Stress-dependent SPT-based estimation of small-strain shear modulus for marine sand in central Vietnam and application to high-speed railway foundations

Abstract

This study evaluates the influence of effective stress on the small-strain shear modulus (G₀) of coastal sand in central Vietnam using data obtained from downhole seismic and standard penetration tests (SPT).  A database consisting of 50 paired SPT–downhole test intervals collected from the investigated boreholes was analyzed to develop a stress-dependent correlation of the form G₀ = A(σ’v)mNn, where G₀ is expressed in MPa and σ’v in kPa. Regression analysis indicates that the contribution of SPT resistance (n ≈ 0.40) is substantially greater than that of effective stress (m ≈ 0.20), suggesting that soil shear stiffness is governed primarily by penetration resistance. The weak stress dependency explains why conventional G₀–N correlations provide reasonable predictions for the investigated deposits. The proposed formulation offers a practical framework for preliminary stiffness estimation and dynamic behavior assessment of coastal sandy soils under field conditions. To demonstrate its engineering applicability, the estimated G₀ profiles were incorporated into a layered elastic railway foundation model based on the analytical approach proposed by Sheng et al. (1999). Representative loading conditions corresponding to the future North–South High-Speed Railway in Vietnam, including passenger and freight axle loads, were considered. Parametric analyses were conducted to evaluate the effects of train speed, and axle load on the vertical displacement response of the foundation system. The results indicate that the proposed correlation can provide realistic stiffness profiles for preliminary evaluation of railway foundation performance and can serve as a practical tool for geotechnical assessment in regions where direct geophysical measurements are unavailable.