Improvement in soft ground tunnelling using an innovative technique

ABSTRACT: The primary stabilization of tunnel face in soft ground tunnelling by means of soil nailing has been found to be an effective and economical method. This paper focuses on some aspects of the performance of two different techniques for the stabilization of the tunnel face and surrounding ground, with reference to a real tunnel project in Southern Italy. The difficult conditions met during tunnelling, due to poor quality of the rock mass and presence of high pore water pressures, required a design solution featuring a preliminary ground improvement and a heavy tunnel support. An innovative technique for ground improvement was applied using special soil nail consisting of a fibreglass bar element and an external sheath devised to contain the injected grout, which can also be integrated with a coaxial drain. The high strength of the nails, evaluated by field pull out tests, and the ability to reduce the pore water pressures ahead of the tunnel face resulted in an effective increase of the stability during excavation.