In a recent article, we addressed a picture sent in by an installer where he or an employee was shown in a trench excavation where the sidewalls extended above his head without any visible protection such as shoring, sloping or means of escape (ladder). The rationale of the installer was that the soils were cohesive enough that he did not have to provide protection; so we reviewed the Occupational Safety and Health Administration soil classification system and the required sloping protections for the different classifications.
About the time the column was slated to run, we read about a sewer crew working in an unprotected trench that collapsed, killing all three of them. These types of accidents are totally preventable with a little understanding about what can go wrong when excavating in soils and planning to avoid the problems.
When we in the industry talk about soil, we refer to naturally occurring material that is made up of some mixture of sand, silt and clay particles. We use the Department of Agriculture classification system to describe these mixtures because it best reflects the ability of the soil to accept water. OSHA places these materials into three classes instead of 12 based on their strength and cohesiveness. Soil conditions can vary from one site to the next and even within a site. Soil may be loose or partially cemented, organic or inorganic. These characteristics need to be evaluated at each site before excavation begins.
MOST COMMON FAILURES
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. Sliding soil failure can occur in all types of excavations including sloped trenches, but when safety precautions are taken, employees can be protected. This is one reason why OSHA looks for the “competent” person on site who makes sure safety precautions are in place.
Soil stresses and deformations can occur in an open cut or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation. Look for tension cracks in the soil away from the trench edge. These cracks usually form at a horizontal distance of one-half to three-quarters times the depth of the trench, measured from the top of the vertical face of the trench. Soil from between these cracks and the edge of the excavation slides into the trench void. Similarly, the soil at the top edge of the trench along the tension crack line can topple into excavation.
The installer should look for these cracks while the excavation is proceeding. It is part of the reason why spoil piles from the excavation need to be more than 2 feet away from the edge of the trench. It is hard to see the cracks if they are covered by the spoil material. The other reason, of course, is to make sure the spoil material does not fall back into the trench.
In some cases, an unsupported excavation can create an unbalanced stress in the soil. This causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench. Think of it as the trench wall blowing out due to the stress.
There are a few additional conditions that can occur in onsite work, but they are less common given our typical excavation depths. They are bottom-heaving and boiling. Bottom-heaving is caused by downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the excavation. Heaving can occur even when shoring has been properly installed. If this condition is observed, the trench should be vacated until additional safety measures such as sloping are in place.
SOIL IS HEAVY
Boiling occurs where the excavation extends below a water table and water “boils” upward into the bottom of the excavation, creating a very unstable condition even with shoring. Since we certainly should keep our systems above the water table, this should not be a condition that we would see a lot. Although it could happen where tanks or parts of the piping need to be installed at elevations at or near water table levels.
A sobering thought to remember: Soil is heavy! Depending on moisture content, 1 cubic foot of soil can weigh 110 to 140 pounds. When soil falls on you, if you are even partially buried, it is difficult to get yourself or someone else out! In addition, if you are buried, it is impossible to breath; time available to rescue someone is very short.
A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which sliding will not occur. The unique mixtures of the different types of soil (sand, clay, silt and rock) necessitate different safe slopes for every excavation site. This is why OSHA developed the three-soil-type classification system with different sloping criteria.
Other complicating factors can result in sliding soil failures. During an excavation, visibly different layers of soil may be uncovered. For instance, each of those layers may call for different safe slopes. It is essential to plan your excavation around the most gradual (rather than steepest) safe slope for different soil types and layers encountered during the excavation.
We will explore other complicating factors and soil conditions in next month’s column.
