Precipitations in the Tunnel Drainage System - Optimized Shotcrete Mix-Design

摘要

In central Europe double-shell tunnel lining is a well proven technology. The construction consists of a primary lining (e.g shotcrete) and an inner lining (e.g. cast-in-situ concrete). For serviceability reasons and the durability of the tunnel, the structure has to be protected against water ingress. One possibility is to drain off the water depressurized ("umbrella insulation"). The water is collected at the edges of the tunnel sidewalls in drainage pipes, so that the tunnel lining is relieved from water pressure. On its way from the surrounding ground to the drainage pipe the groundwater gets in contact with cement bound materials, might change pressure, temperature, pH-value and finally comes in contact with the atmosphere. The water composition, lime-carbonic acid equilibrium, carbonate hardness itself as well as leaching of calcium hydroxide can lead to precipitation of calcium carbonate. With such precipitations the drainage pipes can get clogged. Subsequently uncontrolled water draining can occur or the water pressure on the tunnel lining may increase and the construction can be damaged. From time to time the drainage system must therefore be maintained. To minimize the tunnel maintenance work in traffic tunnels it is therefore crucial that precipitations are reduced to a minimum. The aim of an ongoing Bavarian research project is the improvement of shotcrete mix-design regarding reduced leaching of calcium hydroxide and hence reducing precipitation potential. The leaching behavior of new and already known binder-compositions is investigated. Commonly leaching of calcium hydroxide can be reduced by replacing part of the cement in the binder with supplementary cementitious materials (SCM). But especially in case of shotcrete the early strength development should not be influenced substantially. This paper presents results from lab tests and sprayed mortar tests with known and new cementitious materials. Furthermore, influences on the test procedure as well as basic principles of the precipitation mechanism will be discussed. In that context a method for the simulation of calcium diffusion in cement based materials will be briefly presented.