The Influence of Compaction on the Mechanical Behaviour of a Silty Sand

Abstract

The paper analyses the mechanical behaviour of a silty sand, as it was measured in laboratory tests on samples prepared with different density and moisture contents. After a description of the physical characteristics of the material, the methods used to prepare compacted and slurry-consolidated soil samples are reported, followed by a description of the apparatus and experimental procedures adopted. The compressibility characteristics were analysed by isotropic tests at ordinary stress levels (p'<1 MPa) and by oedometer tests at stress levels up to 40 MPa. The shear behaviour was investigated by means of triaxial compression tests following conventional undrained or drained paths with variable directions ; also, the initial shear stiffness was measured by resonant column tests. The basic behaviour of the slurry-consolidated silty sand appeared consistent with the theoretical framework of Critical State Soil Mechanics ; this soil could therefore be used as a reference material to which the mechanical properties of the compacted samples were compared throughout the paper. Two volumetric state loci were taken as reference curves to investigate the behaviour of the compacted material : (i) the isotropic compression line of the remoulded soil, and (ii) the critical state line, which proved to be independent of the soil preparation procedure. Whatever the preparation technique, the mechanical behaviour of the silty sand under triaxial compression was found to be stress-path dependent from the very beginning of the stress-strain curve. The net effect of the compaction could be assessed by comparing the stress-strain behaviour of dynamically compacted soil to that of remoulded material brought to comparable density through a mechanical overconsolidation process. The stress-strain behaviour of the compacted silty sand proved to be more fragile and showed lower initial stiffness than the remoulded soil. It was therefore observed that, even though the compaction may be considerably beneficial to compression behaviour, it does not necessarily optimise the stress-strain response under shear loads.