Assessment of the Significance of Moisture Variations in Construction Soils Investigated Using a Light Weight Deflectometer: an Experimental Study on Fine Sands and Sandy Silts/Sandy Clays

Abstract

In situ investigations of construction subsoil are a key component of engineering site investigation for infrastructure and building projects. The light weight deflectometer method (LWD) is becoming increasingly popular owing to its rapid measurement procedure, ease of use, and high mobility during investigations. This study focused on determining the influence of moisture content in the near-surface soil layer, down to approximately 1 m below ground level, on subsoil stiffness, expressed as the dynamic deformation modulus (Evd). The investigations were carried out in two measurement campaigns, before and after artificial irrigation equivalent to 6 mm of rainfall, on two experimental plots composed of coarse-grained fine sand (fSa) and fine-grained sandy silt/sandy clay (saSi/saCl). The obtained results demonstrated a clear influence of moisture content on the Evd modulus. For the fSa plot, Evd decreased by 18%, with an average increase in moisture content of approximately 8 percentage points. In contrast, for the plot composed of saSi/saCl soils, Evd decreased by approximately 41%, with an average change in moisture content of approximately 2 percentage points. These contrasting results indicate a stronger response of fine-grained soils to even small changes in their moisture content, as well as their different sensitivity during LWD testing. The study provides a controlled, plot-scale quantification of moisture-induced changes in LWD-derived stiffness in two contrasting natural subsoils. Furthermore, the findings show that near-surface moisture conditions should be considered during geological and engineering site investigations to avoid an incorrect assessment of foundation conditions.