Unsaturated Soils: Advances in Geo-Engineering – Toll et al. (eds) © 2008Taylor & Francis Group, London, ISBN 978-0-415-47692-8 Stability of a tailings dam considering the hydro-mechanical behaviour of tailings and climate factors M.T. Zandarín & L. Oldecop Instituto de Investigaciones Antisísmicas ‘‘Ing. Aldo Bruschi’’, Facultad de Ingeniería, Universidad Nacional de San Juan, San Juan, Argentina R.R. Pacheco Departamento de Química. Facultad de Ciencias. Universitat de Girona. Campus Montilvi, Girona, Spain ABSTRACT: Tailings storage facilities are complex geotechnical structures. The present paper focuses on the study of a case-history, a tailing dam from the nickel industry from Cuba, with the objective of gaining knowledge about the geotechnical behaviour of such structures. The dam was modelled by means of a coupled hydro-mechanical finite element formulation. Due to the low permeability of tailings, the phreatic levels in the deposit remain high during and after its construction. Steady-state flow regime would be reached only after several decades after closure. Moreover, capillary rise causes the degree of saturation to stay high in the whole storage. Under the action of rain storms, phreatic levels rise quickly due to the presence of capillary water. At the end of the storm, phreatic levels fall slowly because of the low hydraulic conductivity. The results of the analysis show that the stability of the dam strongly depends on its hydraulic operation. 1 INTRODUCTION Tailings storage facilities are complex geotechnical structures. They are commonly built by the discharge of slurry within an impoundment. Tailings are fine- grained non-plastic materials. Their permeability is low and unsaturated phenomena play a significant role in their behaviour. A number of factors may influ- ence the stability of tailings dams, such as the pluvial regime, evaporation, capillary rise, construction rate, drainage, consolidation and rain storms. The present paper focuses on the study of a case-history, with the aim of identifying which of those factors are relevant for the structural safety of tailings dams. The tailings dam studied is one of the facilities at Pedro Sotto Alba nickel mine, located in the province of Moa, in the southeast of Cuba. The impound- ment is located on a low lying flood plain in the south bank of Moa River (Figure 1a). It is founded on alluvial sediments of quaternary age, underlain by stiff clays and cretaceous ultramafic serpentine (Figure 1b). (Chalkley et al. 2002; Greenaway et al. 2002). The climate at the site is tropical. Temperatures vary between 23 ◦ C and 27 ◦ C and the annual average rainfall is 2830 mm. The rainfall intensity can reach between 2 and 3 mm per minute, for rainfall events with 5 to 20 minutes duration, having a period of recurrence of 100 years (Moya, 1998). Two extraordinary rainstorms occurred in 1996 and 1998 (Chalkley et al. 2002). The first lasted 48 h with a total rain volume of 722 mm and a maximum intensity of 190 mm in 90 minutes. The second lasted 12 h with a 690 mm total precipitation. The annual average evaporation is 2296 mm. Figure 2 shows the monthly average rainfall and evaporation at the site. The operation of the facility began in 1970. Tail- ings were discharged in slurry form behind a laterite embankment built in stages, applying the upstream construction method. The mean rise rate of the impoundment was about a half metre per year until 1987. The facility had a system of decant pipelines to allow the drainage of the excess water accumulated in the decant pond. The section 1-1 shown in Figure 1b experienced a number of episodes of slope instability. Such episodes involved the release of important amounts of tailings covering the flood plain of the Moa River and Los Lirios Creek. Tailing relicts can be found even in the opposite bank (north) of Moa River, indicating that at some time the slurry flow was large enough to cross the main channel of the river. During the 1990 s a number of works were undertaken in order to rehabilitate the facility (Chalkley et al. 2002, Greenaway et al. 2002). In the present work, section 1-1 was modelled with the objective of gaining knowledge about the influence 925