The meaning and laboratory measurement of swelling pressure are examin
ed 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 pr
essure relationship, rather than a single value of swelling pressure,
and is interpreted by means of constant volume, free swell, and contro
lled load or controlled deformation unloading, tests. A concept of uns
aturated fissured shale is introduced and is used to explain the magni
tude 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 suppo
rt 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 th
e lining.