The Influence of Selected Parameters of Cyclic Process on Cohesive Soils Shear Characteristics at Small Strain / Wpływ Wybranych Parametrów Procesu Cyklicznego Na Charakterystyki Scinania Gruntów Spoistych W Zakresie Małych Odkształcen

The subject of the paper comprises a cohesive soil response to a cyclic loading applied in the range of small strains (10^-5÷ 10^-3). To this end tests of undrained cyclic shear in a triaxial compression apparatus were carried out on homogeneous material - kaoline from Tułowice. The tests were carried out on a modernised test bed, enabling full saturation of samples using the back pressure method, as well as a precise, intra-chamber measurement of small strains. Maintaining a constant deviatoric stress amplitude for NC and OC soils, the effect of ist size (A = 0.75 Δq or A = 0.375 Δq) as well as the influence of strain rate on material characteristics “deviatoric stress (excess pore water pressure) - axial strain” and effective stress paths were tested. While analysing the results obtained, a phenomenon of closing and stabilising initially open and moving loops were found, in contrast to proposal by Jardine [8]. The observed increments in the axial strain during cyclic loading operation, at the same levels of lateral effective stress, were greater for normally consolidated than for overconsolidated soils. At the same time, at each next cycle, these increments were smaller and smaller, assuming even the value equal to zero for the tenth cycle. Similar relationships occurred during the increase in the pore water pressure during the cyclic load action. For the set number of cycles n = 10 they were that small - max. 46% (and decreasing with each consecutive cycle) that they did not result in weakening of the material. Taking the trend of decreasing Δu increments into account it was possible to accept that the conclusion considered was right irrespective of the cycles’ number.