Types of Flowslide Failures and Possible Failure Mechanisms
Flowslide is a common type of failure of granular soil slopes. Although many flowslides can be explained using static liquefaction or instability behaviour of sand under undrained conditions, some of the failure might have occurred under essentially drain
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TYPES OF FLOWSLIDE FAILURES AND POSSIBLE FAILURE MECHANISMS Dariusz Wanatowski School ofCivil Engineering, The University ofNottingham, University Park, Nottingham, NG7 2RD, United Kingdom Jian Chu School ofCivil and Environmental Engineering, Nanyang Technological University, Elk NI, 50 Nanyang Avenue, Singapore 639798 Robert S.C. Lo School ofAerospace Civil and Mechanical Engineering, UNSW at ADFA, Canberra, ACT 2600, Australia Flowslide is a common type of failure of granular soil slopes. Although many flowslides can be explained using static liquefaction or instability behaviour of sand under undrained conditions, some of the failure might have occurred under essentially drained conditions. In this paper, some flowslide case histories are reviewed. Typical flowslides and the possible failure mechanisms behind each flowslide are identified. Based on the laboratory studies, new failure mechanisms are proposed and used to offer new interpretations to some of the failure cases. INTRODUCTION
Landslides are commonly classified as falls, topples, spreads and flowslides based on their ways of movement (Sassa, 1989). Since flow failures are characterised by a sudden loss of strength followed by a very rapid development of large deformations, their effects are often much more dramatic and devastating than other types of landslides. Examples of such flow failures include Fort Peck Dam (Casagrande, 1965), Helsinki Harbour (Andresen and Bjerrurn, 1976), the Mississippi river bank (Torrey and Weaver 1984), Nerlerk berm (Sladen et al., 1985), Wachusett Dam (Olson et al., 2000), and the failures of some tailings dams such as the Merriespruit Tailings Dam in South Africa (Fourie et aI., 2001). Most of these failures are considered to be triggered by static liquefaction or instability of granular soils occurring under undrained conditions. Here, static liquefaction is defined as a phenomenon in which loose sand loses a large percentage of its shear resistance and flows in a liquid manner when it is subjected to undrained monotonic loading, and instability as a behaviour in which large plastic strains are generated rapidly due to the inability of a soil element to sustain a given stress. Under undrained conditions, failure occurs at a stress state much lower than that
determined by the effective failure envelope . For this reason, most of the above mentioned flowslides should have remained stable according to conventional slope stability analysis. Although it is a common understanding that static liquefaction or instability only occurs for loose granular material under undrained conditions, this may not be the case in practice . Slope failure in dilating soil has been observed in several cases by Been et al. (1988). The Fort Peck Dam (Casagrande 1965) case is one of such failures. Flow failures in Mississippi riverbanks occurring in dense sand were also described by Torrey and Weaver (1984). Furthermore, Been et al. (1987) argued that a well-documented Nerlerk berm case (Sladen et al., 1985) might also have occurred for dilatin
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