7 essentials to better trench rescue: Part 2
These shoring devices and techniques will make rescuers safer and more effective
By Dalan Zartman
Part 1 of this series presented some general considerations and practices to enhance both the efficiency and safety of our rescue operations. Those steps took us up to the trench and allow our sheeting material to be placed.
Next comes the challenge of shoring. Here are the remaining three key elements imperative to safe and efficient shoring operations.
- Acquire low-pressure trench bags for slough operations and trench wall deviations.
- Stop using timber and mechanical shoring. Pneumatics and hydraulics provide an array of solutions for different budgets that are faster and more effective.
- Use techniques and tools that allow the trench to be completely shored from the top.
These essentials require training and resources but yield tremendous returns if the objective is a viable trench rescue. Here's a closer look at each.
Low-pressure trench bag
We've already discussed the importance of panel sets that are near vertical and the use of bridging to establish a straight edge or template for straight panels. The next step is to analyze the voids between the straight panels and the trench wall.
When these voids exist, the typical practice is to allow the panel to move out of alignment and marry up with the trench wall as it is configured unless the void is radical. This dramatically increases the likelihood of shoring failure should a secondary collapse occur.
There are three choices to fill these voids and create horizontal load columns that will maximize the shoring's effectiveness: fill the void with soil, cribbing or a medium.
Filling the void with soil is commonly referred to as backfilling. The advantage of this choice is that soil is readily available on every trench rescue scene, simplistic, and conforms and loads well.
The disadvantage is that it is difficult to control and relatively time consuming and labor intensive. This is particularly problematic with the first pair of panels. If there is nothing on each side of the initial panels to contain the soil, then the soil being dumped into the void will fall to the bottom of the gap, build up to the maximal angle of repose, and then spill out around the panels into the trench and onto the victim.
Cohesive and wet soils may be difficult to break up and result in large clumps with extensive voids in the soil pack. The most common way to pull off backfilling is to get a lot of buckets and shovels and start filling them from the spoil pile and dumping them behind the panel.
Filling the void with cribbing provides more control over the placement of the backfill product and can be done quicker than soil buckets typically. However, careful attention has to be paid to ensure that the cribbing, wedges, etc. are placed appropriately for good load columns; soil backfilling is usually required to fill in the remaining voids. Additionally, the cribbing can be unstable initially and roll or shift until it settles into the trench wall once pressurized.
Filling the void with a medium — low-pressure airbags — is the best option for large slough zones. These airbags have flat loading surfaces as well as 3- to 4-foot diameters to fill the void behind the panel.
These bags are only 2 inches tall when deflated and up to 40 inches when inflated and have known load capacities which reduce the impact of variables when using more dynamic resources like soil and cribbing.
To use this option, lower the trench bags with accessory ropes into position behind the panel and inflate them until they make contact with the panel, but do not displace the panel. The panels are worked up to pressurization by bouncing back and forth between pressurizing the shore and adding air to the bag.
The key is to maintain the position of the panel until the shore is up to desired operating pressure. We have used pillow cushions or high-pressure bags in this application. They are highly unstable and have limited inflation height, so they are not a very feasible option.
Low-pressure airbags are expensive but have other lifting applications for heavy rescue that make them a versatile investment and worth their weight in gold for trench operations.
Pneumatic and hydraulic shores
Traditional shoring applications using wood, pipe jacks and screw jacks are slow, weak, complicated and typically under engineered. All shoring models should follow tabulated data charts.
Based on OSHA's tabulated data, the allowances for 4 x 4 material to be used in functional spacing configurations are very limited. Only a relatively shallow and narrow trench can accept this shoring material. The required upgrade is to increase the diameter of the timber and close the spacing.
Both of these options are bad in a rescue scenario and become worse when you add the significant increase in time and risk required to construct wood shoring systems. Hydraulic and pneumatic shores will take your trench shoring operations to another level.
Hydraulic shores, commonly referenced as speed shores, are aluminum or steel cylinders with a hydraulic piston and affix to large base plates, rails and other accessories. In the last article we touched on the debate regarding spot shoring techniques.
Hydraulic shores are often used in these applications due to their larger base plates and rails, which the manufacturers advocate being placed directly against the soil. This depends on the classification of the soil and is extremely risky in rescues. Once a trench is compromised, it should degrade in soil classification by standard rule.
Also, the reduced risk and added strength and engineering achieved with strong backs and panels far outweigh the minimal time saved with spot shoring. These shores have a swivel end and a rigid end with minimal allowance for deflection.
If the base plates on the strong backs, or rails in lieu of strong backs, do not form near 90-degree angles with the shores, the pressurization process may result in catastrophic failure of the shoring system. The manufactures also advocate a 4-foot spacing allowance from the bottom of the trench due to the increased load cone created by the larger base plate (12 x 6 inches).
Hydraulic pumps with large fluid reservoirs transfer fluid from the pumps, through a hose, to the shore to bring it into the desired pressure range for the type of soil. Many of these shores are not equipped with mechanical or secondary captures, which results in a one dimensional safety of pressure maintenance that is solely dependent upon valves and seals.
These shores are very fast and easy to operate and install, but become somewhat time consuming and complex when altering their length. The throw or stroke of the strut is a constant, which is based on the one-size-fits-all piston base.
The overall length is altered by pulling pins and exchanging outer sleeve cylinders and reinserting pins. Lining up the holes and keeping track of the pins can be cumbersome. Finally, extreme discipline is needed to manage the valve controls to ensure that pressure is not inadvertently lost or gained.
Pneumatic shores are aluminum or steel cylinders that can be pneumatically pressurized and mechanically captured. These shores are radically different from hydraulics in that a residual pressure within the cylinder will stroke it out until it meets a resistance of greater force.
Conversely, the hydraulic shores stroke out based only on the quantity of fluid pushed into them and will stop stroking out when fluid is not pushed. This means that pneumatic shores can separate and discharge if panels shift or if the ends are not captured making them more hazardous.
So, be disciplined about managing the pneumatic operating system and remain aware of the hazard level. Pneumatic shores have smaller base plates (6 inches), which eliminate the option for extended spacing.
Generally, these shores are vertically placed a maximum two feet from the bottom and top of the trench with no more than four feet in between struts.
There are variables for mechanical capture. Some have a self-locking strut; others have a threaded collar that must be spun down to capture progress. Some have a rotating collar and pinning system that must be applied to capture progress. Each option present advantages and disadvantages and should be fully understood before selecting a shoring system.
Pneumatic shores are lowered into position and pressurized with air from 150 to 250 psi. This pressurization is linked to the soil classification. I am an advocate of treating every compromised trench as a Class C soil composition to avoid under engineering.
These struts have a wide array of extensions and the swivel base plates allow for more significant deflections, which play a large role in advanced trench applications. Pneumatic shores are more expensive than hydraulic shores but are more versatile and efficient when the dimensions of the trench require augmenting the staged length of the struts.
Build from the top side
Select shoring systems and accessories that can be placed and manipulated from the top of the trench. Any time we can shore up a trench adequately before we place rescue personnel in harm's way, we are succeeding. There are some tricks of the trade and essential accessories that facilitate top side shoring.
Always have plenty of accessory ropes with suitable snap links. This can be purchased at any hardware store and should be a diameter that is easy to handle and commensurate with the loads applied. These ropes can be wrapped around, girthed or connected to the rigging rings on the struts.
One rope should be attached to each end of the strut and a rescuer assigned to each line. Use the top of the strong backs as a centering and friction point for the rope and lower the strut into position. Once the strut is locked, wrap the ropes around the strong backs so they are not trip hazards.
Pre-mark the strong backs at 1- to 2-foot increments with fluorescent or identifiable marking paint. This allows the trench boss and the rescuers lowering the strut to have visual benchmarks that aid in quickly placing the strut correctly from the top.
Purchase or manufacture long-handle tools that mechanically capture the struts. Hydraulic shoring tools should have a hook as well as a coupling release device. This allows struts to be lowered with the hook and the hoses to be released from the struts without entering the trench.
Pneumatic tools should be capable of spinning collars and if applicable, releasing self-locking collars. An easy manufactured version involves purchasing a long general-purpose, telescoping handle from the hardware store.
Wrap the tool attachment end of the handle with accessory rope with 2-inch gaps. Then apply a single-layer wrap of duct tape to keep the rope in place and another layer of reverse duct tape so that the adhesive is outboard.
Take your trench team out and see if these don't increase the speed, safety and efficiency of your operation. I'd love to hear how this impacts your approach to trench rescue.