The round length has a huge influence on the technical and economical requirements, but it is still determined based on the intuition and experience of engineers. In this study, the determination of round length is researched for the tunnelling in weak rock, where rock mass behaviour is not governed by the discontinuities. Therefore the failure is mainly caused by over-stressing, not by the geometry of discontinuities or blasting damage. Regarding the depth of tunnel, this study focuses on shallow to medium depth so that squeezing or rock burst is not concerned.
The behaviour mode of the face and unsupported span was investigated by a series of small scale model tests and PFC3D analyses. Total five (5) types of behaviour mode are suggested for the planning of excavation and support. Based on the reference models of PFC3D analyses, each behaviour mode was simulated in FDM analysis by use of the relative shear stress (RSS) in the elastic stress field. It is found out that the maximum relative shear stress (MRSS) has a relation to the behaviour mode. The reference MRSS indicating the behaviour modes is determined in comparison to the reference models of the PFC3D analyses. The safety factor for the face stability is defined by the concept of the ‘critical cohesion’. It can consider the influence of friction angle, cohesion, tunnel diameter and overburden on the face stability.
A parametric study has been performed with various values of cohesion, friction angle and overburden for 5, 10 and 15m diameter tunnels. The safety factor for the face stability is adopted as an indicator for the behaviour mode. The safety factor for the face stability can be calculated by use of a simple equation which is derived by fitting the results of the parametric study. All the results of parametric study are illustrated in the ‘Conditional chart for excavation plan in weak rock tunnelling’.
With the detail construction information, the optimization of excavation can be carried out deterministically and probabilistically in the design stage. According to the conditional chart, the behaviour mode can be determined with the applicable range of round length. The optimum round length can be determined under consideration of a specific site and contractual conditions. For the practicable application during the construction stage, the prediction of the system behaviour is required in consideration of the influence of the round length.
Although the proposed method has some restrictions under some conditions, this method can provide useful information for the optimization of the excavation, especially in the design stage.