Macropore structure and permeability of clay fill samples from a historic clay fill earthwork

Near surface macropores and macro features (e.g. cracks and fissures) provide pathways for rapid water infiltration into the core of clay fill earthworks. However it is more difficult to measure the size and distribution of macropores located below the weathered soil surface (>1.5 m depth) and hence assess their influence on water flow through the clay fill core of an earthwork. This paper explores the influence of macropores on the rate of water flow within the core of a historic railway earthwork. Samples were excavated from the core (1.5 m – 6.5 m depth) of a clay fill railway embankment and subjected to laboratory saturated hydraulic conductivity testing. The samples were scanned using X -ray computed tomography (XCT) before and after laboratory testing. XCT was used to measure the size and distribution of macropores (>63 ×10-6 m) within the samples and compare with the saturated hydraulic conductivity measurements. The results showed that the distribution of macropores and the saturated hydraulic conductivity of the samples from the embankment core was not dependant on the depth of excavation. The total macroporosity of the samples was very small relative to the total porosity (less than 10%). The saturated hydraulic conductivity of the samples was more closely related to the connectivity of the macropores (mean length) than to the total porosity or the total macroporosity. The macropores were variably distributed within the core of the clay fill embankment, they did not show a clear relationship with depth and they were connected over relatively short lengths (the mean macropore length was not greater than 1.6 ×10-3 m). Therefore water flow through the core of the embankment is likely to be through the clay fill matrix, rather than through the connected macropore pathways which allow rapid water infiltration at the near soil surface (<1.5 m depth).