The Elastic Modulus Variation During the Shotcrete Curing Jointly Investigated by the Convergence-Confinement and the Hyperstatic Reaction Methods

Induced stresses in sprayed concrete (or shotcrete) are quite complex to evaluate and depend on many factors such as the size and depth of the tunnel, the geomechanical characteristics of the surrounding ground in which the tunnel is excavated, the type of shotcrete, the evolution of its mechanical parameters over time and the excavation face advance rate. In particular, the evolution of the mechanical properties of the shotcrete is crucial regarding the interaction with the tunnel wall and the development of the bending moments and the normal forces which occur along the circumference of the lining. In this research, a new calculation procedure based on the combined use of two calculation methods, the convergence confinement and the hyperstatic reaction methods, is presented. Thanks to this procedure, it is possible to progressively apply the load acting on the lining as the curing phase of the concrete progresses and therefore with the evolution of its mechanical parameters. This procedure has been applied to several examples of calculation, obtaining useful considerations regarding the mechanical behavior of the shotcrete lining when some fundamental parameters of the calculation change. It is possible to achieve bending moments and forces in the lining with the progress of the load steps. It is also possible to determine the trend of the lining safety factor over time and at the end of the loading phase, allowing a proper design of the support, with particular attention to the type of shotcrete and the thickness of the lining.