Improving the stability of slopes using a spaced piling technique

Discrete piles are used to stabilise infrastructure slopes, especially where there is insufficient land to allow construction of large toe berms or regrading of the slope. In addition to providing a technique that can be used to steepen slopes with minimum landtake, spaced piling can be used for the permanent and cost-effective reinstatement and repair of unstable or failed clay slopes. This is particularly relevant to many motorway slopes, which are prone to shallow failures and are now reaching a critical age for deep-seated failure to occur. The situation is also exacerbated by the increasing incidence of heavy rainfall and flooding that has occurred in recent years due to the effects of climate change, which will add to the risk of softening of the clay. For these reasons, the Highways Agency initiated a comprehensive programme of research with TRL to investigate the potential of the piling technique. This Insight Report discusses the findings from a literature review, centrifuge and analytical studies, and two instrumented case history studies of the remediation of clay slopes on the highway network. The implications of the findings are discussed and appropriate design advice recommended.