Soil-geosynthetic interaction: interface behaviour

Knowledge of soil vs. geosynthetic and geosynthetic vs. geosynthetic interface shear behaviour is of fundamental importance to designers. This paper considers factors influencing measured behaviour, summarizes methods of measurement including specifications, presents data that quantifies variability, details methods for obtaining characteristic interface shear strength parameters for use in design and defines key questions to be answered by engineers. It is shown that the design of direct shear apparatus is the main reason for observed large variability of measured interface strengths from inter-laboratory comparison testing pro-grams. The need to carry out repeat tests at each normal stress is established. The use of global databases of measured interface strengths to inform selection of strength parameters is discouraged. A recommendation is given to use the results of repeatability testing programs to support calculation of characteristic interface strength parameters. Using the example of landfill lining design, guidance is provided on selection of strength parameters in conjunction with relevant factors of safety, consequences of failure, selection of the controlling interface and minimizing interface displacements

Development of a MSW classification system for the evaluation of mechanical properties

To date, sparse information is available on the mechanical properties of municipal solid waste and the results of published work are often hard to compare due to differences in waste composition and therefore properties. To allow comparison, a unified classification system for waste is deemed crucial. Existing classification systems are presented and discussed. For a geotechnical classification, mechanical properties, size, shape and degradability potential of waste components have to be taken into account. A new and improved classification system for waste components is proposed, which complies with the requirements of a geotechnical classification system. It classifies waste components based on (1) their material engineering properties (e.g. shear, compressive and tensile strength), (2) a size distribution of the components, (3) the component shape (reinforcing, compressible and incompressible) and (4) the degree of degradability. The proposed classification system is applied to data from literature and methods for presenting classification information are demonstrated. Further work required to develop a full classification system for waste bodies is highlighted

Acoustic emission behaviour of dense sands

Interpretation of acoustic emission (AE) generated by particulate materials has to date been qualitative. The objective of this study was to move the discipline towards quantitative interpretation of AE to: enable early warning of serviceability and ultimate limit state failures in the field, and enhance the instrumentation of element and physical model tests in the laboratory. Results from a programme of drained triaxial tests on dense sands show that: AE generation is proportional to the imposed stress level, imposed strain rate, fabric coordination number and boundary work done; there are two types of AE response at the transition from contractive to dilative behaviour, which was governed by the mean particle size; and AE activity in particulate materials is negligible until the current stress conditions (compression and/or shear) exceed the maximum that has been experienced in the past. Relationships have been quantified between AE and boundary work (i.e. AE generated per Joule) for a unit volume of sand under isotropic compression and shear, and between AE and shear strain rate. An example interpretation framework demonstrates how AE measurements could be used to identify the transition from contractive to dilative behaviour, mobilisation of peak shear strength and quantify accelerating deformation behaviour that typically accompanies shear zone development

Acoustic emission generated by glass beads in compression and shearing

Acoustic emission (AE) monitoring offers the potential to sense particle-scale interactions that lead to macro-scale responses of granular materials. This paper presents results from a programme of drained triaxial tests performed on densely packed glass beads to establish quantitative interpretation of AE during isotropic compression, shearing and associated stick–slip events. Relationships have been quantified between: AE and boundary work (i.e. AE generated per Joule) for a unit volume of glass beads under isotropic compression and shear; AE and shear displacement rate; and the amplitude of deviator stress cycles and AE activity during stick–slip events. In shear, AE generation increased with shear strain and reached peak values that were maintained from volume minimum (i.e. the transition from contractive to dilative behaviour) to peak dilatancy, whereupon AE generation gradually reduced and then remained around a constant mean value with further increments of shear strain. In each stick–slip event, AE activity increased during shear strength mobilisation, particle climbing and dilation, and then reduced with the subsequent deviator stress drop during particle sliding and contraction. The amplitude of these cycles in AE activity were governed by the amplitude of deviator stress cycles during stick–slip events, which were also proportional to the imposed stress level and inversely proportional to particle size

Quantification of landslide velocity from active waveguide generated acoustic emission

Acoustic emission (AE) has become an established approach to monitor stability of soil slopes. However, the challenge has been to develop strategies to interpret and quantify deformation behaviour from the measured AE. AE monitoring of soil slopes commonly utilises an active waveguide which is installed in a borehole through the slope and comprises a metal waveguide rod or tube with a granular backfill surround. When the host slope deforms, the column of granular backfill also deforms and this generates AE that can propagate along the waveguide. Presented in the paper are results from the commissioning of dynamic shear apparatus used to subject full scale active waveguide models to simulated slope movements. The results confirm that AE rates generated are proportional to the rate of deformation, and the coefficient of proportionality that defines the relationship has been quantified (e.g. 4.4 x 105 for the angular gravel examined). The authors demonstrate that slope velocities can be quantified continuously in real-time through monitoring active waveguide generated AE during a slope failure simulation. The results show that the technique can quantify landslide velocity to better than an order of magnitude (i.e. consistent with standard landslide movement classification) and can therefore be used to provide an early warning of slope instability through detecting and quantifying accelerations of slope movement

Quantification of slope displacement rates using acoustic emission monitoring

In soil slopes, developing shear surfaces generate acoustic emission (AE). The Authors have previously proposed the use of active waveguides for monitoring the stability of such slopes. Active waveguides comprise of a steel tube installed in a preformed borehole through a slope with coarse grained soil backfill placed in the annulus around the tube. Deformation of the host soil generates AE in the active waveguide. Field trials of this system reported previously have shown that AE rates are linked to slope deformation rates. This paper extends the study by detailing a method for quantifying slope movement rates using an active waveguide. A series of laboratory experiments are presented and used to define the relationship between AE event count rate and displacement rate. The method was shown to differentiate rates within an order of magnitude, which is consistent with standard landslide movement classification (i.e. 1 to 0.001 mm per minute), using a relationship derived between the gradient of the event count rate with time and deformation rate. In addition, it was possible to detect a change in displacement rate within two minutes of it occurring even at very slow rates (i.e. 0.0018mm/min). Knowledge of changes in displacement rate is important in situations where slope movements are suddenly triggered or displacements accelerate in response to a destabilising event. Field trials of a realtime AE monitoring system are currently in progress to compare performance against traditional instrumentation

Climate change and slope stability in the UK: challenges and approaches

It is now widely accepted that climate change is occurring and that this will affect the processes and parameters that determine the stability of slopes. There remains, however, significant uncertainty in forecasting these changes in the long term. This issue was addressed in a series of workshops, organized as part of a UK-wide network on CLimate Impact Forecasting For Slopes (CLIFFS). The major outcomes from the workshop discussions provide a focus for the modelling environment relevant to long-term forecasting of slope stability that include better definition of material properties, improved understanding of processes (notably an upgrading from the site-specific to the regional scale) and more effective communication to achieve synergies of understanding in this multidisciplinary research environment

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

Numerical 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

Deterministic and reliability-based design: veneer cover soil stability

A design chart is a graphical tool that provides solutions to different scenarios of a system. In this paper, two types of design chart are developed based on deterministic and reliability-based analyses for determining the interface shear strength required for stability of a cover system to achieve a target safety factor of 1.5 and failure probability of 1 x 10(-2). The deterministic design chart assists in the selection of different types of geosynthetic for lining materials based on the required interface shear strength for stability, and the reliability-based design chart enhances decision-making by taking into account the uncertainties in the design parameters, such as the variability of interface shear strength parameters. Additionally, the latter chart can also be used to determine the optimum slope angle for a containment facility that will satisfy both the target factor of safety and acceptable failure probability. Examples are provided to illustrate the use of the design charts in estimating the minimum required interface shear strength and their allowable variability for a given veneer cover, and the optimum slope inclination corresponding to different interface shear strengths and their associated variability

A multi consumer-grade fixed camera set-up with poorly determined camera geometry for precise change detection [abstract]

A multi consumer-grade fixed camera set-up with poorly determined camera geometry for precise change detection [abstract