Integrity Assessment Of Earthen Dams And Levees Using Cross-Plot Analysis Of Seismic Refraction And Electrical Resistivity Tomograms

Geophysical methods provide a rapid, economical, non-invasive, and spatial coverage of the subsurface in terms of geophysical properties. On the other hand, geophysical methods can generate multiple geophysical anomalies. An anomaly on a seismic refraction or an electrical resistivity tomogram is an area that has different values compared to its surrounding. Geophysical anomalies in dams and levees can be due to the overall heterogeneity of the subsurface, structures such as principal spillways, artifacts of inversion software, or to a true compromised location, such as an air void due to internal erosion or seepage. Therefore, there is uncertainty involved with using geophysical methods where an anomaly does not necessarily represent a true compromised zone. Identification of true compromised zones requires an invasive geotechnical investigation, such as drilling. To identify anomalies that are associated with true compromised zones, multiple types of geophysical surveys are commonly conducted. Although the use of multiple geophysical methods and qualitative side-by-side interpretation can reduce this problem to some degree, a more quantitative analysis in identifying the type of compromised zones is required. Such analysis can be achieved with the application of cross-plot analysis. With the use of cross-plot analysis, it is possible to relate and map results from multiple geophysical surveys to more commonly used geotechnical terms such as porosity and moisture content. This research develops the use of cross-plot analysis using time-lapse seismic refraction tomography and electrical resistivity tomography for the assessment of earthen dams and levees. The focus of this research is on the development of a method for quantifying the bounding seismic velocity and electrical resistivity values, which then divide the subsurface integrity conditions into different groups. A new approach of using preliminary and laboratory geophysical measurements to define cross-plot constraints is presented. This work will separately target different types of compromised zones, such as sand zones and dry compacted clay zones, by incorporating their unique seismic and electrical resistivity attributes into the cross-plot analysis. A new approach of cross-plot analysis using external physical constraints derived from geophysical surveys and theoretical models at the Francis Levee Site is also presented

Seepage monitoring and diagnosis of distresses in an earth embankment dam using probability methods

Failure of embankment dams may result in catastrophic consequences. Considering seepage and internal erosion are accounted as one of the major causes of failure in earth embankment dams, it is essential to detect any concentrated seepage and sources of distress at early stages. A number of investigation and monitoring methods exist for the detection of seepage, with varying degrees of technological and implementation complexity. This research, focuses on the Electrical Resistivity Monitoring Method (ERM), and develops a condition assessment process that allows 1) the identification of potential seepage areas and progress through visual observation and flow measurement, and 2) the determination of the most likely paths where piping may have occurred. In particular, two separate statistical studies are carried out to identify the existence of and quantify the probability of potential seepage sources in earth embankment dams. The testing and evaluation of the accuracy and reliability of the ERM method in seepage detection in earthen hydraulic structures is also undertaken as a result of the correlation of the field measurements of flow rates and ERM outputs. An earth dam suffering from seepage is studied and monitored visually and with the ERM to discover and locate the potential sources and paths of seepages, detected and observed at the downstream toe over time. A Bayesian network model is developed to evaluate the potential sources and related paths associated with the detected flows downstream. The model is completed by developing an approach to estimate the rate of erosion and predict the potential failure time of the dam with empirical and theoretical methods

Combined electrical resistivity tomography and ground penetrating radar to map Eurasian badger (Meles Meles) burrows in clay-rich flood embankments (levees)

Globally, earth embankments are used to protect against flooding. Raised above the surrounding water table, these embankments make ideal habitats for many burrowing animals whose burrows can impact their structural integrity. Ground Penetrating Radar (GPR) is commonly used to identify and map animal burrows and other small cavities. However, the depth of investigation of a GPR survey can be severely limited in saline and clay-rich environments, soil properties commonly associated with flood embankments. In contrast, Electrical Resistivity Tomography (ERT) can image subsurface voids in conductive ground conditions but has been rarely used to image animal burrows. Here we aim to assess the efficacy of ERT and GPR to image two badger burrow networks, called ‘setts’, located in clay embankments on the River Ouse, Yorkshire, UK. The two setts were excavated to validate the geophysical results, and the soil was characterised through logging and geotechnical analysis to develop a ground model of the site. We find that ERT can accurately resolve tunnels down to 1.5 m depth, map the structure of a multi-entrance badger sett and successfully identify the end of the tunnels. This result compares favourably to the GPR surveys, which mapped all but the deepest tunnels, limited by its penetration depth due to clay soils. Our results show that ERT can be used as a primary survey tool for animal burrows in clay-rich environments and can be validated using co-located GPR surveys if penetration depth is sufficient. The implications of this study may allow embankment managers to map burrow networks, assess flood embankment stability, minimise repair costs, and reduce unexpected failures during flood events. Additionally, a better understanding of how, for example, local heterogeneities impact badgers' burrow geometry may be achievable using these geophysical methods, as they provide a non-destructive, repeatable method for imaging setts

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

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

A Review on Applications of Time-Lapse Electrical Resistivity Tomography Over the Last 30 Years : Perspectives for Mining Waste Monitoring

"Mining operations generate large amounts of wastes which are usually stored into large-scale storage facilities which pose major environmental concerns and must be properly monitored to manage the risk of catastrophic failures and also to control the generation of contaminated mine drainage. In this context, non-invasive monitoring techniques such as time-lapse electrical resistivity tomography (TL-ERT) are promising since they provide large-scale subsurface information that complements surface observations (walkover, aerial photogrammetry or remote sensing) and traditional monitoring tools, which often sample a tiny proportion of the mining waste storage facilities. The purposes of this review are as follows: (i) to understand the current state of research on TL-ERT for various applications; (ii) to create a reference library for future research on TL-ERT and geoelectrical monitoring mining waste; and (iii) to identify promising areas of development and future research needs on this issue according to our experience. This review describes the theoretical basis of geoelectrical monitoring and provides an overview of TL-ERT applications and developments over the last 30 years from a database of over 650 case studies, not limited to mining operations (e.g., landslide, permafrost). In particular, the review focuses on the applications of ERT for mining waste characterization and monitoring and a database of 150 case studies is used to identify promising applications for long-term autonomous geoelectrical monitoring of the geotechnical and geochemical stability of mining wastes. Potential challenges that could emerge from a broader adoption of TL-ERT monitoring for mining wastes are discussed. The review also considers recent advances in instrumentation, data acquisition, processing and interpretation for long-term monitoring and draws future research perspectives and promising avenues which could help improve the design and accuracy of future geoelectric monitoring programs in mining wastes.

Moisture monitoring in clay embankments using electrical resistivity tomography

Systems and methods are described for monitoring temporal and spatial moisture content changes in clay embankments using electrical resistivity tomography (ERT) imaging. The methodology is based upon development of a robust relationship between fill resistivity and moisture content and its use in the transformation of resistivity image differences in terms of relative moisture content changes. Moisture level and moisture content movement applications are exemplified using two case histories from the UK. The first is the BIONICS embankment, near Newcastle (NE England), which was constructed in 2005 using varying degrees of compaction of a medium plasticity sandy, silty clay derived from the Durham Till. The second is a Victorian embankment south of Nottingham (Central England), constructed in 1897 using end tipping of Late Triassic siltstone and mudstone taken from local cuttings. Climate change forecasts for the UK suggest that transportation earthworks will be subjected to more sustained, higher temperatures and increased intensity of rainfall. Within the context of preventative geotechnical asset maintenance, ERT imaging can provide a monitoring framework to manage moisture movement and identify failure trigger conditions within embankments, thus supporting on demand inspection scheduling and low cost early interventions