The paper focuses on the development of an underground chromite mine beneath an open pit mine which has already reached its ultimate pit limit. Both the host rock and ore body are weak and friable. However, it is estimated that approximately 30 million tonnes of ore is trapped in the geologic formation. The paper presents a part of a detailed study for designing a novel mining method in such friable ground conditions keeping cost-effectiveness and safety in mind. It is imperative that the support systems as well as dimensions of the decline, drives and cross-cuts, stope(s) and pillars are of great significance. Rock classification and characterization has been conducted in order to determine the proper tunnelling technique and ground support method. Finite element modelling is used to determine the required support capacity and the maximum length of unsupported zone while constructing the decline and other underground excavations. The paper presents the description of the proposed underground mine and elaborates on support design of the decline in such a friable rock mass condition.


The chromite ore deposit at a surface mine located in eastern India has reached its ultimate pit limit (UPL) leaving huge amount of the ore trapped underneath which can only be extracted by underground mining method. However, both limonitic host rock and ore body are weak and friable in nature, and hence, it requires careful design of support systems. The rock hosting the chromite ore is weathered sedimentary deposits rich in limonite mineral. The uniaxial compressive strength (UCS) of the limonitic rock is as low as 300 kPa having cohesion of 80 kPa and friction angle of 34°. The UCS of chromite ore ranges between 170 kPa and 1.0 MPa. Needless to say, developing underground structures in such an ore body and host rock is a major challenge. The GSI (Geological Strength Index) value is estimated to be in the range of 25 to 30. It is estimated that the stand-up time of such rocks is very low and any delay in supporting can result in collapse of the structure. In Fig. 1, the green star shows the mine location in the support chart proposed by NGI (Norwegian Geotechnical Institute) in the year 2015.

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