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

Dixon, N.; Crosby, C.J.; Stirling, R.; Hughes, P.N.; Smethurst, J.; Briggs, K.; Hughes, D.; Gunn, D.; Hobbs, P.; Loveridge, F.; Glendinning, S.; Dijkstra, T.; Hudson, A.. 2019 In situ measurements of near-surface hydraulic conductivity in engineered clay slopes. Quarterly Journal of Engineering Geology and Hydrogeology, 52 (1). 123-135. https://doi.org/10.1144/qjegh2017-059

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Abstract/Summary

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 owing 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 (0–0.3 m depth), decreasing by four orders of magnitude between 0.3 and 1.2 m 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 dependence of a two orders of magnitude decrease from 0.2 to 0.6 m depth. The main factor controlling the large range is found to be spatial variability in the soil macrostructure 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. Infrastructure slopes are complex structures made up of a composite of soil, water, air and vegetation. The mechanical and hydraulic properties of the in situ (cuttings) and compacted (embankments) materials play a controlling role in the stability of earthwork slopes (O'Brien 2013). The UK experiences infrastructure slope failures that have primarily been triggered by changes in soil hydrology owing to rainfall (e.g. Springman et al. 2003; Xue & Gavin 2007; Hughes et al. 2009; Glendinning et al. 2014; Briggs et al. 2017). Slope instability causes significant disruption to the UK's road (Anderson & Kneale 1980; Garrett & Wale 1985) and rail (Birch & Dewar 2002; Ridley et al. 2004; Loveridge et al. 2010) networks. Large numbers of slope failures were recorded during periods of high precipitation in the winters of 2000, 2001, 2007 and 2014 and the summer of 2012. Cyclic seasonal effects, potentially influenced by a changing climate, also affect slope structures. Dry summer periods remove water, leading to shrinkage and cracking; prolonged and intense rainfall events cause swelling and increased porewater pressures (Hughes et al. 2009; Loveridge et al. 2010; Smethurst et al. 2012; Briggs et al. 2013; O'Brien 2013; Glendinning et al. 2014). Repeated shrink–swell cycles can lead to accumulation of shear strains resulting in strain softening and progressive failure (O'Brien et al. 2004; Vaughan et al. 2004; Loveridge et al. 2010; Take & Bolton 2011; O'Brien 2013). The spatial and temporal distribution of hydraulic conductivity of the soil (this term has been used with the same meaning as coefficient of permeability) governs the distribution, magnitude and rate of change of porewater pressures within a slope. The size and distribution of these porewater pressure cycles, and hence effective stress cycles, control the progressive failure mechanism. For example, soil with higher hydraulic conductivity, although still low compared with coarse-grained soils, can lead to more rapid and larger changes in porewater pressure and effective stress at depth (Nyambayo et al. 2004; O'Brien et al. 2004; O'Brien 2013), promoting progressive failure of a slope, often after many years of stability (Briggs et al. 2017). Knowledge of hydraulic conductivity and how it varies with depth and over time is therefore needed if the movement of water and its influence on slope stability is to be quantified.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1144/qjegh2017-059
ISSN:	1470-9236
Date made live:	15 Mar 2019 09:21 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/522530

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