Cave-ins pose the greatest risk of injury or death when trenching or excavating. Other hazards include striking utilities, being struck by vehicles, or being exposed to hazardous or toxic gases and vapors. For this discussion, trenches are included in references made to excavations. One cubic yard of soil can weigh as much as a car. An unprotected excavation can be an early grave.
What is the typical point of failure in an excavation? Soil failure is defined as the collapse of part or all of an excavation wall. The most common soil failure is typically described as an unexpected settlement, or cave-in, of an excavation. Soil sliding is the most common factor leading to soil failure.
Preventing excavation collapse is a simple matter of physics, since excavation walls want to collapse. When they do, it happens quickly (soil can move as fast as 25 feet per second) and the results can be fatal or, if you survive, result in potentially irreversible internal damage to body parts. It doesn’t take much soil to trap and crush a worker, which is why it is important that you take the proper precautions during all excavations – especially for those situations that require an excavation to be deeper than 5 feet. At 5 feet of depth, a protective system is required by law! At shallower depths, it is up to the Competent Person.General Excavation Rules
- Know where underground utilities are located. Connecticut along with other states require the location and identification of all underground utilities (electric, gas, water, etc.) prior to commencing with excavation operations (in Connecticut it is “Call Before You Dig” – call 811).
- Know your minimum approach distance (MAD)to above ground utilities to avoid electrocution. The electric utility is your best friend and they will provide you with safe distances for you to maintain. Use qualified operators and spotters!
- Control the work area around the excavation operations to protect workers and the public. Workers need to stay clear of striking and struck-by hazards and out of the equipment operator’s blind spots or exclusion zones. Traffic control also includes vehicles driving by your work site and pedestrians that may be walking nearby. Consult the Manual of Uniformed Traffic Devices (MUTCD) requirements.
- Plan the work and work the plan. Establish a Site Safety Plan for the work and conduct daily briefings before work commences and during the day if there are changes to workers, equipment, means and methods or conditions.
- Plan for emergencies including rescue. We don’t want to deal with emergencies, but if we do, we want to have an Emergency Action Plan already in place that we can follow. This includes arrangements with emergency services such as police, fire, ambulance and CT DEEP for environmental releases. Consider who else you may need to call for emergencies involving utilities or for situations that occur beyond your capability or jurisdiction (intruders, threats by others, other contractors working too close to you).
- Keep the Surface Clean - Excavation collapse occurs when the trench walls can no longer contain the large amount of pressure put on them by the surrounding soil. While this can be a problem at any depth, it is made worse when excavated materials are piled at the edge of the trench. To reduce some of the pressure put on excavation walls, keep heavy equipment away from excavation edges.
- Keep surcharge loads at least 2 feet (0.6 meters) from excavation edges.
- Test for low oxygen, hazardous and toxic gases and vapors. Remember to test every 4 feet of vertical depth first. Some hazardous and toxic gases and vapors are heavier than air and be at lower depths than others.
- Make sure a Competent Person is present to inspect excavations at the start of each shift and at any other time when conditions of the soil or environment change from such events as weather or nearby activities causing vibration. Some companies require a trench permit to be completed and approved before commencing excavation operations.
- Pile all excavated materials at least 2 feet back from the edge of the excavation. If there is not enough room to allow at least 2 feet, remove excavated materials from the immediate location.
- Do not work around the edge of the excavation when others are below.
- Keep equipment away from the edge of the excavation. Not only can it cause cave-ins but there is also a chance that it could fall on those working below. Slope for Stability - Another way to reduce the pressure put on excavation walls is to use a sloping or benching system.
- Sloped Walls – When digging into disturbed material such as backfill or into sandy soil, a 34- degree slope should be used when digging to prevent a section near the top from giving out and burying the bottom of the trench.
- Benching – When there is enough space available, and the soil exhibits an unconfined cohesive strength greater than .5 tons per foot (type B soil) benching allows a trench to be dug in a series of steps that slowly descend to the deepest point.
- Reinforce excavation walls once a trench has been dug. The walls should be braced in a way that will protect those working in the area if a cave-in does occur. Construct a support system (shoring system) made with posts, beans, shores or planking and hydraulic jacks. Consult the OSHA Trenching and Excavation Standard (1926 Subpart P) tabulated data in the Appendices for use in depths up to 20 feet. Systems can be purchased as engineered systems as well. These systems are engineered for use up to specific depths based on the soil type (A, B, or C). Use of shoring systems and other methods of protection (trench shields and sloping) at depths greater than 20 feet require site-specific designed system by an engineer registered within the state you are installing such protective measures.
- Trench Shields - A trench shield can be used as a convenient alternative to building a support or shoring system directly into an excavation. However, for the trench shield to be used to provide the proper protection, it must be used properly – check the manufacturer’s tabulated data for that specific information. Always place the trench shield or shoring system before entering the trench. Follow the manufacturer’s installation instructions and never allow anyone in the excavation before the trench shield or shoring system is installed.
- Never excavate more than 2 feet past the bottom of the support system.
- Enter directly into the protective system and exit the same way. Use a ladder or ramp. For ladders, they need to be in place at depths of 4 feet or more and extend 3 feet above grade.
- Make sure there is always a safe exit route within 25 feet of where you are working in the excavation. The exit route must be protected and within the protective system you are working in. It doesn’t make much sense to leave a protective system and use a ladder that is not within the protective system.
- Never move the trench shield while workers are in the excavation.
- Never perform work in the excavation outside of the protective system.
- Secure the work area at the end of the work day so pedestrian and vehicular traffic are not exposed to slip, trip or fall hazards.
- Go home the way you came to work!
Michael Ziskin is President and CEO of Field Safety Corporation, North Branford, CT