The extent and an analysis of shallow failures on the slopes of highway earthworks

Abstract

The reported incidences of shallow failures on the slopes of highway earthworks have increased in recent years. This Thesis includes a study of the extent of this problem, the likely factors contributing to failure, and presents the results of empirical design and the analysis of the likely mechanism of failure. A survey was conducted, covering a total length of 570km of selected lengths of motorway in England and Wales, which included the principal geologies encountered on the British motorway system, in particular, areas where over-consolidated clays predominate. From the survey, the basic factors that have contributed to shallow failures on the side slopes of embankments and cuttings can be deduced, and attempts are made to quantify any long-term problems. The results show a high incidence of failure associated with the major influences of geology, age of earthworks and geometry of slope, with many more failures occurring on embankments than on cuttings. The slope angles recommended are empirically derived and can be used both in the design of new earthwork side slopes and to identify slopes at risk of failure in existing earthworks. An estimate is made of the extent of failures in the future which suggests that three times as many slopes are likely to fail than have failed so far. To study the mechanisms of failure and the behaviour of over- consolidated clays at extremely low effective stresses, an analytical method is developed which includes a detailed study of non-linear failure envelopes and the fitting of the most representative curve to peak strength data. Also a new rigorous slope stability analysis method is developed which incorporates this type of failure envelope. Back-analyses are conducted for several embankment slopes from which samples have been tested in the laboratory. Results indicate that the critical state strength rather than the peak strength governs the formation of shallow failures.