Direct Shear Testing of Various Hard Soils and Weak Rocks from Greece

Hard soils-weak rocks combine characteristics of soil behavior and rock behavior. Perhaps the most distinct among them is brittleness reducing shear strength significantly after failure with very little additional shear displacement combined with intense dilation. Several hard soils-weak rocks were tested under drained direct shear conditions following consolidation to various vertical stress values. All of them exhibited very high reduction of shear strength after rupture, ranging from 32 to 71% taking place in as little as 0.1 mm of additional shear displacement. This loss of strength resulted in very large decrease of cohesion intercept and for all soils significant decrease in the angle of shearing resistance. Dilation was abrupt during rupture and was as high as 1.2 mm for the coarsest of the soils. Dilation was exhibited for all vertical stress values applied, never switching to contraction even for the highest vertical stress applied except one point for one soil. Percentage loss of peak shear strength after failure remained very high even for the highest vertical stress values applied, with points of percentage loss of peak strength with vertical stress falling in a very tight region for all soils. This is in clear difference to both the position of points falling on the same plot for a preconsolidated clayey soil and the higher rate of strength loss decrease with increasing vertical stress compared to the hard soils-weak rocks tested. The test results presented add to the existing understanding of the behavior of hard soils-weak rocks under drained direct shear and constitute examples of behavior of very brittle hard soils-weak rocks.