Switzerland – Lausanne: Excavation nearly under the Cathedral

The new M2 Metropolitan Railway Line crosses Lausanne/CH from South to North and will extend the existing network from Ouchy out to the northern Lausanne suburb of Epalinges. The 275 m long tunnel Viret, which is part of the M2 Line, had to be excavated nearly underneath the cathedral. Since there are ground water lenses just above the tunnel, a waterproofing method was looked for, which could be employed to minimize the risk of any water drainage into the tunnel to avoid soil subsidence and damage to the cathedral. In addition the overall construction speed was seen as essential parameter.
The original design called for a sheet membrane for waterproofing and a 30 cm thick cast-in-place concrete inner lining. The alternative technical solution proposed by MEYCO and the contractor involved a fully bonded spray applied waterproofing membrane and an 18 cm thick inner lining of fibre-reinforced sprayed concrete (see title). This was chosen because the fully bonded membrane minimized the risk of any lowering of the water table, because no water migration was possible along both interfaces between the concrete and the membrane. Additional benefits like avoiding any formworks for the inner lining and considerable time savings were also taken into account.
Tunnelling was carried out using a road header for excavation and conventional sprayed concrete for the primary support. The invert support was constructed with a cast-in-place concrete lining. The construction of the permanent waterproof inner lining was done as follows:
– Water ingress through the sprayed concrete was sealed by injecting acrylic gel MEYCO® MP 308 in combination with temporary drainage holes
– Application of the 3 mm MASTERSEAL® 345 spray applied water proofing membrane
– Application of the 18 cm thick inner lining of steel-fibre reinforced sprayed concrete.
Quality checks of the applied membrane comprised:
– systematic spot checks of the thickness during application
– visual checks
– cutting out of already hardened membrane pieces (5 x 5 cm, destructive test, which was simple to repair)
– measurement of the curing status of the membrane before sprayed concrete application
– core samples after inner lining application to test quality and bond.
The composite inner lining made of sprayed concrete and a fully bonded membrane proved to be successful and flexible to use. Time savings were considerable and amounted to about 2 months.