Strategies for rock slope failure early warning using acoustic emission monitoring

Research over the last two decades has led to development of a system for soil slopes monitoring based on the concept of measuring Acoustic Emission (AE). A feature of the system is the use of waveguides installed within unstable soil slopes. It has been demonstrated that the AE measured through this technique are proportional to soil displacement rate. Attention has now been focused on the prospect of using the system within rock materials. The different nature of the slope material to be monitored and its setting means that different acoustic trends are measured, and development of new approaches for their interpretation are required. A total of six sensors have been installed in two pilot sites, firstly in Italy, for monitoring of a stratified limestone slope which can threaten a nationally important road, and secondly in Austria, for monitoring of a conglomerate slope that can endanger a section of the local railway. In this paper an outline of the two trial sites is given and AE data collected are compared with other physical measurements (i.e. rainfall and temperature) and traditional geotechnical instrumentation, to give an overview of recurring AE trends. These include clear AE signatures generated by stress changes linked to increased ground water levels and high energy events generated by freeze-thaw of the rock mass. © Published under licence by IOP Publishing Ltd

Modified stress and temperature-controlled direct shear apparatus on soil-geosynthetics interfaces

In this paper, a bespoke stress and temperature controlled direct shear apparatus to test soil-geosynthetics interfaces is introduced. By adopting the apparatus, a series of ‘rapid loading’ shear tests and creep tests were conducted on the Clay – Geosynthetic Drainage layer (GDL) interfaces to assess the functionality of the apparatus. The experimental results indicate that, the modified apparatus can allow the shear deformation behaviour of soil-geosynthetics interfaces under environmental stress during thermal and drying-wetting cycles to be investigated, with a reliable performance. The resistance of Clay-GDL interfaces to shear deformation under the rapid loading of shear stress decreases after drying-wetting cycle and at elevated temperature. In the creep tests, the interfaces subjected to drying-wetting cycles and thermal cycles fail under a lower shear stress level than that of the interfaces without experiencing drying-wetting cycles and thermal cycles, respectively. The impacts of drying cycles on the horizontal displacement is significantly larger than that of wetting cycles. The first drying cycle has the largest impacts on the horizontal displacement than those of the following drying cycles. The impacts of drying alone on the horizontal displacement of Clay-GDL interfaces during drying cycles are small, and the main influence factor is the elevated temperature

Modelling deformation during the construction of wrapped geogrid reinforced structures

Although geogrids and geotextiles have been successfully used for over a quarter of a century to reinforce soil, there are currently no commonly agreed analytical methods to model their deformation behaviour. The Serviceability Limit State is becoming an ever more important design consideration, as structures are built with increasingly tighter tolerances. While there are many deformation databases and design charts available, providing information and guidance on the sensitivity to certain design variables, these are largely focused on facets such as height, shear strength and geogrid ultimate strength and do not consider construction method. Following a review of existing analytical and empirical guidance, this paper presents numerical modelling derived guidance for flexible faced Geogrid Reinforced Structures constructed using cohesionless fill that incorporates installation methods. The modelling approach is validated against measured results from three varied case studies, before analysing the changes in deformation distribution resulting from two different construction methods (layer by layer and full height construction). For the conditions analysed, including height of the structure, the lateral deformation resulting from layer by layer construction, was shown to be consistently greater, than for full height construction. In contrast, an analysis of post-construction deformation, for each of the construction methods, found full height construction to be more sensitive to post-construction loading, for the conditions considered. For low wall height structures constructed using the layer by layer method, <5 metres, this study indicates that horizontal face deformations are underestimated by current guidance

Measuring deformation performance of geogrid reinforced structures using a terrestrial laser scanner

Geogrid Reinforced Structures (GRS) are inherently flexible and although the design for ul-timate limit state is relatively mature, GRS are often defined by their deformation performance, in the serviceability limit state (Koerner and Koerner, 2013). Currently, serviceability design protocol does not determine or prescribe deformation limits for the built wall or slope, but rather imposes limits on the theo-retical mobilised strain of geogrid reinforcement. Current understanding of the principle mechanisms for GRS deformation is weak and often the only way to assess the serviceability of structures is by the observational method. Typically this has been done with external surveying instruments such as total stations or internally using strain gauges, extensometers and inclinometers. Laser scanning has previously been used to measure the serviceability performance of conventional geotechnical structures and slopes and provided useful information (Mechelke et al., 2007) but has not yet been used on GRS. This paper assesses the potential of a Terrestrial Laser Scanner (TLS) to rapidly survey GRS. This assessment covers a range of structures including a 6.5 m high steel mesh faced retaining wall and a 3.6 m wrap faced structure. The measured behaviour obtained from this range of structures demonstrates the importance of facing stiffness on controlling deformations. Terrestrial laser scanning has potential because it is unobtrusive, only requiring lines of sight to the face and does not use targets located on the GRS. The system can be used to measure the position of the GRS face to within a noise range of ±5 mm (Kersten et al., 2008), across a large surface area from a single observation point in minutes. This paper assesses the application of using TLS to measure deformations during construction and in-service and proposes a standard scanning procedure. It also details experience gained surveying GRS constructed with a range of face systems and discusses accuracy and repeatability issues. It con-cludes with possible implications of using the TLS method for routine monitoring of GRS

Mechanical behaviour of soil under drying-wetting cycles and vertical confining pressure

A system for preparing soil specimens subjected to drying-wetting cycles whilst under vertical confining pressures is introduced with Clayey soil specimens subjected to different drying-wetting histories, then the relative performance tested using consolidated undrained triaxial shear tests. Meanwhile, their soil water retention properties were also measured. The experimental results indicate that drying-wetting cycles lead to a rise in the matric suction for soil with high moisture content, and the decrease of matric suction for soil with low moisture content. Partly owing to the higher pore water pressure, peak shear strength reduces gradually during drying-wetting cycles. The impacts of drying-wetting cycles on hydro-mechanical properties of soil specimens during the first cycle are larger than those during the second and the third cycles as the highest matric suction of soil occurs during the first cycle. Vertical confining pressure is shown to limit the impact of drying-wetting cycles on the hydro-mechanical properties of soil effectively because of its restricting effects on the volumetric deformation of soil during the cycles

Numerical modelling of landfill lining system–waste interaction: implications of parameter variability

This article was published in the journal, Geosynthetics International [ICE Publishing / © Thomas Telford Ltd.]. The definitive version is available at: http://www.icevirtuallibrary.com/content/serial/geinNumerical modelling techniques can be used to examine the serviceability limit states of landfill side-slope lining systems in response to waste placement. A study has been conducted in which the variability of significant model input parameters have been investigated within a probabilistic framework using Monte Carlo simulation. Key model parameters are treated as random variables, and the statistical information required to describe their distributions has been derived from a laboratory repeatability testing programme, a literature survey and an expert consultation process. Model outputs include relative shear displacements between lining components, and tensile strains in the geosynthetic layers that occur in response to staged placement of waste against the side slope. It was found that analyses including input parameter variability were able to identify mechanisms influencing liner performance and their probability of occurrence. These mechanisms include large (i.e. ≫100 mm) relative displacements at interfaces that can generate post-peak strengths, and mobilised tensile strains in the geomembrane and geotextile layers. Additionally, it was found that relative displacements at the controlling (i.e. weakest) liner interface are greater for landfills with a steep side slope, for stiffer waste and thicker waste lifts, while tensile strains in the geosynthetic elements are greater for steep side slopes, more compressible waste and thinner waste lifts. Outputs from probabilistic analyses such as that used in this study can guide engineers regarding geometries and materials that could produce waste-settlement-generated serviceability limit state failures, and hence can be used to support more reliable designs

Modelling deformation during the construction of wrapped Geogrid-reinforced structures

Although geogrids and geotextiles have been successfully used for over a quarter of a century to reinforce soil, there are currently no commonly agreed analytical methods to model their deformation behaviour. The serviceability limit state is becoming an ever more important design consideration, as structures are built with increasingly tighter tolerances. Although there are many deformation databases and design charts available, providing information and guidance on the sensitivity to certain design variables, these are largely focused on facets such as height, shear strength and geogrid ultimate strength and do not consider construction method. Following a review of existing analytical and empirical guidance, this paper presents numerical modelling-derived guidance for flexible faced geogrid-reinforced structures constructed using cohesionless fill that incorporates installation methods. The modelling approach is validated against measured results from three varied case studies, before analysing the changes in deformation distribution resulting from two different construction methods (layer-by-layer and full height construction). For the conditions analysed, including height of the structure, the lateral deformation resulting from layer-by-layer construction, was shown to be consistently greater, than for full height construction. In contrast, an analysis of postconstruction deformation, for each of the construction methods, found full height construction to be more sensitive to post-construction loading, for the conditions considered. For low wall height structures constructed using the layer-by-layer method, <5 m, the present study indicates that horizontal face deformations are underestimated by current guidance. © 2015 Thomas Telford Ltd

Internal instability in soils : a critical review of the fundamentals and ramifications

Seepage induced fine particles migration that leads to a change in hydraulic conductivity of a soil matrix is referred to as internal instability. This could jeopardize the structural integrity of the soil matrix by initiating suffusion (or suffosion): a form of internal erosion. Susceptibility to suffusion has been studied mostly under extreme laboratory conditions to develop empirical design criteria, which are typically based on the particle size distribution. The physics governing the process has not been comprehensively uncovered in the classical studies due to experimental limitations. The mainstream evaluation methods often over-idealize the suffusion process, holding a probabilistic perspective for estimating constriction sizes and fines migration. Prospective studies on constitutive modelling techniques and modern computational techniques have allowed a more representative evaluation and deeper insight into the problem. Recent advancements in the sensing technologies, visualisation and tracking techniques have equally enriched the quality of the gathered data on suffusion. This paper sets out to present the long-standing knowledge on the internal instability phenomenon in soils. An attempt is made to pinpoint ambiguities and underscore research gaps. The classical empirical studies and modern visualising techniques are integrated with particle-based numerical simulations to strengthen the theoretical understanding of the phenomenon

Numerical modeling of the nonlinear mechanical behavior of multilayer geosynthetic system for piggyback landfill expansions

This study examines the extent to which the results of numerical calculations can be influenced both by the differing compressive and tensile behavior of multiple geosynthetics GSYs and by the assumption of strain softening at interfaces between GSYs. Several numerical models are implemented using the finite-difference code FLAC 2D on a typical piggyback landfill expansion (PBLE) that involves four GSYs and six interfaces. The present work applies comprehensive, state-of-the-art numerical modeling to study the interactions between multiple layers of GSYs. It also investigates the nonlinear axial stiffness of GSYs through a series of uniaxial tensile tests. The numerical results show that, if the GSY axial compressive and tensile characteristics are the same, then tensile force is minimized, which induces significant compressive force in the GSYs. The results also indicate that neglecting strain softening at the interface between GSYs affects interface shear stresses, displacements of GSYs at the interface, and the GSY force distribution, potentially rendering the model unrealistic. Including strain softening, however, allows the assessment (location) of unstable areas along the interface where large displacements occur. © 2016 Elsevier Lt