Permanent shotcrete linings are nothing new. Numerous examples can be cited in the fields of water gallery, cavern and traffic tunnel construction. There are examples of "genuine" single-shell shotcrete linings, i.e. the primary lining serves as permanent lining, and there are cases in which the outer lining is reinforced by a second layer of shotcrete. In his patent specification of 1948 Prof. Rabcewicz already points out that under certain circumstances there may be no need for an inner lining. No satisfactory solutions have yet been found as to the structural bond of two shotcrete linings and watertightness. The outer and inner shotcrete lining show different utilization factors and/or stresses and strains. Because of the desired close bond of the two linings, there are concerns regarding safety considerations especially ii the outer shotcrete lining is already damaged. The pronounced creep capacity of shotcrete facilitates the "interaction" of the two shotcrete linings.
