Applicability of Hyperbolic Method for the Prediction of Shear Strength Parameters from Multistage Direct Shear Tests

Abstract

Measurement of shear strength through a conventional direct box shear test involves the requirement of at least three identical soil specimens. The collection of samples and carrying out a number of tests is very expensive and time consuming. Multistage shear strength test provides a faster method for the determination of shear strength parameters of a soil through tests on a single sample. Earlier studies conclusively proved the effective use of multistage triaxial compression test to predict the shear strength of soils. In this paper an attempt is made to study the possibility of using multistage box shear tests on a single soil sample instead of the conventional box shear tests to predict the shear strength. Undrained direct shear tests conducted on three different soil types – medium sand, air dried Cochin marine clay and red earth showed very good agreement between the results of multistage and conventional box shear tests. It has been brought out that the stress-strain curve in the shear box test follows the hyperbolic form throughout the test. Hence, it is possible to predict the failure shear stress, knowing the stress-strain relationship for the initial portion only. Making use of this behavior, multistage tests were carried out on single sample changing the normal load after obtaining initial portion of stress- strain behaviour. It has been brought out that the conventional box shear test could be approximated to multistage box shear test using only one soil sample, avoiding the variability between three or four soil samples used in a conventional test. The test procedure has the distinct advantage of requiring only one sample coupled with large saving in time without much compromise on the accuracy.