General Requirements

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Trenching and Excavation Operations

GENERAL REQUIREMENTS

OBJECTIVES

1. Upon completion of this section, the participant should be able to:

2. Understand the Competent person’s responsibility regarding the safety concerns listed in the General Requirements.

3. Develop a safety checklist, which will cover all of these safety concerns.

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INSPECTION OF THE TRENCH

The following is a discussion of a General Requirement items found in CFR 1926.651. As you will see, much of the responsibility for the evaluation of each of these issues falls upon the competent person. The competent person has the responsibility to inspect the trench and the area around the trench for any hazard which could cause a cavein or other hazard to the employees and the keep the employees out of the trench until the safety problem has been remedied. This person must inspect the trench area for any evidence of a cavein hazard, which might include: • Fissures • Tension cracks • Sloughing • Undercutting • Water seepage • Bulging at the bottom • The protection system integrity should also be evaluated. Unlike any other worker, the competent person may stand on the shoring system during the inspection process. Like the other employees, the competent person may not enter a trench for inspection until all appropriate protective systems are in place.

Daily inspections should evaluate the following: • Visual and manual soil classification surface hazards (encumbrances) • Water hazards • Atmospheric hazards • Vibration hazards • Access and egress • Spoil pile location • Barricade and traffic control structure placement • Utilities, underground structures location and protection • Integrity of the protective system (handson and visual) • Vehicular and other mobile surface hazards

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Also included in this report should be

• The name and signature of the Competent person

• Weather conditions

• Date and time of evaluation

While not required by OSHA, a trench log sheet should be filled out each of time an evaluation of the trench is made. THE COMPETENT PERSON IS REQUIRED TO INSPECT THE TRENCH AND EVALUATE THE HAZARDS PRIOR TO THE START OF WORK EACH DAY. ADDITIONAL INSPECTIONS ARE REQUIRED AFTER ANY RAIN STORM OR AFTER ANY HAZARD POTENTIALINCREASING EVENT (E.G. RAIN, VIBRATION, EARTHQUAKE, ETC.

ATMOSPHERIC TESTING The Competent person must test the atmosphere for hazards if they could reasonably have been expected to exist. This analysis would be required if the trench were being dug in a landfill; to repair broken sewer or gas utility lines, near leaking underground storage tanks, or anyplace that the soil could reasonably be expected to contain this type of hazard. Testing should be made prior to entry into the trench. Further testing should continue as conditions warrant.

Acceptable atmospheres for trenching operations are:

• Oxygen Level: 19.5% 23.5%

• Flammability: Less than 20% of LEL (Lower Explosive Limit)

• Toxicity: Less than the PEL (Permissible Exposure Limit for that chemical)

Ventilation may be required to keep the atmosphere parameters within acceptable limits. Extra caution and continuous monitoring should be provided in the situation where continuous ventilation is required to keep the hazardous atmosphere from building back.

Many people wonder if the area within a trench could also be a confined space. OSHA officials have stated that the only time that a trench area would be considered a permitrequired confined space is when there is an actual or potential hazardous atmosphere present. In that case, the trench may also be

©HMTRI 2000 Page 76 Trenching REV1 Trenching and Excavation Operations classified as a permitrequired confined space. IT SHOULD BE NOTED THAT IF THE TRENCH IS ALSO CLASSIFIED AS A PERMITREQUIRED CONFINED SPACE, ENTRY INTO THE TRENCH IS PROHIBITED UNTIL A CONFINED SPACE ENTRY PERMIT IS FILLED OUT. This too would probably be the responsibility of the competent person.

One other point about the confined space issue. Remember that the hazard limit for flammable atmosphere is 10% of LEL for a permitrequired confined space, not 20% of LEL as in the trenching regulation.

WATER HAZARDS Water is one of the greatest hazards in trench work. It can undermine the trench walls or decrease the soil's ability to bear weight. Further, transmitted water pressure can turn the bottom of the trench into quicksand. In any case, the probability of wall failure is greatly enhanced by accumulating or seeping water. No worker is allowed in a trench where water is seeping or accumulating unless adequate precautions against the water hazard are in place. Therefore, if work must take place in the trench, the competent person has the responsibility to design and monitor the water removal process.

WATER REMOVAL In many cases, the water that accumulates in a trench is water that is natural drainage and seepage from the groundwater table. Factors determining how deep the groundwater table will be include soil type(s), elevation of the trenching site, recent rainfall, and natural or manmade mechanisms of drainage (tile). The groundwater table should ideally be kept at least 2 feet from the floor of the trench. Dealing with water from below the soil surface can be accomplished in the following ways:

Local Pumping In many cases, the water inside the trench can easily be pumped from the trench. The end of the suction line is placed in the lowest portion of the accumulated water and water is drawn through the pump and out a discharge line. The advantage of this system is its simplicity. One disadvantage of local pumping is that while it will remove gross standing water, it will not leave the bottom dry. Rather, the bottom will be mud. Also, be sure the discharge line is dumping the pumped water far enough from the trench that it does not run right back in.

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Most pumps used for dewatering are centrifugal pumps, meaning that they rely on the vacuum created within the pump for the drawing power. Centrifugal pumps do not "suck" water. Rather, they decrease the pressure within the pump to allow the weight of the atmosphere (atmospheric pressure) to push the water out.

Since there is only so much weight to the atmosphere (around 15 psi), the pump can lift water only so far. A perfect pump could theoretically lift water just over 30 feet. Most pumps in real life can lift water only 1/3 that high. A good rule of thumb is the greater the vertical distance between the pump and the water surface, the less the pump will discharge. Here are some other tips on pump placement:

Keep the suction intake clean;

Have as few bends in piping as possible;

Use a grounded circuitry for electrical pumps;

Be sure exhaust fumes do not enter trench.

Wellpoints In wet locations, it may be preferable to install Wellpoints. A wellpoint is a conduit with a screenlike opening near the end, which is driven into the ground below the water table. This type of system works best in granular soils. The other end of the conduit is connected to a pipe that serves as a vacuum manifold for a series of well points. The end of the vacuum manifold is a combined vacuum and centrifugal pump. When in operation, the pump creates a vacuum within the system, allowing water to flow from the ground into the system. Once in the manifold system, the water is removed via the centrifugal pump. This system is analogous to a child sucking the juice from a snow cone using a straw.

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The advantage of this system is that the water is being removed before it enters the trench. This leaves the bottom of the trench relatively dry. Disadvantages include cost, time and expense of setup, and it may be in the way if construction near the trench is contemplated.

Enclosed Excavations Special tongueandgrove timber planking or steel piling can form a watertight barrier. This type of material will form the sides of the trench and will have to be able to withstand both the lateral pressures of the soil and water together. This type of water control is commonly used when the trench area is in saturated or submerged soil. Since the bottom is still soil, the water table will also have to be controlled or local pumping will have to remove any water seeping up from the bottom.

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PreExisting Tile Systems In many locations, tile systems used to control or supplement natural drainage. Locating these tiles before digging may prevent their being disturbed. Tile systems that are cut and empty into the excavation can exacerbate water removal problems, especially during wet periods.

WATER DIVERSION Water can also enter the trench from the surface of the ground. It can come from a preexisting body of water near the trench or from rain runoff. In most cases, this water will be controlled through diversion and diking.

Diversion Diversion of water means to redirect the natural drainage of surface water in a way that keeps it from entering the trench or weakening the walls of the trench. If an excavation intersects a streamed, the stream can be rerouted by digging an artificial waterway.

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Dikes and Berms Berms also can keep the excavation area free of surface water by placing a barrier between the excavation area and the surface water. Berms can be used to reroute bodies of water or can be constructed around the excavation to keep surface water from entering.

The berm should be constructed so as to resist water infiltration. The use of wellcompacted impervious materials, which are free of large rocks or clumps, is desired. It should be sloped at a 2H: 1V rate to prevent sloughing. This means that the width of the dike will be four times its height. Erosion should also be controlled with an outer covering of rock or riprap.

The berm should be constructed high enough to accommodate all of the anticipated water. Water running over the top of the berm will quickly cause the berm to fail. The height of the berm should be 1.5 times the anticipated height of the water. Therefore, if a berm is to hold back 2 feet of water, the it should be at least 3 feet high and 12 feet wide.

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STABILITY OF ADJACENT STRUCTURES Anytime excavation operations are close to or under the foundation or footing of an existing structure, the stability of that structure could be jeopardized. Therefore, when such an operation is contemplated, the competent person must assure the stability of the structure and worker safety by providing support systems that will assure that the structure will not collapse. That support may include underpinning, bracing, shoring, cribbing, or some other effective means. This may also be the time to solicit the advice of the RPE.

The RPE must be consulted if the excavation will involve removal of soil from below a base or footing of a foundation or retaining wall if that removal could reasonably pose a hazard to workers. The exception to this is if the excavation is made in stable rock or a support system has already been provided. In either case, the advice of a RPE would still be a good idea. Removal of soil from under a sidewalk, pavement or other surface structure is also prohibited unless appropriate protective systems are being used.

©HMTRI 2000 Page 82 Trenching REV1 Trenching and Excavation Operations UNDERGROUND INSTALLATIONS Probably the largest unexpected cost to a contractor is the inadvertent cutting of a utility pipe or communication cable. What's worse, most insurance companies will not pay for this type of damage if the location of the utility or cable had not been previously identified. Many explosions have occurred as well when natural gas lines are ruptured. It then makes good safety sense and economic sense to locate utility and communication assets prior to digging.

Digging may not take place until an attempt at locating these installations is made unless an inordinate time period has passed since the time of request for location. This time period is somewhat dependent upon local custom, but a reasonable time period (usually 24 to 48 hours) should be granted prior to digging. If the time period has passed and no location has been established, digging may commence, but caution must be used. Contact is not necessary if the work is to be done in a remote area where utilities are not likely and no aboveground features suggest utility presence.

Once these installations are encountered and uncovered, they must be protected, supported, or removed. Further, if soil removal near the installation is required, it should be done by hand if possible.

EXPOSURE TO VEHICULAR TRAFFIC AND BARRICADING If a worker is exposed to vehicular traffic on a public thoroughfare or construction site, that employee must wear a reflective vest or some other suitable, highly visible garment. This is true for workers who will be on the top of the excavation all of the time and for those who will be traversing the vehicle exposure area.

The area around the excavation must be appropriately barricaded and protected while the excavation is open. The barricades should be at least 6 feet away from the hazard. Please consult the Construction Standard for Signs, Signals, and Barricades (CFR 1926.200. 203). It also must be backfilled as soon as the work within the excavation has been completed. This is true for the primary excavation and ancillary temporary excavations (temporary well, pit, shafts, etc.) as well.

©HMTRI 2000 Page 83 Trenching REV1 Trenching and Excavation Operations EXPOSURE TO FALLING LOADS One very dangerous component of the trenching operation is lifting, lowering, and dumping very heavy material. The competent person has the responsibility of assuring that nothing is dropped upon a worker, whether in the trench or outside of the trench.

As the trench is being dug, large bucketloads of soil or rock are being lifted and piled, either on the ground or in a dump truck. Workers are not permitted to be close to the loads being lifted. The exception is the dump truck operator. They are permitted to stay in their cab if it has the appropriate protection. If the spoil is to be stockpiled for backfill, the spoil pile toe may be no closer than 2 feet from the edge of the trench. Not only does this prevent the soil from sliding back into the trench; it also decreases the surcharge load on the walls of the trench. If it is not possible to move the spoil pile back, a retaining barricade must be installed to prevent the material from falling. Any scale or loose clumps of material that are unstable on the face of the trench shall be removed. No worker shall work above another worker on the sloped face of a trench. If crumbling of the sides cannot be initially removed, a protective barrier will be placed between the scale and the workers.

Surface encumbrances are items, which have the possibility of rolling into the trench and falling on workers. Ideally, all construction materials and vehicles should be positioned as far away from the trench wall as possible. Other non mobile items such as large rocks should be immobilized so they cannot roll into the trench.

If mobile equipment (like a dump truck) is to be operated near the edge of the trench, safeguards must be in place to assure that the vehicle does not enter the trench. If the driver does not have a clear view of the edge of the trench, a spotter using hand signals barricades, stop logs, or mechanical systems are required to assure that the vehicle will stop before nearing the edge of the trench.

Finally, no worker is allowed under a load that is being lowered into a trench. This may be difficult especially when attempting to guide a section of pipe into place. Further, all workers must evacuate the trench when trench box is moved vertically.

ACCESS AND EGRESS The Competent person must insure that all workers will have a safe means of entering and exiting the excavation. A worker is not allowed to enter or exit the trench by climbing on the protective system (shoring). Therefore, some other means such as ladders, stairways, and ramps are needed. Ladders and ramps are the usual methods of entry and exit, and the Standard has requirements for both.

Ramps Ramps can be made manmade or can be excavated (earthen ramp). If the ramp is manmade, the amount of engineering design expertise is dependent upon the load that it must bear. A Competent person may design a ramp which is used solely for worker entry and exit, whereas a RPE or Competent person qualified in structural design is required to design a ramp which will be used for equipment. The ramps must be constructed in so that there is a nonskid surface to be walked on. If the ramps are made of more than one piece, the cleats or braces which hold the parts together shall be located and fastened so that no one will trip over them.

Ramps and runways shall be constructed of uniform thickness material to prevent a tripping hazard. Further, if a walkway or bridge is to span a 6 feet or more deep trench which is at least 15 inches wide, it must also have an approved handrail and be of approved width. Please consult the Construction Standard for Stairways and Ladders CFR 1026.1050. 1053 for specific details on handrail requirements.

Excavated ramps are considered safe only if the employee can walk unaided (no knotted ropes) up and down in an upright position. The ramp slope, soil type, environmental conditions, and obstructions need to be considered by the competent person.

Ladders Ladders can be either manufactured or job built, as long as they comply with OSHA standards. Please consult the Construction Standard for Stairways and Ladders (CFR 1926.1050. 1053) for further details on specifications for ladders.

Ladders should be secured at the top of the excavation by staking or tying the tip to an immovable object. Ladders may be tied to the protection system only if the effectiveness of that system is not impaired. The ladder should not be tied to a vehicle.

In all excavations greater than 4 feet in depth, a safe means of egress is required such that a worker would not have to go further than 25 feet to reach this means of egress.

EMERGENCY EQUIPMENT Hazardous atmospheres and dangerous trench designs require that the workers entering the trench wear equipment which will safeguard them from the atmosphere within, will affect their rapid removal, and/or will assist in their emergency medical treatment. This type of equipment must be well maintained and must be attended when in use.

SOIL TESTING One of the most important jobs that the competent person performs is soil testing. As we have seen, the results of soil testing are a major determining factor in the design of some types of protection systems.