Slope stability analysis at Siilinjärvi Mine

Abstract

Siilinjarvi phosphate mine in eastern Finland has been in operation as an open pit since 1979. The current main pit, the Sarkijarvi open pit, is 2,900 m in length, 235 m in depth and at its widest part 750 m wide. The ore feed to the mill is close to 11 Mt/a. The stripping ratio in current LOM (mine life to 2034) is on average 0.6 but a major extension program is ongoing and for the next nine years the stripping ratio will be around 1.2. Part of the main pit extension program involves new infill drillings, altogether 39 km in 2012–14. The updated geological and rock mechanics data is and will be used in slope stability analysis. Engineering company Poyry Finland Oy is developing rock mechanics analyses and delivering guidance’s to the mine engineering team. The Siilinjarvi carbonatite complex is a steeply dipping Archean vein intrusion that has intruded into granite gneiss. The complex is approximately 16 km long. The main rock types are calcite carbonatite, glimmerite and their varieties depending on the amount of apatite and calcite. The carbonatite is surrounded by a fenite halo. The geology in the mine is challenging as seen from a rock mechanics perspective. The glimmerite rock has poor rock quality and a low uniaxial compressive strength. The glimmerite has a similar major joint direction as the intrusion general strike and dip which is noted as a weakness plane for the rock mass. The glimmerite rock mass was partly highly sheared due to late/post intrusion tectonics. The shear zones consist of an extremely weak rock mass and have high water loss values. Geotechnical characterisation of the rock mass has been carried out using Q – and GSI systems. The rock quality was mapped directly from the open pit walls and from drill holes. Also 3D-photogrammetry was used to map joints and shear zones in the pit. The stability analysis was performed using the distinct element program, 3DEC. The rock mass was modelled as a Mohr-Coulomb elastic, perfectly plastic continuum. Analysis used a shear strength reduction approach: the shear strength envelope of material was reduced by a Factor of Safety until the velocities in the model are not balanced. Because of limited groundwater data simulations were made with drained and almost fully saturated conditions. The lowest safety factor was found to be in the weak rock mass area where the safety factor for overall stability was 1.6 in saturated slope and about 1.9 in drained slope. In individual benches the lowest safety factors varied from <1 to 1.2. For the detailed parametric study 2D analysis was also made with Phase 2. The cross section was taken through the middle part of the open pit. In 2D analysis the extent of the disturbance zone and groundwater table was studied. The stability analysis was performed in order to identify potentially unstable areas. From those areas collection of groundwater data will be substantially increased. Also additional geotechnical core logging and mapping will be executed. To monitor the stability of the open pit, purchasing of slope stability radar is under consideration by the mine. In 2013 the extension of the open pit will be analysed with 3DEC. After completion of the drilling program in 2014, new open pit optimisation will be carried out; at the same time the geotechnical study will be updated using 3DEC.