Measuring soil moisture content can be important for a variety of reasons.
In placing underground electrical equipment or digging tunnels, it can be essential to know exactly what soil moisture conditions look like at specific depths.
Early CPT test procedures used the standard CPT output data of cone resistance, sleeve friction and friction ratio to identify all of the parameters underground. When it comes to soils that have some moisture content or are saturated, it can be helpful to use a boring rig to obtain soil samples at depth close to the first CPT sounding. This enables you to ‘calibrate’ your rig to the site to ensure that the interpretations of the test data are accurate.
Because establishing subsurface moisture content can be safety-critical in certain cases, Cone Penetration Testing methodologies have evolved to provide relative soil moisture content data. It is now possible to measure soil moisture more directly at the cone head vs. inferring what the moisture might be through interpreted sounding data. One method of measuring the presence of water is with a ‘piezocone’. This is a CPT cone that is fitted with a device that measures pore pressure. As the cone penetrates into saturated soils, hydraulic (water) pressure is exerted on the instrumented cone. By watching this pressure increase and decrease as the cone is driven deeper into the ground, it is possible to measure the presence of moisture at depth. This type of approach is better suited to soil conditions in which it is expected for the soil to be fairly wet to saturated conditions.
Another method of establishing the extent of the presence of water is by using electrical sensors such as a dielectric probe, which measures soil electrical conductivity. This can be a useful practice and can be helpful in soils with less moist conditions. One factor to remember is that since the salinity of the water can affect conductivity, that the test again should be calibrated to ensure that the data being interpreted is relevant to the actual site conditions. It has been found that generally the salinity is consistent within a site, so one calibration boring should be sufficient to ensure accuracy.