Soil Testing Can Help Keep Job Sites Safe

Besides shoring, trench safety relies on effective soil testing and best practices for running your heavy equipment on the job site

Soil Testing Can Help Keep Job Sites Safe

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In this article, we highlight OSHA soil standards and the need to provide worker protection during excavations by discussing some complicating factors that contractors may encounter and providing a quick reminder of field test methods used to identify the OSHA soil classes.

Among complicating factors to be aware of, soil composition may vary significantly from one area of a project to another. As the soil composition changes during an excavation, the safe slope for trench wall excavation also changes. The slope of the bank may need to be different.

Sliding and other failures can also occur in soils that are not densely compacted. For example, a trench that is made close to a previously dug trench is very unstable. If uncompacted soil is discovered, the normal safe slope for dense soil will not be enough to prevent sliding. Bracing or further sloping may be necessary.

If cracks are observed in rocky types of soil, sliding has already occurred. These cracks should signal the need for a more gradual slope for excavation because the rocky soil is very susceptible to slides and other types of failure.

Excavations that have been stable for long periods are also subject to sliding types of failure. After prolonged exposure to the elements, the moisture content in the soil may increase. This increase in moisture may be due to various causes, such as rainfall or a broken waterline. The extra soil moisture tends to speed up sliding soil failures.

Soil failure can occur for any number of reasons. Factors that increase the chances of soil failure are excessive vibration, surface encumbrances and weather conditions.

WATCH FOR SITE DISRUPTION

Anything that causes extra stress on the soil can result in soil failure. This can include vibration from moving your heavy machinery near the excavation. Other construction traffic can also cause this, which means your work site should be protected from this additional traffic. Sandy soils in general are more susceptible to failures due to vibration. Add water to sands with the vibration and they will flow and slide.

Similarly, having heavy loads next to the excavation can result in failure. This can be the presence of your equipment, or stockpiles of rock and other materials. They can place more stress on the sides of the excavation. The best approach is to keep these materials away from the excavation. Where space is limited and heavy loads must be located near an excavation, the trench walls should be braced or shored.

Weather is an important factor in determining soil conditions. Excess water from rain or melting snow increases pressure on the excavation. If there have been heavy precipitation events and the excavation was left open, additional precautions (shoring or sloping) may be needed before work continues. Treatment trenches should be allowed to dry before working on them again.  

Dry conditions can also be dangerous. As moisture content decreases, some dry soils lose their ability to stick together. This lack of cohesion may result in a sliding type of soil failure. With soils high in sand and silt content, the lack of cohesion between particles, and where soil structure is not well-developed, sliding failures may occur.  

FIELD METHODS EXPLAINED

Contractors may follow field methods to determine soil type (A, B or C) and requirement for sloping. Field methods are use of a pocket penetrometer, thumb test, dry strength test and wet thread test.

Pocket penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated, cohesive soils. Pushed into the soil, an indicator sleeve displays the reading. They are calibrated by tons per square foot, providing a direct reading to use to determine sloping requirements. The downside is penetrometers have large error rates in the range of plus or minus 20% to 40%.

The thumb penetration procedure involves pressing the thumb firmly into the soil in question. If the thumb makes an indentation in the soil only with great difficulty, the soil is probably Type A. If the thumb penetrates no further than the length of the thumbnail, it is probably Type B soil. And if the thumb penetrates the full length, it is Type C soil. The thumb test is probably the least accurate, but absent better information, it can be useful.

Dry soil that crumbles freely or with moderate pressure into individual grains is granular. Dry soil that falls into clumps that subsequently break into smaller clumps (and the smaller clumps can be broken only with difficulty) is probably clay in combination with gravel, sand or silt. If the soil breaks into clumps that do not break into smaller clumps (and the soil can be broken only with difficulty), the soil is considered Type A unless there is visual indication of cracking.

The wet thread test is conducted by molding a moist soil sample into a ball and attempting to roll it into a thin thread approximately 1/8 inch (3 millimeters) in diameter (thick) by 2 inches (50 millimeters) in length. This is similar to our field test to see if the soil is dry enough to excavate for sewage treatment trenches. The soil sample is held by one end. If the sample does not break or tear, the soil is considered cohesive.

In addition to tests to determine cohesiveness, the contractor should conduct a visual evaluation of the site. A visual test is a qualitative evaluation of conditions around the site. In a visual test, the entire excavation site is observed, including the soil adjacent to the site and the soil being excavated. The contractor should note potential problems before excavation begins.

Hopefully this article has provided useful information about OSHA requirements and will save lives. 



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