The pressure exerted on its surroundings by water held in pore spaces in rock or soil. The pressure is positive when a soil is fully saturated, and is then proportional to the height of the water measured in an open tube (a piezometer) above the point of interest. A buoyancy effect is achieved and the shear strength of the soil is reduced. The pressure is zero when the soil voids are filled with air, and is negative when the voids are partly filled with water (in which case surface-tension forces operate to achieve a suction effect and the shear strength of the soil is increased). Compare pore fluid pressure.