Shear Strength and Stiffness of Expansive Soil and Rubber (ESR) Mixtures in Undrained Axisymmetric Compression

The shear strength and stiffness of expansive soil and rubber (ESR) mixtures were evaluated systematically in this study in undrained axisymmetric compression. The materials tested included the Pierre shale residual soil from the Front Range of Colorado and granulated rubber with 6.7-mm nominal maximum particle size. The three mass-based rubber contents used were 0, 10 and 20%. All specimens subjected to element testing were consistently prepared at soil states defined for a single level of relative compaction equal to 95% and optimum water content, based on the standard Proctor compaction curves of each mixture. The effects of the rubber content and mean effective stress were systematically evaluated by performing isotropically consolidated undrained triaxial tests at mean effective stresses equal to 50, 100 and 200 kPa. Bender element tests were used to determine the small-strain stiffness of the Pierre shale residual soil and its ESR mixtures at mean effective stresses of 50, 100 and 200 kPa. The critical-state friction angle increased with increasing rubber content for specimens compacted to and tested at similar initial states. Conversely, the stiffness of the ESR mixtures decreased with increasing rubber content for all three levels of mean effective stress used in the tests. The drop in stiffness was more significant at low mean effective stresses.