Design and construction of underground cavern involves fixing the size, layout and orientation of cavern, design of rock support, blasting pattern, excavation sequence, rock support, instrumentation etc. This is a case study paper on planning, design and construction aspects of Transformer cavern of the Pykara Ultimate Stage Hydro Electric project (3x50MW) already commissioned in the Nilgiris District of Tamil Nadu, India. In this project, a separate transformer cavern of 68m long, 12m wide, 15m high to house 3 three phase transformers orienting parallel and downstream to the Power house cavern was constructed. The Transformer cavern is accessible from Access tunnel, cable tunnel as well as Power house cavern. Bus ducts from three units are routed through three bus duct tunnels independently from power house to the transformer cavern. An escape tunnel around the transformer cavern was provided as an escape route in the event of emergency. The power house and transformer caverns are separated by a rock pillar of 30m wide. The transformer cavern is located in a medium of fresh and massive Charnokite rock. The rock cover over the cavern is about 500m. The orientation of major axis of the cavern was chosen parallel to the direction of in-situ major horizontal principle stress and parallel to the major axis of the power house cavern. The excavation of the cavern was commenced from crown and continued in stages and proper sequence up to the service bay floor level. The rock support consisting of rock bolts and shotcrete was designed based on the rock mass classification and 3D numerical modeling of entire power house complex. Each stage was supported before commencing next stage. The cavern was dry. No seepage of water was encountered. However, holes were drilled to drain seepage water if any during operation stage. Presence of shear zone and fracture zone were noticed in the floor of the cavern at service bay level. Such zones were treated properly before laying of foundation for the Transformers and columns supporting cable spreading floor. Multipoint borehole extensometers were installed in the crown as well as side walls of the cavern to measure rock deformation. Instrumentation was continued for two years to assess stability of the cavern as the cavern contains numerous cavities. The cavern is stable for the past six years. This paper attempts to describe planning, design and construction aspects of the Transformer cavern and problems encountered and will be much useful for the Engineers involved in planning, design and construction of underground caverns.
Pykara Ultimate Stage Hydroelectric Project was constructed commissioned already in the Nilgiris district of Tamil Nadu to utilize available flows of Pykara River and its tributaries for power generation. The main components of the project are as indicated below. The general layout of the project is shown in Fig. –1. A Headrace tunnel (2.1x2.42 m) of 377 m long from existing Glenmorgan forebay dam A Gate shaft (5 m dia.) and a Surge shaft (8 m dia.)
