Drying effect on the microstructure, water retention and stiffness of compacted salinised loess

Abstract

Salt-induced deterioration is a major issue for earthen structures on the Chinese Loess Plateau. This study investigated the soil-water retention curve (SWRC) of loess with 2% Na₂SO₄ (C0S2) or NaCl (C2S0) using the contact filter paper method and a chilled-mirror dew-point hygrometer (WP4C). Mercury intrusion porosimetry (MIP) was used to examine pore size distribution (PSD) evolution during drying, while the small-strain shear modulus (G_max) was measured via bender element tests. The results showed that salt-free loess and Na₂SO₄-amended loess followed a “bimodal-trimodal-bimodal” PSD evolution during drying, whereas NaCl-amended loess maintained a consistent bimodal pattern. Before crystallisation, Na₂SO₄ reduced micro- and meso-pores while expanding macro-pores, altering the pore structure. In contrast, NaCl maintained a more uniform pore size distribution, preserving the soil microstructure. NaCl also generated higher osmotic suction than Na₂SO₄, and the difference between both increased as Na₂SO₄ crystallised earlier. However, the influence of osmotic suction weakened as drying progressed. Before crystallisation, matric suction enhanced interparticle attraction, increasing G_max, while osmotic suction provided additional tensile forces. As salts crystallised, osmotic suction declined, and pore filling stabilised meso-pore structures. In the final drying stage, osmotic suction became negligible, matric suction dominated, and Na₂SO₄ crystallisation expansion in C0S2 disrupted particle arrangements, potentially reducing G_max.