What instrument measures the unconfined compressive strength of soil?

The pocket penetrometer is an instrument specifically designed for measuring the unconfined compressive strength of cohesive soil. It operates by inserting a small, hand-held device into the soil and applying a load until a failure is observed. The dial on the penetrometer provides a direct reading of the compressive strength, with the results indicative of the soil's consistency and strength characteristics. This is particularly useful in field conditions where quick assessments are needed.

In the context of soil testing, the other instruments listed have different functions. A load cell is typically used for measuring force or weight and is not designed specifically for soil strength testing. The shear vane is utilized primarily to measure the shear strength of saturated soft soil but does not provide an unconfined compressive strength value. The standard penetrometer is often used for determining soil resistance during driving tests but is not directly correlated to measuring unconfined compressive strength. Thus, the pocket penetrometer is the most appropriate choice for assessing unconfined compressive strength in soil.