Sill pillar recovery at the Louvicourt mine
This paper describes the challenges of mining a sill pillar under various types of paste backfill quality. From 1994 to 2002, Louvicourt mine produced approximately 12,600,000 t grading 3.5% Cu, 1.5% Zn, 27 g/mt Ag, and 0.85 g/mt Au. The mining method is transverse blasthole stoping, mining primary and secondary stopes. The production of this 4,300 t/d operation was accomplished using two mining horizons: from the 655 m level to the 415 m level and from the 860 m level to the 680 m level. The main sill pillar is between the 680 m level and the 655 m level.
Since 2002, production has gradually decreased and mining activities are expected to end in mid-2005. The sill pillar production will represent almost 25% of the total mine production. To recover the sill pillars, more than 1,500 m of development was planned in or under paste backfill.
Using two mining horizons for production created two sill pillars. The main sill pillar is between the 680 m level and the 655 m level. Another much smaller sill pillar remains between the 885 m level and the 860 m level. For the final three years (2003, 2004, and 2005), 25% of production will be coming from the sill pillars largely under paste backfilled stopes.
Dilution and recovery are measured using a cavity monitoring system (CMS). Since 1995, over 300 stopes have been measured using CMS representing more than 90% of all mined out stopes. Historically, dilution for the primary stope is 7% and 13% for the secondary stope. In the January 1, 2004 ore reserves evaluation, the sill pillar dilution was estimated to be at 10% for the primary stope and 20% in the secondary stope.
Recovery, based on the CMS survey, is 96%. A 95% recovery was used for the reserves evaluation for all stopes. In the 680 m to 655 m level sill pillar stopes, the recovery factor is based on backfill cohesion and rock mass quality. The average recovery factor used for the sill pillars stopes is 59.6%.
It was decided to develop under paste backfill on the 655 m level and apply transverse blasthole stoping as the mining method for the sill pillars. The objective of developing directly under paste backfill was to excavate in the shadow of the above backfilled stopes (eliminating the rockburst occurrence). This method will safely recover the sill pillar stopes away from the in situ stress.
Standards of drifting in or under competent paste backfill were initiated in 1999. In 2001 and the first part of 2002, development standards were modified several times to improve the drilling, blasting, and, most importantly, the ground support under various type of backfill and unconsolidated rock. The basic ground support consists of two layers of shotcrete with size 9 mesh between them.
After development under paste backfill and using reinforced rockburst support on the 680 m level, the stopes are designed as normal blasthole stopes with the exception of additional holes for pre-shearing on both sides. These pre-shearing holes are 1 m apart and are blasted before the mass blast. The production holes are surveyed with a Flexit instrument and the mass blast is taken using electronic detonators.
Experience and results of the first sill pillar stopes are presented in this paper.