Execution of one of the special works included in Section 1 of the GRT gaz “Arc de Dierrey” gas pipeline project. At design stage, the crossing was not supposed to be executed by means of a trenchless technology. During the works, the excavation of an open trench in the wetland of the Aronde river turned out to be extremely difficult due to the presence of peat. Therefore, execution method was changed from open trench to microtunnelling, in order to cross under the river valley. The microtunnel length is 794 m, the internal diameter 2.000 mm and the external diameter 2.500 mm.