Analysis of pore water stress in Lake Agassiz clay through a separation of the volumetric and deviatoric components of stress

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Krasaesindhu, Prasop
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The maximum resistance to shear on any plane in the soil is a function, not of the total normal stress acting on the plane, but of the difference between the total normal stress and the pore pressure so that the study of the relationship between stress and pore pressure is very important. Skempton established the concept of pore pressure parameters to relate changes in pore pressure with changes in stress. He related the change in pore pressure to the minor principal stress and to the maximum principal stress difference. This approach does not take into account the intermediate principal stress nor does it unequivocally separate effects of normal and shearing stresses. This paper describes a method for determining the pore pressure at any state of stress. This is done by separating the mechanics of the soil behavior into a volumetric and a deviatoric component. These components refer to the stress-strain relationship involving changes in the mean principal stress and changes in the deviatoric components of stress... The relationship between the octahedral normal stress and the pore pressure was determined by means of isotropic compression tests... The relationship between the induced pore pressure and the octahedral shear stress, or the pore pressure parameter ~, was investigated by means of constant octahedral normal triaxial compression tests... To assess the validity of this method, a comparison was made between measured pore pressure in the conventional triaxial compression tests and pore pressure computed on the bases of changes in volumetric and deviatoric stresses. Very good agreement was found between the predicted and the observed pore pressures. These results indicate that predicting pore pressure through a separation of the volumetric and deviatoric components appears to be valid.