M. Kirkham

4D electrical resistivity tomography monitoring of soil moisture dynamics in an operational railway embankment

The internal moisture dynamics of an aged (> 100 years old) railway earthwork embankment, which is still in use, are investigated using 2D and 3D resistivity monitoring. A methodology was employed that included automated 3D ERT data capture and telemetric transfer with on-site power generation, the correction of resistivity models for seasonal temperature changes and the translation of subsurface resistivity distributions into moisture content based on petrophysical relationships developed for the embankment material. Visualization of the data as 2D sections, 3D tomograms and time series plots for different zones of the embankment enabled the development of seasonal wetting fronts within the embankment to be monitored at a high-spatial resolution and the respective distributions of moisture in the flanks, crest and toes of the embankment to be assessed. Although the embankment considered here is at no immediate risk of failure, the approach developed for this study is equally applicable to other more high-risk earthworks and natural slopes.

3D ground model development for an active landslide in Lias mudrocks using geophysical, remote sensing and geotechnical methods

A ground model of an active and complex landslide system in instability prone Lias mudrocks of North Yorkshire, UK is developed through an integrated approach, utilising geophysical, geotechnical and remote sensing investigative methods. Surface geomorphology is mapped and interpreted using immersive 3D visualisation software to interpret airborne light detection and ranging data and aerial photographs. Subsurface structure is determined by core logging and 3D electrical resistivity tomography (ERT), which is deployed at two scales of resolution to provide a means of volumetrically characterising the subsurface expression of both site scale (tens of metres) geological structure, and finer (metre to sub-metre) scale earth-flow related structures. Petrophysical analysis of the borehole core samples is used to develop relationships between the electrical and physical formation properties, to aid calibration and interpretation of 3D ERT images. Results of the landslide investigation reveal that an integrated approach centred on volumetric geophysical imaging successfully achieves a detailed understanding of structure and lithology of a complex landslide system, which cannot be achieved through the use of remotely sensed data or discrete intrusive sampling alone.

Testing the utility of structure-from-motion photogrammetry reconstructions using small unmanned aerial vehicles and ground photography to estimate the extent of upland soil erosion

Quantifying the extent of soil erosion at a fine spatial resolution can be time consuming and costly; however, proximal remote sensing approaches to collect topographic data present an emerging alternative for quantifying soil volumes lost via erosion. Herein we compare terrestrial laser scanning (TLS), and both unmanned aerial vehicle (UAV) and ground photography (GP) structure-from-motion (SfM) derived topography. We compare the cost-effectiveness and accuracy of both SfM techniques to TLS for erosion gully surveying in upland landscapes, treating TLS as a benchmark. Further, we quantify volumetric soil loss estimates from upland gullies using digital surface models derived by each technique and subtracted from an interpolated pre-erosion surface. Soil loss estimates from UAV and GP SfM reconstructions were comparable to those from TLS, whereby the slopes of the relationship between all three techniques were not significantly different from 1:1 line. Only for the TLS to GP comparison was the intercept significantly different from zero, showing that GP is more capable of measuring the volumes of very small erosion features. In terms of cost-effectiveness in data collection and processing time, both UAV and GP were comparable with the TLS on a per-site basis (13.4 and 8.2 person-hours versus 13.4 for TLS); however, GP was less suitable for surveying larger areas (127 person-hours per ha(-1) versus 4.5 for UAV and 3.9 for TLS). Annual repeat surveys using GP were capable of detecting mean vertical erosion change on peaty soils. These first published estimates of whole gully erosion rates (0.077 m a(-1)) suggest that combined erosion rates on gully floors and walls are around three times the value of previous estimates, which largely characterize wind and rainsplash erosion of gully walls. Copyright (c) 2017 John Wiley & Sons, Ltd.

Shrinkage limit test results and interpretation for clay soils

The shrinkage limit is one of the Atterberg limits and is a fundamental geotechnical parameter used to assess the settlement of engineering soils containing clays, yet is rarely tested for as part of ground investigation. This paper describes shrinkage limit test results on a variety of soils from Britain and overseas obtained using an improved laboratory testing procedure developed at the British Geological Survey. The co-relationships with the other Atterberg limits and with density are explored. In particular, the coincidence of the shrinkage limit with the water content at the peak bulk density achieved in the test is examined. The shrinkage behaviour for undisturbed and remoulded states and a three-way relationship between water content, density and suction are demonstrated. Some tropical residual and highly smectitic soils show a very wide range of shrinkage behaviour, albeit for a small dataset, when compared with the larger dataset of temperate soils tested. Consideration is given to limitations of the new and existing test methods.

Archaeological and Resources Assessment of Sand and Gravel Deposits Using 3D ERT and Automated Interface Detection

River terrace deposits form one of the major sources for Palaeolithic artefacts and are of economic importance for the groundwater and aggregates industry. Their depositional history and thus structure can be highly complex. We present automated interface detection techniques applied to 3D geoelectrical data in order to guide archaeological prospecting and mining activities. Three different interface detection techniques were applied, which were based on resistivity iso-surfaces defined from clustering techniques and comparison to intrusive investigation, and on the steepest gradient in resistivity depth sections. Their results were compared to intrusive investigations. All three methods successfully imaged an incised channel underlying 3 – 8 m of sand and gravel deposits. The results showed good agreement with bedrock depths obtained from boreholes. The extracted bedrock topography was used to define archaeological trenching locations, within which Palaeolithic artefacts were recovered. The remaining sand and gravel resource of this extraction phase was estimated to be more than 1 350 000 m3.

Volumetric Remote Monitoring of an Earth Embankment: Part II - Visualisation and Condition Assessment

We describe a study in which time-laspe electrical resistivity tomography (ERT) has been used to investigate an aged (>100 years old) section of earth embankment on an operational railway (Great Central Railway (GCR), Nottingham, UK). Monitoring has been undertaken using a permanently installed ERT array extending across the embankment from toe to toe. Part I of the study (Munro et al., 2012) describes our approach to ERT data collection and processing, including corrections for seasonal temperature changes, and the implementation of a procedure to translate time-lapse resistivity models into gravimetric moisture content. Here, in Part II of the study, we use the derived models of gravimetric moisture content to assess the internal condition of the earth embankment. These results show that this can be monitored noninvasively, and at a high spatial resolution, using ERT.

Volumetric Remote Monitoring of an Earth Embankment: Part I - Geophysical – Geotechnical Property Translations

In this work we describe a study where automated time-lapse electrical resistivity tomography (ALERT) monitoring technology has been installed on a section of Victorian embankment on the Great Central Railway (Nottingham, United Kingdom). Raw datasets collected by the ALERT system have been processed/filtered, and inverted to yield a 3D resistivity distribution which is temperature corrected and converted to gravimetric moisture content using a relationship established by laboratory testing. Electrical resistivity tomography monitoring has been used to characterize the internal structure of the embankment, and image moisture content changes and wetting front development at a high spatial resolution. Monitoring has been undertaken at the test site to determine seasonal temperature changes in the subsurface; these data have been used to correct for temperature effects. We fitted the resistivity data as a function of gravimetric moisture content by modifying the Waxman-Smits model. Using results from laboratory testing, a best fit is computed and used to establish a resistivity, gravimetric moisture content relationship, used to facilitate property translation from temperature corrected resistivity to gravimetric moisture content. These results indicate that ERT has potential to identify structures and processes related to instability at an early stage in their development.