Gary Fowmes Geotechnical Research Engineer, Loughborough University and Golder Associates (UK) Ltd, UK Neil Dixon Professor, Loughborough University, UK Russell Jones Principal and Senior Geotechnical Engineer, Golder Associates (UK) Ltd, Stanton-on-the-Wolds, UK Proceedings of the Institution of Civil Engineers Waste and Resource Management 160 May 2007 Issue WR2 Pages 51–61 doi: 10.1680/warm.2007.160.2.51 Paper 7200010 Received 17/05/2007 Accepted 31/07/2007 Keywords: geotextiles, membranes & geogrids/failures/landfill Landﬁll stability and integrity: the UK design approach G. Fowmes BSc, MSc, FGS, N. Dixon PhD, FGS and D. R. V. Jones PhD, CEng, MICE, MCIWM, FCIArb, FGS 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. 1. INTRODUCTION A 2003 report on UK landﬁlls identiﬁed over 85 failures. 1 Although the majority of these failures were recorded during landﬁll construction and could thus be easily remediated, there were still signiﬁcant cost implications. The failures were primarily attributed to inadequate site investigation, uncontrolled groundwater and inappropriate design. This highlights the need for guidance on the design of landﬁll lining systems. A literature review (report TR1) 2 and a guidance document (report TR2) 3 for assessment of the stability and integrity of landﬁll lining systems have been produced for the Environment Agency. Application of the guidance provides a framework for the assessment of six aspects of landﬁll stability and integrity: subgrade, basal lining systems, shallow-slope lining systems, steep-slope lining systems, waste slope stability and capping lining stability. In England and Wales, operation of landﬁlls requires a pollution prevention and control (PPC) permit. The stability risk assessment requirements for the permit application are based on guidance TR2. 3 This paper presents the design considerations for each of the six aspects and discusses the key considerations. To date, over 600 copies of guidance TR2 3 have been distributed by the Environment Agency in response to orders from designers, operators and researchers both in the UK and overseas. This paper considers the key aspects of landﬁll design and provides an updated summary of current design approaches. Areas for future consideration are highlighted. 1.1. Typical lining system A typical lining system comprises barrier, protection and drainage layers formed from geological (e.g. clay and gravel) and geosynthetic (e.g. geomembrane and geotextile) materials. Fig. 1 shows a typical lining system. A barrier layer is required to limit the leakage of ﬂuids (both liquid and gas) from the waste mass into the surrounding environment. Examples of barrier layers include compacted clay, bentonite-enriched soil (BES), colliery spoil, polymeric geomembranes and geosynthetic clay liners (GCLs). For non-hazardous landﬁlls (e.g. those taking municipal solid waste), the 1999 EC Landﬁll Directive, 4 enforced in the UK through the 2002 Landﬁll Regulations, 5 requires a geological barrier to aid attenuation of contaminants. With properties equivalent to a thickness of 1 m and hydraulic conductivity of 1 : 0  10 ÿ9 m/s, the barrier is required along the base and up the sides of landﬁll sites. The thickness can be reduced to a minimum of 500 mm if the permeability of the barrier layer is decreased. In the UK, the geological barrier is typically formed from compacted clay or BES (artiﬁcially established geological barriers) and may also include low permeability in situ materials (natural geological barriers). In order to further reduce leakage from a landﬁll, composite liners can be used where a combination of barrier materials provides a greatly reduced hydraulic conductivity. The most common composite lining system involves a compacted mineral liner overlain by a polymeric geomembrane, where the geomembrane is in close contact with the mineral liner; hence any ﬂuids migrating through defects in the geomembrane must still pass through the mineral liner whilst the hydraulic head across the mineral liner is signiﬁcantly reduced by the presence of the geomembrane. It is important to protect the geomembrane from damage both during and following installation. A geotextile protection layer is typically placed above a geomembrane to protect the membrane from the overlying materials. The performance of a lining system is improved by control of the leachate head acting on it, and it is common practice to include a drainage layer above the lining system. The drainage layer can be a granular mineral layer or a polymeric drainage composite. 2. DESIGN ISSUES 2.1. General Site investigation should provide designers with conﬁdence in parameters selected for use in analyses. Inadequate site investigation may not only lead to inappropriate design parameters being selected but can also lead to critical failure Waste and Resource Management 160 Issue WR2 Landﬁll stability and integrity: the UK design approach Fowmes et al. 51