Evolution of structure and anisotropic shear stiffness of compacted loess during compression

Different compaction conditions (water content and density) may induce various soil structures. The influence of these structures on small strain shear stiffness G seems contradictory and is not understood (e.g., denser specimens may have larger or smaller G than looser specimens after compression). Furthermore, the influence of compaction condition on stiffness anisotropy remains unclear. This study investigated the evolution of structure and anisotropic stiffness of saturated and compacted loess during isotropic compression. Specimens compacted at different water contents and densities were explored. The measured G was normalised by a void ratio function (f(e)) to eliminate density effects. Before yielding, G/f(e) increases with decreasing compaction water content and increasing density. These two trends are reversed at large stresses (2 to 3 times yield stress), implying that an initially softer structure becomes stiffer. Based on MIP, SM and SEM results, the trend reversal is likely because interparticle contacts are more strengthened and pores are more compressed in the initially softer specimens. Furthermore, the stiffness anisotropy becomes more significant with decreasing compaction water content and increasing density because of more orientated fabrics, as evidenced by the particle/aggregate directional distribution results.