Behaviour of a compacted silty sand during suction controlled tests

The paper reports the results of an extensive experimental study carried out on a silty sand from the core of the Castagnara dam (Italy), whose properties are representative of a large variety of earth construction materials. The soil has been dynamically compacted at the optimum water content by the modified Proctor procedure and then tested in suction-controlled conditions. For this purpose a stress path triaxial cell (Bishop and Wesley type) and an oedometer (Wissa type) have been used, both modified in order to test partially saturated soils. Altogether the experimental program consists of 23 tests, carried out in the suction range of 0÷400 kPa. The results permit the complete characterisation of the soil behaviour. The influence of suction on compressibility, stiffness and shear strength has been clearly and continuously quantified; the mechanical properties of the soil improve with suction, following an exponential law with decreasing gradient. The experimental data clearly show the existence of a threshold mean net stress of 150÷200 kPa, above which the compacted silty sand collapses. The soil behaviour along variable suction stress-paths is consistent with the general framework deduced from constant suction tests