Açık Ocak ile Yer Altı İşletmesi Arasında Bırakılan Eşik Topuğun Üretilmesi ile Meydana Gelen Duraylılık Koşullarının Sayısal Analizi: Bir Maden Özelinde İnceleme

Abstract

Crown pillar (CP) separates underground mining operations from surface or open pit operations. It protects an underground mine against the risks of flooding and rockfalls, all while improving ventilation and maintaining pit slope stability to ensure the safety of personnel and machinery. Accurately determining the optimal CP thickness to guarantee the stability of the surrounding rock mass is crucial and requires thorough analysis. The ore contained within the CP is often considered as a loss, yet occasionally, the need to extract high-grade ore from the CP is realized. The extraction of the high-grade ore in the CP may result in unstable pit slopes and the backfill may not achieve the same level of strength or support as a naturally occurring rock mass. During the mine closure phase, these challenges are addressed, particularly focusing on factors that impact the long-term sustainability of the pit. This includes considering environmental benefits for the local community and ecosystem as a priority. This doctoral thesis employs an integrated approach that combines a case study with both continuum and discontinuum numerical methods to analyse the stability of the CP during its initial establishment and subsequent extraction. In order to accomplish the objective, a case study obtained from Bauhinia Creek Mine, a gold mine located in Tanzania, is employed in conjunction with Itasca codes, namely FLAC3D and 3DEC. The case study incorporates various geometric and geomechanical properties, including in-situ stresses and geotechnical data, which are utilized to construct numerical models in FLAC3D and 3DEC. These models serve multiple purposes, such as validating the thickness of the CP estimated earlier with an empirical scaled span method, simulating the extraction process of the CP, and conducting stability assessments. Subsequently, the deformed part of the FLAC3D model is replicated in 3DEC to analyse the influence of discontinuities and verify the failure mechanism. Ultimately, this thesis suggests the creation of a sustainable pit lake during mine closure and rehabilitation phase, including fill materials that form a base of the pit lake. Generally, this thesis demonstrates that extracting CP significantly increases the extent of failure in the hanging wall and pit slope. Unstable pit slopes and loose rocks, particularly near the pit floor, can contaminate water in the potential pit lake, thereby contributing to Acid Mine Drainage (AMD). The thesis concludes by suggesting the filling of the pit to a specific level where the slopes of the pit attain a state of relative stability, using well-packed rock materials of appropriate sizes. The author hopes that the objective of this thesis will make a substantial contribution to the understanding of CP estimation and extraction, which represents a crucial connection between mining practices and environmental concerns.