Influence of NaCl Content on the Small-Strain Shear Modulus and Microstructure of Saline Soil: Resonant Column and SEM Analysis

Xinjiang’s inland arid climate has led to widespread saline–alkali soils. Along with frequent strong earthquakes, they pose challenges to foundation and infrastructure construction. So, understanding the small-strain dynamic properties of saline–alkali soils is of scientifical and engineering significance. This study used resonant column and SEM tests to investigate the influence of microstructure on the small-strain shear modulus (G0) of saline soil, considering variations in dry density and NaCl content. This study makes three distinctive contributions: (1) it provides quantitative evidence that NaCl (rather than Na2SO4) reduces G0 in Taklamakan saline silty clay, with a critical inflection point at 10%—a finding opposite to the behavior of sulfate saline soils reported in previous studies; (2) it establishes a direct quantitative link between NaCl-induced macroporosity increase and G0 reduction via the Young–Laplace equation, providing a mechanistic explanation at the pore scale; (3) it proposes a modified Hardin equation that explicitly incorporates NaCl content as a state variable, enabling practical prediction of G0 for chloride saline soils in engineering design. Overall, these findings provide essential technical guidance for engineering design in regions affected by saline–alkali soils.