Kriechverhalten gefüllter Gesteinstrennflächen
Creep deformations of a rock mass occur predominantly along its discontinuities, preferably on those filled with gouge material. Since so far only little Information has been available on the creep behaviour of filled rock discontinuities a detailed experimental study has been conducted to investigate the critical parameters and to quantify their influence.
Creep tests on filled rock discontinuities were performed on sandwichtype samples in modified direct shear frames at different boundary conditions. Factors of influence investigated were type of filier material, shear stress, normal stress, and size of the specimen. Smooth saw-cut surfaces were used as rock discontinuities since these proved to represent boundary conditions which are similar even to those at rough rock surfaces, where the material in the roughness troughs is not involved in the creep movement.
The results clearly show for the primary creep phase an exponential dependence of the creep velocity on time, whereby the exponential exponent m is equal to -1 for lower shear stresses, but it increases towards zero for higher shear stresses. The initial creep velocity is directly proportional to the thickness of the filier layer and related to the clay content of the filier by an exponential function. The exponent m was found to be dependent on the clay content, the layer thickness, and the shear stress level related to the residual shear strength. The parameters of these empirical functions were derived from the experimental data.
None of the creep tests showed the existence of a secondary stationary creep phase but an immediate transition from the primary to the tertiary, accelerating creep phase was observed. For the higher shear stress levels investigated an exponential decrease of the time of transition with increasing clay content was found. The clay content of the filier, i.e. the content of particles smaller than 2 μm, thus represents the critical material parameter which determines the primary and tertiary creep behavior of a filled discontinuity.
Other potential factors of influence, such as normal stress and size of the discontinuity, are automatically taken into consideration if, aspracticed in this study, the creep data are given as a function of the relative shear stress, i.e. the ratio of actual shear stress to residual shear strength. In all tests the consistency index of the filier was kept constant and close to the lower bound of values observed in critical slopes with sliding on filled discontinuities.
Potential reasons for the direct dependence of the creep behaviour in the primary and tertiary phase on the clay content of the filier are discussed on the basis of a simplified physical model.
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