Witznau power station, grouting tests for upgrading the headrace tunnel.

In response to spalling of concrete parts, the headrace tunnel was to be strengthened section by section and made fit for the years to come. Since little information was available on the condition of the tunnel and the cause of the spalling, possible damage patterns and causes were analyzed and grouting in combination with concrete rehabilitation were defined as strengthening measures. During a break in operation of approx. 2 months in June 2020, cement injections were tested in grouting trials in different variants to determine their suitability and optimize the parameters and the work to be carried out. The tests and the investigations carried out revealed interesting findings about the condition of the tunnel. The grouting was carried out at low pressure, at different distances and drilling depths, testing important parameters for the subsequent execution and enabling optimization. The porosity of the lining was significantly reduced by grouting, thus improving the quality of the shell. A before‐and‐after measurement with georadar investigation demonstrated the improvement of the lining due to the grouting. A remarkable finding is the combination of the relatively poor quality of the lining concrete, which is due to the construction during World War II, with the practically uncracked and relatively tight lining. It is a testimony to a high level of construction skill that it was possible to create such durable structures with the means available at the time. [ABSTRACT FROM AUTHOR]