Trench Rescue: Understanding Soil

By OLIVER-Denzil TAYLOR, S. MARIE LaBAW, and RON ZAWLOCKI

There are volumes of books, research papers, and articles written on the topic of soil mechanics and construction-based trench and excavation shoring. However, there is very little information available specific to rescuers.

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For the past four decades, the fire service has relied on the Occupational Safety and Health Administration (OSHA) and shoring manufacturers for guidance. The problem is that neither OSHA nor the shoring manufacturers adequately address the soil conditions and forces that first responders encounter at most trench rescue incidents. Although firefighters don’t need to be geotechnical engineers to properly shore a trench for rescue, a working knowledge of current research in soil mechanics and trench collapse best-practice shoring requirements can help us avoid dangerous mistakes. This article will explore soil theory for rescuers and trench rescue shoring requirements.

Soil Theory for Rescuers

One of the most important pieces of soil-related information that rescuers need to know is how to determine the amount of force that their shoring system must confine and support to protect themselves and any trapped victims. The theories and soil pressure diagrams used by civil engineers, shoring manufacturers, and OSHA to determine the theoretical lateral earth pressure in soil retention systems do not accurately account for the soil forces generated by an active soil failure in a shallow trench (less than 20 feet deep).

A simple and accurate method for determining earth pressure is now available for use at trench rescue incidents based on actual case histories and laboratory investigations into unconstrained soils (trench walls). The soil theories used by OSHA, shoring manufacturers, and many civil engineers do not account for the soil conditions commonly encountered at trench rescue incidents. Note that the tabulated data associated with those theories should not be used by first responders.

In 2017, a group of urban search and rescue (USAR) engineers began looking into all aspects of trench rescue operations. The initial findings were that the absence of geotechnical and structural engineering principles has resulted in haphazard trench rescue shoring practices that, in some cases, can actually increase the dangers to the rescuers and the trapped victims. The group started by developing a method for first responders to rapidly and accurately determine soil forces specific to a trench collapse. Group members Dr. Oliver Taylor (P.E.) and Dr. Marie LaBaw (P.E.) used modern geotechnical principles to develop a forensic-based lateral earth pressure determination called the “T-L method” that accurately represents actual earth pressures at trench collapses.¹

Photo 1 shows three active soil failures at one trench. All things being equal, the failure having the largest mass (cubic feet) will exert the most force on the shoring system. Rather than expecting first responders to analyze and classify soil to determine different lateral earth pressures, Taylor and LaBaw used relevant soil property parameters to determine a true worst-case active earth pressure. The extent to which worst-case active earth pressure can exist was determined after conducting 400,000 simulations using unsaturated soil mechanics methods. With those worst-case forces applied, first responders with minimal training (four hours) and minimal equipment (a tape measure and the MUSAR Trench Rescue Shoring Operations Guide) are now able to rapidly and accurately determine maximum soil forces and the load that a shoring system must support. The T-L method has been peer reviewed by the professional civil engineering community and validated with laboratory experiments and multiple real-world case studies.

The results of the T-L method research concluded that worst-case soil exerts 68.75 pounds per cubic foot (pcf). It is common practice for rescuers to install 4- × 8-foot panels with two struts. Using that configuration, each strut supports a 4- × 4-foot area of trench wall behind each “half panel.” When 68.75 pcf is applied to that 4- × 4-foot (16 square-foot) area, we get a force of 1,100 pounds for each 16 cubic feet of soil.

Figures 1 and 2 show how the T-L Method determines the force of a failing section of soil that is being supported by panels and struts. Each photo shows these failures (in these cases, the fissures or tension cracks). The “SL” designations are abbreviations for the term “Simple L,” which is the distance measured in feet farthest from failure extent to the trench face. Note that, if no surcharge is present, the Simple L equals the total L or “L.” The drawings show the number of cubic feet contained in the failures that are pushing on the shoring system.

Photo 2 shows Simple L measurements on a real trench. Simple L is always the measure of the largest failure. In this case, although the trench has four different failures, the farthest distance from the trench face to a tension crack is 3 feet 4 inches. Always round up; in this case, Total L is 4, or L-4.

The second order of business for the USAR engineers was developing tabulated data for shoring equipment that is specifically designed for soil conditions found at trench rescue incidents. Using the forensic-based T-L method, group members Craig Dashner (P.E.), George “Donnie” Barrier (P.E.), and Dr. Janos Gergley (P.E.) tabulated data that is uniquely designed for trench rescue shoring operations. The design loads provide a minimum 2:1 factor of safety for the worst-case active soil forces. To make things easier for first responders, the factors of safety are always included in the tabulated data.

Using the tabulated data from page 14 of the MUSAR Trench Rescue Shoring Operations Guide, 5th edition (Table 1), we can determine which panel and strongback configurations will safely support a determined Simple L. For the L-3 trench in Figure 2, all panel assemblies listed, except the non-composite ¾-inch CDX plywood with 2- × 12-inch wood strong-backs, are strong enough to support this small load (3,300 loads per 4- × 4-foot section of shoring).

When heavy objects or piles of soil are close to the trench lip and/or within 12 feet of the shored area, they can cause additional pressure on the shoring system; those loads are called “surcharge loads.” Typical surcharge loads can include spoil piles, pipes, and heavy equipment such as excavators, loaders, and backhoes. The additional force from surcharge loads must be accounted for in the selection of the shoring equipment. The T-L method allows first responders to safely determine and add surcharges (ScL) to the Simple L (SL) to determine the Total L (L).

Surcharge L (ScL). Additional surface loads that are within the area between the original trench faces and the farthest point of soil failure (Simple L) measured in feet perpendicular to the trench wall.

Total L (L). The Simple L (SL) plus the Surcharge L (ScL) if one exists.

In Figure 4, the spoil pile is three feet from the trench face (wall). The Simple L (the farthest extent of the failure) is seven feet (SL-7) from the trench face. Therefore, four feet of the spoil pile is within the Simple L, and that load must be added to the SL-7. To find out how much additional force this will exert on the shoring, look at the surcharge chart (Table 2). Under the SPOIL column, four feet within the Simple L and right next to that in the Add to SL column we see that we need to add 2 to the SL-7. In this case, the Total L is 9 (L-9). To determine an equipment surcharge load, we repeat this process using the “Equipment” column in Table 2.

For the much less likely trench rescue incidents that do not involve any type of soil failure, an engineered conversion chart (Table 3) has been developed. The chart uses a correlation between the trench depth and the farthest extent of potential soil failures.

In Figure 5, there is no evidence of a soil failure. To determine the converted Simple L (SL), we need to measure the depth of the trench and then refer to Table 3. In this case, we find the trench depth (10 feet) in the “Trench Depth” column and follow that row to the Simple L Equivalent column where we find that the Simple L is 7 (SL-7). If there is no surcharge load, the Total L is 7 (L-7). If there is a surcharge load within 7 feet of the trench wall, then we would use Simple L-7 and repeat the process shown in the example above.

By using the existing soil failure evidence and the worst-case soil properties that are incorporated in the T-L Method, an accurate and reliable soil force can be determined. To make that determination, first responders only need to determine the Total L and then refer to the associated tabulated data to select the shoring designs and equipment needed to efficiently make the rescue area safe. The tabulated data included in the MUSAR Trench Rescue Shoring Operations Guide includes the items listed in the Table of Contents (Figure 6).

Trench Rescue Shoring Requirements

There is much confusion in the fire service generated from a lack of engineering principles designed for shoring soil failures and subsequent collapse conditions. Some strut manufacturers have added to that confusion by giving guidance for shoring soil that is “at rest.” In that guidance, it is usually mentioned that panels are only needed to confine the soil if signs of soil movement are present. They typically mention only two—sloughing and raveling—of several possible signs of soil movement. They also fail to mention other signs such as fissures, bulging, and subsidence and how arriving on scene to a collapse is a clear indication that the soil must be considered dynamic and unstable. As a result, firefighters have been taught to use shoring equipment that may support stable soil but cannot confine and support a significant soil failure.

Effective trench rescue teams do not carry a variety of shoring equipment or use different shoring techniques based on soil types. Instead, effective teams use equipment and techniques designed to handle worst-case soil conditions and learn how to apply it to all incidents. The goal of every trench rescue team should be to become proficient at installing shoring equipment in all typical trench shapes, depths, and widths with all the common soil failure void patterns. That goal can be achieved and begins with the following guidance.

To ensure that victims and rescuers are protected from progressive collapse, the trench rescue shoring system must use equipment that will collect/confine, transfer, distribute, and resist a minimum of twice the potential soil forces of worst-case soil conditions.

Collect/confine loads. The only way to ensure that the soil will be confined is to always use strong panels and close sheeting (panel edges touching or overlapping).