Influence of joint roughness on discontinuity shear strength

The results of a study on the influence of the joint roughness, considered to be a geometric parameter only, on the evaluation of the shear strength of the two making surfaces are presented. Reference is made to casts of natural discontinuities in order to carry out a number of shear tests with increasing normal stress on surfaces with the same roughness, thus avoiding the problem of altering the initial discontinuity roughness during the test. A good agreement was found between the values of the second order angles i (Patton, 1966), that were measured with an automatic mechanical profilometer, the values of the angles of first displacement ? obtained from a tilt test, and the values of i=arctg(?/?)-?b, obtained from direct shear tests. This allows one to write a relation between the value of the angles i, measured by the mechanical profilometer, and the horizontal strain given by the percentage of the relative displacement of the two surfaces of a discontinuity. In this manner a reliable prediction can be made in an early stage of the analysis, when only the data on the discontinuity morphology and the tilt test results are available.