Abstract
In some large‐scale hydroelectric power projects, there are more than 3 tunnels that are too close to each other to eliminate the mutual influence during the excavation period, especially for large‐scale tunnel groups. In this paper, aimed at analyzing the Bukun hydropower station consisting of 8 tunnels in Malaysia, the displacement, stress, and plastic zone of the surrounding rock mass are analyzed to study the effect of the excavation sequence on the stability of the surrounding rock mass for large‐scale tunnel groups. On the one hand, the in situ monitoring of the surface displacement of the rock mass surrounding the tunnel using extensometers is performed to obtain the deformation characteristics on the excavation limit under the typical excavation sequence. On the other hand, a series of elastic‐plastic 3D numerical experiments are carried out to explore the displacement characteristics, stability of the large‐scale tunnel groups, and safety of the initial supporting system. The results show that the tendencies of the displacement increase corresponding to the tunnel face movement are similar for the three excavation sequences. The displacement under initial excavation sequence 2 (IES2) is the smallest among the three sequences; the area of the plastic zone under IES2 is the smallest among them; and the stresses in the shotcrete layer and axial forces in the rock bolt under the three excavation sequences are within the safety limitation. Initial excavation sequence 2 is an optimized excavation sequence, in which tunnels #1 and #5 are excavated first; after an advance of 3 times the diameter of the tunnel, tunnels #3 and #7 are excavated; tunnels #2 and #6 are excavated after an advance of 3 times diameters; and tunnels #4 and #8 are excavated after an advance of 3 times diameters.
Highlights
Tunnels are one of the main structures that allow water to flow from reservoirs to power plants in hydroelectric projects.ere are always two or more power tunnels due to the electric generating unit in a large-scale project [1, 2]
A case of closely spaced twin tunnels excavated beneath other closely spaced existing twin tunnels is presented, and the settlements of the existing tunnels and the ground surfaces associated with the new tunnel construction were systematically monitored [11]. ese results show that a close existing tunnel has a substantial influence on the newly constructed tunnel, and the new tunnel excavation has a remarkable influence on the deformation and stress of the old tunnel in the surrounding rock mass. e failure of the support structure is the main problem for old tunnels. e stability of the surrounding rock mass is the main problem for newly constructed tunnels, especially for large-scale tunnel groups
Comparing the displacements under the three Initial Excavation Sequence (IES), the smallest displacement is produced under initial excavation sequence 2 (IES2), and the largest displacement is produced under Initial Excavation Sequences 3 (IES3). us, IES2 is the best initial excavation sequence for the surrounding rock mass stability of the tunnel among the three IESs
Summary
Tunnels are one of the main structures that allow water to flow from reservoirs to power plants in hydroelectric projects. A parametric study is performed based on Advances in Civil Engineering numerical methods for a three-dimensional model to explore the complex interaction of the different parameters, which may affect the tunnel response on the open-pit mining sequence of existing tunnels [6] In another example, the deformation of an existing tunnel is analyzed by 3D finite element methods (FEM), and the settlement of the overburden is obviously influenced by the structure of the existing tunnels through the plastic development of the soil layer [7, 8]. To obtain more information on the surrounding rock mass and supporting system, a systemic series of elastic-plastic 3D numerical experiments are carried out by FEM to explore the deformation characteristics and stability of the rock mass surrounding the tunnel under different excavation sequences based on the Bukun hydropower station in eastern Malaysia. An optimized excavation sequence is suggested for the Bukun hydropower station
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