BEHAVIOUR OF CLAYEY GOUGE MATERIAL ALONG DISCONTINUITY SURFACES IN ROCK MASS

Abstract

The Rock masses near slopes behave as discontinuum due to the presence of discontinuities such as joints, faults, shear zones, thrust zones, bedding planes etc.. The most important factor is the shear strength of potential failure planes. The strength deformation relationship and particle orientation in the gouge govern the behaviour of rock mass, when it approaches the state of limit equilibrium. The presence of clay gouge material influences significantly the shear strength of joints. The available data on behaviour of joints or fracture planes filled with clay gouge is inadeguate and earlier workers have used direct shear tests on artificial joints. So the investigations were carried out to study the infl uence of gouge thickness, joint roughness,amplitude or asperities on strength of joints infilled with clay gouge material from tri axial tests. The pore water pressure in joints was also measured. The degree of particle breakdown due to shear displacement was studied. Samples of shale collected from joints and shear zones of Kaliasaur landslide were mixed and pulverised to simulate joints with clay gouge in field situations. Similarly joints with clay gouge were simulated for testing on samples of Kohima clay, Ball clay and China clay. Unconsolidated-Undrained and Ko Consolidated Undrained triaxial tests were carried out at various joint dip angles of 5°,20°,30°,45°and 50° with Kaliasaur clay gouge of 5mm, VII 10mm, 20mm, 30mm and 40mm thickness at speed of shearing 5,10,20, 40 and 80 mm/hr for both undulating and planar type of joints. Ko C-U tests were carried out at speed of shearing 5, 20 and 80 mm/hr. U-U tests with Kohima clay, Ball clay and China clay were carried out at dip angle of 30° for comparison. The Consolidated Undrained tests on samples of the gouge materials were conducted by multistage triaxial testing method. The particle size distribution of gouge materials after cond ucting triaxial tests were obtained by automated laser particle size analyser to assess the degree of particle breakdown due to shear displacement. The degree of particle breakdown was compared with the original grain size distribution of the gouge materials. Experimental results indicated that evaluation of shear stre ngth parameters should be carried out at 20 to 40mm/hr controlled rate of strain in triaxial tests for undulating and planar joints filled with clay gouge. The results also revealed the lowest strength at dip angles 30°-45° irrespective of strain rate. With increase of gouge thickness (t/a > 1.25), exponential decrease in strength was observed upto about t/a = 10 and thereafter, it indicated shear strength of gouge material only. PWP factor (ru) during shearing for filled discontinuities may be used to estab lish the factor of safety for risk analysis of failure along joint surfaces. Test results indicated that the deviator stress which cont rols the shear failure is a better criterion for evaluating shear strength of joints with thick gouge (t >> a). So modifications in VI ij Barton's equation have been proposed suggesting the correlation factor for thickness. The correlation, to predict the shear modulus of joints filled with gouge, have also been proposed. The shear strength of clay gouge material is gradually reduced due to progressive realignment of clay particles during shear displacement along the slip surfaces. Thus a rock slope which is stable over several decades, may fail due to the develo pment of slickensides within the clay gouge with clay fraction > 25 % . Evidences of particle breakdown studied through laser particle size analyser and electron microscope are in agreement with earlier work and may be a simple technique to evaluate the residual shear strength for post failure slope analysis. IX