Pore water pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores)
Pore water pressure is vital in calculating the stress state in the ground soil mechanics, from Terzaghi's expression for the effective stress of a soil.
Pore fluid pressures that develop within soil masses as a result of both mechanical and physio-chemical effects influence the magnitude of the inter granular or effective stresses.
The intergranular stresses control, in many cases, soil behavior in shear and compression. If the opposite conditions prevail, there might also occur negative pore water pressure.