Training: Spot shoring
Spot shoring has gained popularity with some trench rescue teams and is being taught by a few technical rescue training organisations. The current draft of the National Fire Protection Association (NFPA) 1006 Standard (2021 edition) includes the use of spot shoring. Several civil engineers who are involved with the Federal Emergency Management Agency (FEMA)/US Army Corps of Engineers (USACE) Structure Specialists group were asked to examine and comment on the use of spot shoring. None of them would endorse or condone the use of spot shoring for rescue operations. Additionally, in a recent survey conducted, none of the manufacturers of pneumatic struts would condone the use of spot shores at trenches that have collapsed or are potentially unstable. Spot shoring is loosely based on a theory that is best used for soil retention systems in very firm and stable soil conditions.
Fire fighters are not trained and equipped to accurately make the type of soil analysis that could determine that soil condition. In fact, any trench collapse situation would preclude soil from being considered firm and stable. Spot shoring for trench rescue operations was invented and promoted by fire fighters. I have been unable to find any academic support from the geotechnical engineering community that supports this technique for trench rescue situations. As Craig Dashner explains below the concept of spot shoring was extrapolated (Definition: extend the application of a method to an unknown situation by assuming that existing trends will continue or similar methods will be applicable.) from soil theories, conditions and retention methods that are completely different than the shoring we do at trench rescue incidents.
Below is a condensed summary from Craig Dashner, one of the FEMA/USACE group members.
“The spot shoring that is being used for trench rescue shoring applications relies on the theory of “Soil Arching”. The idea behind that theory is that the properties of the soil, will transfer soil pressures to the spot shores placed in a trench by developing arching between the struts.
The theory (and supporting research) of soil arching has primarily been used in piling and drilled shaft arrangements, the most common being a soldier pile and lagging wall. This type of system installs the piling in the ground before any excavation takes place and installs the lagging as the excavation progresses. The analysis of this system assumes a reduced soil pressure on the lagging in between piles. This reduced pressure carries the soil that is under the “arch” that is being counted on in the system. The arch develops in a two-dimensional (2D) arrangement spanning between the installed piles.
Spot shoring used for trench rescue shoring extrapolates beyond that theory and the research behind it, by assuming that soil arching can be achieved in three dimensions AFTER a trench is excavated and presumably has failed. In the case of a failed trench, the soil has already moved and can no longer be assumed to be a competent, in-situ, soil. This is important because disturbed soil will not arch as readily as undisturbed soils. For spot struts to work the soil must readily arch between the struts (point supports). Furthermore, the soil must arch in three dimensions (more like a dome than an arch) to deliver the earth pressure to the struts and to deliver the strut resistance to the soil mass.
Spot shoring is extrapolating on two areas, that arching can develop in the walls of an excavated (and presumably failed) trench and that the soil will arch in 3D (dome rather than arch). This concept is not supported by any significant engineering practice or research. If this isn’t concerning enough, it completely neglects the fact that the soil under the arch (or dome) is completely unsupported. If a 4’ horizontal and vertical strut spacing was considered, the (unsupported) soil under the arch would represent 16-cubic feet of soil (if treated as a half sphere) with a weight of about 2 200 pounds. If the soil was strong and developed a flatter dome, you would still have 1 000 pounds areas between each strut. That is still enough weight to cause great harm to someone in a trench. Spot Shoring ignores this unsupported soil and risk.”
Source: Trench Rescue Shoring