Pérez, C, Pérez, V, Beale, G, Ríos, D & Soto, F 2013, 'Water management for slope stabilisation – an example from Peru', in PM Dight (ed.), Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering
, Australian Centre for Geomechanics, Perth, pp. 381-391, https://doi.org/10.36487/ACG_rep/1308_23_Perez
Peru is one of the world’s leading mining countries and includes a number of large open pits such as Antamina, Toquepala and Yanacocha. The Yanacocha operation comprises six large pits including El Tapado where mining has been ongoing since 1999. Instability of the north wall of the El Tapado pit occurred as the slope was mined down. Initial site investigations identified high water pressures in the permeable silica alunite material behind the slope and also in strongly altered propylitic clays at shallower levels within the slope.
Minera Yanacocha created a multi-disciplinary team of mining engineers, geotechnical and hydrogeological staff to develop a mitigation plan to allow mining of the slope to continue. The failure main surface was identified at the silica alunite contact within the overlying clays using piezometer and inclinometer installations. The silica alunite had a permeability range from 10-3 to 10-6 m/s and it was possible to install pumping wells to reduce the water pressures. The overlying propilitic clay had a much lower permeability (10-7 to 10-8 m/s) and the plan was to depressurise the material using a combination of surface water controls, under-drainage by pumping from the silica alunite and horizontal drain holes. The first stage of the program was to install a joint monitoring network which could measure the both the pore pressure and slope movement response to the initial dewatering wells in the silica alunite. The second stage was to install the surface water controls to minimise ongoing infiltration and recharge to the clay units during the wet season. The third stage was the horizontal drain hole program to increase the rate of depressurisation of the propyllitic clays and to evaluate the coupled hydromechanical response associated with the ongoing excavation of the pit wall. This paper explains the joint methodology performed by the project team to stabilise the slope and allow mining to near the full planned depth of the pit.
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