Determination of the round length for tunnel excavation in weak rock

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• 作者：Young-Zoo Lee ; Wulf Schubert
• 关键词：Round length ; Weak rock tunnel ; Behaviour mode ; Safety factor ; Optimization
• 刊名：Tunneling and Underground Space Technology
• 出版年：2008
• 出版时间：May 2008
• 年：2008
• 卷：23
• 期：3
• 页码：221-231
• 全文大小：2773 K

文摘

Although the round length has a major technical and economical impact in conventional tunnelling, no coherent procedure is available for its determination. In this study, the determination of the round length was investigated for tunnels in weak rock, where the behaviour is not governed by discontinuities. This study focuses on shallow or medium depth tunnels 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 types of behaviour modes are suggested for the planning of excavation and support. Based on the results from the PFC3D analyses, the equivalent models were analyzed by a FDM code, using elastic material behaviour. Using the relative shear stress (RSS) concept, a correlation between the maximum relative shear stress (MRSS) and the different behaviour modes was investigated. The safety factor for the face stability is defined by the concept of the ‘critical cohesion’ and this safety factor for the face stability is adopted as an indicator for the behaviour mode. The results are illustrated in the ‘Conditional chart for excavation plan in weak rock tunnelling’ which shows the relation of the safety factor and relevant behaviour mode as the round length varies. With detailed construction information, such as cycle time, unit price of materials etc., the optimization of excavation can be carried out deterministically and probabilistically in the design stage. Depending on the site conditions, round lengths causing a limited volume of overbreak can be considered in the excavation plan. Although the proposed method has some restrictions, it can provide useful information for the optimization of the excavation, especially in design stage.