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  Excavation of a large underground surge pool benching with a glory hole technique-A case study

  Excavation of large underground cavern is always a time taking process and expensive, when ramps are used for mucking during different levels of benching. To complete the excavation of surge pool cavern timely and cost-effective, a glory hole technique was applied for mucking purpose during different levels of benching. Surge pool 375 m long, 25 m wide and 67.6 m high constructed in the Telangana State of India for lift irrigation purpose. Engineering geological mapping of heading portion and third bench foundation was carried out on 1:200 scale. Based on engineering geological classification of rock mass of heading, bench foundation and bore holes data of detail project report stage, good to very good rock mass was anticipated and a glory hole was designed. A 2.5 diameter vertical glory hole was dug up from El+ 119 m to El+140 m for 21 m height at chainage 200 m. Mucking was done from El+110 m level, which is connected from draft tubes and main tunnel. Rock type exposed is mainly grey and pinkish granite belonging to Peninsular Gneissic Complex of Archean age. Maximum five joint sets plus random joints observed during the geological mapping. Prediction of rock mass based on heading portion,foundation mapping and bore hole data geology for different benching levels is challenging for large underground cavern and if not anticipate properly may problematic during excavation. In this case, predicted rock mass was observed during the excavation of a glory hole and benching. This benching excavation methodology may helpful for the planning of similar large underground cavern if rock mass can be predicted properly.Engineering geological mapping, Detail project report, Peninsular gneissic complex, Archean age, Bore holes data and A glory hole.

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