Three Important Excavation Safety Concepts

For several years now there have been a variety of “off the shelf” shoring systems readily available that have proven to be more cost effective, easier to use, and more efficient than timber shoring. As a result, across the industry there has been the understandable decline in timber shoring usage as well as the time spent studying Appendix C. However, in Appendix C there are some valuable excavation safety concepts. This article will address three of those important concepts. They are the “adjacent area”, sloping to reduce the effective depth of the excavation, and the lateral earth pressure represented by OSHA’s Type A, B, and C soils.

The “adjacent area” is the area out from the edge of the excavation that is equal to the depth. So if an excavation is 10 feet deep, the adjacent area would be the area out from the lip (edge) a distance of 10 feet. This is also sometimes referred to as the “affected zone” or “area of influence”. There are two major reasons why this area is significant. One has to do with surcharge load limitations, the other has to do with structures that are positioned in this area. Today we’ll address surcharge loading.

The reason that surcharge loads are an important issue is that the loads in the adjacent area contribute to lateral earth pressure applied to a protective system, and enough pressure could cause any protective system to fail. The strength of a timber shoring system is primarily based on the size and the spacing of the shoring members. If there is too much of a surcharge load in the adjacent area, the size and spacing of the shoring members as specified in the tables in Appendix C would not be considered adequate. The surcharge load limits (in the adjacent area) found in Appendix C are the equivalent of a two-foot soil surcharge or 20,000 lbs.

Important to note that the surcharge load limits of a manufactured system such as a trench shield or hydraulic shoring could be found in manufacturer’s tabulated data or by contacting the manufacturer. Those load limits may or may not be the same as the loading limitations found for timber shoring in Appendix C.

The point is that the load limits specified in Appendix C apply only to Appendix C. OSHA’s limitations for Appendix C does not establish surcharge load limits for the industry. Some manufacturers have set spoil pile limits in the adjacent area at three feet high and they go with a 36,000 lb. weight limit.

The second issue we are discussing involves sloping above a protective system. Simply put, when we use basic sloping configurations, such as those outlined in Appendix B, above a protective system it does not reduce the effective depth of the excavation. In other words, standard sloping does not reduce the pressure on the protective system. In order to slope enough soil away to reduce the pressure on a protective system, it must be sloped on a ratio of 3:1.

As an example, if an eight-foot tall shield is positioned at the bottom of a 12’ deep trench, we would have a couple of options on how to treat the upper unprotected four feet. One option is to stack another shield on top of it, the other is to slope the upper portion of the excavation.

Either way we can expect the shield placed at the bottom to experience the earth pressure equivalent to 12’ deep. And we emphasize that is the case even if we slope the upper portion of the trench, starting 18” below the top of the shield. In this scenario the soil pressure is still calculated to be 12’ deep. Yes, it’s still considered 12’ deep even though some soil has been sloped away from the top.

The only way we can slope the upper portion and reduce the soil pressure is to slope the upper portion at a ratio of 3:1, which would be a lot of soil. Using the above example, an eight-foot tall shield in a 12’ deep trench would have four feet of soil above the shield. If we drop down 18 inches below the top of the trench shield to begin our slope, it would mean we would have to slope out 16.5 feet on each side of the shield to reduce the pressure. We arrive at the 16.5 feet number by taking the four feet above the shield, add 18 inches below the top of the shield which gives us five and a half feet. Multiply that by three and we get 16.5 feet.

The point is that in our scenario above, the only way we can reduce the effective depth is to slope 3:1 above the protective system. The standard sloping guidelines found in Appendix B will not be enough to reduce the effective depth of the excavation.

Someone might wonder why that concept would be found in Appendix C. Good question. The answer is that as we dig deeper the soil pressure increases and therefore the size of the shoring members specified in Appendix C are larger and the spacing between the shoring members is reduced. And someone excavating might come up with the idea that by sloping the upper portion it would reduce the effective depth of the excavation and therefore the size and the spacing of the shoring would be the same as if it was a shallower excavation. OSHA says no, unless that upper portion is sloped 3:1.

The third concept we are discussing has to do with the lateral earth pressure associated with the different soil classifications. In Appendix C we find that Type A soil is described as having 25psf, Type B soil 45psf, and Type C soil 80psf. It is interesting that this is the only place in the excavation standard where one can find those numbers.

It is noteworthy that the soil that is described in Appendix A as Type C is literally anything that does not meet the requirements and is not good enough for Type B all the way down to a flowing or submerged Type C soil. That is a very wide range of soil stability.

That is why the manufacturers of protective systems have a designation for C-60 soil. Even though C-60 soil is not as stable as a Type B, it is generally defined by the manufacturers as a soil that is not flowing and is not submerged. What is more, C-60 soil can be excavated with vertical sides and will stand unsupported long enough that a shoring or shield system can be installed. In other words, the C-60 soil designation helps us to separate the wide range of Type C soils into two categories, better Type C and worse Type C. What makes C-60 better is that it will stand unsupported when excavated, which allows for the installation of protective systems such as vertical shores. Additionally, if the excavation doesn’t have water issues the soil will be more stable than if it is freely seeping water.

The competent person should review the manufacturer’s tabulated data for the exact C-60 soil definition when using it with their equipment to make sure nothing is overlooked. The manufacturer may require the competent person to monitor the excavation to make sure the soil stability does not deteriorate due to water or any other factors.

One final point is that since Appendix C does not have an allowance for C-60 soil, that explains why the timber shoring requirements for Type C is so much different from Type B. The differences in soil stability explain such things as the difference in the requirements for the upright spacing between the two soil types, and also the “tight sheeting” versus “close sheeting” discussion.

One more interesting point to make about Appendix C, if you are still reading this, is that it contains the only reference in Subpart P to a protective system called “soil freezing”. Now that is a very cool topic. Sorry, I couldn’t resist.

For more information about excavation safety I refer the reader to “Trench and Excavation Safety by the Book”. It can be found at www.trenchandexcavationsafety.com