Stability of slopes of an operative mine with a large pre-stripping excavated in hard soil/soft rock

Resumen

As metal prices drop in the commodity markets and mineral grades also begin to lessen, there is a need for mining companies to attempt to optimise their pit operations and designs, reducing stripping ratios via increases in pit slope angles. The requirement for stability in these pit slopes can be accomplished using different philosophies. At Las Cruces mine in Southern Spain a zero harm policy defines and defends the general approach to slope stability. As a modern Spanish mine, Las Cruces is committed to the use of best available technology and practices with the aim of guaranteeing the production of ore in a safe environment. From a practical perspective, this involves high quality pit design, ongoing geotechnical mapping of exposed slopes to ensure conformity with design, and use of leading geotechnical surveillance technology to complement on-going monitoring. The purpose of this thesis is to map out and identify the process of ensuring stability and geotechnical safety, at Las Cruces, since first stripping activities of open pit slope stability maintenance and improvements, highlighting the following crucial elements: 1. Material Characterisations – Fundamental to any design are the parameters utilised in analysis. This thesis investigates the practical investigation of material characteristics over the mine life to date and the methods utilised for the periodic update of this information. The methods of determining these characteristics were predominately:  Intrusive investigation work comprising bespoke geotechnical drillholes.  Laboratory testing on samples obtained from drillholes and bulk samples obtained from the pit slopes during excavation development.  Visual observations and direct rock competence determinations undertaken during slope cartography.  Correlations of various rock lithologies with known similar rock types in other mines and more specifically in mines of the pyritic belt. 2. Optimised Pit Design – Using the geo-mechanical data from the site investigation based on boreholes and laboratory and in-situ testing, improvements were made to the initial design of the slopes undertaken for the mine, Final pit shell configurations were utilised to provide a geometry for: 2D limit-state analysis, with Rocscience commercial programme SLIDE. This consisted of the optimisation of the original pit design and a review of that design incorporating:  Known bedding planes and other geological structures.  Segregation of the upper and lower marls in terms of resistance parameters.  Improved resistance parameters from less disturbed bulk samples obtained directly from the pit slopes and floor.  Variations in the Palaeozoic, including the challenging footwall shales adjacent to the south side of mineral ore body. STABILITY OF SLOPES OF AN OPERATIVE MINE WITH LARGE PRE-STRIPPING EXCAVATED IN HARD SOIL/SOFT ROCK VIII FLAC3D finite difference three-dimensional phased analysis. The analysis was divided into a south side and a north side to reduce processing requirements and time frames. With respect to the marls, an important hydro-mechanical coupling process was fully incorporated which was able to effectively model pore pressure dissipations as a result of principle stress rotation near the excavation slopes and an overall reduction in stress. FLAC2D finite difference two-dimensional phased analysis. A specific worst case geometry was analysed on the north side of the pit, again taking into account a coupling of the hydrological and mechanical elements. However the two dimensional nature of the analysis enabled a refinement of the mesh from 5m x 5m down to 0.5m x 0.5m. This allowed an improved modelling of the hydrological elements of the analysed section. The section also enabled the modelling of the mines adjacent north dump in its final configuration to check both its local stability state and also the effect on the pit. Phase 2D finite element two-dimensional phased analyses. This was undertaken as a method of checking the FLAC2D analysis for QAQC purposes. As such a completely unrelated stress-strain deformation analysis programme was utilised, PHASE2D. In addition the hydromechanical coupling method was not implemented to provide emphasis on the importance of coupling these interdependent factors for mining pit slope optimisation in large open pits. 3. Geotechnical Blast Optimisation – Optimisation of the blast activities was undertaken. This approach was distinct to that of the mine blast engineer with attention being focused upon laboratory data and in pit geotechnical mapping in order to better determine the characteristics of each lithotype present in the mining pit. A methodology was developed to derive, from this laboratory and field data, rock mass dynamic elasticity and strengths parameters. From these parameters, corresponding blast propagation parameters were also determined and cross-checked against in field blast signature testing, with favourable results. Estimations of the blast induced damage depth into the rock mass were made utilising the propagation and rock resistance parameters. Finally further development of the use of the Hoek-Brown damage factor was proposed and a back analysis was undertaken on previously blasted slopes in order to emulate observed failure modes. A comparison with onsite observations was favourable and the methodology was subsequently utilised to analyse and improve blasting for slope construction in the deeper areas of the mine, with highly effective demonstrable results. Observation reconciliation of the effects of this blasting was undertaken via the incorporation of the Stacey blast reconciliation methodology in geotechnical mapping of formed slopes. 4. Pit monitoring – Mapping and vigilance systems. Intensive monitoring of the slopes is conducted from the very first stages of pre-stripping using inclinometers, piezometers and a robotic topographical total station. In the mine, the author has emphasised the difference between:  An appropriate on-paper design and its correct implementation which provides a high probability of slope stability during excavation works.  On-going slope and instrumentation monitoring, usually undertaken by an onsite geotechnical engineer which ensures that any potential instabilities are tracked and highlighted to the operational team such that safety can be maintained. This distinction between slope stability (the first defence for safe mining operations) and slope monitoring (the last defence) is considered fundamental for good mining geotechnical performance. This thesis explores these aspects, implementing innovating new techniques over those used in historic mine practices and provides improvements that could with ease be implemented in other mining operations in the future to ensure that best available practices are in place.