Landslides and Engineered Slopes – Chen et al. (eds) © 2008Taylor & Francis Group, London, ISBN 978-0-415-41196-7 Numerical modelling of the thermo-mechanical behaviour of soils in catastrophic landslides F. Cecinato & A. Zervos School of Civil Engineering & Environment, University of Southampton, UK E. Veveakis & I. Vardoulakis Faculty of Applied Science, National Technical University of Athens, Greece ABSTRACT: A new landslide model is proposed by improving on an existing one, which is able to interpret using a simple 1-D mechanism the post-failure sliding regime of catastrophic landslides and rockslides consisting of a coherent mass sliding on a thin clayey layer. The model takes into account frictional heating and subsequent pore pressure build-up, leading to the vanishing of shear resistance and unconstrained acceleration. First, an existing thermo-elasto-plastic constitutive model for clays is discussed, and modified by re-formulating it in a general stress space and taking into account thermal softening. The soil constitutive model is then employed into an existing landslide model. The resulting model equations are shown to be well-posed, and then are discretised and integrated numerically to back-analyse the final stage of the well-documented case history of Vajont that occurred in Italy in 1963. Finally, the results are used to highlight the possible importance of thermal softening in the development of catastrophic failure. 1 INTRODUCTION The Vajont landslide of October 9, 1963, has been the subject of numerous geological and geomechani- cal investigations, due both to its potential contribution to slope stability analysis and to the social and legal implications of the disaster. The landslide moved approximately 2.7 × 10 8 m 3 of rock into an artificial reservoir of about 1.5×10 8 m 3 , impounding the Vajont deep gorge. The slide moved an 120 m thick (on aver- age) compact rock mass over a front of 1850 m for a maximum slip of 450–500 m (Hendron and Patton, 1985) and at a final slip rate of about 25–30 m/s. The abrupt filling of the reservoir with debris produced a giant wave (4.8 × 10 7 m 3 ) that propagated up and down the valley, overflowing the dam and wiping out the village of Longarone, located 2 km west. Habib (1975) proposed that the high slip veloc- ity achieved by the Vajont landslide was due to the conversion of mechanical energy into heat during frictional sliding, which should lead to the ‘‘vapor- ization’’ of pore water and hence to a cushion of zero friction. Temperature increase in the slipping zone may also have led to pressurization of pore water with the same effect on the shear strength of the slope (Anderson, 1980; Voight and Faust, 1982; Vardoulakis, 2000, 2002). Total loss of strength by thermal pressurization has also been claimed for the Jiufengershan rock and soil avalanche triggered by the Chi-Chi (Taiwan) 1999 earthquake (Chang et al., 2005a, 2005b). Vardoulakis (2000, 2002) analyzed the pressuriza- tion phase of the Vajont slide, when thermal pres- surization sets in, during which the slide accelerates rapidly. He proposed a one-degree-of-freedom, fric- tional pendulum model, employing a Mohr-Coulomb constitutive model for the soil and assuming that frictional heating triggered pore water pressurization inside a shear band of the order of 1 mm. This analysis showed that the catastrophic pressurization phase of the Vajont slide should not have taken more than a few seconds to develop in full. In this paper we extend the above study by using a more general thermo-elasto-plastic constitu- tive model, based on the one recently proposed by Laloui et al. (2005). Furthermore we investigate the impact of thermal softening, which some clays exhibit, in the development of the catastrophic mechanism. In the following, we present in section 2 the land- slide model developed by Vardoulakis (2000, 2002), and in section 3 the new thermo-elasto-plastic consti- tutive model. Section 4 deals with the modification of the landslide model to include this new constitutive law. Finally, in section 5 some computational results are presented and discussed and conclusions are drawn in section 6. 615