Investigation on shear strength parameters of soil and soft rock material in the low stress range

The use of the Mohr-Coulomb shear parameters, friction angle (φʹ) and cohesion (cʹ), to define the expected shear strength of the soil is a widespread approach in the verification of the stability of a wide variety of structures. Due to technical limitations, these parameters are generally determined in the lab at excessively higher stress levels compared to field stress levels. The aim of this work is to investigate material behaviour in the lower stress range in the laboratory and to illustrate the development of the failure surface in these areas. For this purpose, special tests were carried out in which, by increasing the pore water pressure, a reduction in the effective stresses in the specimen could be achieved. The effective stresses were reduced until the specimen failed. By performing this test - starting from a normal consolidation stress the shear strength can be investigated in the low stress level.