Moving towards a performance-based design approach for cutting slopes in high PI clays

The design of new, or the remediation of existing, cutting slopes in high-plasticity clay geologies is of considerable economic interest in the UK because they comprise a significant percentage of the UK’s existing and proposed linear transportation infrastructure.The key metrics which asset owners need to understand to make informed decisions about these designs are capital cost and whole life cost (including the operational impact of future interventions or absence of interventions). Design is undertaken in a framework of code compliance which incorrectly gives the impression that different solutions either pass or fail, but it does address the question of comparative capital cost. It doesn’t, however, allow the designer to express the relative merits of different design options, because it is very difficult to quantify the functional design life of different designs, and therefore their whole-life cost. Some designs ‘pass’ more easily than others, but there is no tangible way of defining what this means in terms of those key asset management metrics.This paper reviews the latest science on the performance of cutting slopes in high plasticity clays from the perspective of design. It shows that the potential impacts of long-term weather conditions add significant uncertainty to established, simplified, design approaches. It then proposes that there is an emerging opportunity to refocus design on performance, quantified through the concept of effective design life, rather than purely on compliance with calibrated partial factor models. The benefit of this is that design could be presented in terms of key asset management metrics and would become immediately more tangible to all stakeholders, with a consequence of facilitating more informed, and therefore, better decision-making.