Analysis of steep sided landfill lining systems

The EC Landfill Directive (1999), which is enforced in England and Wales through the Landfill (England and Wales) Regulations (2002), has increased the technical challenge associated with the design and construction of landfill containment systems, in particular those on steep side slopes. Increased numbers of lining system components, varied configurations, and complex loading scenarios require advanced analysis tools to facilitate design. This project involved the development of advanced numerical modelling techniques, based on the FLAC finite difference modelling code. The analysis toolbox can be used to predict the behaviour of multilayered geosynthetic and soil lining systems, during and after staged construction. The model can include non-linear interface and geosynthetic axial properties, represent complex loading, including downdrag from the waste mass, whilst retaining the flexibility to represent varied geometries and include engineered support structures. Whilst numerical modelling is becoming increasingly commonplace in commercial design, there is little evidence of the validation of numerical models with field or experimental data. Validation of the analysis toolbox described in this document was conducted by back analysis of published data, modelling of landfill failure mechanisms, and comparisons to large scale laboratory testing. Design of field scale instrumentation has also been carried out as part of this project. The influence of interface shear strength variability has been assessed through the compilation of a comprehensive database, and the effect of this variability on lining system behaviour assessed through reliability based analyses. This has shown probability of failures may be higher than proposed limiting values when adopting traditional accepted factors of safety. A key area of interest identified during the project was the requirement for support, potentially through reinforcement, of the geological barrier. The inclusion of randomly reinforced fibres in bentonite enhanced soil has shown the potential for increased strength, without adverse effects on hydraulic barrier performance. ii Additionally, the influence of geomembrane seams on lining system integrity has been investigated, showing that fusion welded seams can result in stress concentration and extruded seams can cause significant stress concentration

Permanent capping of temporary waste slopes: the challenge of declining waste streams

With diminishing waste streams many sites now face closure before filling to intended profiles. As a result, slopes which were intended as temporary waste slopes now require permanent capping solutions to be installed. These slopes are often steep and challenging to cap. Various stability and integrity issues arise with solutions including the use of reinforcement within the capping layers. However, on long slopes the total forces are often large and anchorage on benches can be problematic. Tapering the cover soils can achieve a stable solution; however, large soil volumes may be required to achieve a satisfactory resistance to failure. This paper considers a case study of a 72m high temporary waste slope requiring a permanent capping solution. Interface shear strength is critical on such steep slopes and sampling, testing and interpretation requires careful consideration. A number of shear strength tests were carried out as part of the project and this paper shall consider the designer's concerns around the interpretation of these tests for use in stability analysis. As part of construction of the regulating soil, variable material was delivered to site, some with moisture content higher than specified, thus potentially reducing the strength and introducing a weak layer. This paper considers the challenges in stability risk assessment, data interpretation and future instrumentation requirements at the site

Mixing and compaction of fibre- and lime-modified cohesive soil

Fibre reinforcement is a versatile method of increasing the shear strength of soils for earthwork applications. However, research to date has encountered a number of problems when utilising cohesive host soils. The aim of this study was to develop a suitable site-applicable method of mixing fibre into cohesive host soils. Intermediate plasticity clay reinforced with monofilament polypropylene fibres was used in the laboratory investigations. In order to mix the fibres successfully, the initial moisture content of the host soil was increased prior to the introduction of fibres. By introducing quicklime, excess moisture was removed through the hydration process, and a portion of free water was effectively held within aggregations of flocculated clay particles, thereby having little influence on the dynamic boundaries. Fibrous inclusions within the clay clods resisted compactive effort, forming an interlocked structure. As a result, the optimum moisture content increased and the maximum dry density decreased. This trend was heavily dependent on the interfacial shear resistance along the fibre boundary, which consequently decreased as the water content increased or the compactive effort was increased. Results from strength tests confirmed that both peak and post-peak shear strength increased, creating a more ductile material capable of maintaining shear strength at high levels of strain

Global challenges, geosynthetic solutions and counting carbon

The earth is experiencing unprecedented change driven by increasing population, industrialisation and urbanisation. This is leading to rapid climate change and scarcity of resources. There is growing agreement globally of the need to deliver sustainable development to improve the lives of millions of people in low and middle income countries through provision of clean water, sanitation, energy and transport solutions. The response of the international community to this challenge is via the United Nations programme (published in January 2016), which establishes 17 Sustainable Development Goals (SDGs) including response to climate change. These SDGs will guide decisions taken by nations and organisations over the next 15 years. This paper is the written version of the opening keynote lecture delivered to the 3rd Pan American Conference on Geosynthetics in Miami Beach, USA, in April 2016; it considers the role that geosynthetics can make in achieving the SDGs. Scientific evidence for climate change is presented, and the value and uncertainty in available climate change information is discussed to inform its use in design. International agreements on reducing greenhouse gas emissions are based on country specific action plans for mitigation and adaptation against climate change, and the potential for geosynthetics to help achieve these targets is identified. Finally, approaches for calculating embodied carbon for solutions incorporating geosynthetics are introduced and case studies that provide evidence for the ‘sustainability’ case for geosynthetics are summarised. The geosynthetics community is challenged to play a leading role in helping to deliver the SDGs and hence a better future for populations worldwide

Early detection of first-time slope failures using acoustic emission measurements : large-scale physical modelling

Early warning systems for slope instability need to alert users of accelerating slope deformation behaviour to enable safety-critical decisions to be made. This study shows that acoustic emission (AE) monitoring of active waveguides (i.e. a steel tube with a granular backfill surround installed through a slope) can both detect shear surface development and quantify increasing rates of movement during slope failure, thereby providing an early detection of slope instability. A large-scale physical model was designed and built to simulate slope failures on elements of soil, through which full-scale active waveguides were installed. A shear surface develops in each test and the sliding mass accelerates during failure, reaching velocities greater than 300 mm/h and shear deformations of 50 mm. Continuous measurementswere obtained to examine the behaviour of activewaveguides subjected to first-time slope failure dynamics (i.e. development of new shear surfaces and accelerating deformation behaviour). Comparisons with continuous subsurface deformation measurements show that AE detection began during shear surface formation, and AE rates increased proportionally with displacement rates as failure occurred. Empirical AE rate–slope velocity relationships are presented for three granular backfill types, which demonstrate that generic AE rate–slope velocity relationships can be obtained for groups of backfill types; these relationships allow displacement rates to be quantified from measured AE rates to provide early detection of slope instability. © 2017, ICE Publishing. All rights reserved

Analysis of acoustic emission patterns for monitoring of rock slope deformation mechanisms

Acoustic emission (AE) is generated in soil and rock materials by rearrangement of particles during displacement or increasing damage in the microstructure preceding a collapse; therefore AE is appropriate for estimation of slope degradation. To overcome the high attenuation that characterise geological materials and thus to be able to monitor AE activity, a system that makes use of a waveguide to transmit AE waves from a deforming zone to a piezoelectric transducer was developed. The system quantifies acoustic activity as Ring Down Count (RDC) rates. In soil applications RDC rates have been correlated with the rate of deformation, whereas the recent application to rock slopes requires new interpretation strategies. In order to develop new strategies the system was installed at two rock slope trial sites in Italy and Austria. RDC rates from these sites, which have been measured over 5 and 1.5 years respectively, are analysed and clear and recurring trends are identified. The comparison of AE trends with response from a series of traditional instruments available at the sites allows correlation with changes in external slope loading and internal stress changes. AE signatures from the large rock slope in Italy have been identified as generated in response to variations in the groundwater level and snow loading. At the slope in Austria, AE signatures include the detachment of small boulders from the slope surface caused by the succession of freeze-thaw cycles during winter time. The work reported in this paper is contributing to the development of AE monitoring and interpretation strategies for rock slopes. The longer-term aim is to identify approaching failures and derive rules for setting thresholds that can be used to give warning of rock slope failures in time to enable action to be taken

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

Landfill stability and integrity: the UK design approach

This paper highlights the design considerations, in terms of stability and integrity, for EC Landfill Directive compliant sites. The paper details a design chart based on research and development reports produced for the Environment Agency (England and Wales) intended to guide designers and highlight areas for consideration in each of six aspects of landfill construction: subgrade, basal lining system, shallow-slope lining system, steep-slope lining system, waste slopes and capping lining systems. The paper is not intended to offer design methodologies in terms of which calculation methods should be adopted, but to provide designers with a framework in which to apply engineering skill and judgement and to highlight challenges

Monitoring buried pipe deformation using acoustic emission: quantification of attenuation

Deformation of soil bodies and soil-structure systems generates acoustic emission (AE), which are high-frequency stress waves. Listening to this AE by coupling sensors to structural elements can provide information on asset condition and early warning of accelerating deformation behaviour. There is a need for experimentation to model the propagation of AE in buried pipe systems to enhance understanding of real behaviour. Analytical solutions are often based on many assumptions (e.g. homogeneity, isotropy, boundary conditions and material properties) and cannot exactly represent the behaviour of the in situ system. This paper details a series of experiments conducted on buried pipes to investigate AE attenuation in pipes due to couplings and soil surround. The attenuation coefficients reported provide guidance to engineers for designing sensor spacing along buried pipes for monitoring ground deformations, and active waveguide installation depths for slope deformation monitoring. Attenuation coefficients have been quantified for both air–pipe–air and air–pipe–soil trilayer systems for the frequency range of 20–30 kHz

An acoustic emission slope displacement rate sensor: Comparisons with established instrumentation

An acoustic emission slope displacement rate sensor: Comparisons with established instrumentatio