The effective shear strength of artificially fissured overconsolidated clays.

Abstract

The effective shear strength of artificially overconsolidated clays with continuous fissures, or with discontinuous or partial fissuring, has been discussed from both the experimental and numerical points of view.Direct shear and triaxial tests have been conducted on a range of unfissured, partially and fully fissured specimens of artificially overconsolidated clay samples in the laboratory. Specimens subjected to direct shear tests have been prepared in three different preconsolidation pressures and two or three different Overconsolidation ratios (OCR) for unfissured, partially and fully fissured specimens. Specimens subjected to triaxial tests also were prepared for three different preconsolidation pressures and overconsolidation ratios. In order to investigate the effect of orientation of fissures, artificially overconsolidated fissured triaxial specimens were prepared in three different orientations at 30 degrees, 45 degrees and 60 degrees to the direction of minimum principal stress (sigma[subscript]3).For both direct shear and triaxial tests, special tools and devices were designed and constructed to prepare unfissured, partially and fully fissured specimens.Taking into account the number of parameters which influence the effective shear strength of overconsolidated clays, and the time which is needed to artificially prepare the overconsolidated specimens and to run drained tests, as well as the impossibility or impracticality of the laboratory simulations for some specific cases, numerical methods were used to complement the experimental component of the investigation.Numerical modelling of direct shear and triaxial specimens utilised the FLAC (Fast Lagrangian Analysis of Continua, Itasca, 1993) program for two dimensional simulation of the direct shear tests and the ANSYS (1996) program for three dimensional simulation of triaxial tests. The experimental results have been used to calibrate the coefficients of the numerical models and to verify the results obtained from numerical models.Strain softening behaviour was simulated numerically for unfissured and fully fissured specimens subjected to direct shear tests. Using the obtained experimental and numerical results of the study of direct shear tests with respect to the effects of different parameters on the effective shear strength of the spacing subjected to direct shear tests and also FLAC programming, FLACish (FISH), a model was written designated as the Homogenised Strain Softening Model (HSSM). In this model the effects of different parameters discussed in this thesis, are applied to the Mohr Coulomb parameters (c'[subscript]u and phi'[subscript]u) of unfissured specimens. This model was used to predict the effective shear strength of cases in which laboratory simulation was impractical or not feasible.The advantage of this model (HSSM) is that it relates the effective shear strength of the fissured mass to the corresponding Coulomb parameters (c'[subscript]u and phi'[subscript]u) of the intact or unfissured overconsolidated clay specimen with reduction functions relating to the parameters discussed in this thesis.The numerical models developed by ANSYS were calibrated and verified by the experimental results, and then used to predict or estimate the effects of confining pressure, orientation of fissures on the three dimensional modelling of the partially and fully fissured overconsolidated triaxial specimens.In this thesis the effects of the type of clay, preconsolidation (P'[subscript]c) pressure, Overconsolidation ratio (OCR), size of sample, rate of shearing and fissure parameters, such as spacing, width and orientation of fissure were discussed and identified or quantified to estimate the effective shear strength of the artificially overconsolidated fissured samples. These results are applicable for the estimation of the effective shear strength of the naturally overconsolidated fissured mass by homogenising the effects of the parameters on the Mohr Coulomb parameters (c'[subscript]u and phi'[subscript]u) of the intact or unfissured clay mass.