In Monaco, creating a new road through the cliff requires a 145 m long tunnel with cross section 7 m wide by 8 m high. The tunnel is located close to an underground railway station, residential premises and underground services. The rock is a compact, dolomitic limestone containing microscopic fracture planes resulting in physical characteristics that correspond more closely to a rock with uniaxial compressive strength ranging from 60 to 100 MPa. The need to keep vibration levels low and the hard nature of the local rock means that the choice of an appropriate excavation method is critical to the success of the project.
An ER 3000-2
mounted on a Liebherr 954 excavator is the key piece of equipment used to keep vibration levels within the extremely low tolerances allowed. The first 33 m of tunnel from the easterly portal and 12 m from the westerly portal are excavated exclusively with the Erkat rotary drum cutter
. In the hard rock conditions, production rates of 3.5 m3
per hour are achieved resulting in an advance of 3.5 m per five-day working week. For the rest of the tunnel, a combination of drum cutter and drill and blast methods are used. To minimize vibration and protect the most vulnerable sections of the tunnel, the drum cutter is used to excavate voids in the face.