Effects of Principal Stress Direction and the Intermediate Principal Stress on the Stress-Strain-Strength Behavior of a Cross-Anisotropic Fine Sand Deposit
Variations in the principal stress inclination and the intermediate principal stress ratio can significantly affect the behavior of cross-anisotropic soils. The main focus of this investigation is to explore the stress-strain-strength behavior of a cross-anisotropic granular soil by performing a series of drained tests on air pluviated specimens of fine Nevada Sand.A newly developed Hollow Torsional Shear Apparatus was used to shear specimens to failure following a predetermined stress path that maintained constant values of principal stress inclination, principal stress ratio, and mean normal effective stress.Tests showed that soil stiffness decreased with increasing principal stress inclinations up to 67.5 degrees after which a moderate increase in stiffness was observed to 90 degrees. Peak shear strength values followed a similar trend for mid range b-values. However, the trough occurring at 67.5 degrees was not observed near the end conditions of b = 0 or 1. Increasing b-values from 0 to 0.5 resulted in significantly increased stiffnesses. A more subtle increase in stiffness occurred from 0.5 to 1. Peak shear strength increased from b = 0 to 0.5 and decreased from 0.5 to 1. However, the strength at b = 1 was somewhat higher than b = 0.During the elastic portion of the test, the strain increment directions were several degrees higher than the stress directions for tests with principal stress inclinations of 22.5 degrees and 45 degrees while at 67.5 degrees the strain increments were several degrees lower than the stress directions. All strain increment directions approached the stress directions at failure.Shear bands propagating parallel to the specimen wall intersected the rigid end platens and were generally thin and ubiquitous. At both high and low b-values shear bands propagated perpendicular to the specimen wall. These shear bands, which intersected flexible membranes rather than rigid end platens, were typically thick and well defined.The inclination of shear bands propagating parallel to the specimen wall agreed well with Coulomb's theoretical prediction. It was not possible to measure the inclination of shear bands propagating perpendicular to the specimen wall.
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