In situ measurements of near-surface hydraulic conductivity in engineered clay slopes

In situ measurements of near saturated hydraulic conductivity in fine grained soils have been made at six exemplar UK transport earthwork sites: three embankment and three cutting slopes. This paper reports 143 individual measurements and considers the factors that influence the spatial and temporal variability obtained. The test methods employed produce near saturated conditions and flow under constant head. Full saturation is probably not achieved due to preferential and by-pass flow occurring in these desiccated soils. For an embankment, hydraulic conductivity was found to vary by five orders of magnitude in the slope near surface (0 to 0.3 metres depth), decreasing by four orders of magnitude between 0 and 1.2 metres depth. This extremely high variability is in part due to seasonal temporal changes controlled by soil moisture content (up to 1.5 orders of magnitude). Measurements of hydraulic conductivity at a cutting also indicated a four orders of magnitude range of hydraulic conductivity for the near surface, with strong depth dependency of a two orders of magnitude decrease from 0.2 to 0.6 metres depth. The main factor controlling the large range is found to be spatial variability in the soil macro structure generated by wetting/drying cycle driven desiccation and roots. The measurements of hydraulic conductivity reported in this paper were carried out because they are an important parameters required to undertake numerical modelling of weather/vegetation/infiltration/soil interaction mechanisms that can drive deterioration, and hence instability of earthwork slopes, including potential impacts of a changing climate.