INNOVATIVE DESIGN OF OPEN CAST MINES IN WEAK ROCKS

A clear definition for open cast mine is ground surface cut, which is typically open for the duration of pit life, made to extract minerals or other geological materials of economic interest to the miner. The primary goal of mine design is the maximisation of the profit, which is expressed as ore extracted vs the excavating cost. The selection of physical design parameters and the mine scheduling program are two critical decisions that have a significant impact on the financial return (profit) of the mining programme. Therefore, it can be easily stated that the planning of an open cast mine is a financial problem that is constrained by engineering aspects.

The design of the slopes is the most challenging one of the engineering aspects. The slopes must be built focusing on two factors, safety of workers and financial return of the mining program, which are usually in contrast since hazard and profit are both increasing with the steepness of the overall slope angle. Therefore, the problem requires a detailed optimisation, where the final design of a pit slope is a balance of the two factors mentioned above. This procedure will lead to a working environment with an acceptable level of risk and slope angles as steep as possible.
An important point that must be underlined is that the final slope design is not a mere theoretical exercise, but the entire mining process must be considered (safety, equipment used to realise the mine, mining stages and maximum level of risk that can be accepted).

In the last decade, the development of the mining equipment allowed the possibility of creating mines in the presence of a thick layer of weak rocks. They are "rocks with intact strength ranging from extremely weak to weak and with uniaxial compressive ranging from 250 kPa to 25 MPa" (Martin and Stacey, 2018). Therefore, slope angles are typically milder relative to those adopted in hard rocks, and this reduction in slope angle has a significant economic impact on the profit of the mining program. The main reason for this difference is that the design of a slope is driven by the rock strength, in case of weak rocks, and by the presence of joints and discontinuities in hard rocks.

Recently Professor Utili and co-workers have developed a novel theoretical framework based on limit analysis for the design of non-linear slope profiles that demonstrate improved performance relative to the traditional planar profile. By using a non-linear profile, the stability factor could be improved up to 49%, meaning that the non-planar shape can reach steeper overall slope angles than the planar one.

The research stated above has never been applied to a real case study in the world because additional development is needed to design a real open pit slope. Specifically, the main missing features that must be improved are the rock heterogeneity, groundwater pressure, seismic action, presence of berms and haul roads and usage of multiple failure criteria.

This PhD project will be a further step of the available framework and will achieve steeper overall slope angles from the ones that are currently developed in weak rocks by mine designers. Therefore, it will provide a significant improvement for the mining industry because it can be directly translated into a considerable increment of the profit. Lastly, the application to a case study may serve as a model to further researches which will be practised on real cases.

In order to fill the research gap presented above, this PhD project aims to answer how to design an optimal profile for open pit slopes in weak rocks. To support this research, question some objectives have been set. In particular, the primary objective will be the creation of a user-friendly software package capable of implementing the non-linear slope profile for open pit mines in weak rocks. After that, the economic impact on the mine profit will be evaluated in order to quantify the benefits of this new design methodology.