Using the rock mass as a load bearing element for high pressure hydro-tunnels can lead to considerable reductions in the wall thickness of the steel pipe. For a safe and economical design it is imperative to correctly assess the mechanical properties of the rock mass. The Feldsee project of the Austrian hydro-power company KELAG included an extensive large scale field test program, aiming at the identification of these parameters. The plate load tests were conducted in the tunnel, measuring the force, the absolute displacements of both plates and the displacements in the surrounding rock mass at various depths. Since the results of this kind of test are subject to various influences (problem geometry, loadplate stiffness, mortar bedding), the application of usual closed-form solutions for determining the elastic parameters is questionable. Coupling of a fully three-dimensional numerical model with an optimization routine was used to backanalyze the elastic parameters. The measured data featured asymmetric displacement patterns (and thus: locally differing stiffness) inherent to a heterogeneous rock mass which cannot be properly accounted for in a homogenous numerical model. A simple smoothing and averaging technique was applied to filter out the influences of the heterogeneity and measurement noise. Using the commercial code FLAC3D and its built-in language FISH, the model has been parameterized and coupled with the optimization shell INVERSE. The obtained results are discussed and compared to the currently available semi-empirical equations.

1. INTRODUCTION

The construction of the Feldsee project of the Austrian hydropower company KELAG started in 2006 and is scheduled to be completed by the end of 2008. The basic idea is connecting two existing reservoirs, Feldsee at 2200 m a.s.l. and Wurten at 1700 m a.s.l. in a daily pump storage scheme with a peak power output of 70 MW. Since the startup and shutdown procedures of the turbine have to be rapid (due to the chosen operation scheme and capacity of the reservoirs), the dynamic head reaches 820 m at the lowest part of the pressure tunnel. Motivated by the great cost-saving potential, the design of the pressure tunnel lining treated the surrounding rock mass as an integral and primary load bearing element. This approach is justified when having the following facts in mind: the rock mass is composed of massive, unweathered gneiss with widely spaced joints of low persistence and the overburden is high enough to allow the development of tensile cracks induced by the internal pressure loading without endangering the global stability of the slope. Assuming the lateral pressure coefficient of 0,4, the calculations have shown that the crack depth under peak internal load does not exceed 10 meters. This is well within boundaries deduced from the hydropower project Häusling, successfully constructed in similar ground conditions. The final solution features pre-cast concrete elements with a thin steel lining in the lowest part of the tunnel and pre-cast concrete elements with a GFRP (Glass Fiber Reinforced Polymer) lining in the upper part (Figure 1).

Figure 1: The longitudinal section of the Feldsee project(available in full paper)

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