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Excavation of Large Underground Power House Cavern & Surge Shaft in Adverse Geological Features Like Sheared/Fractured Zone: a Case Study on Mangdechhu H.E. Project, Bhutan

Authors

Shrish Dubey, Prashant Jaiswal, Pradeep Kumar Garnayak, Deepak Verma
 

DOI

Abstract

The complex geological region and unpredictable nature of the Himalayas pose major challenges for construction of large underground structures. This paper presents the excavation of Power House Cavern & Surge Shaft of Mangdechhu Hydroelectric Project in poor geological features like sheared/fractured zones. The Cavern was proposed to be explored through a 410m long drift, however only 36.85m of Cavern was explored since the excavated drift got flooded with silt due to ‘Aila’ flood in May, 2009 and balance portion of cavern was investigated by two drill holes and Hydrofrac test was carried out to reveal the in-situ stress conditions. During the excavation of the central gullet of the underground caverns, a 1.5 m to 2.0 m thick sheared /fractured zone was encountered in oblique direction to the caverns. Due to its low dipping profile, this zone intersected both upstream and downstream walls of cavern and also partially affecting the crown of Transformer Cavern. This sheared/fractured zone could not be deciphered previously due to limited length of drift in the cavern. The geology of the caverns was again explored through additional vertical drill holes and both in-situ shear & deformability tests and laboratory tests were conducted for deciphering the rock mass properties of the sheared/fractured zone. Similarly, 152m deep Surge Shaft was explored by two drill holes revealing bedrock to be encountered at 15 to 18m below the ground level. The encountered strata comprised of highly fractured, highly weathered to decomposed rock mass with low in-situ strength.
Based on the numerical analysis of above structures, the excavation methodology and rock support system of Cavern and Shaft were updated and an elaborate instrumentation program was devised. This proved to be beneficial for the safe excavation of Power House Cavern & Surge Shaft in geologically challenging conditions, resulting in timely commissioning of Project.

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