The invention relates to a device and method consisting in inserting a membrane cell (7) into a borehole in the ground. A pressurised gas is supplied to the cell and the increase in the volume of the cell (7) is subsequently measured as a function of the pressure exerted. A vertical tube (10) located just above the membrane cell (7) is provided with an inclined revolver (8) having a bullet in the form of a hypodermic needle provided with a sensor (5), which is shot into the ground as far as the point of maximum deformation (6) of the membrane cell and close thereto. The interstitial pressure is measured as a function of the pressure exerted and the volume of the membrane cell.
