By RON ZAWLOCKI
A variety of shoring designs for trench rescue have managed to find their way into the fire service. Few have been developed or reviewed by professional engineers, and fewer yet have been tested in their position of function (i.e., in a trench). Several of these designs have been developed on premises that are not founded on engineering principles (science).
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Designing systems to retain soil must be left to professional engineers. Firefighters have ventured “way out of their lanes” by promoting unproven shoring designs like spot shoring and thrust block systems. Following, we will explore two such designs.
Spot Shoring Design Myths
Recently, an unscientific practice has begun to permeate the fire and rescue service. The premise is that shoring without the use of panels, known as spot shoring, can be a safe practice at trench rescue incidents. There are no engineering principles or practices and no shoring manufacturer’s tabulated data that support the use of spot shoring at the unstable and dynamic soil conditions that are associated with trench rescue incidents.
Although spot shoring may be effective in limited soil conditions, those conditions cannot be determined by rapid visual or field testing means. Despite these facts, the National Fire Protection Association (NFPA) misguidedly published the following job performance requirement in NFPA 1006, Standard for Technical Rescue Personnel Professional Qualifications, 2021 edition: “12.3.7, Utilize spot shoring techniques to support soil without incorporating uprights or panels as part of the shoring plan, given a trench incident, trench rescue toolbox, tabulated data, and trench shoring plan, so that the soil is prevented from collapse.”
The spot shoring myth assumes that spot shoring will only be used when soil conditions are appropriate. For spot shoring to be considered safe for use, a highly trained and experienced member must conduct an extensive soil analysis. First responders can make a visual analysis of conditions that can rapidly and accurately determine when soil conditions for shoring without incorporating uprights or panels (spot shoring) are not safe.
(1, 2) When applied to the unstable and dynamic soil conditions found at most trench rescue incidents, spot shores and orange spray paint are equally unreliable and dangerous. (Photos courtesy of author.)
These conditions include any signs of active (moving) soil, such as trench wall collapse, bulging, heaving, flowing, sloughing, and raveling. Unfortunately, when those signs are not readily apparent, it is not possible to perform the type of soil analysis needed to rapidly and accurately determine when conditions are safe for spot shoring techniques.
Fire service personnel will never have enough experience and will never be exposed to enough varying soil types to effectively conduct the kinds of soil analysis that are needed to determine those soil types. This also leads to situations where first responders are making highly technical decisions that affect the life safety of rescuers and trapped victims without the proper training, experience, and equipment. The wrong decision can be deadly.
Spot Shoring Science
Shoring designs are required, by law, to be produced by professional engineers (PEs). Spot shoring is a shoring design. To date, we have been unable to find a single PE who has produced a spot shoring design for use in the weak and unstable soil conditions found at a trench collapse (rescue) incident. The following reasons for first responders not to use spot shoring have been compiled from PEs who are trench rescue shoring subject matter experts.
Geotechnical principles. Dr. Oliver Taylor (PE/PhD), a research geotechnical engineer with the U.S. Army Engineer Research and Development Center, conducted extensive research on spot shoring. His research spanned nearly 75 years of geotechnical academic publications and found no support for the use of spot shores in collapsed (weak/unstable) soil conditions. In fact, Taylor concluded that published and ongoing research strongly advises against using spot shores without strongbacks (uprights) or panels in trench rescue scenarios. In 2019, he said, “Geotechnical design criteria along with various State Departments of Transportation [DOTs] do not allow for the use of soil nails or spot shores for Type-C soil conditions, specifically for soils susceptible to collapse [i.e., a trench failure condition].”
Manufacturer’s tabulated data. Following a review of tabulated data from the four largest manufacturers of pneumatic struts, PE Craig Dashner concluded, “The only two manufacturers that specify tabulated data for spot shoring are Hurst/Airshore and Prospan. In their tabulated data, both of these products require the use of sheeting/paneling if in a Type C soil or if raveling is present. Since a trench wall failure would be considered raveling, the Hurst/Airshore and Prospan tabulated data essentially requires sheeting in all trench collapse cases. Paratech and ResQtec do not recommend the use of spot shores in trenches that have collapsed and have no tabulated data for that use. The use of spot shores at trench collapse incidents directly conflicts with all of the manufacturer’s tabulated data.”
Causing collapse. Research conducted by Dr. Marie LaBaw (PE/PhD) in 2009 found that introducing strut activation forces over small surface areas (spot shore bases and rails) directly to weak trench walls can cause—rather than prevent—soil failure (collapse). Struts applied directly to a trench face (spot shore) can create an additional failure and make the trench less stable than if no strut were applied. The strut alone is not sufficient to hold up an excavation wall because it only applies that resistive force over a very small area (strut base) and not the whole wall. This means that, without appropriate sheeting, there are exposed and unaccounted for loads at the trench face. That exposed force is likely to blow out (i.e., collapse) the trench wall if the soil’s internal strength is not enough to counteract it. In a trench with unstable and dynamic soil conditions, the result is likely to be progressive trench wall failure.
Strut intervals. Spot shores leave unsupported soil between the struts. Dashner says, “The soil in between the spot shores is unsupported and in weak soil conditions can collapse.” Taylor adds, “In order to create the safest possible environment for rescuer operations, trench shoring must resolve lateral earth pressure, thrust pressure, and passive pressures induced by the shores themselves. Spot shores without strongbacks and/or panels cannot support lateral earth pressure, do not satisfy equilibrium equations, and cannot resolve thrust pressures. Additionally, they leave unresolved moments at the exposed trench face where forces are not obvious or verifiable.” Lastly, Taylor says, “There is no safety code, design guidance, or sound geotechnical engineering principle to support the use of spot shoring in the manner stated in the NFPA 1006 standard.”
There are very few soil conditions that can be safely shored using “spot shores”; those soil conditions are dependent on a large number of soil characteristic properties—for example, grain size, angularity, roundness, and distribution; stiffness and rigidity on the intact soil; and environmental conditions such as saturation, temperature, material heterogeneity, and surcharge weights from spoil piles and excavation equipment and materials that cannot rapidly be determined by visual or field testing means and are beyond the ability of engineers, much less first responders, to determine those variables at a collapsed trench site.
Thrust Block Shoring Design Myths
Firefighters have been taught the following myths regarding thrust block shoring designs:
- Diagonal struts are needed to support failing soil and collapse at the inside corner.
- Diagonal struts provide support to trench wall collapse at the outside corner and the outside walls.
- The activation forces of the struts installed perpendicular to the trench walls can provide enough resistance to support the lateral soil forces being transferred to them by the angled struts used in the thrust block system.
Thrust Block Shoring Design Science
Following are the scientific truths about thrust block shoring design.
Diagonal struts are not needed to support the inside corner. The most common collapse associated with an L-shaped trench is the inside corner, which is the weakest section of the trench because it is where two unsupported soil faces (walls) meet each other. The failure begins with fissures along the surface of the ground at about a 45° angle between the two walls. Gravity then acts to create a fracture that continues from the surface down into the trench, causing the wedge of soil to collapse into the trench in an angular fashion.
Figure 1. Simple Shoring Design for Inside Corners
By using quality composite panels and engineered struts, this simple shoring design will support an inside corner failure without introducing the sliding forces (vectors) associated with angled struts.
(Figure by Craig Dashner, PE.)
Figure 2. Thrust Block Shoring Design (Not Recommended)
Figure by Craig Dashner, PE.
Figure 1 shows the vectors of a wedge failure on the inside corner of an intersecting trench. This simple shoring design captures the inside corner soil by using adjoining panels at the inside corner with panels directly across the trench (outside walls) and struts installed perpendicular to the walls. If you are using panels and struts that are strong enough to support the load, there is no need for diagonal struts to support the inside corner. Installing thrust blocks and diagonal struts to support an inside corner is a waste of time, equipment, and effort. To make matters worse, the vectors associated with any forces on diagonal struts can cause sliding of the panels on the inside corner and can result in a complete shoring system failure.
The load from the outside corner area of an L-shaped trench is simple, but when it reaches the angled “thrust block” struts, the vectors (direction and magnitude) become considerably more complicated. Figure 2 shows the load path for an outside wall failure on a “thrust block” shoring system. Angled struts and thrust blocks develop both bearing and sliding forces. When the sliding forces overcome the resistance, wales, panels, and struts slide and provide little to no resistance from collapse. Once the shoring equipment starts to slide, the friction resistance decreases significantly, meaning the wales, panels, and struts will keep sliding until the system fails.
Typically, the wales are in direct contact at their ends in the corner. However, when they are not in direct contact, the outside thrust blocks make an indirect contact between the wales, which results in the unwanted movement of both wales and the struts attached to them. Whenever a system is loaded, there is deflection. Any deflection at the end of wale #1 will push wale #2 down the wall, and any deflection of wale #2 will push wale #1 down the wall.
(3) The thrust block shoring method has several design errors that can result in system failures.
Photo 4 shows the result of the load pushing the wales from left to right. The load begins in the soil of the outside corner wall and transfers into the panels, then into wale #1. The outside thrust block then transfers the load, partially to the diagonal struts and partially to wale #2 in the directions shown by the red arrows. The wales (top and bottom) slide and travel to the right, taking the struts with them.
(4) The dislodged struts resulting from the sliding wales leave this entire wall unprotected from collapse.
The bottom line is that the activation force of the struts that are perpendicular to the trench walls and the friction coefficient cannot provide enough resistance to adequately prevent the wales and the struts themselves from sliding down the wall. To help understand why, consider that most strut activation forces are about 1,000 pounds of force. This being the case, you might guess that each perpendicular strut could resist 1,000 pounds of force coming from the side or end wall. However, the panels behind those struts contacting the trench wall have a slippery surface. They typically have a friction coefficient on soil of about 0.5, which means only half of that force from the strut activation will resist a load coming from the side. So, each perpendicular strut installed at 1,000 pounds of activation force will resist only 500 pounds of sliding force.
Since all rescue shoring systems must have at least a 2:1 safety factor, we can only count on each of those struts to resist 250 pounds of sliding force. At best, the thrust block shoring system can only resist a few hundred pounds of lateral soil force from an outside wall collapse; this is a major design flaw of the thrust block system.
Figure 3. Engineered L-Trench Shoring Design (Recommended)
Figure by Craig Dashner, PE.
Current geotechnical lateral soil force, which includes surcharged loads, can reach forces much higher than that—22,000 pounds over a 4- × 4-foot section of trench wall. Additional resistance to the sliding force on both the inside and outside corner walls is essential to the safe use of the thrust block system, but it is seldom taught or provided because it is a difficult and very time-consuming process. Resistance to those sliding forces can be resolved with a series of properly placed pickets, but pickets can only be used if the trench is wide enough (picket length and enough room to swing sledgehammers) to install them.
Unfortunately, this information is not included by most of the misinformed instructors who teach the thrust block method. When we compare the required system strength to the resistance strength of the thrust block system, it should be apparent that we must stop teaching and stop using the thrust block shoring design.
A simple solution that eliminates the sliding forces implicit with angled struts is shown in Figure 1. The capacity of that shoring design can be increased easily by shortening the length of the cantilever section of the wales, as shown in Figure 3. The capacity of this shoring system can be increased by 150% by spacing the wales at 2 feet (horizontal) intervals (Table 1). This engineered and tested shoring design with 7- × 7-inch laminated veneer lumber (LVL) wales is a “best practice” for L-shaped trenches.