ABSTRACT

Some serious leakage problems have occurred in the underground structures of hydroelectric projects where irregular topography had given rise to inadequate confinement of high pressure water conductor systems. The problem can be overcome through adequate rock mass testing during construction stage by checking and ensuring sufficient magnitude of in-situ stresses. Head race tunnel for Nathpa Jhakri Hydroelectric Project in the Himalayan region is a high pressure tunnel which encountered low cover reach of 9 m where steel liner of 8.5 m diameter has been provided. Location of start of steel liner had undergone considerable change in comparison to that fixed initially based on the minimum cover criteria. The locations of start and end of the steel liner in the head race tunnel were fixed finally on the basis of in-situ stress measurement using hydraulic fracturing method. Results of in-situ stress measurements at four locations for the start of steel liner and at two locations for the end of steel liner were used to reach at the final conclusions. A lot of money and time, which are always are crucial for the successful execution of a project, were saved on the basis of hydraulic fracturing in-situ stress measurement.

1 INTRODUCTION

The 1500 MW Nathpa Jhakri Hydroelectric Project is located across the river Sutluj, downstream of the existing Bhaba underground power station (120MW) in Kinnaur and Shimla districts of Himachal Pradesh state in India. The Project was commissioned in the year 2003 and has been successfully operating since then. A 60.5 m high concrete gravity dam along with four underground desilting chambers (525 m long, 16.3 m wide and 27.5 m high each) are important components of the project. The project has a 10.15 m diameter and 27374 m long head race tunnel (HRT) terminating in a 21.6 m diameter and 301 m deep surge shaft; underground powerhouse with a cavern size of 222 m length, 20 width and 49 m height houses six Francis type turbine units of 250 MW each with a total installed capacity of 1500 MW for utilizing a design discharge of 405 cumecs with a rated head of 428 m; a 10.15 m diameter tail race tunnel for conveying the water back into the river Sutluj. The layout plan of the project is shown in Fig. 1.

In view of the fact that the rock cover is less than the required thickness in certain reaches of the tunnel under reference, and also based on the available geological features, it was proposed to go for steel liner of 8.5 m diameter at two locations in the doubtful reaches of head race tunnel.

(Figure in full paper)

The Central Soil and Materials Research Station (CSMRS) conducted hydraulic fracturing tests in drill holes at different depths at six locations using Minifrac System and verified that the in-situ state of stress inside the tunnel was more than the required hydrostatic pressure of up to 3.08 MPa in these reaches so as to decide whether steel liner was required for the entire reach.

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