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Dixon N.,Loughborough University | Smith A.,Loughborough University | Spriggs M.,Loughborough University | Ridley A.,Geotechnical Observations Ltd | And 2 more authors.
Proceedings of the Institution of Civil Engineers: Geotechnical Engineering | Year: 2015

The paper details the use of acoustic emission generated by active waveguide subsurface instrumentation to monitor the stability of a rail soil cutting slope failure. Operation of the active waveguide, unitary battery-operated acoustic emission sensor and warning communication system are described. Previous field trials reported by the authors demonstrate that acoustic emission rates generated by active waveguides are proportional to the velocity of slope movement, and can therefore be used to detect changes in rates of movement in response to destabilising and stabilising effects, such as rainfall and remediation, respectively. The paper presents a field trial of the acoustic emission monitoring system at a reactivated rail-cutting slope failure at Players Crescent, Totton, Southampton, UK. The results of the monitoring are compared with both periodic and continuous deformation measurements. The study demonstrated that acoustic emission monitoring can provide continuous information on displacement rates, with high temporal resolution. The ability of the monitoring system to detect slope movements and disseminate warnings by way of text messages is presented. The monitoring approach is shown to provide real-time information that could be used by operators to make decisions on traffic safety © 2015, Thomas Telford Services Ltd. All rights reserved.

Smethurst J.A.,University of Southampton | Briggs K.M.,University of Bath | Powrie W.,University of Southampton | Ridley A.,Geotechnical Observations Ltd
Geotechnique | Year: 2015

Seasonal shrinkage and swelling of clay fill railway embankments can disturb the track geometry, resulting in train speed restrictions that disrupt normal operations. Such movements are exacerbated by vegetation, but reliable analytical descriptions of the effects of trees on embankment behaviour are not yet established. This paper presents and analyses the results of a field experiment, carried out on a heavily vegetated clay railway embankment to investigate quantitatively the influence of trees. After the first year of monitoring, the mature trees initially present on the upper two-thirds of the embankment slopes were removed. The field monitoring data are used to assess and understand the mechanisms of soil water content and pore water pressure changes before and after tree removal, and their influence on the vertical and lateral displacements of the embankment slopes. Removal of the vegetation stopped seasonal volume changes in the clay fill at the crest of the earthwork, but also resulted in the loss of the deep-seated suctions generally beneficial to embankment stability. The wider implications for the management of vegetation on embankment slopes are discussed. © 2015, ICE Publishing. All rights reserved.

Monroy R.,Technical University of Cartagena | Zdravkovic L.,Imperial College London | Ridley A.,Geotechnical Observations Ltd.
Geotechnical Testing Journal | Year: 2012

The miniature (high-capacity) tensiometer is increasingly being used both in practice and in research to determine the matric potential in unsaturated soils. Although the potential which can be recorded with this type of device tends to be limited to a maximum of approximately 1500 kPa (a function of the air entry value of the porous element), simplicity of use, together with the ability to measure potentials under atmospheric conditions, makes this a unique piece of equipment for investigating and characterizing the hydro-mechanical response of unsaturated soils in the laboratory and in the field. The use of the miniature tensiometer is now well established and documented, and the particulars of a number of such devices, together with modifications to standard laboratory equipment to accommodate them, have appeared in the literature. The measurement of potential with the tensiometer, like any other measurement of a physical quantity, is not exempt from a certain degree of uncertainty. Additionally, the device is usually calibrated by applying a positive water pressure; however, when in use, the fluid filling the internal reservoir experiences a tensile stress. Although extrapolation of the calibration curve to negative values of water pressure is justified, it is nevertheless interesting to assess the level of uncertainty associated with a single measurement of potential in a soil. To this effect, data on gravimetric water content and matric potential from a statistically representative set of clay samples, carefully prepared and compacted in a similar manner to the same initial conditions, were analysed. This paper presents details and results from this exercise, together with the statistical treatment of the data.

Monroy R.,Heriot - Watt University Dubai | Zdravkovic L.,Imperial College London | Ridley A.M.,Geotechnical Observations Ltd
Geotechnical Testing Journal | Year: 2014

A variety of radial stress measuring systems are available in the literature, all of which allow the determination of horizontal stresses with a suitably modified odometer. However, most of the experience with such systems has been on saturated samples. A new osmotic odometer was developed to test unsaturated soils under atmospheric conditions. The device incorporated an active radial stress measuring system to determine the full state of stress during a test. Suction control was achieved by means of an osmotic system, whereas suction measurements were made with miniature tensiometers. The main odometer ring incorporated four small strain-gauged diaphragms, together with a compensating system, to ensure that radial deformations were kept to a minimum. To evaluate the performance of the system, data obtained from a number of tests on compacted clay have been analyzed. Results show that the system yields plausible measurements in the majority of cases; however, there were instances when unusual results were observed. These cast some doubt on the viability of using an odometer equipped with a radial stress measuring system-such as the one presented in the paper-to measure horizontal stresses in unsaturated soils. It is recommended that similar tests are performed with different equipment, to verify whether some of the anomalous results presented in this paper represent actual soil response, or are simply an artifact consequence of the chosen testing method. © ASTM Int'l.

Monroy R.,Heriot - Watt University Dubai | Zdravkovic L.,Imperial College London | Ridley A.M.,Geotechnical Observations Ltd
Engineering Geology | Year: 2015

The mechanical response of unsaturated soils with significant amounts of active clay minerals can be highly stress path dependent. Traditionally, the Axis Translation Procedure has been used to study these materials in the laboratory. This technique, however, does not fully replicate conditions in the field, nor is it able to test soils during the important process of desaturation and resaturation. A novel osmotic oedometer has been developed at Imperial College London to test unsaturated soils under atmospheric pressure. With this equipment, it has been possible to continuously record changes in vertical and radial stress, gravimetric water content, degree of saturation, matrix suction, and void ratio, throughout a test. The Paper presents results from tests carried on samples of compacted London clay using the new oedometer and standard oedometers. The full data set gives an insight into the mechanical response of unsaturated expansive clay along complex stress paths. Results are interpreted using an existing framework for unsaturated expansive clays. Because of inherent limitations in the method of testing, some of the data needs to be interpreted with care. Nevertheless, the response recorded along different stress paths was found to be consistent and in agreement with framework predictions. © 2015 Elsevier B.V.

Monroy R.,Georgia Institute of Technology | Zdravkovic L.,Imperial College London | Ridley A.,Geotechnical Observations Ltd
Geotechnique | Year: 2010

The influence of fabric on mechanical behaviour is explicitly considered in some of the current constitutive models for unsaturated soils. These are based on assumptions regarding the interaction between different levels of structure, which still require experimental validation. A study has been carried out to investigate the evolution of fabric in a compacted natural clay during wetting and loading, and the results are presented in this paper. Samples of London Clay were statically compacted to the sae initial conditions, dry of optimum moisture content in a Proctor plot, and subsequently taken along complex stress paths involving wetting, loading, or a combination of both. Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM) microstructure investigations were carried out to observe and quantify the change in fabric associated with each path. The soil fabric was observed to change from an aggregate to a matrix type structure along all wetting paths. This transition was found to take place only when the suction was reduced to a value close to zero kPa. Results also showed that fabric changes during yielding were stress path-dependent. It was not possible to find a correspondence between the volume of free porosity and the volume of intra-aggregate pores, as suggested by some authors. Finally, the volume of pore water was found to agree closely with the volume of intra-aggregate pores, providing support to the assumption that in an unsaturated aggregate microstructure the clay aggregations are saturated.

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