BY JOHN RYAN
Hydro-excavation offers another option in rescuing a buried victim in a trench collapse incident. A truck-mounted hydro-excavator system uses a pressurized stream of water to wash the soil away from the victim. The soil and water are then removed simultaneously from the excavation, using a vacuum hose or tube. Although the technique is similar to that of using pneumatic excavators (vacuum trucks), the main difference is that in hydro-excavation, a pressurized water stream replaces a high-pressure air stream.
Originally developed as a nondestructive excavation method, hydro-excavation uses a highly controlled, variable-pressure stream of water of up to 2,000 psi to reduce the soil to a state in which it can be removed by a high-volume vacuum system. A typical hydro-excavator truck consists of a variable-flow water pressure pump, a vacuum system, and separate holding tanks for water and debris collection.
Hydro-excavation offers significant advantages over other methods typically used in trench rescue, according to Ken Sugawara, the general manager for hydro-excavation for Specialized Maintenance Services, Inc. (Pasadena, Texas), which performs pneumatic and hydro-excavation.
First, it can move a significant amount of soil far more quickly than other means. Cave-ins occurring at depths of eight feet or more often result in several yards of material covering the victim. Historically, trench rescue instruction advised rescue personnel to anticipate one hour of manual excavation for each vertical foot of soil to be excavated. For a victim suffering from trauma, restricted breathing, or blood flow, time is of the essence. Tragically, some rescuers who despaired of moving the soil manually in time to save a victim have attempted to use backhoes or excavators to do so, resulting in severely injuring or killing the victim.
Depending on the size of the hydro-excavator unit and the type of soil to be excavated, an operator can move between two and seven cubic yards per hour. Clay-rich soil, which entraps the greatest number of cave-in victims, is the most difficult soil for pneumatic excavators to handle. Hydro-excavation can remove clay soil at three times the rate of pneumatic excavation by reducing it to a solution instead of having to physically break it up.
Second, hydro-excavation does not require the more physically demanding “stab and pop” technique used in pneumatic excavation. Longer pressure wands can be more easily controlled and used with hydro units. Shafts of up to 65 feet in depth have been excavated completely from the top of the excavation by increasing the length of the pressure and vacuum hoses.
Third, the more powerful vacuum units found on hydro-excavators allow for a greater standoff distance from the excavation, which reduces surcharge loading and vibration of the trench walls. The relatively low-power vacuum units typically found on pneumatic units require that they be used in fairly close proximity to the trench, which may prove problematic if the area around the trench is muddy or obstructed. Further, the vacuum hoses, which move a fluid-like mix of soil and water on a hydro unit, are less subject to clogging than those on pneumatic units.
DEPLOYMENT
Once hydro-excavation is determined to be the best rescue method available to ensure rescuer and victim safety, the rescue team would proceed with standard trench rescue site preparation prior to the hydro-excavator’s arrival (e.g., setting safety zones, shoring or shielding the trench, and removing or limiting destabilizing influences such as traffic).
On arrival, the hydro-excavator is slowly positioned forward or rearward of the collapsed zone at a distance from the edge of the trench equal to at least twice the depth (Figure 1). The vacuum and pressure hoses are then extended to the critical area.
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After determining the victim’s position, rescuers identify the best place to excavate a sump, into which water and soil washed away in the process flow to be vacuumed away within a few seconds, eliminating pooling or seepage into the soil. The technicians or rescue personnel will activate the vacuum and pressure-jetting systems and begin to hydro-excavate the soil from atop the excavation.
When sufficient soil has been removed to allow for the extrication of the victim, the technician would vacuum up the residual water and wet soil. Rescue personnel would then enter the trench to remove the victim, using the most appropriate means and methods.
JOHN RYAN is a retired rescue instructor who taught firefighting and rescue training programs for the Texas Engineering Extension Service of the Texas A&M University System for 16 years.
