The Stadtwerke München (Munich Department of Public Works) is making great efforts to ensure the continuing and future quality of Munich's drinking water. One of the measures it has undertaken is the renewal of the conduits from Upper Bavarian Mangfall Valley, which are over 120 years old. A 17.6-kilometer-long section, the "Hofoldinger Stollen", of the 30-kilometer-long water tunnel was to be excavated by machine. Some challenging features of this job were the mixed geology, the high pressure resistance of the conglomerates of up to 60Mpa and the chunks of rock up to 70cm in size.
Due to the nature of the geology, a machine with partial face excavation with roadheader was employed for tunnelling in open mode. Existing voids between the segments' outer edges and the rock, and the round space around the steel pipe, were filled with a special mortar. A cement mortar lining applied afterwards ensures that the steel pipes remain hygienic and corrosion-proof.