The effects of soil macroporosity on the hydrology of infrastructure slopes

  • David Muddle

Student thesis: Doctoral ThesisPhD


In order for infrastructure owners to be able to properly assess the risk of slope failures within their network of assets it is essential that the hydrological processes within slopes can be accurately modelled. Hydrological modelling is, in turn, dependent on a fundamental understanding of soil physics and flow at the soil structure scale.

It is well known that flow within soils is strongly related to a soil’s physical structure and pore space architecture and is therefore affected by the presence and interconnectivity of macropores. However, macropore influence on governing flow has traditionally been hard to define within laboratory experiments, particularly on representative and undisturbed samples. Additionally, quantitative descriptions of the dynamic nature of soil pore architecture and the effects of saturation on altering internal pore networks have proven elusive.

This thesis uses detailed numerical modelling of a case study infrastructure slope in conjunction with field data to assess the key influences on flow within infrastructure slopes. The results of this investigation reinforce the importance of determining how the hydraulic conductivity of clay soils varies with depth and with saturation, in order to be able to correctly model the hydrological response of earthworks to climate conditions. Determining the extent to which pore structure and connectivity influence hydraulic conductivity and the evolution of this pore architecture with changes in saturation and depth is therefore of great importance.

This thesis builds on recent developments in X-ray computed tomography (CT) in order to progress the technique as a means of visualising and quantifying macropore characteristics in a non-intrusive manner. A microCT scanning technique which allows for the scanning of large undisturbed clay fill samples is developed, as well as an image analysis procedure that allows for the quantification of internal macropore architecture. It is shown that 100 mm diameter clay cores are at the limit of microCT capabilities as a result of achievable spatial resolution and phase contrast. The use of subsampling and image improvement techniques allows for the pores above 63 microns in size present within the samples to be visualized and quantified.

A further novel development within this thesis is the assessment of the evolution of the internal macropore structure of undisturbed clay fill samples with saturation. Scans were conducted on 100 mm diameter clay fill samples at different states of saturation at microCT resolution for the first time. It was found that the saturation procedure reduced overall measured total macroporosity as well as the number of macropores within the samples. Additionally, the saturation procedure was observed to decrease the size of largest macropores within the samples and to make the samples more uniform in structure throughout their height.

MicroCT determined macropore property metrics of the clay fill samples were compared to saturated hydraulic conductivity tests of the samples. Saturated hydraulic conductivity was found to correlate strongly with microCT derived mean macropore length, which represents the connectivity of the macropores within the samples. The results also indicate that the length of the macropores within a sample has more influence on the saturated hydraulic conductivity than the quantity of macropores (total macroporosity).
Date of Award21 Mar 2018
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorKevin Briggs (Supervisor)

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