SRK carried out a feasibility level geotechnical design for an underground and open pit copper project in southern Morocco. The study included a site visit to access drifts dating from the 1970s to gather geotechnical data.
The study included various mining methods down to a depth of 600m, such as drift and fill, drift and pillar, room and pillar, open stoping and a similar to- AVOCA method. A combination of empirical and numerical approaches was used to estimate stresses on pillars and abutments, evaluate pillar strength, assess the impact of surface subsidence and determine backfill strength and stiffness requirements.
The tributary area and Mathews methods were used for a first-pass design of pillars and spans; however, they could not be used to optimise the designs and improve extraction ratios. Therefore a simple model was constructed in FLAC3D to estimate stresses in an elastic homogeneous material. Code written in the FISH scripting language allowed several layouts, varying spans, pillar and panel sizes to be analysed with ease and makes it possible to automate the output of stress values for postprocessing with MS Excel. Output of stress contours for reporting was also automated through FLAC’s in-built Python capabilities.
Complementing the above, a conceptual monitoring system was designed with the aim of validating and calibrating the models used in the geotechnical analyses; such a ‘design by measurement ‘rationale can help to manage risks, improve safety and productivity, optimise designs and reduce costs. The system was directed specifically at optimization and roof support optimisation; the former can be achieved by measuring the loads developed within pillars as mining progresses through the use of CSIRO hollow inclusion cells. Borehole extensometers were recommended to assess how the roofs of excavations behave with respect to distance from the face and excavation dimensions.
