At sequenced tunnel drives, next to the rock mass conditions, the advance rates of the single excavation stages and the moment of ring closure determine the performance of shotcrete linings. Especially in heterogeneous rock masses, the investigation of the optimum ring closure moment resulting in no damage of the lining challenges the engineers. If the ring closure is too early, the lining may experience compressive failure because the building up of ground loads is too large. A late ring closure may allow for too much tunnel displacement. To investigate the effects of various ring closure moments on the utilization of the tunnel lining and the tunnel crown convergences, this study performs 3D finite element simulations. The results at a specific measuring cross section and for the assumed ground conditions suggest that a delayed ring closure can lead to higher utilizations compared to an early ring closure.
Sequential tunnel excavations are based upon the concept that the ground in the vicinity of the tunnel is not only a load element but also acts as a load bearing element. The excavation and support activities are adjusted in real time to suit the ground conditions considering the design requirements (Galler et al. 2009). A balance must be maintained between the construction speed and the need to expose a minimal area of ground at a time to ensure reduced deformation in the unsupported zone (Clayton et al. 2000). To stabilize the tunnel structure and the surrounding ground, a closed ring of tunnel lining is introduced. The chosen construction speed and construction sequences, determining the moment of ring closure, influence the utilization of the tunnel lining as shown by Stärk & Zachow (2015). To maintain a balance between the need for a cost intensive strong support and low safety margins for lighter support structures, the closure length should generally be less than one diameter as concluded by the studies of Awaji et al. (2016).
In this paper, trends for tunnel crown convergences and lining utilizations in dependency of various ring closure moments are evaluated. To achieve this task, 3D finite element models are established, and two design parameters are varied: first, the advance rate of the excavation and second, the length of excavation stretches in top heading and bench. A limitation regarding the simulation of the shotcrete behavior is made upon the phenomena of creep, shrinkage and softening which are not included in the models.
