Ground treatments for soft clays below the water table

Water in tunnels represents a challenge in any geological condition. When dealing with cohesive material, this aspect might bring additional problems linked to the low permeability, which does not allow effective and quick drainage to stabilize the ground around the tunnel. Ordinary drainage systems are an effective method, especially in higher permeability grounds, but these are losing efficacy in soft clays. This paper focuses on an innovative technique for stabilizing the tunnel face and surrounding ground by using a special soil nail consisting of a fiberglass bar element and an external sheath devised to contain the injected grout, which can also be integrated with a coaxial drain. The advantage of this technique is not only a decrease in the pore water pressure but also an increase of cohesion and elastic modulus over time thanks to the induced consolidation process of the clay and a consequent improvement of the mechanical characteristics.