Meaning, measurement and field application of swelling pressure of clay shales

The meaning and laboratory measurement of swelling pressure are examined by swelling and swelling pressure tests on undisturbed specimens of Taylor shale from San Antonio, Texas. Swelling pressure response of a shale element is defined in terms of a void ratio against swelling pressure relationship, rather than a single value of swelling pressure, and is interpreted by means of constant volume, free swell, and controlled load or controlled deformation unloading, tests. A concept of unsaturated fissured shale is introduced and is used to explain the magnitude and time-rate of swelling pressures measured in laboratory tests and mobilized in the field. The magnitude of swelling pressure that a shale surface exposed in an excavation mobilizes against a stiff support is expressed in terms of an expansion of the shale mass adjacent to the excavation. The expansion of shale is mainly through the opening of joints and fissures during excavation and before installation of the lining.