Practical application of non-destructive test methods at a single-shell tunnel lining

Non-destructive test methods are often used in tunnel construction to check the required lining thicknesses. Hereby, mostly ground penetrating radar, impact-echo and ultrasonic echo are applied. However, the current knowledge and experience is largely limited to testing the concrete thickness of double-shell tunnels. Problems with the application of non-destructive test methods can arise, when definitive interfaces are not present, cracks or honeycombing influence the results or the lining is soaked with water. As a sophisticated case, a 2550m section of a power tunnel concrete liner in Nepal was inspected with non-destructive test methods. The specific feature of the power tunnel were its challenging boundary conditions: constructed as single-shell tunnel, layer identification was complex; a synthetic foil for sealing, which usually acts as impedance contrast between grout and sprayed concrete, did not exist; the lining was not cast with watertight concrete and was therefore accordingly wet. The current limits of non-destructive testing applied at tunnel linings were explored and evaluated in the project. In the paper, the non-destructive investigation and its case specific problems are described. An emphasis is put on the Impact-Echo method, as it was mainly used. A detailed account is given on the following aspects: selection of the most appropriate test methods and testing procedures, considering the above mentioned preconditions; analysis and interpretation of the acquired data; evaluation of the applicability and limits of the used test methods.