Mix proportioning of underground cemented tailings backfill M. Fall a, * , M. Benzaazoua b , E.G. Saa c a Department of Civil Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ont., Canada K1N 6K7 b University of Quebec in Abitibi-Temiscamingue (UQAT), Rouyn-Noranda, Que., Canada, J9X 5E4 c Ecole Polytechnique Fe ´de ´ rale de Lausane, Switzerland Received 27 August 2005; received in revised form 6 August 2006; accepted 12 August 2006 Available online 20 February 2007 Abstract The usage of cemented tailings backfill (cemented paste backfill) in the underground by mining industry is becoming increasingly important. However, until now, the mix proportioning of CTB has been mainly based on the realization of extensive laboratory tests on a large number of CTB mixes. Therefore, this paper presents a design method for mix proportioning of CTB to minimize the number of trial mixes and provide an appropriate mix proportion. This method is based on the pairing of the response surface method (RSM) and the desirability approach. First, the RSM was used to develop predictive models for the performance properties of CTB. The pre- dicted properties in question are the uniaxial compression strength (UCS), the slump, the solid concentration (solid percent, %Solid) and the cost (based on cement cost) of the CTB. The predictive models that were developed were able to accurately represent the relationships between the physical and chemical characteristics of the CTB components (tailings, binder, water) and the above properties. The results of the modeling phase were then used as input data in the optimization phase (based on desirability approach) to develop optimal recipes for the studied CTBs. This study has demonstrated that the combination of the RSM and desirability approach represents an effective tool for CTB mix proportioning. It has also shown that the mix parameters (cement content, water-to-cement ratio, tailings fineness and density) affect the performance properties of CTB. The results of this research provide a more comprehensive engineering approach to CTB mix proportioning. The developed design method can be useful in reducing the laboratory test protocol needed for the determina- tion of the optimal mix composition. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Tailings; Cemented backfill; Optimization; Mine; Response surface method; Desirability; Strength 1. Introduction Cemented tailings backfill (cemented paste backfill stud- ied in this paper) is a heterogeneous material in which tail- ings are held in place by a hardened cement paste binder. Its components (tailings, water, cement) are combined and mixed in a plant usually located on the surface and transported (by gravity and/or pumping) to the under- ground mine (Fig. 1). Cemented tailings backfill (CTB) is extensively used in Canadian underground mines and in many parts of the world and is following an increasing trend as it offers a number of technical and economical benefits (Lerche and Renetzeder, 1984; Landriault et al., 1997; Hassani and Archibald, 1998; Fall and Benzaazoua, 2003a). Indeed, the CTB technology is considered superior to conventional mine backfill methods in terms of cost- effectiveness (Hassani and Archibald, 1998; Fall and Ben- zaazoua, 2003b). It is especially important in ensuring the stability of underground mine openings and in maximizing the safe recovery of ore. In addition, the maximum under- ground disposal of mill tailings is a significant environmen- tal advantage (Huynh et al., 2006). In order for the CTB to assume the aforementioned roles in a safe and cost-effective manner in underground mining, proper proportioning of the CTB mixtures is nec- essary. In other words, the mix proportioning of CTB is a vital step in obtaining a CTB that meets desired technical 0886-7798/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2006.08.005 * Corresponding author. Tel.: +1 613 562 5800x6558. E-mail address: mfall@eng.uottawa.ca (M. Fall). www.elsevier.com/locate/tust Available online at www.sciencedirect.com Tunnelling and Underground Space Technology 23 (2008) 80–90 Tunnelling and Underground Space Technology incorporating Trenchless Technology Research