Cone Penetrometer Testing or CPT

The cone penetrometer is a brute-force instrument of some delicacy. A pointed steel pipe is shoved straight down into the ground, as deep as 100 meters or until it meets bedrock. Sensors at the tip take two measurements: the pressure against the cone tip (resistance) and the drag on the pipe just behind the cone (friction). Clay, for instance, has low resistance and high friction. The ratio of these two numbers is used to map sediment types. There's a tiny hole near the cone that allows the pore water pressure to be measured, which can affect the resistance. The pipe can also take down with it gravity meters, pore-water samplers, soil samplers and electromagnetic sensors. Cone penetrometer testing (CPT) is widely used in the construction and environmental industries.

This CPT exercise was mapping the soil to a depth of about 20 meters near the trace of the Green Valley fault near Fairfield, California. Each hole took less than an hour.