DOTHAN FIRE DEPARTMENT Chief Dennis L. Rubin Personnel: 174 career firefighters Apparatus: 12 paramedic engines, two ladder trucks, one heavy rescue, two battalion chiefs, one rehab unit, one air-and-light truck, one rescue trailer, one customer-service unit Population: 60,000 Area: 83...
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Chief Dennis L. Rubin
Personnel: 174 career firefighters
Apparatus: 12 paramedic engines, two ladder trucks, one heavy rescue, two battalion chiefs, one rehab unit, one air-and-light truck, one rescue trailer, one customer-service unit
Area: 83 square miles
On Sunday, May 24, 1998, the Dothan Fire Department was called to a tragic construction accident. A worker installing a sewer line was trapped beneath nine feet of earth when a "shear wall" collapse occurred. He and another man were working on the line over the Memorial Day holiday weekend to complete a project that would tie a new shopping center into the city's sewer system.
About The Trench
The trench was about 120 feet in length, two feet wide and nine feet deep. The earth had been opened for a few days before the cave-in and the area was in mild drought conditions. The last reportable rainfall had occurred 20 days prior to the incident. The very dry soil added to the danger of the operation and the instability of the trench.
Photo courtesy of the Dothan Fire Department
A worker installing a sewer line was trapped beneath nine feet of earth after a shear wall collapse.
It is difficult for the average person to comprehend, but soil is a heavy material - in fact, it weighs more than 100 pounds per cubic foot. Consider that a single cubic yard of soil (measuring three feet by three feet by three feet) contains 27 cubic feet of material, weighing in at over 2,700 pounds. A rule-of-thumb measurement is that a cubic yard of space is about the size of a typical full-size automobile trunk. After reviewing these facts, it is easy to see that the material at a trench cave-in has a tremendous amount of weight and, therefore, force.
Soils are classified by types, ranging from A (most stable) to C (least stable). The classification system is used to help determine the likelihood of a collapse and degree of difficulty of working with a particular soil. A measurement of unconfined compressive strength is used in determining these classifications. In layman's terms, under how much pressure will the soil crumble and break apart?
- Type A soils are cohesive with an unconfined compressive strength of 11/2 tons per square foot or greater. Examples are clay, silty clay and clay loam.
- Type B soils are cohesive and have an unconfined compressive strength greater than a half ton but less than 11/2 tons per square foot. Examples are angular gravel, silt or silt loam.
- Type C soils are cohesive soils with an unconfined compressive strength of a half ton or less per square foot. Examples are sand, gravel and sandy loam. Soils that have been previously disturbed, or which water is freely seeping, or that have collapsed are classified as Type C.
Photo courtesy of the Dothan Fire Department
The trench was 120 feet in length, two feet wide and nine feet deep.
Trenches can collapse in three ways: lip slide, wall slough ("belly-in") and side wall shear. The lip slide is usually caused by piling the excavated spoil too close to the edge of the opening. This is the most common type of collapse and can be shored relatively easily. The wall slough is caused when a large section of soil falls from within the side of the trench. This may leave an overhang of material, which is extremely dangerous. In this case, the remaining uneven surfaces are difficult to shore. The side wall shear occurs when an entire section of the earth shears away and collapses into the trench. This is a dangerous situation because of the amount of material that has been disturbed.
The edges around the opening are called the trench lips. The removed material from trench is "spoil." The spoil pile should always be placed at least two feet from the lip because of the pressure that is exerted on the wall of the trench. Also, the spoil pile can fall into the opening and injure workers below. The "belly" is the side walls and the bottom is called the trench floor or toe.