Mechanical-hydraulic interaction in the lining cracking process of pressure tunnels

Pressure tunnels are in operation subjected to internal water pressure. When the hoop tensile stress acting at the lining intrados exceeds the tensile strength of concrete, longitudinal cracks occur in the concrete lining. As a consequence of crack openings, the internal water pressure will act at the lining extrados and cause high local water losses. If left untreated, these losses will induce the washing out of joint fillings and increase the risk of hydro-jacking of the surrounding rock mass. When pressure tunnels are situated close to valley slopes, excessive water losses can endanger the stability of the rock mass and provoke landslide. Whether or not the internal water pressure is fully effective at the lining extrados, it depends predominantly on the number of cracks and the width of crack openings. The width of cracks can be estimated based on the total circumferential deformation of the rock mass, which is governed not only by mechanical boundary pressures, but also by seepage pressures. In turn, seepage pressures generate water losses from the tunnel, which are depending not only on the permeability of the rock mass, the grouted zone, and the concrete lining, but also, if any, on the width of the crack openings. The estimation of pressures transmitted to the rock mass requires therefore solutions using iterative methods dealing with this coupling behaviour. This paper presents a method to estimate the distribution of seepage pressures and water losses around concretelined pressure tunnels pre-stressed by grouting, which considers the lining cracking process due to a high internal water pressure. The mechanical and hydraulic behaviour of cracked concrete-lined pressure tunnels is presented so as a first step for more elaborate numerical studies.