ANISOTROPIC STRENGTH BEHAVIOR OF A FISSURED OVERCONSOLIDATED CLAY IN RELATION TO SAALIAN GLACIAL DIRECTIONS

The studied Pot Clay described in this paper occurs in the northern Ne therlands as a lacustro-glacial infilling in glacial channels (Elsteri an glaciation) and is overconsolidated as a result of an ice load (Saa lian glaciation). Previously observed past preconsolidation stresses s how an increasing gradient with depth (maximum sigma(vm)'approximate t o 2100 kPa) 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 mineralogic al and geotechnical research has been performed on undisturbed and ori entated tube samples of the clay. In unconfined, consolidated-undraine d and unconsolidated-undrained triaxial tests, anisotropy in strength behaviour, expressed as two distinct groups of dip directions of failu re surfaces - SW and NW to N - has been observed and these could be de monstrated to be related to the directions of the early and late Saali an glacial events in The Netherlands. These directions have been infer red previously by others from glaciogenic and geomorphological phenome na. The SW anisotropy occurs in the triaxial tests at confining stress es sigma(3)' < 400 kPa and connected to a generally contractant deform ation behaviour and related to directions of the clay matrix, which ar e thought to be formed by a thrusting force of the moving ice in an ea rly phase of the glaciation, with the clay at depositional water conte nt and normally consolidated. The NW to N anisotropy occurs at confini ng stresses sigma(3)' > 400 kPa and is connected to a dilational defor mation behaviour along pre-existing fissures, which are thought to hav e been formed with the clay in an overconsolidated and stiffer state i n a later phase of the glaciation. (C) 1998 Elsevier Science B.V.