Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome Angelo Doglioni (1) , Vincenzo Simeone (2) Recovery of strength along shear surfaces in clay soils (1) Technical University of Bari, Engineering Faculty of Taranto, Taranto, viale del Turismo 8, Italy, +390994733204 Abstract The residual shear strength as operative strength along the shear surface is commonly used for analyzing the stability of reactivated landslides. On this base, it is not possible to have brittle failure or progressive failure mechanisms. In fact, these kinds of phenomena claim for a recovery of strength along the shear surface, which is due to ageing effects during the quiescence time. Several tests on clay samples were performed using Bromhead ring shear apparatus to verify the presence of shear strength recovery. Tests showed that recovery of strength are present along the shear surface. Moreover, they proved the stress-stain behavior is characterized by a brittle failure after that the strength, along the shear surface, falls again to the residual value. Keywords Residual strength, ageing, recovery of strength, shear surface, quiescent landslide, clayey soils, thixotropy Introduction The study of stability of reactivated landslides is usually based on the assumption that along the failure surface the operative strength is the residual one or a near value. In fact, during previous landslide movements, the strength along the shear surface falls down from the peak value to the residual one (Skempton, 1964, 1985). According to this assumption, the reactivation of quiescent landslides does not allow for progressive failures, since the minimum shear strength shows along the whole shear surface. So it is not possible that reactivation of quiescent landslides can occur with brittle failure and progressive failure mechanisms. In fact, along all the shearing surfaces the minimum strength has just been reached. It is thus necessary to assume a recovery of strength along the shear surface of landslide during the quiescence time to explain how reactivation can occur with brittle o progressive failure. Then, it is possible also to explain how landslide close to limit equilibrium conditions can be stable for long periods, without being subject to continuous reactivation (Simeone, 1993a, 1993b). Recovery of strength along shear surface also contributes to explain the observed delay (D’Elia et al., 1985; Cotecchia, 1986) during the reactivation of old landslide in clayey materials after seismic shocks. In fact the delay can be considered as the time necessary to the development of progressive failure. It is well know that remolded clay left quiescent is subject to an increment of stiffness and shear strength with time (Skempton and Northey, 1952). This also happens if clay material is not subject to variation of volume or water content. When residual strength conditions occur, clayey particles assume an iso-oriented texture, corresponding to the lower value of shear strength (Skempton, 1964, 1985, Lupini et al. 1981). Therefore, it is necessary to suppose a structural rearrangement during the quiescence period, to get recovery of strength along shear surfaces where residual strength has been reached. Along a shear surface in clay soils, when the landslide movement stops, clay particles can flocculate into a more stable different texture with a low energy level. Flocculating clay particles evolve into a different structure, which can have a higher value of shear strength. This allows for a recovery of strength along the shear surface. During the quiescence period arrangements of clay particles evolve toward the minimum energy texture, which can be different with respect to the minimum strength texture, as function of the clay mineralogy and of the chemistry of pore water. The phenomenon of shear strength and stiffness increasing with time is broadly defined as thixotropy by modern scientific literature. However, the word Dzage hardeningdz should be strictly used (Mitchell, 1961; Schmertmann, 1992) to describe this phenomenon. This is typical of many soils, when they have been reworked and then left resting. During their experimental study on thixotropic phenomena, Osipov at al. (1984) observed recovery of strength at constant volume and water content conditions in diluted suspensions of clay; comparing the value with the lowest strength measured when the system undergoes vibrations. Simeone (1993a, 1993b, 2007) and Guerricchio et al. (1996) using a Bromhead ring shear apparatus observed recovery along shear surface in clayey soil for different clays from South Italy. Also Gibo et al. (2002), Angeli and Gasparetto (2003), Angeli et al. (2004), and Carrubba and Dal Fabbro (2006) got similar behaviours in clayey samples. Aim of this work is to better evaluate this phenomenon by mean of laboratory tests as well as its main variables.