A large number of geotechnical, agricultural and environmen-tal site investigations need knowledge of the distribution of hydraulic properties of the unsaturated soils and their temporal changes. Unsaturated hydraulic properties are also important in climate modelling and in the study of biological and chemical processes in the vadose zone. The ability of the soil to retain or transmit water and its constituents is reflected by these properties. In hillslopes, the flow of water, controlled by the hydraulic properties of the soil, is crucial for water resource management and for predicting slope failure caused by heavy rainfall. One of the most important hydraulic properties is the water retention curve or the soil-water characteristic curve (SWCC), which relates volumetric water content or degree of water saturation to matric suction. Water content and matric suction are related to other important soil properties including: hydraulic conductivity function, cohesion and internal friction (e.g., Guo, 1985; Lu et al., 2012; Robinson et al., 2017). The stability of an unsaturated soil slope or a river embankment subjected to rainfall is determined to a great extent by SWCC. Recently, it has been shown that changes in seismic shear-wave velocity (VS) in unsaturated soils can be used to monitor in situ the distribution of water and the dynamic water transportation in the vadose zone (Suzaki et al., 2017). In this research, among various models for SWCC, the van Genuchten (1980) model was assumed: (1) where s is matric suction, SW is degree of water saturation, a and n are van Genuchten parameters that depend on the soil-type. These describe, respectively, the air-entry suction and the slope of SWCC. A Bishop-type model for effective stress was coupled with a well-tested form of the stress-suction-saturation-depend-ence of the small-strain shear modulus G0 (Sawangsuriya et al., 2009; Han and Vanapalli, 2016), which gave (2) where x is a sample point representing a certain spatial location in the subsurface, and A and B are fitting parameters that depend on the soil-type. More recently, it has been possible to invert the SWCC parameters a and n from G0, SW and bulk density rb obtained from geophysical measurements (Suzaki et al., 2019a, 2019b). In this article, we first illustrate the accuracy in the result of SWCC inversion that one might expect when the input data is realistically noisy. Once the in-situ SWCC is obtained by inver-sion, in the next stage, considering a projected time series for the future rainfall, the distribution of hydraulic properties, viz SW and s, within the soil embankment at different times in the future is calculated through seepage analysis. This provides an important piece of information to assess the vulnerability of a dyke in the future, given an anticipated rainfall. We analyse the prediction error and its cause. Considering Coulomb failure for a sand embankment, we examine the predicted factor of safety (FOS) and its temporal evolution in comparison with the true values.@en

The soil-water characteristic curve (SWCC) is crucial in order to characterize the hydraulic and mechanical properties of unsaturated soils. To overcome the problem of error/uncertainties in laboratory estimates due to sample disturbances and to incorporate spatial heterogeneities, in-situ SWCC inversion using geophysical measurements appears to be a worthwhile, albeit challenging, goal. For SWCC inversion for a dyke under rainfall or rising water level or for a dyke containing low-permeable soils, accounting for the non-hydrostatic condition is crucial. Furthermore, the geometry of a dyke is important in determining the internal stress state. In this research, we model the shear modulus in an unsaturated dyke including the effect of the dyke geometry. This is achieved by solving the static deformation problem using FEM. Our newly developed inversion scheme assumes availability of integrated geophysical measurements to obtain saturation, bulk density and shear-wave velocity. We illustrate numerically the significance of the estimation of confining stress distribution using FEM for modelling the shear modulus and for SWCC inversion. Our inversion scheme can correctly handle the non-hydrostatic condition. This can have a large impact on diverse applications related to stability of earth-retaining structures as well as in problems involving dynamic transportation of fluids.@en

The Cocoa Grove Group is planning a housing development on approximately 5.5 acres in the area of Saphire, Diamond, St Lucia. Directly downhill of the property is a WASCO (Water and Sewage Company) Spring that has supplied drinking water to the town of Soufriere, going back to historic times. This potable water is of high purity. Concern was therefore expressed by WASCO as to the advisability of implementing any development directly above the emanation of the spring. The obvious concerns are the possible change in flow volumes and chemical and biological contamination if a housing development was completed. F.D. Morgan and the St Lucia Water Resources and personnel representing the Cocoa Grove Group discussed whether or not geophysical methods could be used to map the direction of groundwater flow into the spring. The group agreed that geophysical methods could be used in an attempt to delineate the underground flow paths into the spring. But it was made clear that the investigations could produce results that could be either favorable or unfavorable to the proposed development. Consequently, geophysical surveys were planned and executed in the area of concern, to measure resistivity and self potentials (SP). The geophysics did not indicate evidence, as expected, of substantial fluid flow into the spring. However, we were able to locate the main water supply from the top of Terre Blanche down towards the spring. The flow comes downhill via a significant geological fracture that is hidden from sight in the dense tree cover. Recommendations were made that it was relatively safe to develop the site as intended, with respect to possible water contamination. In conclusion, we also recommend that WASCO could take water from another point of a topographic step on Terre Blanche. Doing so, would reduce the contamination risk essentially to zero from the planned housing development area.@en

The soil-water characteristic curve (SWCC) plays an important role in determining the hydraulic and mechanical properties of the unsaturated soils. SWCC is vital to evaluating the stability of a slope or a river embankment. The sensitivity of seismic shear-wave velocity to SWCC has recently been quantified. However, the inversion of SWCC from field-observed VS is challenging. In this research, we have explored the possibility of integrating shear-wave velocity, water saturation and bulk density obtained from independent geophysical measurements and utilizing the underlying physics of shear-wave in terms of suction, saturation and stress, to invert SWCC. We derive a formulation for the shear modulus by combining van Genuchten model with stress-, suction- and saturationdependence of shear modulus. The stress is estimated using FEM, thereby taking into account the effect of geometry of the embankment. Tests on realistic synthetic data indicates that the inversion is very stable. This new scheme allows, for the first time, to estimate in-situ SWCC from integrated geophysical measurements. This can potentially have a large impact in diverse applications related to stability of earth-retaining structures, natural slopes, as well as in problems involving dynamic transportation of fluids in the vadose zone.@en

Understanding the effect of saturation is important in assessing the failure mechanism on the land-side of a river-dyke due to rising water level in a river after heavy rainfall. S-wave velocity is controlled by soil suction and degree of saturation. Therefore, there is a possibility to estimate the unsaturated soil properties from the temporal changes in S-wave velocity. For this purpose, we model the temporal changes in S-wave velocity due to seepage of water in a dyke under rainfall. We propose a new approach for interpolating/extrapolating experimental data, in order to obtain shear modulus as a function of suction and confining stress. The seepage analysis of a river dyke under heavy rainfall shows that the temporal change in the S-wave velocity is determined more by the shear modulus than by the density. Furthermore, the S-wave velocity at shallow depths is more sensitive to seepage than the S-wave velocity at greater depths. These, together with the fact that a simple relationship exists between the shear modulus, suction and saturation, lead us to the new possibility of predicting shear modulus as a function of suction from the time-lapse S-wave velocity monitoring. @en