Abstract: The point, lower-end region, and shaft of a penetrometer tube set are respectively connected to lower, intermediate, and upper members of a sensor head. A cap member which slidably receives the upper member is struck by the penetrometer anvil to drive the tube set into the ground. The lower and intermediate members are respectively slidable in the intermediate and upper members, and upper, intermediate, and lower strain gauges are provided between the members to produce outputs corresponding to the forces exerted by them on each other. A group of pawls can lock the upper member onto the shaft of the tube set and the intermediate member is limitedly axially displaceable in the cap member so that the intermediate strain gauge is compressed on pulling-out of the tube set.