Training: Trench rescue shoring: spot shores
A technique used to support soil without incorporating uprights or panels, is being taught in parts of the US. Together with a group of highly educated and experienced civil engineers I have begun research into the use of this technique at trench collapse incidents. To date, we have not been able to find a professional engineer who recommends its use at trench collapse incidents or who has developed a shoring design for it. Additionally, we have not found any manufacturer of pneumatic rescue struts that has developed tabulated data for spot shoring collapsed trench conditions nor have we found any geotechnical principles that support this technique in those conditions. Please reply with rebuttals to any of the information listed below.
Geotechnical Principles: Dr Oliver Taylor (PE/PhD) a research geotechnical engineer with the US Army Engineer Research and Development Centre 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, Dr Taylor concluded that, “Published and ongoing research strongly advises against using spot shores without strongbacks (uprights) or panels in trench rescue scenarios.”
Manufacturer’s Tabulated Data: Fire fighters who are trained to interpret and use tabulated data (usually Trench Rescue Technicians) can legally utilise the prescriptive engineered tabulated data provided by manufacturers of shoring equipment. Following a review of tabulated data from the four largest manufacturers of pneumatic struts, Craig Dashner (PE licensed in three states) 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/panelling if in a Type C soil or if ravelling is present. Since a trench wall failure would be considered ravelling, the tabulated data would require sheeting in all cases. The two other manufacturers of pneumatic struts (Paratech and ResQtec) do not recommend the use of spot shores in trenches that have collapsed and have no tabulated data for that use. Spot shores (without uprights or panels) directly conflicts with all of the manufacturer’s tabulated data.”
Engineered Designs: Shoring designs are required, by law, to be produced by registered professional engineers (PE). Spot shoring is a shoring design. To date I 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. Using OSHA soil classifications, Type-C soil is considered weak, unstable and most likely to collapse. “Geotechnical design criteria along with various State Department of Transportations (DOTs) do not allow for the use of soil nails or spot shores for Type-C soil conditions, specifically for soils susceptible to collapse (ie a trench failure condition).” – Dr Oliver Taylor (2019).
Causes Collapse: A study conducted by Dr Marie LaBaw in 2009 (PE/PhD) 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). “A good rule to remember is every action has an equal and opposite reaction. Simply put: if we push on the soil, it pushes back. Elastic models show force distributing back into the soil in the shape of a half ellipsoid with greater diameter than the original induced strut load. That means that without appropriate sheeting, there is exposed and unaccounted for load at the exposed trench face. In real world application that exposed force could very well blow out the trench wall if the soil's internal friction and cohesion are not enough to counteract it. Perhaps under certain conditions inducing strut forces could compress destabilized soil. The more likely scenario in a compromised trench (already known to have cohesion problems) is progressive failure.”- (Dr Marie LaBaw) Dr Taylor supports Dr LaBaw’s conclusion, “During a rescue operation in an already compromised trench, spot shores, in fact, are less likely to be helpful and more likely to induce progressive failure.”
Strut Intervals: Spot shores leave unsupported soil between the struts. “A collapse most likely indicates OSHA Type C soil, which is relatively weak. The soil in between the spot shores is still unsupported and in weak soil conditions can collapse.”- Craig Dashner (PE)
“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.”
Dr Oliver Taylor (PE/PhD)
Soil Analysis: Spot shoring can be used in strong and stable soil conditions. Spot shoring should not be used in weak and unstable soil conditions. OSHA requires a Competent Person to make that determination and classify the soil. A qualified Competent Person must be highly trained and experienced with soil. “Expecting any first response personnel to be a qualified Competent Person is a fallacy at best. To truly be a Competent Person requires training AND regular and repeated use of the equipment and extensive experience with many different soil types. Fire service personnel will never have enough experience and will never be exposed to enough varying soil types to be an effective Competent Person. This leads to a situation where an inexperienced but likely confident, team member will make a determination that affects the safety of the victim and the rescuers with no one on site to verify their decision. To expect a fire service Competent Person to correctly classify a soil under pressure is unrealistic. The wrong decision could lead to a shoring system being installed with insufficient capacity.” Craig Dashner (PE)
Summary: “The resolution of earth and thrust pressures, under trench failure conditions, can only be accomplished through the use of predetermined strut locations along the bracing panels in conjunction with strongbacks. Spot shores cannot meet these design criteria and resolve the required physics necessary for retaining lateral earth pressures. There is no safety code, design guidance or sound geotechnical engineering principle to support the use of spot shoring at trench collapse (rescue) incidents.” Dr Oliver Taylor
Source: Trench Rescue Shoring