Anisotropic strength behaviour of a fissured overconsolidated clay in relation to Saalian glacial directions

Abstract

The studied Pot Clay described in this paper occurs in the northern Netherlands as a lacustro-glacial infilling in glacial channels (Elsterian glaciation) and is overconsolidated as a result of an ice load (Saalian glaciation). Previously observed past preconsolidation stresses show an increasing gradient with depth (maximum σ′ vm≈2100kPa and observed sub-glacial deformation phenomena consist of intensive shearing, matrix deformation and the formation of fissures. In three boreholes, up to 125 m in depth, geological, clay mineralogical and geotechnical research has been performed on undisturbed and oriented tube samples of the clay. In unconfined, consolidated-undrained and unconsolidated-undrained triaxial tests, anisotropy in strength behaviour, expressed as two distinct groups of dip directions of failure surfaces - SW and NW to N - has been observed and these could be demonstrated to be related to the directions of the early and late Saalian glacial events in The Netherlands. These directions have been inferred previously by others from glaciogenic and geomorphological phenomena. The SW anisotropy occurs in the triaxial tests at confining stresses σ′ 3 < 400 kPa and connected to a generally contractant deformation behaviour and related to directions of the clay matrix, which are thought to be formed by a thrusting force of the moving ice in an early phase of the glaciation, with the clay at depositional water content and normally consolidated. The NW to N anisotropy occurs at confining stresses σ′ 3 ⩽ 400 kPa and is connected to a dilational deformation behaviour along pre-existing fissures, which are thought to have been formed with the clay in an overconsolidated and stiffer state in a later phase of the glaciation.