The effect of saturation and salts on the rigidity of fine-grained material

Abstract

Repeated multi-channel analysis of surface waves (MASW) and horizontal to vertical spectral ratio (HVSR) experiments were conducted on a hummock within a large landslide in east-central Utah adjacent to a small wetland. Stakes were placed into the ground to ensure identical array geometries and geophone locations. The landslide is an admixture of fine-grained material (clay, quartz and Na-plagioclase) and intermediate-composition volcanic clasts. A prominent crust of Na-salts covered the hummock surface, produced by evapotranspiration of shallow groundwater associated with the adjacent fen.The experiments produced three primary observations. First, on any given day, the Rayleigh wave frequency spectra were highly reproducible between individual experiments. Second, from May to October the spectra exhibited high variability between experiments conducted weeks apart, especially at frequencies <20 Hz. Third, a persistent subsurface boundary exists across which Vs increases from ~175 to ~300 m/s. However, that boundary migrated over time from a depth of ~5 to ~4 m.The low velocity of the upper ~5 m subsurface layer represents poorly-compacted landslide material, perhaps weakened from high Na+ concentrations. However, the variability in frequency spectra and upward migration of the boundary must have arisen from changing moisture and salt content over time. The fen was instrumented with a piezometer that showed an overall ~1 m decline in water levels from May to October with small, superimposed recharge events resulting from summer monsoon rains.This study leads to two important conclusions. Specific to this site and perhaps others, variable moisture content can affect capillary forces and the proportion of evaporite cements in the shallow subsurface that are manifested in strikingly different frequency spectra and migrating stiffness (Vs) boundaries. More generally, moisture and salt content can affect estimates of subsurface stiffness, including the position of apparent boundaries within unconsolidated materials.