Abstract Moisture-induced landslides are a global geohazard; mitigating the risk posed by landslides requires an understanding of the hydrological and geological conditions present within a given slope. Recently, numerous geophysical studies have been attempted to characterise slow-moving landslides, with an emphasis on developing geoelectrical methods as a hydrological monitoring tool. However, landslides pose specific challenges for processing geoelectrical data in long-term monitoring contexts as the sensor arrays can move with slope movements. Here we present an approach for processing long-term (over 8 years) geoelectrical monitoring data from an active slow-moving landslide, Hollin Hill, situated in Lias rocks in the southern Howardian Hills, UK. These slope movements distorted the initial setup of the monitoring array and need to be incorporated into a time-lapse resistivity processing workflow to avoid imaging artefacts. We retrospectively sourced seven digital terrain models to inform the topography of our imaging volumes, which were acquired by either Unmanned Aerial Vehicle (UAV)-based photogrammetry or terrestrial laser ranging systems. An irregular grid of wooden pegs was periodically surveyed with a global position system, from which distortions to the terrain model and electrode positions can be modelled with thin plate splines. In order to effectively model the time-series electrical resistivity images, a baseline constraint is applied within the inversion scheme; the result of the study is a time-lapse series of resistivity volumes which also incorporate slope movements. The workflow presented here should be adaptable for other studies focussed on geophysical/geotechnical monitoring of unstable slopes.
The paper describes results to date of a continuing monitoring study of coastal ‘soft cliff’ recession at the British Geological Survey's (BGS's) Coastal Landslide Observatory (CLO) on the east coast of England at Aldbrough, East Riding of Yorkshire. The cliffed site, part of the 50 km long Holderness coast, consists of glacial deposits, and is one of the most rapidly eroding coastlines in Europe. This rapid rate of erosion provides an ideal opportunity for observation and process understanding because it facilitates the collection of data over periods of time encompassing significant new landslide events at the same location. The results of two approaches are reported: first, terrestrial Light Detection and Ranging (LiDAR) surveying (TLS); second, the installation of instrumented boreholes. The aim of the research is to combine these to investigate the role of landslides and their pre-conditioning factors and the influence of geology, geotechnics, topography and environmental factors on cliff recession. To date, an average recession rate of 1.8 m a −1 and a maximum rate of 3.4 m a −1 have been recorded for the site. The establishment of the CLO and its conceptual geological–geotechnical model are described in a related paper.
Abstract Salt marshes are globally distributed, vegetated intertidal wetlands and marsh edge erosion is common on many shores. To understand how and why marsh edge erosion occurs, the response of salt marsh substrates to applied shear and vertical stress must first be quantified. This response is likely influenced by marsh substrate biological, geochemical and sedimentological composition. However, currently there is little systematic research into the between‐marsh variability in these properties and how they affect both marsh edge erosion processes and the ability of a marsh to maintain its position vertically within the tidal frame. This paper compares two marshes of contrasting sedimentology at Tillingham marsh, East England and Warton marsh, Northwest England. Soil shear strength and compressibility are determined by applying geotechnical methods to determine marsh resistance to shear and vertical effective stresses. This research was able to isolate the influence of roots on substrate shear strength in a three‐dimensional sample. In response to vertical effective stress, both the expected displacement magnitude and the vertical recovery potential of a marsh substrate are affected by past stress conditions on the marsh, particularly those resulting from desiccation. The substrate response to vertical effective stress also influences substrate shear strength through the effect of consolidation on the void ratio (or bulk density). We present evidence for the connection between marsh composition and substrate behaviour under applied stress. The results shed light on potential determinants of marsh resistance to edge erosion, which is ultimately essential for the informed implementation of both nature‐based coastal flood defences and coastal restoration schemes.
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.
A new apparatus for the determination of shrinkage limit is described. Two versions have been produced: a manually operated prototype ‘version1' followed by an automated version named SHRINKiT. Test results using the former for British and overseas clay soils are described and comparisons made with the British Standards preferred method. A further set of test results is described for SHRINKiT. However, it was not possible to compare these with the BS 1377 method owing to the introduction of a ban on the use of mercury in the British Geological Survey's geotechnical laboratories. The new method is set in the context of the huge cost of shrink/swell-related subsidence damage in Britain and the relative disuse of both BS 1377 methods for shrinkage limit, for reasons of safety. The shrinkage behaviour of different soils types and sample states is discussed, in addition to the advantages and disadvantages of the new method.
The paper describes the establishment of the British Geological Survey (BGS)’s field-based ‘Coastal Landslide Observatory’, designed for the study of coastal ‘soft cliff’ recession on the east coast of England at Aldbrough, East Riding of Yorkshire. The cliffed site is part of the 50 km long Holderness coast which is one of the most rapidly eroding coastlines in Europe. The site features a 16–17 m high sea-cliff formed by Late Devensian (18–13 ka) glacial deposits dominated by tills. The aim of the research is to investigate the role of landslides and their pre-conditioning factors, including geology, geotechnics and topography, in cliff recession with a view to developing coastal landslide forecasting capability. Two research approaches are encompassed in the observatory: firstly, terrestrial LiDAR surveying, from 2001 to present, to create digital elevation models of the cliff and, secondly, installation of instrumented boreholes from 2012. The design, methods, conceptual ground model, implementation of the observatory and geotechnical laboratory test results are described in this article. The interpretation of results and the final ground model are presented in a related article in this volume.
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