By Ron Zawlocki and Craig Dashner
Based on current lateral soil force calculation developed specifically for trench rescue shoring conditions by Dr. Marie LaBaw (P.E.) and Dr. Oliver Taylor (P.E.), forces on each 4- × 4-foot half-panel area of shoring in a typical trench, 20 feet deep or less, can reach 22,000 pounds when typical surcharged loads are present.
Trench Rescue Shoring: Myths or Science? Part 1 | Part 2
To safely shore those soil forces, firefighters need panels, struts, wales, and system designs with ultimate strengths of at least 44,000 pounds of force to maintain a minimum factor of safety of 2:1. Most fire service trench rescue teams are not using the equipment and shoring techniques that can safely resist those kinds of forces.
Absent a fire service consensus on standard trench rescue shoring specifics, many firefighters have chosen to accept the use of nominal sawn lumber and plywood material to shore trenches. Nominal sawn lumber is commonly used by rescuers for wales, strongbacks, uprights, and struts. Common plywood such as ¾-inch CDX is typically used for ground pads, which is not shoring material. However, when plywood is used for panel construction, it is shoring material, which requires a factor of safety. Nominal lumber sizes commonly used for wales are 4- × 4-inch, 6- × 6-inch, and 8- × 8-inch. Nominal lumber used for uprights and strongbacks are typically 2- × 12-inch and 2- × 10-inch.
On one end of the spectrum, when used to shore the unstable and dynamic conditions found at most trench rescue incidents, these types of nominal lumbers do not have the capacities needed to provide minimum 2:1 factors of safety. On the other end of the spectrum, if we follow the Occupational Safety and Health Administration (OSHA) timber shoring guidance (1926 Subpart P App C—Timber Shoring for Trenches), the sizes of the lumber required for Type C soil such as 8- × 10-inch cross braces, 4- × 6-inch uprights, and 12- × 12-inch wales result in shoring material weight that is extremely difficult or impossible to be manually installed by rescuers.
Recently, we have seen engineered guidance purporting the use of nominal lumber as shoring material. However, closer examination of that guidance typically reveals that it is based on a soil “at rest” calculation or an application of reduced lateral soil forces resulting from soil arching theory. As is explained in Part 1 of this article (Fire Engineering, March 2023), neither of those calculations or theories is applicable to the soil conditions that are prevalent at trench rescue incidents.
Panel Myths
First, firefighters have embellished the concept and have created the myth that panels do not need to be strong and are only needed if sloughing or raveling is seen. That may be the result of misinterpreting shoring manufacturer tabulated data, which provides guidance to trench construction workers and is based on the premise that the soil is at rest during the installation of shoring. It is often stated in that tabulated data, “Sheeting has no structural value and is only needed when sloughing or raveling is occurring.” Taking that statement at face value and neglecting to apply the real-world soil conditions found at trench rescue incidents has led to a misunderstanding of importance of “close sheeting” using panels, which have the capacity needed to support unstable and active soil.
Another myth that has circulated through the fire service is that panels must have full contact with the trench walls to be effective. This concept originated from proponents of “soil arching” and “cone of pressure” theories. Although full contact between the soil and the supports is one of many requirements needed for soil arching to occur, full contact between panels and trench walls at a trench collapse site is virtually impossible and is not needed when engineered panel construction is used.
The third “panel myth” purports the installation of strut bases directly on FinnForm™ sheeting—i.e., without the use of strongbacks. Although that practice may be appropriate for “at rest” soils and trench walls without voids and undulations, it is a dangerous practice for shoring in the significantly different soil conditions found at trench rescue incidents.
Panel Science
Structural value. The safest way to shore or brace unstable and active trench walls is to install a shoring system that is capable of collecting/confining, transferring, distributing, and resisting at least two times the load of the lateral soil forces present. The component of such a shoring system that collects, confines, and distributes those loads is sheeting. Close sheeting—no gaps between panels—provides the best means of protection from collapse provided the panels are built with premium (engineered) plywood and strongbacks and are composite construction.
Confusion occurs when firefighters apply shoring guidance that has been developed for construction workers digging fresh trenches with stable and at-rest soil conditions to the typically unstable and dynamic soil conditions associated with trench rescue incidents. An example of this is found in an OSHA directive issued in 1993, which states that two layers of ¾-inch-thick plywood can be used to hold back raveling when used with an aluminum hydraulic wale and that the two layers of plywood cannot be used as a substitute for any primary load carrying structural component because even two layers of nominal plywood do not have the strength capacity to resist typical lateral soil forces associated with active soil conditions. Firefighters have latched onto the part of the OSHA guidance that states, “Two layers of ¾-inch-thick plywood can be used” and have applied that to shoring soil conditions that are not included in the full statement.
What is not sufficiently understood is that, although two layers of ¾-inch plywood may have the capacity needed to contain the minimal soil force of raveled soil, when the soil has failed and collapses or when failure is likely, the soil forces on the sheeting are significantly greater, which requires engineered panels to safely support moving sections of trench walls and for those conditions panels that are required components of trench rescue shoring systems. These engineered panels are composite constructions that have the capacities needed to support collapsing trench walls. It is important for us to understand that the factor of safety that is needed for shoring “worst-case” soil cannot be achieved with nominal plywood, even with two layers, and nominal lumber used for strongbacks (Table 1). Not only is the quality of the sheeting and strongback material important, but the manner in which they are attached to each other is extremely important. Panels typically used by firefighters are constructed with sheeting such as common plywood or FinnForm and connected to strongbacks with bolts or nails. When this type of panel construction is used, the plywood and the strongbacks act independently, causing them to bend at different rates when exposed to lateral soil forces. This causes the strongback and sheeting to break at different bending moments. These panels, called “noncomposite construction,” although capable of holding back small amounts of sloughing or raveling soil, are not capable of resisting the forces of dynamic soil conditions such as collapse.
When strong and ductile sheeting such as a minimum of ¾-inch Finnform is properly glued to strongbacks, it combines the two members into one new member with considerably more strength. These types of panels, called “composite construction,” are much stronger than just the sum of the strength of the individual parts. With composite panels built with engineered sheeting like FinnForm and engineered strongbacks such as LVL, the structural value or capacity becomes high enough to safely resist the lateral soil forces associated with trench collapse conditions. The sheeting is a large part of the overall capacity of a panel assembly and serves an important function to “collect and confine the load,” which is failing soil.
Table 1 is an engineered comparative analysis of common trench rescue panel construction methods based on the 4-foot horizontal and 4-foot vertical strut spacing commonly used for trench rescue shoring.
Panel-to-trench wall contact. When trench walls have an even and regular surface that is free from perceptible projections, lumps, or indentations; full or near-full contact between panels and the trench wall is possible. Freshly cut trenches in stable soil provide the best chance of having walls with even and regular surfaces. The shoring guidance found in strut manufacturer tabulated data assumes that construction workers are installing shoring shortly after or during the digging process, which allows for the installation of panels on trench walls before soil failures begin to occur. Panels installed on stable trench walls with even and regular surfaces can allow for full or near-full surface contact. In those cases, the strength of the panel may only have to be enough to resist small soil failures, such as sloughing and raveling, from the trench face, so plywood sheeting without strongbacks may be sufficient. However, rescuers rarely encounter those trench wall conditions, and in rescue situations panels typically do not have full contact with the trench walls.
The indentations, voids, projections, and lumps on trench walls that are common at trench collapse rescue incidents prevent full and sometimes even good panel contact. Those indentations, voids, projections, and lumps can result in unwanted soil movement behind the panel and increased lateral soil forces. Since there is no timely method to smooth the trench walls out, and since backfilling techniques rarely result in full contact between the panels and the trench walls, the only practical solution comes from very strong panel construction. Using panels with capacities that are only strong enough to resist sloughing, raveling, and other small soil failures is a dangerous proposition. As seen in Table 1, composite panels constructed with ¾-inch FinnForm and 2- × 12-inch LVL strongbacks have the strength to safely resist the potential soil forces associated with the most common trench and excavation rescues, which occur at depths less than 20 feet.
Panels without strongbacks. In the unrealistic world of full surface or panel-to-trench wall contact, panels without strongbacks might suffice. However, the uneven and irregular trench wall surfaces and the large lateral soil forces associated with trench collapse conditions require panels to have significant structural value. A sheet of ¾-inch FinnForm without a strongback and with struts installed at 4-foot spacing has a safe working load of 1,900 pounds of force. However, a sheet of ¾-inch FinnForm with a 2- × 12-inch LVL strongback (composite construction) has a safe working load of 24,300 pounds of force. Which one are you taking to your next trench rescue? The strength of a bare (without strongback) FinnForm panel is essentially negligible. The Michigan Urban Search and Rescue (MUSAR) Training Foundation’s destructive testing has shown strut bases will punch through FinnForm without strongbacks with forces that are within the range of the soil forces expected at trench rescue incidents. Using FinnForm sheeting without strongbacks at the unstable and dynamic soil conditions associated with trench rescue conditions is a shoring myth and a dangerous practice.
Noncomposite panels. This describes any combination of sheeting and strongbacks that is not glued together with proper adhesive. Noncomposite panels include sheeting and strongbacks that are bolted, nailed, or simply placed on top of each other. Mechanical connectors like nails, screws, and bolts hold the pieces together, but they are not sufficient to prevent the slip between the two pieces that are necessary to significantly increase strength.
Composite panels. Construction adhesives, properly designed and applied, can provide sufficient capacity against slips between individual members and make the two pieces of wood act like a single piece with much more strength than the two pieces. Gluing them together and preventing the slip between them under bending forces causes the two pieces to work together. Engineers call this composite construction. We recommend 1¾- × 12-inch laminated veneer lumber, with a minimum bending stress of 3,100 pounds per square inch (psi) for strongbacks and 4- × 8-foot sheets of 14-ply laminated Arctic birch FinnForm for the sheeting panel. Having both the panel and strongback made of engineered laminates adds strength and longevity and provides a much more consistent breaking strength from panel to panel in your cache. The area on the FinnForm where the strongback will be attached must have the phenolic coating removed with sandpaper on a belt sander for the glue to adhere properly. Use a heavy-duty exterior construction grade adhesive that has an ultimate strength of at least 4,000 psi. We have used Titebond III adhesive with very good results. The Finnform is then screwed to the LVL to hold the strongback tight to the sheeting while the glue cures.
Wale Basics
Wales are commonly used in trench shoring like headers are used in structural collapse shoring. For trench rescue shoring, common spans between supports such as struts on wales are 8 feet. Spans of up to 12 feet are used on occasion but are much less common. Wales are beams, and the capacity of beams depends on the size of the beam, the material’s bending strength, the way the supports are connected to the beam, and whether the load is distributed or concentrated. A “simply supported” beam allows the beam ends to rotate as the beam is loaded and bends between the supports. A simply supported beam essentially just sits on the supports. On the other hand, a “fixed beam” does not allow the beam ends to rotate as the beam is loaded. This changes how the beam can bend and increases the strength of the beam. Examples of fixed beams include beams welded to columns at steel construction buildings and beams cast monolithically with columns at concrete construction buildings.
Fixed beams have significantly more capacity and strength than simply supported beams. Although we can’t weld the connections points of wales and struts, we can do some things to make the wale/strut interface act more like a fixed connection and can help distribute rather than concentrate the load. By maintaining a minimum wale overhang that goes at least 12 inches past the supporting struts, and by using wood fillers to fill the gaps between the sheeting and the wale at the overhanging ends, a fixed-end behavior can be attained. Additionally, applying equal strut activation forces (1,000-1,250 pounds of force) to the struts on both ends of the wale reduces sliding and beam rotation, which helps our connections act more like fixed rather than simply supported connections. The best practices for the use of wales at a trench rescue incident include using the strongest wales that can be manually installed (Table 2), distributing the load across the wale, and obtaining fixed beam behavior at the supports or struts.
Wale Myths
Firefighters have used 6- × 6-inch sawn lumber for wales for many years without knowing the capacity of a 6- × 6-inch when used as a beam or wale. Firefighters have also used 6- × 6-inch and sometimes 4- × 4-inch sawn lumber with 8 feet or greater spans without any consideration of the loads that the soil could place on the wales (beams) and without knowing the beam capacities and safe working load limits.
The most common use of a wale by rescue teams consists of three-panel sets with wales spaced 4 feet on center vertically and with struts spaced 8 feet on center horizontally.
In this configuration, the wales only contact the strongbacks; this means that, in the area between the strut supports, there is only one point of contact, which is concentrated at the wale/strongback interface at the center of the wale. Because of firefighters’ lack of understanding of the importance of distributing a load across a wale, they continue to do this to improve the beam’s capacity. Placing the load on the wale in a concentrated area is not a best practice, but it continues to be done that way because, “That’s the way we’ve always done it.”
Wale Science
The safe working load of a 6- × 6-inch sawn lumber wale with an 8-foot span between struts is only 1,870 pounds of force when the load is concentrated. That is only enough capacity to safely support a very small (e.g., 1.5- × 4- × 4-foot) soil failure, which could create about 1,650 pounds of lateral force. A 4- × 4-inch sawn lumber wale with an 8-foot span between struts can support only 660 pounds of force. The size of the soil failure that a 4- × 4-inch wale can support is so small that our PEs declined to include it in the tabulated data found in the MUSAR Trench Shoring Operations Guide.
The capacity of sawn lumber wales will vary dramatically based on the lumber age and condition such as large knots, loose knots, splitting, checking, twisting, and so on (see Table 2 lumber note). Testing has shown some members break at less than half the expected capacity. The rated capacities of engineered lumber such as laminated veneer lumber and extruded aluminum wales are much more reliable than sawn lumber. Most importantly, even when they are in the best possible condition, with optimum loading that is distributed and has fixed beam behavior, 4- × 4-inch and 6- × 6-inch sawn lumber beams and wales do not provide enough strength (capacity) to safely support the range of lateral soil forces associated with most trench rescue soil conditions.
If the only point of contact between supports or struts is a 2- × 12-inch strongback, the load (i.e., lateral soil forces) pushing from behind the panel will create a concentrated load on the wale. The red areas on the wales in photo 1 show the concentrated loads that are placed on the wales when there is soil movement behind the center panel in a typical wale shoring design with 8-foot spans between struts. A wale with a distributed load has about 33% more capacity than the same wale with a concentrated load. Placing the load on a wale exclusively at the strongback/wale interface is a common but potentially dangerous practice. Photo 2 shows a simple and rapid technique that improves the distribution of the load, which increases the safe working loads of all wales. The technique simply fills in the gaps between the wales and the FinnForm with 2× lumber material. This method changes the end support conditions and increases the capacity. Table 2 shows the differences in the safe working loads of wales with concentrated loads and distributed loads in their position of function.
(1) Concentrated loads on wales. (Photos by Ron Zawlocki.)
(2) Wood spacers help distribute the load and increase the wale’s safe working load.
Trench rescue is a low-frequency/high-risk event. Firefighters and their families are entitled to the enhanced protection that premium shoring materials and equipment can provide. Firefighters should no longer accept the use of low-quality, nominal “big box store” lumber for shoring trenches. The investment of a little more time for proper construction and a little more money for premium materials can pay big dividends in firefighter safety and equipment longevity.
Almost two decades ago, Professional Engineering published guidance for shoring collapsed structures. That structural collapse shoring guidance developed by the Federal Emergency Management Agency and the United States Army Corps of Engineers has become “the consensus standard.” The shoring materials and designs found in that guide are significantly different than the OSHA shoring guidance used during the construction of buildings because the scope and purpose of rescue shoring are significantly different than construction shoring. The structural collapse shoring guidance was developed using engineering principles and has been validated destructive testing results. Unfortunately, such a consensus standard for trench and excavation rescue operations is still not available.
In her 2009 thesis, LaBaw wrote, “There has traditionally been a lack of engineering analysis of technical rescue systems including trench rescue shoring. Being a derivative of the fire service, the technical rescue industry has trained most personnel to make do with available tools. Prior to current value engineering trends in most construction fields, this tactic was typically effective. Engineering analysis is becoming increasingly important to evaluate existing rescue system effectiveness in the face of less redundant systems used by the construction industry at large. As education and technology increasingly pervade the technical rescue industry and the fire service, the potential to research, evaluate, and recommend changes to existing rescue systems is becoming a reality.”
The lack of an engineered shoring standard for trench and excavation rescue incidents has resulted in the use of shoring materials and shoring designs that are not based on engineering principles and testing. These are the trench rescue shoring myths that have spread throughout the fire service. Firefighters should no longer accept the idea that they should “make do” with available tools and nominal shoring materials. Firefighters should also no longer accept shoring designs and tabulated data that have not been specifically engineered for the unstable and dynamic soil conditions that they face at trench rescue emergency incidents. Fortunately, such engineered tabulated data is now available in the MUSAR Trench Rescue Shoring Operations Guide.
RON ZAWLOCKI is a 49-year fire/rescue service veteran. He has responded to several trench and excavation rescue incidents and has shored more than 1,000 live trenches across the United States. Zawlocki has pioneered destructive testing methods for trench rescue shoring equipment and designs and has used the test result along with professional engineering guidance to provide safe and efficient shoring methods to trench rescuers around the world.
CRAIG DASHNER, PE, is the lead structures specialist with MI-TF1, which resulted in Trench Rescue Shoring (TRS) involvement resulting in many changes to best practices in TRS and new methods for shoring complex excavations and removal of historic shoring methods that were found to be unsafe. He has deployed on complex trench collapse incidents and search and rescue incidents.