EXPERIMENTAL LABORATORY DETERMINATION OF THE STEADY STATE OF SANDS

DIEGO C. F. LO PRESTI, IGNAZIO PUClI, ORONZO PALLARA, ROBERTO MANISCALCO and SERGIO PEDRONI

ABSTRACT Some experimental problems encountered when determining Steady-State (SS) or Phase Transformation (PT) envelopes and their projections in the state diagram are studied; 1) accurate assessment of the void ratio (e) at the end of the consolidation process; 2) the effects due to the membrane compliance; 3) effectiveness of lubrication ends and 4) influence of specimen slenderness on the lateral strain uniformity. The specimen's dimensions after its formation were continuously monitored by measuring axial and radial strains locally. Both conventional undrained triaxial tests and constant volume tests with continuous correction for the membrane penetration effects were performed at constant cell pressure using loose saturated Toyoura sand. The adopted lubrication system worked effectively up to an axial strain of 40/0 for specimens with a height to diameter ratio of H/D=2, while for H/D=1 lateral strains were uniform until the end of the tests (e_a=25%). The SS and PT envelope and their projection in the state diagram (SS or PT line) were established based on results from 29 tests. The condition which triggers the softening behaviour, i.e. the so called Critical Stress Ratio (CSR) envelope was also determined. The test results showed the importance of careful control of the test factors mentiononed above determining the undrained behaviour of saturated very loose sand under truly constant conditions

key words: lubricated ends, membrane compliance, sand, triaxial compression test, undrained shear (IGC: D6/D7)