Unit Weight of Soils
This refers to the weight of one unit of a particular soil. The weight of soil varies with type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and one cubic meter (35.3 cubic feet) of soil can weigh more than 3,000 pounds.
A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which sliding will not occur. The unique mixtures of the different types of soil (sand, clay, silt, and rock) necessitate different safe slopes from one excavation site to the next.
There are other complicating factors that can result in sliding soil failures. During an excavation, visibly different layers of soil may be uncovered. Each of those layers may call for different safe slopes. It is essential to plan your excavation around the most gradual (rather than steepest) safe slope for all of the different soil types and layers encountered during the excavation.
Another complicating factor is that soil composition mixtures may vary significantly from one area of the project to another. During an excavation, as the soil composition changes, the safe slope for trench wall excavation also changes. Thus, across an excavation site, the slope of the bank may need to be different to provide a safe working environment.
Sliding and other modes of failure 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 non-compacted 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 that a more gradual slope for excavation is needed 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 water line. The extra soil moisture tends to speed up sliding soil failures.
Determining the correct safe slope can be quite difficult for certain types of soil. The OSHA standard has developed a simple method of determining safe excavation bank slopes for different soil types. This method will be discussed in more detail in a later section of this document.
Soil failure can occur for any number of reasons.
Factors that increase the chances of soil failure are:
- excessive vibration – heavy equipment movement, earthquakes
- surface encumbrances – obstructions, broken water lines
- weather conditions – prolonged periods of rain