Realization of the shore approach by microtunnelling technique, aimed to avoid the excavation of contaminated soil at the top layer.
Sidra designed and carried out the construction work for the pipeline in a joint venture with Saipem. Additionally, Sidra engineered and executed the shore approach using the ‘microtunnelling’ technique.
Construction of a jacking shaft
The shaft reached a depth of 11.5 m and was realised by driving 22 m long sheet piles into the soil embankment to protect the excavation and by drilling reinforced concrete piles with a diameter of 1,000 mm into the soil embankment for the anchorage of the shaft.
Excavation of an offshore trench
A 700 m offshore trench was excavated by means of a grab dredger. This trench provided a transition between the microtunnel exit and the natural seabed, thus providing the shore approach of the gas pipeline.
The microtunnel was realised by a Tunnel Boring Machine (TBM) with a diameter of 2.5 m. In the Arno River spillway canal 258 concrete pipes were jacked into the soil behind the TBM.
Sheaves with a diameter of 2 m were installed inside the jacking shaft to guide the pulling cable towards the 300-tonne linear winch. A messenger line was installed inside the microtunnel until the end of the tunnel for the subsequent pull in of the gas pipeline.
The recovery of the TBM was carried out by a heavy lift crane barge with the assistance of divers.
Pull in of the gas pipeline
Once the messenger line and the gas pipeline were connected, the pull in of the gas pipeline took place by means of a 300-tonne linear winch.
Filling of the microtunnel
The cavity wall was filled between the microtunnel and the gas pipeline by a cement bentonite mix and then the trench was backfilled.