Earth Work Excavations

Although there are a number of benefits derived from performing quantitative soil analyses, operations and field workers typically prefer to use qualitative methods. Perhaps the biggest reason for this preference is that qualitative analyses do not require the use of a measuring device, which can easily be lost and is expensive to replace.

There are also other benefits to conducting qualitative soil analyses. A competent person trained in this type of analysis can ascertain soil behaviors – such as those of wet and dry soil, clay and sand – instead of just the unconfined compressive strength. This may enable him or her to make better decisions about how to address the challenges of working in various environments.

A competent person shall make a soil classification based on the results of at least one visual and one manual analysis. Before performing either analysis, one should consider the size of the excavation. Samples should be collected from various depths. In addition, depending on how long or wide the excavation site is, the competent person should also confirm that the soil conditions are consistent throughout the proposed excavation area. If they are not, additional safeguards may be required. The safeguards to be used will depend on the depth of the excavation and the soil classification.

Next, the visual analysis can begin. This process is rather straightforward; the competent person literally looks for clues about soil characteristics. For example, he or she can determine if the soil is wet or dry, cohesive or fissured, granular like sand or smooth like clay. We will get into why these attributes are important a little later in this article. I recommend beginning the visual analysis process when earth-moving equipment is used to begin the excavation. Doing so will enable the competent person to get a better idea of gravity’s effect on the soil at the excavation site, determine how water will run off or settle, and discover if the soil has different characteristics at different layers.

Once the visual analysis is complete, the manual soil analysis can begin. Many individuals begin a manual analysis by collecting a soil sample and rolling it in their hands, adding or removing soil until the sample is about the size of a golf ball. Some then choose to use the thumb test, in which – as the name implies – you push your thumb into the ball in order to estimate the soil’s unconfined compressive strength. Others choose to roll the ball into 1/8-inch (3.175-mm) threads. After rolling the threads, the competent person dangles roughly 2 inches (50 mm) of 1/8-inch thread to determine if the soil will tear.