Lateral oedometer testing of anisotropic clay as affected by surface roughness

Abstract

To examine the state of anisotropy of a natural (glaciated) clay sample lateral stress oedometer tests are carried out to determine the magnitude and direction of the principal stresses. Initially, this was done by assuming no slippage along the boundary, thus for a perfectly rough oedometric ring with uniform anisotropic stress state and zero in-plane deformation. Here the other extreme case, the perfectly smooth oedometric ring, is examined by accounting for the analytical solution of a circular cylindrical continuum for isotropic linear elasticity, using appropriate boundary conditions. After determining experimentally the frictional coefficient, this solution is used in a semi-empirical interpolation method for estimating the influence of the roughness of the oedometric ring. For the assumption of a rough oedometric ring for the investigated Potclay the anisotropy ratio decreases in the range 0·58 to 0·30 with increasing axial effective stress in the range 355 ≤ σax ≤ 1860 kPa. For the observed reduced wall roughness the anisotropy ratio is shown to increase to the range 0·70 to 0·40, which is a relative increase of 21% to 34%. The relative error in this estimated anisotropy ratio is estimated in the range ±9% to ±13%, increasing with increasing axial effective stress. The orientation of the in-plane anisotropy is shown to be independent of wall roughness.