Geotechnical Engineering Applications to Enhance Open-Pit Gem Mining Operations in Ratnapura District, Sri Lanka

Abstract

Ratnapura district in Sri Lanka has been famous across the world for high quality gems found over the decades from alluvial deposits. The source of origin is considered as “Samanala” mountain complex from which four major rivers in the country originate. These alluvial deposits are found from shallow to deeper levels across the district. The traditional way of mining these deposits are by digging the pits for shallow alluvial deposits, and timber supported pits followed by cross-cuts for deeper deposits which are also supported by selected special timber varieties. Even though this process is a slow operation and less productive, it has been the choice of many miners and investors in the gem mining industry as per the needs in the past. Subsurface ventilation had been the most difficult they had faced and many deposits had been given up due to this practical difficulty. As per the needs of modern society, there is a new trend on starting large-scale open-pit mines. In this transition, the industry has faced a lot of practical problems due to not having a good focus on stability of earthen slopes. Hence, most of the sites experience even fatal accidents due to sudden collapse of earthen slopes. The current open-pit mining operations have not properly taken the inputs by the geotechnical engineering professionals to enhance the operational safety of the workers and the excavations. The main reason for these issues could be as most of the miners prefer to maintain the confidentiality of the resources on their sites, and hence visits by the outsiders are not entertained. This study is well focused to identify the operational safety issues related slope stability in open-pit gem mining, and the fieldwork covered visiting of ten active open-pit mining operations across Ratnapura district. The outcome of the study showed that most of the slopes on these sites are unstable, however that an optimized benched slope design that maintains an 1:2 ratio and bench heights of 3m to 5m can successfully attain a safe Factor of Safety greater than 1.25. Further, the proposed ‘Divide and Stabilize’ method has been adopted to successfully counterbalance lateral pressure and dewatering effects through the use of localized and reusable steel sheet piles and the progressive cut and fill sequence. Based on the outcome of this study, timber-shoring can be recommended for shallow open-pit mine operations as a low cost slope stability method for small–scale mines. However, for large-scale operations where the investors are concerned of high productivity, sheet piling is recommended as a reliable and safe earth supporting system. Application of piling for open-pit gem mining has another advantage of opening up the opportunity for the investors to exploit a maximum possible land volume, without leaving a huge space to maintain stability of unsupported earthen slopes. The rehabilitation together with pile supported excavations are recommended to enhance the quality of current mining practices in open-pit gem mines.

• Alluvial deposits
• Divide and stabilize
• Gem mining
• Geotechnical techniques
• Slope stability