Influence of geology and geological processes on the geotechnical properties of a plastic clay

Abstract

The complex properties of postglacial clay at Winnipeg, Canada have major impact on civil engineering construction in the city. Problems include house foundation movements, low stability of riverbanks, poor highway pavement performance, difficult excavation, and a high incidence of watermain breaks. The research in this paper presents an understanding of the geologic factors that cause these problems, and how to ameliorate their effects in engineering design. The geotechnical properties of the clay were measured from carefully sampled, 76 mm diameter triaxial specimens. The clay is nonhomogeneous, anisotropic, active, overconsolidated and fissured. These properties are related to the depositional and post-depositional geology of the area. The depositional geology of the extensive Lake Agassiz clays is straightforward and well known. However, it does not fully explain the measured geotechnical properties. Attention is then directed towards post-depositional geological processes such as regional groundwater hydrology, cementation, aging, freeze-thaw and desiccation. Consideration of effective stress strengths, quasi-elastic anisotropy, the variation of overconsolidation ratio with depth, yielding and porewater pressure generation, suggests strongly that the clay is cemented by carbonates and sulphates from upwards movement of salt-rich groundwater. Freeze—thaw disturbance causes marked changes in behaviour, especially at low stresses.