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.