Rock Stress Estimation for Unlined Pressure Tunnel Design

Abstract

In underground hydropower projects rock tunnels are used extensively to convey water for the purpose of generating power. As these tunnels represent a major cost element in typical hydropower developments, efforts are made to ensure that the tunnel design is cost-efficient. Under Norwegian tradition the principal cost reducing measure is to keep most of the tunnel length unlined and steel line only a relatively short section of the tunnel. This approach requires that the rock stress in the unlined tunnel sections exceeds the internal water pressure. To avoid hydraulic failure of the tunnel, information on the underground state of stress thus is crucial.

Today, it is generally recognized that in-situ measurement is the only reliable way to estimate rock stresses for the purpose of finding the safe location of the transition between unlined- and steel lined pressure tunnel. The current approach to rock stress estimation typically involves performing stress measurements at relatively few test locations, and to use interpolative techniques to assess stresses between and beyond locations. As the distance between test locations increases, so does the risk of leaving tunnel sections with insufficient stress undetected.

In this thesis, a new methodology for rock stress estimation is suggested to mitigate this undesirable situation, involving a reduction of the distance between test locations by performing measurements more regularly along the entire length of unlined pressure tunnel. For this approach to be financially and practically feasible, measurements must be made more rapid and more efficiently than they are done with the currently available test methods.

A new hydraulic jacking test protocol, the rapid step-rate test (RSRT), is proposed as an alternative to the available methods. This test protocol has been developed through a series of laboratory-controlled experiments conducted in a custom-built true-triaxial test rig, and a successful field verification of the test protocol was later performed inside the tunnel system of the Løkjelsvatn HPP.

Through the field- and laboratory tests the RSRT protocol has demonstrated a promising ability in estimating fracture normal stress, and it is therefore believed that the new test protocol may serve as a rapid alternative to current rock stress estimation methods that is well suited for adoption in the proposed new rock stress testing methodology.