Back analysis of elastic properties of the ground using data from a large scale plate load test

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 back-analyze 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.