Most cave-ins result from a superficial inspection of a site that indicated no apparent chance of a trench collapse. In fact, the vast majority of cave-ins actually occur in clay-based soils (where wall failures are not anticipated), as opposed to sandy conditions where failures normally would be indicated. Also, most cave-ins happen in trenches between five and 15 feet deep, where many managers and workers falsely believe they can quickly escape if a trench wall collapses.
Failure to support or shield the trench as excavation proceeds, even in apparently "good" soil, invariably exposes workers to great danger. For safety's sake, all trenches should be considered potentially lethal, until adequately sloped, shored, or shielded.
You must consider several variables—all vitally important to assuring the selection of a safe and productive trench safety system—when determining the particular trench safety requirements of a particular job:
• Soil conditions
• Depth and width of the trench
• Ground water seepage: saturated or submerged soils
• Nearby utility services or structures
• Surcharge loads: stored material, equipment, traffic, spoil, etc.
• Type of excavator to be used
• Working space requirements
• Potential changes in the weather.
Three alternatives are available for providing a safe working environment for trench workers: sloping, shoring, and shielding. Sloping requires the cutting back of trench walls to an angle that will ensure no collapse into the work area. Shoring devices pre-load the trench walls and provide a positive restraint to soil movement, thus preventing cave-in hazards. Shielding devices are not designed to prevent trench wall collapse, but serve to "shield" workers should a cave-in occur.
SLOPING IS AN EXPENSIVE CHOICE
Although each of the three options are appropriate for specific projects, sloping generally proves the most expensive alternative when compared to shoring or shielding. To properly slope a trench often requires the acquisition of expensive right of way, along with the additional costs of excess excavation, refilling, compaction, and reinstatement. With a vertical cut, few, if any, of these factors are involved.
PRE-ENGINEERED . . . AND RELIABLE
Modern trench shoring and shielding hardware and practices yield three major benefits:
• They increase your job site safety.
• They help meet OSHA and other governmental regulations.
• They reduce costs and improve your crew's productivity.
The first two reasons get a lot of attention from most job site managers. And well they should. But the third point—cost savings—is frequently overlooked. Trench shoring and shielding, properly used, can frequently "pay for themselves," helping your crew work more efficiently and cost effectively. Today's trench shoring and shielding systems are generally pre-engineered, and the designs come with a history of proven reliability. Most modern devices ensure that at no point in your operation—initial excavation, system installation, system removal, back-filling, or compaction—is it necessary for personnel to be exposed to dangers in an unshored trench. Most systems are installed and removed from above the ground. Also, the range of systems available is large, enabling you to have a shield or shore of just about any conceivable combination of trench width and depth delivered right to your job site.
ABOUT THOSE COST REDUCTIONS
In addition to the safety benefits of modern trench shoring and shielding systems, often overlooked advantages include increased productivity and reduced costs. As we discussed, the savings associated with a vertical cut compared to sloping are dramatic (see chart on page 4). Likewise, productivity increases greatly when you integrate the excavation and shoring operations. Pre-assembled systems are quick and easy to use, fewer skilled workers are involved, and your workforce is more productive when working in a well-shored or shielded environment—a place where they can focus on the tasks at hand.
AN EXAMPLE OF COST SAVINGS
The following example is based on numerous similar real-life situations that contractors and crews find themselves in. Suppose that your company is bidding on a 400-linear-foot sewer line that will cut across the parking lot of an existing shopping center. The sewer line must be 12 feet deep. It will not be necessary to remove or cut across any curbs, gutters or sidewalks. In addition, you will not have to remove any light poles, trees or shrubbery. Your choices for trench protection are sloping or using a trench shield. Because you will be working in OSHA "Type B" soil, if you slope the angle will need to 1:1 (H:V). We are assuming that you will need three feet of width at the bottom of the trench to lay the sewer pipe (see drawing).
The chart on page 4 shows what your costs will be to excavate, backfill, and restore the surface versus using the trench shield. Note: This example is of a trench needed for a sewer line, but the same savings apply for virtually any trench, regardless of depth, width, or utility service—water, gas, electrical, telephone, cable-TV, etc.
In conclusion, contractors can increase job-site safety and reduce their costs through proper selection of modern trench shoring and shielding equipment. ip