Development of a new ring shear apparatus for investigating the critical state of sands

The present study investigates the shearing behavior of sands pertinent to liquefaction and critical state soil mechanics using an improved ring shear apparatus designed and constructed at the University of Illinois. Undrained/constant volume and drained triaxial compression and ring shear tests (sheared to about 30 m of shear displacement) were performed on two clean sands and one silty sand. Shear displacements localized when the peak friction angle was mobilized and subsequent shear displacements occur only within the shear band which was (10–14)×D50 thick. Considerable particle damage and crushing was observed within the shear band, particularly for dilative specimens, which led to volumetric contraction in the shear band. The critical state line (corresponding to the critical state after particle damage and crushing was complete) was much steeper and plotted below conventional critical state lines in e – log σ' space measured using triaxial tests. Both dense and loose sands reached this final state. Accordingly, two definitions of critical state of sands with and without particle damage are proposed. The critical state friction angle from the ring shear tests was independent of the initial sand fabric and decreased only slightly with stress level, becoming essentially constant at stresses larger than 100–200 kPa. Particle crushing induced in the ring shear tests increased the critical state friction angle by a few degrees by producing a wider grain size distribution and more angular particles. However, because some of the triaxial specimens likely did not reach a critical state, the mobilized friction angle from triaxial tests was influenced by the initial fabric of the sand. A constant critical shear strength was achieved at very large shear displacements (>750 cm) in the ring shear tests, and before this the shear resistance of sands was dependent on the amount of shear displacement and particle crushing. Yield strength ratios of contractive specimens ranged from 0.15 to 0.31, while the critical strength ratios of both contractive and dilative specimens decreased from a range of 0.04–0.21 (for the original sand) to 0.01–0.07 (for the crushed sand)