Mangone Sambe

Effects of anisotropic stress states on deformation characteristics of sand in small strain range

Hirofumi Toyota

The real ground formed by sands and/or clay has usually anisotropic mechanical behavior. A deep understanding about the anisotropy and the effects on deformation characteristics of soils are very important when analyzing ground deformation in engineering earthworks. Especially, accurate initial shear modulus of soils is a key parameter to design and construct the underground structures rationally.
The effects of anisotropic stress states on deformation characteristics of sand in small strain range (represented by the stiffness = initial shear modulus G0) was treated in this study. The monotonic triaxial compression/extension test on small strain testing apparatus were conducted using Toyoura sand. Specimens were made at different deposition angles of 0, 45, and 90 degrees to investigate the effects of inherent anisotropy. To examine the induced anisotropy, specimens were consolidated at stress ratios (K = σ’h/σ’v) of K= 0.25, 0.4, 0.6 (compressional anisotropic consolidation) and K= 2, 3 (extensional anisotropic consolidation). K of 1 was used for isotropic consolidation. Bender element (BE) and local small strain (LSS) tests, by which precise deformation modulus can be measured, were simultaneously conducted in the same specimens. Shearing was conducted under compression loading and extension loading on specimens at the same conditions for comparison.
Bender element results show that the initial shear modulus was greater when the depositional angle increased from 0° to 90°. Under compressional anisotropic consolidation (K<1), the initial shear modulus decreased with the stress ratio K, from K=0.4 to 1. Then, under extensional anisotropic consolidation (K>1), it increased from K=1 to 2. However, from K=0.4 to 0.25 and K=2 to 3, the initial shear modulus decreased.
Local small strain (LSS) results of compression tests show that the secant shear modulus (Gsec) became greater when the stress ratio was larger, from K=0.25 to K=0.4. Then it decreased from K=0.4 to K=3. LSS results of extension tests show that the secant shear modulus (Gsec) was decreasing when the stress ratio became larger from K=0.25 to K=2. The comparison of results shows that secant shear modulus was almost the same for compression and extension tests at very small strain (smaller than 0.001%). From the strain of 0.001% to 0.1%, the secant shear modulus of extension tests was greater than that obtained in compression tests. The difference between compression and extension became smaller when the stress ratio increased and reached K=1.