Michael Daley describe the ways in which rescuers can train to respond successfully to these emergencies.Today's fire-rescue service must be prepared for all types of emergencies that can arise in the communities they protect. As areas become more populated, construction increases with the...
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Michael Daley describe the ways in which rescuers can train to respond successfully to these emergencies.Today's fire-rescue service must be prepared for all types of emergencies that can arise in the communities they protect. As areas become more populated, construction increases with the production of residential, industrial and mercantile occupancies. One type of emergency that can arise on these sites is a trench collapse, potentially trapping workers beneath tons of soil and trench-related materials. With competent and motivational training, responders can prepare to operate at these types of emergencies efficiently and safely.
Photo by Michael Daley
The trench edge is made safe by applying ground pads around the trench. The spoil pile must be moved away from the lip a minimum of two feet, and ladders are placed at each end to maximize egress for rescuers.
Trenches are temporary excavations in which the length of the floor of the trench exceeds its width, and the width of the trench does not exceed 15 feet at the bottom. It is also common for the height of the trench walls to exceed the floor's width.
Trenches are divided into parts:
- The wall or face - The side of the trench from top to bottom.
- The lip - The top two feet of the trench wall.
- The belly - The center portion of the wall.
- The toe - The bottom two feet of the trench wall.
- The floor - The base of the trench.
Trenches are dug for a variety of reasons - in roads to permit the installation of storm sewers and sanitary lines and to bring electric, gas, water and other utilities into homes and businesses. Underground utility wires are becoming more popular in areas where residents wish to eliminate or avoid the sight of telephone and electrical wires strung from pole to pole. Whatever the purpose the trench serves, it is imperative that rescuers recognize the potential hazards they can become.
The Occupational Safety and Health Administration (OSHA) regulation 1926.650, 651, 652 provides guidelines for ways to make trenches safe for workers to enter. For example, any trench that is deeper than five feet requires shoring or a trench box be installed prior to entry. Trenches that are deeper than 15 feet cannot utilize a sheeting and shoring operation, and they require an engineer-designed trench stabilization system be erected prior to workers entering to perform their duties.
Unfortunately, many safety regulations are modified, or even ignored, for the sake of speed and profit.
Rescue teams must become familiar with the OSHA guidelines that pertain to trenches, excavations, protective systems requirements, soil classifications, sloping and benching, and timber shoring to ensure the successful outcome of emergency incidents.
Why do trenches collapse? Here are some common reasons, just to name a few:
- Digging in soil that has been disturbed, even after the trench has been filled for some time.
- Intersecting trenches with inherently weak areas where the trenches meet.
- Soil from the trench, or the spoil pile, that is placed too close to the lip.
- Vibrations from equipment that is working around the trench area.
- Trenches open for long periods lose moisture that helps bind the soil together. Most commonly, though, the absence of adequate shoring is an invitation to disaster, and by far the most frequent reason for the collapse. The natural tendency of soil is to push to all directions, and the trench is a disruption in the soil that allows it to flow in the path of least resistance. It is vital to match the shoring to the soil that the rescuers will be stabilizing to effect the rescue.
Types of soil can be listed in three categories:
- Type A soil has a comprehensive strength of a minimum of 1.5 tons or greater per square foot, and will hold its cohesive quality when it is compressed. This clay-type soil will hold its shape and bind together well.
- Type B soil has a comprehensive strength between 0.5 and 1.5 tons per square foot. It is a wet, saturated soil that may even show water seeping from the trench walls and floors.
- Type C soil has a comprehensive strength rating of less than 0.5 tons per square foot. It is a very loose, running soil that is free flowing, similar to sugar sand, and can be the most difficult to stabilize.