The identification and management of cuttings at risk from deep-seated, delayed failure on the Highways Agency network

Abstract

The UK trunk road network includes a considerable number of cuttings constructed through high plasticity, overconsolidated clays. These materials are prone to the process of delayed, deep-seated failure whereby instability can occur some considerable time after construction. The timescale between excavation and failure can be anywhere between 20 and 120 years. Many of the recorded deep-seated failures in the literature are from railway cuttings in the UK which are generally much older than cuttings on major highways. The bulk of trunk road construction in the UK occurred during the 1960s and 1970s and many cuttings are at, or approaching, an age when delayed, deep-seated failures might be expected to occur.The Highways Agency (HA) has been aware of this issue and has sought to understand the causes and associated risks of these types of failure. The first phase of a study (concluded in 2007) provided a framework for assessing the likelihood of failure. The selection criteria were based on a combination of cutting height and slope angle. Mott MacDonald was commissioned to undertake the second phase of the study to focus on the application of the Phase 1 recommendations to the HA asset base. This paper describes the work undertaken during this study and conclusions reached.The UK trunk road network includes a considerable number of cuttings constructed through high plasticity, overconsolidated clays. Theses materials are prone to the process of delayed, deep-seated failure whereby instability can occur some considerable time after construction. The timescale between excavation and failure can be anywhere between 20 and 120 years.The term deep-seated failure is defined as those failures extending to more than 2 m depth below the slope surface and these may extend beyond the toe or crest of the slope (Perry et al. 2003). Shallow failures are less than 2 m deep and occur entirely within the cutting side slope. The results of a review of the occurrence and distribution of shallow failures on the Highways Agency (HA) network is contained within the Transport Research Laboratory report RR199 (Perry 1989). At the time of preparing RR199, shallow failures were by far the most common.This study concentrates on first time slides only, that is, it is assumed that there were no pre-existing shear surfaces present prior to construction. Cutting slopes that contain preexisting shear surfaces will behave in a different manner and have been excluded from this study. Many of the recorded deep-seated failures in the literature are from railway cuttings which are generally much older than cuttings on major highways. The bulk of trunk road construction in the UK occurred during the 1960s and 1970s and many cuttings are at, or approaching, an age when delayed, deep-seated failures might be expected to occur.