Large Open Pit Stability

Since 2005, Itasca has been one of the key contributors in the extensive research project on large open-pit stability, the Large Open Pit (LOP) project, initiated by the CSIRO Division of Exploration & Mining.

One of the central objectives in open pit mining is establishing reliable slopes; slopes that are predictable even in failure, in order to prevent severe consequences. Itasca has been an integral part in the study of the relation between rock mass strength and deformability, and slope failure mechanisms.

As part of the LOP project, Itasca developed a completely new software called Slope Model. The software uses the concept of the Synthetic Rock Mass (SRM) approach applied to the specific case of rock slopes. In practice, Slope Model allows a 3D sector of a rock slope to be interactively defined and simulated, or arbitrary pit geometry can be defined and imported. SRM allows movement on joints (sliding and opening) as well as fracture of intact rock. Thus, the large-scale rock mass behavior (which cannot be measured directly) is synthesized from the component behaviors (which can be measured).

Slope Model is designed to simulate rock masses in which overall failure is a combination of slip and opening on joints and intact-rock failure. Non-steady fluid flow and pressure within the network of joint segments and the rock matrix are modeled, and several aspects of fluid-rock interaction are represented, such as effective stress (for sliding behavior) and pressure response to slope deformation (e.g., bench removal).

A case study using Slope Model was conducted for the south wall of Newcrest Mining Ltd’s Cadia Hill Open Pit, which experienced a multi-bench (60,000 t) failure in 2006 while mining at the 505 R1 level. The observed failure mechanism, sometimes called a one-sided wedge, involved a combination of structurally controlled sliding (along the release structure) and rock mass failure (Figure 1). Figure 2 shows velocity vector field at the end of the simulation using Slope Model, conducted in multiple stages. The figure indicates the failure mechanism consistent with the field observations and the conceptual model illustrated in Figure 1.

 

Figure 1 Location and extent of failure on the south wall of Newcrest Mining Ltd’s Cadia Hill Open Pit (Sainsbury et al., 2007).

 

Figure 2 Final velocity field and excavation induced cracks (Sainsbury et al., 2007).

Numerical models have the potential to aid in the understanding of the complex behaviour in large open-pit slopes through the numerical representation of features considered likely to impact slope stability. Given the wide scope of numerical models available today, it is essential that engineers fully understand the varying strengths and limitations inherent in each of the different approaches and methodologies. Slope Model is a significant step towards advancing the use of more complete geological and geotechnical data in assessing rock mass characteristics and slope failure mechanisms.

 

 

References

Cundall, P. (2008) “Recent Advances in Numerical Modelling for Large-Scale Mining Projects,” ACG News., 30, 1-7 (June).

Cundall, P. A., and B. Damjanac. (2009) "A Comprehensive 3D Model for Rock Slopes Based on Micromechanics," in Slope Stability 2009 (Universidad de Los Andes, Santiago, Chile, November 2009).

Jefferies, M., L. Lorig and C. Alvarez. (2008) “Influence of Rock-Strength Spatial Variability on Slope Stability,” in Continuum and Distinct Element Numerical Modeling in Geo-Engineering (Proceedings, 1st International FLAC/DEM Symposium, Minneapolis, August 2008). Paper No. 05-01, R. Hart, C. Detournay, and P. Cundall, Eds. Minneapolis: Itasca Consulting Group, Inc.

Pierce, M. E., and C. Fairhurst. (2012) “Synthetic Rock Mass Applications in Mass Mining,” in Harmonising Rock Engineering and the Environment (Proceedings, 12th ISRM Int. Congress, Beijing, October 2011), pp. 109-114, Q. Qian and Y. Zhou, Eds. London: CRC Press.

Pierce, M., P. Cundall, D. Potyondy and D. Mas Ivars. (2007) “A Synthetic Rock Mass Model for Jointed Rock,” in Rock Mechanics: Meeting Society's Challenges and Demands (1st Canada-U.S. Rock Mechanics Symposium, Vancouver, May 2007), Vol. 1: Fundamentals, New Technologies & New Ideas, pp. 341-349, E. Eberhardt et al., Ed. London: Taylor & Francis Group.

Pierce, M., M. Gaida and D. DeGagne. (2009) “Estimation of Rock Block Strength,” in RockEng 09 --- Rock Engineering in Difficult Conditions (3rd CANUS Rock Mechanics Symposium / 20th Canadian Rock Mechanics Symposium, Toronto, May 2009). Paper No. 4360, M. Diederichs and G. Grasselli, Eds.

Sainsbury, D.P., F. Pothitos, D. Finn and R. Silva. (2007) “Three-Dimensional Discontinuum Analysis of Structurally Controlled Failure Mechanism at the Cadia Hill Open Pit,” in Slope Stability 2007 (Proceedings, Int. Symp. on Rock Slope Stability in Open Pit Mining and Civil Engineering, September 2007, Perth, Western Australia), 163-168. Perth: Australian Centre for Geomechanics.