the wales, and the load transmitted to the ground through additional vertical members from the bottom waler.
When internal combustion engines are used in or near trenches, precautions should be taken against exhaust gases entering the trenches. Where necessary, ducts should be attached to the exhaust to conduct the gases away from the trench.
As soon as work is completed, the trench should be backfilled as the shoring is dismantled. After the trench has been cleared, workers should remove the shoring from the bottom up, taking care to release jacks or braces slowly. In unstable soil, ropes should be used to pull out the jacks or braces from above.
Sometimes the superintendent will have to guard against an unstable excavation bottom, such as below the water level. Sheeting may have to be driven below the bottom of such an excavation to add to the soil stability.
The OSHA Standard requires that diversion dikes and ditches or other suitable means be used to prevent surface water from entering an excavation and to provide adequate drainage of the area adjacent to the excavation. Water causes soil erosion and softening and should not be allowed to accumulate in a trench or excavation.
Excavations, adjacent areas and protective systems must be inspected daily by a competent person prior to start of work. Inspections also are required after rainstorms or any change in condition that can increase the possibility of a cave-in or slide or other hazard. If dangerous ground movements are apparent, such as subsidence or tension cracks, or other hazards noted, all work in the excavation must be stopped and all personnel removed from the excavation until the problem has been corrected.
In case of an emergency, workers must be able to leave the trench quickly. OSHA regulations state when employees are required to be in trenches four feet deep or more, adequate means of exit, such as a ladder or steps, shall be provided and located so as to require no more than 25 feet lateral travel.
Ladders must be in good condition, extend from the floor of the trench to three feet above the top of the excavation, and be secured at the top.
TRENCHING AND EXCAVATIONS – CHECK-LIST
Before Trenching or Excavation
CHECK: Soil conditions or other material to be dug.
CHECK: Proximity to utilities, buildings and sources of vibration.
CHECK: Owners of utilities, service, or transmission piping, etc. and arrange for shutdown or relocating of facilities, if necessary.
CHECK: For previously disturbed ground.
CHECK: For trees, boulders, or other employee hazards.
CHECK: Adequacy and availability of all equipment, including personal protective gear, shoring materials, signs, barricades, and machinery.
During Trenching or Excavation
CHECK: For changing ground conditions, particularly after rainfall.
CHECK: For possible oxygen deficiency or gaseous conditions.
CHECK: Adequacy of shoring and/or sloping as work progresses.
CHECK: For maintenance of entrance and exit facilities.
CHECK: All sheeting, bracing, shoring and underpinning.
CHECK: For changes in vehicular and machinery operational patterns.
After Trenching or Excavation
CHECK: Depth of trench or excavation, its sloping and shoring.
CHECK: Sloping or banks, sides, and walls in relation to depth of cut, water content of soil, vibrations.
CHECK: Entrance and exit facilities.
CHECK: Location of heavy equipment-power shovels, derricks, trucks.
CHECK: That excavated material is two feet or more from edge of opening.
CHECK: The adequacy of portable trench boxes or trench shields, if used.
CHECK: For correct positioning of cross braces or trench jacks to prevent sliding, falling or kickouts.