Stress Dependence of Sand Stiffness

The stress dependence of the Young\u27s modulus of two sands was studied with a computer controlled triaxial apparatus, capable of giving reliable measurements of the stiffness even at very small strains ≈ 0.001 %. It was therefore possible to determine the Young\u27s modulus of the sands for a wide strain range, from very small to very large, up to 20 %. The tested sands were: a uniform fine quartz sand not containing fines (Toyoura sand- TOS), and a well graded coarse to medium carbonatic, crushable sand containing about 2% of fines (Quiou sand - QS). The specimens were reconstituted in the laboratory at a given density by pluvial deposition. Two test series were performed: 1) Compression loading triaxial tests, at a constant strain rate, on specimens consolidated at σ\u27vc = 100kPa and stress ratio K variable between Ko(NC) and 2.0 for TOS. For QS the consolidation stress ratio K ranged from Ko(NC) and 1.0. 2) Compression loading triaxial tests, at a constant strain rate, on specimens of TOS consolidated at σ\u27vc = 100kPa and K variable between Ko(NC) and 2.0. Specimens were prevalently normally - consolidated; however a few of them were subjected to mechanical over consolidation or prestressing at constant K. The main conclusions obtained from the above experiments are: - The small strain Young\u27s modulus E0 depends on the vertical consolidation stress but is independent of the horizontal consolidation stress for both sands; - E0 of the quartz sand is independent of the loading history; this is not true for the carbonatic sand, where E0 increases with increasing of the over consolidation ratio (OCR) values, which is probably due to the sand crushability; -the secant Young\u27s modulus E at larger strains is mainly controlled by the horizontal stress or by the mean stress