INNOVATIVE DESIGN OF OPEN CAST MINES IN WEAK ROCKS

露天矿的一个明确定义是地面开采，通常在矿坑寿命期间露天开采，用于开采对矿工有经济利益的矿物或其他地质材料。矿山设计的首要目标是利润最大化，即开采的矿石与开采的成本之比。物理设计参数的选择和矿山调度方案是对采矿方案的财务回报（利润）有重大影响的两个关键决策。因此，可以很容易地说，露天矿山的规划是一个受工程方面约束的财务问题。斜坡的设计是最具挑战性的工程方面之一。边坡的建设必须关注两个因素，即工人的安全和采矿计划的经济回报，这两个因素通常是相反的，因为风险和利润都随着边坡整体角度的陡度而增加。因此，这个问题需要一个详细的优化，其中一个基坑边坡的最终设计是上述两个因素的平衡。这一程序将使工作环境具有可接受的风险水平和尽可能陡峭的斜坡角。必须强调的重要一点是，最终的边坡设计不仅仅是一个理论练习，而是必须考虑整个采矿过程（安全，用于实现矿山的设备，采矿阶段和可接受的最大风险水平）。在过去的十年里，采矿设备的发展使得在一层厚厚的脆弱岩石中开采矿山成为可能。它们是“具有从极弱到弱的完整强度，单轴压缩范围为250 kPa至25 MPa的岩石”（Martin and Stacey, 2018）。因此，相对于在坚硬岩石中采用的斜角，斜角的减小对采矿方案的利润有显著的经济影响。造成这种差异的主要原因是，边坡的设计是由岩石强度驱动的，如果是弱岩石，以及硬岩石中存在的节理和不连续面。最近，Utili教授及其同事开发了一种基于极限分析的非线性斜坡剖面设计的新理论框架，相对于传统的平面剖面，该框架的性能得到了改善。采用非线性轮廓，稳定系数可提高49%，这意味着非平面形状可以获得比平面形状更陡峭的整体斜率角。上述研究在世界上从未应用于实际的案例研究，因为设计一个真正的露天矿边坡需要额外的开发。具体来说，必须改进的主要缺失特征是岩石非均质性、地下水压力、地震作用、护堤和运输道路的存在以及多种破坏准则的使用。该博士项目将是现有框架的进一步发展，并将实现比矿山设计师目前在软弱岩石中开发的更陡峭的整体斜坡角。因此，它将为采矿业提供显著的改善，因为它可以直接转化为相当大的利润增量。最后，在案例研究中的应用可以作为进一步研究的模型，这些研究将在实际案例中进行。为了填补上述研究空白，本博士项目旨在回答如何设计软弱岩石露天矿边坡的最佳剖面。为了支持这项研究，我们设定了一些目标。特别是，主要目标将是创建一个用户友好的软件包，能够执行软弱岩石中露天矿的非线性斜坡剖面。在此之后，将评估对矿山利润的经济影响，以便量化这种新设计方法的效益。

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.