Use of Geoelectrical Techniques with Numerical Modelling for Surveying and Monitoring of Engineered Water Retaining Structures

Ball, John and Binley, Andrew and Chambers, Jonathan and Wilkinson, Paul (2023) Use of Geoelectrical Techniques with Numerical Modelling for Surveying and Monitoring of Engineered Water Retaining Structures. PhD thesis, Lancaster University.

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Abstract

Water retaining structures are societally and economically important barriers which degrade through various erosional processes over time. Walkover surveys and geotechnical investigations are traditionally used to examine such structures but are limited by a lack of knowledge of internal structure. Near-surface geophysics can provide comprehensive information about the internal structure of embankments, and several techniques exist which can survey and monitor water retaining structures. One such technique is electrical resistivity tomography (ERT), where the resistivity profile of the ground can be linked to moisture content, porosity, and composition, making it a useful tool for use in detecting defects and changes in ground conditions within water retaining structures. However, several uncertainties exist with ERT for use on embankments. A key problem is whether results will be impacted by a 3D effect, where off-line features influence resistivities in the inversion.Such features may be the water body itself, or complex engineering structures within the barrier. This thesis explores the impact of a 3D effect arising from the water body and structural geometry. The work was undertaken using synthetic numerical modelling of an embankment in a tidal setting and a fluctuating water level and resistivity, which was then compared to realERT data. Further synthetic numerical modelling of the Mactaquac dam, Canada, was used as a case study assessing the influence of a large concrete structure within the dam on ERT data. The study also examined the effect of resistivity variation in the headpond of the dam through time. Comparisons between 2D and 3D inversions were also assessed to determine the possibility of 3D inversions mitigating any 3D effects. This was undertaken for sites at Bartley Dam, Birmingham, UK and Paull Holme Strays, Yorkshire, UK. The Bartley Dam case study utilised time-lapse ERT to determine the value of 3D inversions over 2D inversions in a monitoring scheme and to identify whether 3D or 2D inversions could adequately identify water seepage present with changes in ground conditions. The Paull Holme Strays case study focussed on use of crosslines in a 3D inversion for a tidal embankment and compared outcomes to a 2D inversion without use of crosslines. The results of the research shows that 3D effects are likely to be significant when undertaking ERT surveys of a water retaining structure, e.g. artefacts induced by a river with changing water level and resistivity, in addition to the impact of engineering structures that may be present in the embankment. Analysis of time-lapse ERT data at the at Mactaquac Dam site has revealed that changing headpond resistivity can create compensatory effects in an ERT data inversion. No seepage pathways could be reliably identified in time-lapse analysis of Bartley Dam with 2D inversions, likely because of 3D effects and sensitivity issues, whereas 3D inversions had more reliable evidence of seepage pathways. However, analysis of Paull Holme Strays showed that when a large proportion of the measurements have been filtered, there might be artefacts induced by another electrode array along the crest. However, use of crosslines enhanced the ability for a 3D inversion to reduce 3D effects at Paull Holme Strays. This research has shown that 3D effects can be detrimental to ERT surveys, particularly in 2D inversions. However, 3D inversions can mitigate the effect where differences in data filtering between lines are minimal. For further reduction in the impact of the 3D effect it is recommended that smaller crosslines are used between the major electrode lines. Also, results should be compared with geological, geotechnical and hydrological information for understanding the reliability of the inversion. There is a need for further exploration of the impacts of 3D effects on ERT in other water retaining structures and environments, as well as undertaking more comprehensive studies into dynamic changes within embankments and how they impact the 3D effect. By incorporating dynamic change into a synthetic model, a greater understanding of how 3D effects can impact ERT surveys of water retaining structures can be made, especially for timelapse ERT.