Study On Excavation Scheme Comparison And Construction Mechanics Of Highway Tunnel Extension

With the rapid development of highway infrastructure construction, bi-directional and four lane and even bi-directional and six lane freeway tunnels can not meet the needs of increasing of traffic in some developed area. Reconstruction and extension of freeway tunnel has become a pressing issue. Building two new tunnels nearby the existing separated tunnels to form neighborhood tunnels group, which is a basic type of tunnel reconstruction and extension, belongs to complex approaching construction. The construction of new tunnels will change the original multiple stress field to the more multiple stress field in surrounding rock, thus changes the mechanics case of existing tunnels and newly built tunnels, and brings the complex problem of the newly built tunnel and the safety problem to existing tunnels. Relying on one freeway tunnel reconstruction and extension projects, this paper studies on reasonable types of reconstruction and extension, superior excavation scheme, dynamic construction mechanics and support parameters optimization. Contents and resulting achievement of this paper are as follow:1.Considering the influence factors of tunnel approaching construction and location selection of structural opening, the optimal type of reconstruction and extension——build a new tunnel between existing two tunnels and build another new tunnel at the lift side of existing left tunnel, is determined.2. Four excavation schemes of optimal type of reconstruction and extension is simulated. The results presented that, compared with being excavated by cross diaphragm (CRD) method, being excavated by bench method, stress of surrounding rock and supporting structure is comparatively small, and the construction process is more simple and surrounding rock is less disturbed. Bench method is determined as optimal excavation scheme.3. Dynamic construction mechanics of the selected excavation scheme is simulated by three dimensional finite element method. The results presented that stress, strain, and plastic zone of surrounding rock and stress of supporting structure is small.The surrounding rock of newly built tunnels is tended to be stable as achievement of excavation and supporting. The selected type of reconstruction and extension and excavation scheme of newly built tunnels is safe and proper. Meanwhile, influence degree of surrounding rock and supporting structure of existing tunnels under the construction of newly built tunnels is analyzed.4.Based on the numerical simulation analysis, it is feasible that anchor length is shortened from 4.0m to 3.0m and thickness of shot concrete decreased from 26cm to 21cm.The analysis results not only provide scientific foundation for design and construction of this freeway tunnel reconstruction and extension project, and also provide data and experiences for similar freeway tunnel reconstruction and extension project in the future.