Stability evolution of clayey slopes and measurement modelling of mobilizing shearing resistance

Materials in black clays weakly silty of the chaotic complex of "Argille Sc agliose" and in blue clays with silt and weakly sandy of the basin of the U pper Pliocene, in the Bologna area (Italy) were examined. Samples with a different saturation degree for considering significant elas tic modulus and permeability coefficients characteristic of homogeneous rea l unities of slope were tested. With these data it was possible to perform direct shearing resistance measurements with parameters expressed in effect ive and residual terms, to anlyse the stability of slopes formed with diffe rent materials and in evolutionary configuration of erosion. The permeability of these materials measured directly in the country, is ab out 10(-4)cm.s(-1) for blue clay and 10(-5)cm.s(-1) for black clay. In the laboratory in connection 10(-8) homogeneous samples, permeability reaches v alues of 10(-8) cm.s(-1) for both materials. The subsequent tests of shear resistance carried out with the construction of a special speed reducer utilized three different speeds of preconstitute d shear, i.e.: 10(-3) cm.s(-1), 10(-5) cm. s(-1), 10(-7)cm.s(-1). These tes ts have put in evidence higher values of shear resistance for blue clays wi th respect to black clays which reach lower values for the shear speed of 1 0(-7) cm.s(-1). This shear speed was chosen because it allows an almost com plete dissipation of pore pressure. The axial compression tests with latera l deformation in free air revealed different elastic moduli for saturated a nd nonsaturated samples and for the two different materials tested. In part icular: the elastic modulus for blue clays is smaller than that for black c lays whereas saturated samples have an elastic modulus greater than that fo r non saturated samples. These data, introduced in a finite element model t hat utilises triangular mesh with at least six nodes, were analysed with th e automatic system of calculation of plasticity with axis symmetric produce d from "Plaxis". With erosional data collected monthly and annually, projected to 100 and 30 0 years, a slope erosional model was constructed that made it possible to v erify that black clays are unstable with Fs passing from 0.98 to 0.95 over 100 years. It is also possible to see that blue clays move towards stabilit y with Fs passing from 1.32 to 1.34 over 100 years and to 1.39 over 300 yea rs. (C) 2000 Elsevier Science Ltd. All rights reserved.