Geotechnical study on the feasibility of using backfill in steep dip areas of Konkola copper mines (Zambia)

Abstract

Backfill is described as waste material that is used to refill the underground voids left during mining for technical or safety purposes. Backfilling has several applications underground and the use of backfill has increasingly become a fundamental part in the entire underground mining activities all over the world. This has greatly been accelerated by the need to increase ore production as well as maintaining regional stability around mine operations. The ever increasing depth of mining operations has largely increased the use of backfill material in underground operations to allow efficient and safe extraction of the mineral deposits. The need for Konkola Copper Mines (KCM) to develop additional production areas further below its current operations has resulted in the sinking of No 4 shaft to a depth of 1500m below surface. Mining at increased depth may result into increased levels of induced stresses hence this may negatively impact on the stability of the stopes and ultimately can lead to interrupted production. The purpose of this research was to evaluate geotechnically the behaviour of the current and future stopes at increased depth assuming no backfilling is performed and to determine the depth at which backfill may be needed. Numerical modelling using Phase2 computer software and Mathew’s stability graph method was used for the purpose of evaluating, geotechnically, the response of the stopes at different mining depths. Geotechnical investigations was done through core logging, underground structural mapping and laboratory rock tests to determine input parameters into the two evaluation techniques. Numerical modelling and empirical analysis have both indicated that stopes are stable but around 1250mL and below there are chances of increased and excessive instability problems which can lead to increased dilution and risk of collapse. Suggested improvements to reduce costs and improve copper ore production include the increasing of stope strike length from 20m to 30m on 950mL – 1050mL. Stopes are stable at this new length and the option improved overall ore production in the Bancroft area of the mine by 7.14% and a possible reduction in development costs on slot raises and draw-point cross-cuts by 28.6%. Mining with Cemented Hydraulic Fill (CHF) from 1250mL and below within the steep dipping areas (Bancroft areas) has proved to be very economic as gross revenue after an economic analysis increased by 15.5% as compared to leaving substantial amount of ore in stabilising pillars. With CHF, dilution can also be reduced although not quantified in this research.