Abstract: Apparatus and process of determining stress underground, the apparatus having a central standard or shaft provided with a sensing head formed of equally circumferentially spaced, radially extending blades or vanes carried at the lower end portion of the shaft. Sensor elements or cells are carried within each blade, each sensor element having a pair of spaced, deformable, deflection diaphragms separated by a porous interior. The sensor head is inserted directly into the ground or lowered through a hollow auger to project into the ground, therefrom. In other embodiments, the sensor head is carried by a flexible actuator riding in a curved track of a housing. The head, which may be a single blade or a plurality of blades with embedded sensor cells, is recessed in the housing and is projected sidewise from the housing by manipulation of the actuator.