THE INFLUENCE OF AN UNDRAINED CHANGE IN STRESS ON THE PORE PRESSURE IN POROUS MEDIA OF LOW COMPRESSIBILITY

The author describes some earlier work directed at predicting and measuring the pore pressures that are set up by changes in the state of stress of soils, under undrained conditions. He shows how he has derived from first principles an expression of pore porosity ratio in terms of porosity, compressibility of the pore water, compressibility of the soil grains and compressibility of the soil skeleton. It is indicated how the same expression can be derived from Geertsma's equation relating the change in volume of pore space to the changes in total stress and pore pressure. It is stated that the methods of analysis used by Gassman led to conclusions that were consistent with the author's and Geertsma's findings. This is followed by a discussion of the principle of effective stress and, in particular, the importance of using the excess of total stress over pore pressure in the calculations rather than the effective stress relating to volume change. The author then discusses the predicted range of values of pore pressure ratio and concludes with a derivation of the pore pressure parameter as defined by Skempton.