Large and Small Strain Properties of Sands Subjected to Local Void Increase

Abstract

Local strain effects are proposed as a source of shear strength reduction in cemented particulate media, shales, and heterogeneous systems. Shear strength degradation through local straining may arise from particle dissolution, double-layer shrinkage in reactive clay phases, and volume change associated with thermal processes. This study focuses on mechanical property changes produced by local straining in particulate systems made of sand–salt mixtures. Local strains were induced by the dissolution of salt particles. Large-strain properties (angle of shear resistance and dilation rate) were measured using triaxial test methods. Small-strain properties (acoustic wave velocity and attenuation) were measured with a resonant column device and piezocrystals (bender elements). Experimental data showed that large-strain properties are sensitive to changes in aggregate volume; a reduction in the angle of shearing resistance up to 26% was observed for a 90% sand–10% salt mixture after salt dislocation. Acoustic wave velocity and attenuation values changed up to 25% during particle dissolution. Fine sand–salt specimens showed smaller changes in macroscopic parameters, compared to coarse-grained specimens. Changes at the microscale assessed using small-strain measurements are clearly reflected at the macroscale as a reduction in the angle of shearing resistance. Finally, it is shown that changes in macroscale parameters produced by internal volumetric strains can be estimated by considering the change in the void ratio and assuming a random distribution of internal strains. However, small-strain parameters cannot be evaluated using the same approach because the microstructure has a stronger effect on wave propagation parameters (velocity and attenuation) than the macroscopic parameters.