The Bureau of Labor Statistics reports that an average of about 70 people die every year due to cave-ins from trenches and that another thousand or so workers get injured every year from cave-ins. Ninety-five percent of these incidents have one thing in common, they were not properly safeguarded and with so many other unpredictable dangers in the construction industry. Why expose yourself to injury and lost time or even the risk of death; when there are clear and simple trench box safety measures you can take to protect yourself.
The first thing to consider with a trench box; is soil conditions. There are a variety of soil types and ground conditions that will determine the proper course of action for a trench; but before you start take a closer look at the excavated soil, look around the site for things that may cause soil distress. Like nearby vibrating machinery, heavy moving loads, seeping water or rain as well as hot-dry weather. All these things can lead to additional stress on soil where increase precautions would be necessary. One of the most important factors that can help determine the likelihood of a cave-in is the type of soil that you are trenching. Knowing the type of soil you are working with will help determine the proper slope of that trench. There are three main classifications of soil types: A, B and C.
Trench Box Safety and Soil Types
Type A is the most stable soil usually consists of clay. If you don’t have any tools used to determine soil type, a quick way to figure this out is by taking a sample in your hand and pressing your thumb into it. If your thumb sinks only about a quarter of inch you have Type A soil. with type a, the slope of your trench can be no less than three feet horizontal for every four feet vertical. If your thumb goes in about a quarter to three quarters of an inch you probably have Type B soil. This has a medium level of stability in community above silt sandy loam or medium density clay. With Type B soil, your trench slow can be no less than 1 foot horizontal for every one foot vertical. This type of soil can also include unstable dry rock or soil has been previously disturbed. Now if your thumb penetrates more than two inches rather easily you have tape C soil. This is the least stable soil and generally consists of gravel, loamy sand or soft clay. With Type C soil your trench slope can be no less than six feet horizontal for every four feet vertical.
Shoring and shielding are two different Trench box safety methods in protecting workers in a trench from a cave-in and can be used in conjunction with sloping. Shoring is a support provided to prevent the walls of a trench from caving in, this is typically done using hydraulic, pneumatic or timber braces. The specific shoring solution set up that you use depends on the soil type and how deep your trench will go. Trench box safety is always installed from the top down and removed from the bottom up. So for example in a shoring system, the stringers uprights and upper cross braces would be installed first. Then the lower stringers in cross braces are put in. the number rolls of stringers in cross braces depends on the depth of the trench. Upper cross braces should be installed within two feet a ground-level. if the trenches up to eight feet deep, to set up stringers in cross braces are required.in a trench eight to twelve feet deep, three sets of stringers and cross braces is required. In a twelve to fifteen foot ranch requires four sets of stringers in cross braces. In looser soil conditions the uprights should be placed side by side. Plywood can be used in place of some of the uprights as long as the trench is less than nine feet deep, the plywood is three-quarters of an inch thick or thicker, the uprights are installed at no more than 24 inch centers and the cross braces do not bear directly onto the plywood put onto the stringers and uprights. A shielding system on the other hand is designed less to prevent cave-ins and more to protect workers in the event of the cave in. it is typically done with the use of what’s called the trench box or shield box. A trench shoring boxes typically come pre-assembled and are lowered into the trench by machinery. No workers should be present in the trench when a trench box is being lowered or removed.
Other Issues Inside a Trench
Once in a trench, a trench worker should remain aware of his or her surroundings as there are several hazard’s other than cave-ins that can be common in a trench. The first of these is a hazardous atmosphere within the trench this can include flammable as well as toxic vapors. Flammable vapors are the vapor density greater than air can accumulate in low areas of a trench and ignite with any number of ignition sources. Sources of these flammable vapors can be ruptured pipes in the trench, Heavy vapors in the area and materials that are brought in. Toxic vapors can accumulate in trenches through the soil depending on the surrounding ground environments. It’s important to have testing equipment handy to make sure oxygen levels remain normal and toxicity levels remain low. Anytime oxygen levels drop below nineteen and a half percent, respiratory equipment is needed. Once in a trench a worker should always check for unknown utility lines that may be damaged during the digging. Maintaining proper trench box safety to a trench is important as well make sure that any ladder going into a trench extends three feet above that trench.