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Trench Rescue

Below is a brief idea of what we are doing and something you can show the folks in your Department. As a group, we still continue to teach the traditional shoring you see across the country. We usually demonstrate the panel shoring in a variety of environments and discuss its strengths and weaknesses. As for the new techniques, we give the students a sense of time vs. task vs. patient viability. Some shoring attacks are better than others and a rescue crew will have to use their gray matter to select the best or method or combination of methods depending on the type of collapse and the presented hazards. We have used the spot-shoring techniques in a variety of soils up to C60 with significant surcharge and vibration near the lip (vac-trucks). We stress to our members that this is an advanced technique that uses tabulated data as a baseline only. This data is usually cut 50% more conservatively and applied to the particular dynamic of the trench we need to shore quickly (usually around the patient). This can include overhang, significant cracks, single sloughs, shear walls, double sloughs etc. A common global approach for a major collapse is to spot-shore the collapse, and panel shore the flanks. We have found that spot-shoring a collapse is much faster and more effective than a panel shore evolution. The illustration at the left shows a panel shore system of a one-sided belly collapse. Rescue crews spend a significant amount of time setting the panels, building and securing outside waler systems, and inflating low pressure airbags for backfill. During this type of collapse, the patient is usually pushed to the opposite wall and buried or pinned. A secondary collapse on the opposite wall usually will be the finishing blow to a victim. Game over. Many students get wrapped up in building elaborate shoring systems that are manpower and equipment intensive that take 1 to 2 hours before the trench is stabilized and extrication (digging) begins. This is a lot of time for a victim to sit on a floor unprotected from a secondary collapse. Usually layering or soil conditions have dictate as to why an initial collapse occurred and an uncollapsed opposite wall has a high probability of the same inherent weaknesses. We stress to initially shore to make a soon to be non-viable patient into a viable patient. Applying heavily set spot shores (300 to 1000 psi) with an acceptable grid will provide a safe zone for a rescuer to begin clearing the head and chest. The shoring process, however, never stops and a shoring team will advance their shoring, expanding the safe zone from the initial stages to the final stage when a Vactor truck is secured and utilized.

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Trench Rescue

Shoring grids will depend on soil type, what you see and all the other environmental factors that are applicable to trench rescue. Too many details to list here as you know. There are also some circumstances when nothing but spot-shoring can be used. To the left is a photo of a rescue my company had a couple of years ago. Crews had to advance the length of the trench to reach the victim, because of the overhang situation. Once the patient was reached, the shoring evolution progressed to the 14foot level. Spot shoring has to be used with common sense and takes time to become comfortable. The psychological assurances of Finform or plywood are gone. Rescuers cannot apply a typical template grid to any situation as with panel shoring. Rescue group leaders have to approach soil types with a solid understanding and have to read the dynamic of the trench. In other words, shore what you see...cracks, voids, cornices and excessive surcharge loads on the lip. We also shore with consideration to the equivalent weight effect and the inward pressure of the walls. We use the minimum 8x18 spot and back it up, when possible with a 2' x 2' piece of plywood. This seems to conform to the trench angulation in some situations and will reduce sloughing and raveling. However, in some soil types and collapse situations, only the minimum can be used. There are even cases where trench walls have to be shaved to effect a good purchase and reduce the swivel degree of the strut. This past summer, we had a 50-foot long x 13-foot deep trench with severe cracks running down the long axis. One crack went approximately 3-feet back into the trench wall under the spoil pile. Toe collapses were occurring on both sides. Crews used the spot shoring technique on the flanks of the patients shoulders and expanded the zone each direction and over the patients head. They also had to bridge approximately 8-feet outside the trench lip to a more stable area. There are some pictures in the following. The basic advantages of spot shoring are: Protects the victim directly It's fast There is little need for backfill and back-shoring is not necessary You can see the dynamic of the trench You can place them in any direction ­ swivel limits or shave trench walls to shore difficult angles or areas Less staffing is needed to placed shores ­ less people on the lip Multiple shoring teams can work concurrently Less materials ­ Panels etc.

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Trench Rescue

It positively pressurizes the trench walls therefore increasing soil friction and reducing collapse potential. A trapezoidal or elliptical cone of force is distributed into the trench walls directly behind the strut. The dis-advantages are: Loss of psychological effect of panel shoring Longer struts are needed More struts are needed Some sloughing and raveling in the trench if no sheeting is used You have to keep a solid eye on your shores ­ check and re-check. Not part of a "System Shore" for stability ­ Three shores on an upright etc. Many questions arise when discussing panel vs. spot shoring. 1. If rescuers are panel shoring a trench with victim buried to the neck ­ what technique must they use when they have to dig another 6 feet etc for extrication? You don't dig another 8-feet and add another sheet of Finform. Rescuers have to now use smaller incremental shoring as they proceed in depth (spot shoring) to prevent a toe collapse and reach the victim. If you can spot shore below panel shoring, why can't you spot shore above? Typically, in our training, we will dig to 12-14, collapse back to 8, and vac back to 12-14 feet. As rescuers, we don't have the luxury of reshaping the walls with an excavator to insert a trench box. We work with what we have in a delicate environment. 2. In shoring an angulated wall or collapsed area with a panel shore, in essence you are building a trench box. To elaborate, a sheet of Finform may or may not be effectively pressurizing the trench walls behind the struts making the tabulated data suspect. For example, the upper right and lower left corner of the Finform may be the only solid points of contact due to an irregular wall. The only way out of this situation is to backfill, which is time consuming; material intensive and personnel intensive (edge personnel) near the lip of the trench. Regardless, without proper backfill, you are not truly pressurizing the soil to get the cone effect or the elliptical effect engineers rely on. By using backfill, rescuers are trying to get some pressure into the walls and at the very least, are attempting to slow the momentum of a potential collapse. OSHA regulations require a maximum of 6" of gap behind a steel trench box. Apply this standard to our shoring and some problems arise. 3. Another point to consider is an industry trench box is a hell of a lot stronger than plywood or Finform because of its lateral and torsional strength. Think of building shoring that is laced and braced as in a collapse, and then compare it to a system that is not. A secondary collapse on the flank of a panel shore may provide excessive torsional loading to a system. Trench boxes are designed to take a hit directly or indirectly behind the box and catch it. Again, OSHA requires a maximum 6" space behind a trench box. This tells me they don't want a lot of momentum hitting something that's engineered with strength. Is our panel trenchbox as good without effective pressurization into a trench wall?

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Trench Rescue

4. The above brings up another point. In many cases during rescue shoring with heavy angulation, plywood or Finform prohibits the upright from effectively contacting the trench wall in a system shore. This opens the debate as to structural elements of the system. If the structural elements (uprights) are unable to pressurize the walls, do we have a stabilized trench with nonstructural elements? 5. When digging below panel shoring, often times the walls are not truly pressurized as mentioned above either because there is no backfill or ineffective backfill. Not such a big deal in a 6-8 foot trench. However, if you have to dig to 14 or so, you may soon have a significant load (trench wall) at shoulder level un-shored. This could be a hazard that is covered by panels. 6. Is it fast? As fast as a shore team can assemble the struts and shoot them in the walls is all it takes. Rescuers set the bridging and edge protection and start setting shores. Not a lot of cut-table work or fabrication in the beginning. Also, we don't do a lot of spoil removal for the edge. We only clear the area near the patient to reduce surcharge. We had one contractor buddy say that, "firefighters can't shovel worth a shit." He's right. So we do an elevated edge protection & bridge technique that is fast and does not put any weight on the trench lip and sets up a mobile bridge for spot shoring. There are some photos below.

This is just a generic rundown of spot shoring. It's only one technique in the tool bag of many. Again, we train on all the techniques dealing with straight, L, T, and various types of collapses. However, it's the only technique we have used in Seattle because it only seems to be the one that works on the runs we get. In teaching, we use a hell of a lot of other data and techniques not included in this rundown. A great deal of our training focuses on soil theory, equivalent weight effect, tabulated data, and soil pressurization theory in a nasty realistic environment. Vac-Trucks within 20 feet of the lip are part of the scenarios.

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Trench Rescue

Below are some pictures of spot shoring and elevated bridging. Crews had a severe collapse under blacktop. Trench was 14 feet deep and the belly portion was approximately 10 feet. Crews skip shored one side and spot shored the other. This gave them a partial "system" shore for stability. They could not use a direct attack because of the overhang and had to progressively shore to reach the patient. Once reached, crews incrementally set shores to reach depth. This is the opposite wall of the same collapse where the spot shoring is evident. The shoring proceeded laterally under the cornice until the patient was reached. Supplemental spot shoring increased in depth until 14 feet was reached and the patient was removed. Spacing was every 2-feet in B45 soil. Multiple spot shore evolution in a 12-foot deep trench with severe angulation. This was a double belly collapse that was 110 inches wide and 15 feet long. Total trench length was 50 feet.

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Trench Rescue

This was the initial problem for the shoring above. 12 ­ feet deep ­ Belly collapse in the opposite wall, a toe collapse developing a cornice on the patient wall. There was a need for some fast shoring at the 1/3rd weak point.

Rescuers build bridging that is elevated and does not impact the trench lip in an "L" trench.

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Trench Rescue

Multiple spot shores in a 110" wide belly collapse. Rescuers used multiple bridgepoints and several shore teams to stabilize the trench.

Rescuers add additional shoring as a vactruck removes soil. A shore is placed about every 18" when using the Vac-Truck. Vacs work so fast they can outrun the shoring if rescue crews don't keep an eye on it.

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Trench Rescue

Elevated edge protection and multiple bridges for three shoring teams. The point of support for the edge protection is 8-10 feet from the lip of the trench. This was due to a severe wall crack (developing shear) about 3-feet back from the lip that ran approximately 25 feet on the left.

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Trench Rescue

Typical scenarios we give to students. Moving cracks in the belly, a developing cornice that forces crews to come up with elaborate bridging. The victim is usually under the cornice to emphasize quick action and to stabilize the trench to in the fastest manner possible.

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