Tecnologia innovativa di consolidamento in formazioni argillose: analisi numerica e quantificazione dell'effetto migliorativo. = Innovative Ground Treatments in clay formations : numerical analysis and quantitative improvement evaluation

The excavation of tunnels under the water table has always represented a challenge in any geological condition, particularly when dealing with fine-grained soils, or more in general when the presence of fine rules the permeability and consequently affects the consolidation process, strictly connected with the stability of contour and tunnel face over the time. Ordinary drainage systems are an effective method, especially in high permeability grounds, but these are losing efficacy in soft clays. This thesis 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. This kind of reinforcement combines the action of drain, fiberglass element and add the compression of the surrounding soil thanks to grout injection in the geotextile sheath. In Italy the technology is named commercially P.E.R.Ground® (Pressure Earth Reinforcement Ground), but all over the world is identified as ‘x-Nail’, ‘Capsule grouting’ or more generally grouted soil nails. The known advantage of this technique is an increase in the pull-out strength thanks to a better adhesion between the element and the soil, however, to date the quantification of improvement effects has not been sufficiently thorough, leaving a gap in the technical literature useful for design purposes. The goal of this thesis is investigation of some of the expected improvement effects, by the use of numerical methods mainly regarding the rate of decrease in the pore pressure but also the increase of undrained shear strength in the short time. In addition, by forcing the consolidation process of clay the elastic modulus and cohesion over time may increase, leading to better deformability and strength characteristic. The input characteristics of the soil implemented in the numerical model have been obtained by carrying out a geotechnical characterization starting from laboratory and in-situ tests of a real Italian project that is facing difficult geological conditions in clayey soils.