Investigation and measurements of the shear strengths cohesive soils

Abstract

Methods for the determination of the shear strength of cohesive soil have been in process of development since 1935 in the Laboratory for Soil Mechanics at the Federal Institute of Technology in Zurich, and the combined use of the ring shear apparatus and the triaxial apparatus has been specially studied. Through a division of the shear strength in friction and cohesion, and based on tensile tests and cone tests with artificially consolidated samples, it is proved that the cohesive part increases linearly with the pressure. For conditions without stressed pore water the relationship between shear strength and consolidation pressure (a-line) is set up in parallel to the relationship between shear strength and water content. The special conditions which must be observed during shear tests with relieved samples are dictated by the change in volume which takes place during the test. As a complement to the shear strength the residual shear stress is introduced. Through a number of tests. the theoretical relationships are investigated and deviations established. The results of ring shear and triaxial apparatus tests carried out with confined and hydrostatically consolidated samples show a satisfactory confirmation. It is proved that for active lateral pressures the triaxial test gives reliable values even for very small heights of the sample. The practical applications of the test results are demonstrated by calculation of the stability of a dam slope. Special attention is drawn to the creep before failure, which is again related to the progressive failure. The introduction of the residual shear stress makes it possible to estimate, in a simple way, the stability for the progressive failure.