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

Abstract

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 bypass 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 0.3 and 1.2 metres depth. This extremely high variability is in part due to seasonal temporal changes controlled by soil moisture content, which can account for up to 1.5 orders of magnitude of this variability. 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 undertaken to inform and provide a benchmark for the hydraulic parameters used in numerical models of groundwater flow. This is an influential parameter in simulations incorporating the combined weather/vegetation/infiltration/soil interaction mechanisms that are required to assess the performance and deterioration of earthwork slopes in a changing climate.

Emulating computer experiments of transport infrastructure slope stability using Gaussian processes and Bayesian inference

“…For London Clay examples, the TTF contours are consistent with the literature (Ellis and O 'Brien, 2007;Gao et al, 2015). The plots in the lower panel of Figure 3 illustrating TTF for the cuttings in the London-Bristol corridor assumes strength and permeability in agreement with published results (Hight et al, 2003;Dixon et al, 2019). Note that the training data do not cover the upper ranges of slope height and angle (i.e., upper triangle of the plots), thus the TTF predictions made in that region are an extrapolation.…”

“…The five input variables in this study are the slope angle cotangent and height (converted to a single geometry variable—see Supplementary Figure S1) derived from light detection and ranging (LiDAR) survey data provided by project stakeholders (Mott MacDonald and Network Rail), the peak shear strength parameters ( and ) before the material has undergone any strength reduction derived from previous laboratory data and modeling studies (Apted, 1977; Ellis and O’Brien, 2007), and the reference coefficient of permeability of the soil at 1 m depth with respect to water ( ) derived from field measurements (Dixon et al, 2019). For more information on the adopted GM and the values adopted for the remaining input parameters, see FLAC (2016), Rouainia et al (2020), and Postill et al (2021).…”

“…175 and 65 years of age, respectively, the soils from which they are formed will have been subject to a varying number of these seasonal cycles and undergone an indeterminate magnitude of deterioration from their initial state. Here, we focus on replicating these deterioration processes in high-plasticity, over-consolidated clays, particularly London Clay (Hight et al, 2003; Dixon et al, 2019; Rouainia et al, 2020).…”

Strength parameter selection framework for evaluating the design life of clay cut slopes

“…• The variable nature of weather patterns (Alexander and Jones, 2000;Jenkins et al, 2009;Murphy et al, 2019) and how they can effect time to failure (Alonso et al, 2003;Oh and Lu, 2015;Postill et al, 2020Rouainia et al, 2020) • Spatial and temporal heterogeneity of soil (Phoon and Kulhawy, 1999;Griffiths and Fenton, 2004;Dixon et al, 2019;Stirling et al, 2020) • The influence of site-specific problems (e.g. localised drainage failure, the presence and orientation of pre-existing discontinuities or other stratigraphic features and land use changes) • The influence of vegetation and vegetation management (Smethurst et al, 2015;Briggs et al, 2016;Tsiampousi et al, 2017) Therefore, general guidance is not appropriate for all design instances.…”

Climate and Vegetation Impacts on Infrastructure Cuttings and Embankments

“…Seasonal shrink and swell movements in the slope develop increasing plastic strains starting from the toe of the slope, leading eventually to progressive failure. The model also has a gradually decreasing permeability with depth, intended to model the higher permeability close to the slope surface caused by cracking, evidenced by the observations reported by Dixon et al (2018). Figure 9 shows the reduction in factor of safety for a range of London Clay slopes, all 8 m high, and of differing slope angle.…”