Infrastructure has become a buzzword because of the proposed economic stimulus package. A major part of our infrastructure is below ground in the form of utilities, water and sewer systems. Although trenchless technology has become an important way of installing some of these, a vast majority of the work is still done with open trenches, which are commonly dug to lay pipe or place manholes, conduit runs or footings. There will undoubtedly be an increase in underground and utility construction as a result of the stimulus package.
Working in trenches is hazardous and demands adherence to OSHA regulations. According to the Center to Protect Workers' Rights (CPWR), more than 30 workers are killed in trenches each year. A trench is a confined space with many special problems. Most deaths in trenches are from cave-ins. Other risks are falls, electrocution, being struck by falling objects (or a backhoe/excavator), and bad air, which can hurt your breathing, help cause a fire, or poison you. Many workers die trying to rescue other workers in trenches.
If you are involved in any underground construction, you need to become well acquainted with the prevailing OSHA and local governing body rules and regulations. Don't presume that you know the local rules. Several states have recently developed new regulations. For example, Massachusetts promulgated new regulation in 2007 that went into effect January 1, 2009. There are other states that have also upgraded their trench safety requirements. Remember, the OSHA regulations are the minimum requirements. A brief review of the current OSHA regulations follows:
- Do not enter an unprotected trench!
- Trench collapses cause dozens of fatalities and hundreds of injuries each year.
- Trenches 5 feet deep or greater require a protective system.
- Trenches 20 feet deep or greater require that the protective system be designed by a registered professional engineer.
Protective Systems for Trenches
- Sloping protects workers by cutting back the trench wall at an angle inclined away from the excavation.
- Shoring protects workers by installing aluminum, hydraulic or other types of supports to prevent soil movement.
- Shielding protects workers by using trench boxes or other types of supports to prevent soil cave-ins.
OSHA standards require that trenches be inspected daily and as conditions change by a competent person prior to worker entry to ensure elimination of excavation hazards.
- Inspect trenches at the start of each shift, following a rainstorm or after any other hazardous event.
- Test for low oxygen, hazardous fumes and toxic gases before entering a trench.
- Keep heavy equipment and excavation spoils at least two feet away from the trench edge.
- Provide stairways, ladders, ramps or other safe means of access in all trenches 4 feet or deeper (see sidebar).
CPWR Recommendations: Before You Work In A Trench:
Get a competent person to OK it. Make sure all equipment is in good condition. This includes water pumps and ventilators.
You must have a way to get out, like a ladder (within 25 feet of you) if the trench is 4 feet deep or more (see sidebar).
The contractor must have all utilities marked before digging. The contractor must call utility companies and shut off all electricity, gas and water pipes in the trench. Do not use a boom near overhead power lines. If you must operate boomed equipment, ask the competent person to make sure power has been cut off and the lines have been grounded.
If bad air is expected, OSHA says there must be a rescue plan and rescue equipment on the job site. Rescue teams must have special training. The best way is to follow OSHA rules for rescues from confined spaces (see sidebar).
If bad air is expected, a competent person must test the air:
- OSHA says the air must have 19.5 to 23.5 percent oxygen.
- OSHA says substances that can burn or explode — like gasoline or methane — must be at less than 20 percent of the lower explosive limit (or lower flammability limit). The industry says 10 percent.
- Check the air for toxics like chlorine, carbon monoxide, sewer gases, and hydrogen sulfide. These toxics can kill. Carbon monoxide has no smell. Hydrogen sulfide smells like rotten eggs.
The competent person will decide if blowers can keep the air safe.
A competent person must check the soil. This check helps the competent person choose the right worker-protection system.
- A trench can be in stable rock, or type A, type B or type C soil. Stable rock and type A soils are the safest. Most soils are type B. Sand and trenches with water are type C soils.
- Do not work in a trench that contains water until the competent person checks if it is safe.
- Clay can be type A, B or C soil; it depends on how much water is in the clay. Many cave-ins happen in clay although people think it looks safe.
Hazard recognition requires training and experience. The individual will have to be able to determine how various soil conditions will work with the different types of shoring or shielding systems. As in all construction work, the entire job site needs to be designed and laid out so as to provide access for workers, equipment and supplies. Room for maneuvering should be taken into consideration when the site is developed.
It's also important to provide barricades, barriers, fencing, covers, and signs as required.
When You Work In A Trench:
- You will need to use sloping, benching, shoring, or a trench box to prevent injuries or deaths.
- Keep the spoil pile two feet or more from the edge of the trench.
- Prevent materials, rocks or soil from falling into the trench; use barriers, if needed.
- A competent person should test the air as often as needed to make sure it is safe.
- Workers need to be able to get in and out of the trench easily and without creating a hazard for themselves or other workers. The placement of ladders and/or ramps is very important.
If A Trench Caves In:
- Get out of the trench. Call 911 (or emergency services). Help your co-workers from outside the trench, if you can.
- Never go into a trench that is caving in or has bad air — even to rescue coworkers. You can be killed.
NUCA Vice President of Safety George Kennedy says, "There are some basic things every company can do to ensure that employees who work in trenches are protected at all times. Start by training managers and supervisors to ensure that they are competent and capable of identifying trench hazards. Ensure that the competent person knows that he or she has the authority to take corrective action anytime a hazard is observed. Require the competent person to make inspections daily, throughout the work shift as necessary, to ensure that all hazards are eliminated or controlled. Additionally, safety audits should be conducted on a regular basis to determine if anything has been overlooked.
"Provide trench safety training to all employees who work in or around trenches. Train crews to use the options that are available to them and insist that they be used when an exposure exists. Most importantly, instruct all employees to never enter into an unprotected area no matter what the circumstances."
There are three different types of trench support systems: sloping, shielding and shoring.
Sloping or benching is slanting the ground back from an excavation. Sloping does not require special devices. This approach can provide sufficient protection for workers from a cave-in under certain circumstances. Sloping does not safeguard nearby facilities or structures. It also requires adequate space to allow for the sloping. It's not necessarily less expensive because it takes time for the site preparation and restoration after the job has been completed. You need to check OSHA regulations for when and where sloping is acceptable.
Shielding is more cost-effective than sloping and provides better worker protection. Shielding systems are composed of two heavy steel or aluminum panels that stand against trench walls and are held apart by steel cylinders at the top and bottom. However, shielding does not safeguard nearby structure. It works best in relatively shallow excavations because of the difficulty and cost of lowering the systems into trenches. Trench boxes are in fact a type of shielding. As such, they are a passive protective system that allows soil movement while the shield protects against cave-ins.
Shoring offers greater protection than shielding. Shoring systems consist of plating that is held in place with expandable braces. There are two principal kinds of shoring. Passive shoring systems are installed in prepared excavations. Examples are aluminum hydraulic jacks, screw jacks and timber shoring.
Workers can be injured while putting traditional passive shoring systems in the ground during or after excavation. These systems react to earth movement rather than preventing it. The systems do not halt soil slippage that can threaten nearby underground and surface structures. They should be used only on sites where no existing structures need to be protected.
Active shoring is installed prior to or during the excavation process and protects existing and above-ground facilities. Some examples of active shoring systems are beam and plate, slide rail, manhole brace systems, and sheet and brace systems. They are more effective than passive systems in poor soil conditions in that they prevent soil from moving and provide protection for both workers and adjacent facilities and structures.
Active shoring is more appropriate than other methods for underground construction projects that are plagued by problems such as poor soil, groundwater, deep excavations, tight spaces, route-crossing utilities, and long runs of large-diameter pipelines.
Among active shoring alternatives, slide rail systems are gaining popularity with contractors due to their cost-effectiveness. Slide rail saves contractors substantial time and money because the system — comprised of steel panels that slide into tracked rails on vertical posts — is installed and removed quickly and smoothly as the earth is removed and backfilled. Light, modular materials also make slide rail easy to transport, assemble and disassemble, lower into deep trenches, and reuse within a job site.
Obviously we have barely touched the surface on underground construction. In addition to the regulations and trench protection, there is the need to consider the type of equipment you will use for the trenching project. This includes the evaluation of trenchers, excavators and backhoe loaders, for a start. There are also a wide variety of attachments that can be used as a part of trenching operations.
You need to study the project from every aspect and consider your many options. Research the possibilities before making decisions. If you need help, there are dealers, manufacturers and, of course, the Internet.