When dealing with tunnels in weak rock mass and with high overburden, the high displacements imposed on the lining dictate the application of ductile yielding elements with controllable stiffness and yield load. These properties are chosen with two goals in mind: the time-dependent strength of the shotcrete shell must not be exceeded; however the support pressure must be kept reasonably high and controllable. The attainable load-displacement lines of the ductile support elements are almost arbitrary. There are almost countless possible combinations of their stiffness and yield load, thus enabling the development of custom-tailored support systems and leaving considerable room for adapting to the encountered ground conditions. Tunneling in weak ground should be accompanied by increased efforts on monitoring the system behavior, best by a dense pattern of absolute displacement measurements. A simple technique for calculating the shotcrete utilization ratio has been developed. It applies a Newton- Raphson root-finding algorithm to determine the interpolation parameters while obeying the requirements of force equilibrium and fitting the measured displacements. The influence of non- symmetrical displacement behavior caused by heterogeneity and anisotropy of the rock mass, on the lining loading can be quantified and used for support system optimization.
High primary stresses associated with tectonic faulting frequently create problems during construction of Alpine base tunnels. Keeping the displacements in a range which could be sustained by the support would lead to economically unfeasible lining thickness.
Ductile lining systems using in mining cannot be be transferred to traffic tunnels with their requirement of long term stability. First concepts of yielding supports for tunnels date back to the nineteen fifties (Rabcewicz 1950).
The technical requirements posed on a ductile support system are quite clear:
The load-displacement characteristics should be "steerable" within a broad range, allowing the avoidance of overstressing the shotcrete shell, while enabling easy modifications in order to cope with the ground heterogeneity and usually long-lasting displacement increments.
The support resistance has to be reasonably high, allowing a certain amount of control over the displacement magnitude.
(Figure in full paper)
When tunneling in conditions leading to application of integrated yielding elements, great attention should be paid to comprehensive monitoring of the system behaviour. Due to the unreliability of methods measuring the kinetic quantities (e.g. forces or stresses) the measuring of the absolute displacements currently yields the most direct insight on system behaviour. Usually 5 measurement points are being used.