Study On Stability Of Surrounding Rocks And Selection Of Overburden Thickness For Underwater Tunnels

The first underwater pedestrian tunnel that lies through the River Thames Since was built using the shield methed by the British engineer, Brunel, in 1843. From then on, the underwater tunnel has got gradual acceptance of human. Because of none-destroying the shipping, all-weather traffic, short lead, less demolition, multi-hole and so on, the underwater tunnel as a traffic mode which can cross rivers, lakes and straits has become as important as bridges in recent decades. In China, The first underwater tunnel, Dapu tunnel, spans the Huangpu River and was built in 1970. Since then, dozens of underwater tunnels have been built or in planning.The most important and significant differences between underwater tunnels and general mountain tunnels are the presence of overlying water and the inverted longitudinal slope. These two points are closely associated with the overburden layer, a key factor of underwater tunnels. The overburden of underwater tunnel is rock-soil mass between the vault and ground surface. The overburden is not only the percolation path, prevention barrier of water inrush and support structure of stability, but also the key factor of profile design for underwater tunnels which are built with drilling-blasting method or shield methed. So the depth of overburden is an important parameter and key indicator in design and construction of underwater tunnels. Furthmore, the overburden depth restricts construction cost and safety.The core content of this dissertation is study on stability of surrounding rocks and selection of overburden thickness for underwater tunnels. The study is primarily expanded from the following five aspects.(1) Using the conformal mapping function and basic solution provided by Verruijt, the classical problem of an elastic half plane with a circular cavity, loaded arbitrarily on the surface boundary is solved. Then stress and displacement analysis of surrounding rocks of underwater tunnels is performed with this method.(2) Based on the analysis on similarity theory of solid-fluid coupling, the new analogous material for solid-fluid coupling has been found. Extensible 3D test bench and optical fiber monitoring system are developed. Then fluid-solid coupling model test are performed for the Kiaochow Bay Subsea Tunnel. The evolution mechanism of overburden has been analyzed by the monitoring results of multi-field information. The results from model test and numerical calculation are comparatively analysed eventually.(3) An new rock blasting damage constitutive model which can represent damage development under volume compression state is established based on research results of Thorne and Souley, and this model contacts permeability with volumetric strain. In order to be applied in engineering practice, the new constitutive model is embed in strain-hardening/softening model of FLAC3D.(4) The section ZK5+607 of the Kiaochow Bay Subsea Tunnel which is located in the fault f3-1 is chosen as typical cross section, and a three-dimensional model for numerical analysis is established according to geometrical parameters and geological information.of the typical cross section. It is studied use FLAC3D that construction interaction of little distance parallel subsea tunnels owing to excavation effect and blasting vibration influence.(5) Engineering analogy methods used for selecting overburden thickness, for example method from domestic standards, pressure arch theory method, domestic experiences from mining under waterbody, Japanese minimum leakage method, Norwegian subsea tunnel construction experience, minimum displacement method and so on, are expatiated detailed. Then, simple and practical empirical formula are induced. Fitness evaluations are given finally,Through the above analysis, the research results are listed as follows:1. Surrounding rock that lies less twice the tunnel span apart from midline, and from twice the tunnel span bellow floor to ground surface, especially the overburden, has apparent deformation and stress concentration. Additionally, In this region settling tank vary from steep to smooth while the distance to tunnel midline increases.2. Ground subsidence results have obvious layered transmission, ie settlement change gradually weakened from the surface to the vault; and vertical displacement of surrounding rock decreases while the distance to surfaee increases.3. The vertical displacement of covering layer gradually increase with the excavation. The displacement of key point A is about 10.0mm after total excavation under 32.1m water head. Because of the second adjustment of stress field, the vertical stress of rockmass just above the vault decreases, and horizontal tensile stress zone appears. Dramatic changes occur in seepage pressure field, excess pore water pressure dissipates gradually. As the upgrade of water level, displacement of overburden increase substantially, the values of 92.1m water depth are more than twice times of that of 32.1m. tensile stress values increase, and the region expand. Final seepage pressure field changes obviously. Water inflow dramatic increase, the value of 92.1m water depth is at least more than twice times of that of 32.1m.4. Most of the existing blasting damage models are established by stretching damage criterion based on the tensile volumetric strain, but the damage has occurred and accumulated under volume compression state. In addition this will lead to increase of permeability.5. When the little distance parallel underwater tunnels are constructed, the working faces of different tunnels should have some spacing, and there should be time interval between different construction steps. The full-face excavation type and short footage should be chosen firstly if the self-stability of surrounding rock is allowable. In the section of significant mutual influence, arch and invert of service tunnel and the inner wall of constructed tunnel should be concerned in construction and spot measure.6. The charge decouple coefficient of blast holes should be controled strictly when drill and blast construction method is adopted, and it can be determined through the field test if necessary. Hollow holes can be Excavated to Reduce the intensity of blasting vibration. The influence scope of rock overburden under explosive load is within 15m. Spot monitoring measurement should be strengthened and concrete linings should be postponed. The maximal peak values of vibration velocity occur on arch parts of excavating tunnel and walls facing the blasting side of constructed tunnels, while stress concentrations and tension stresses appear at arch parts of excavating tunnel. So the support system shoud be reinforced at these positions.7. Japanese minimum leakage method can be used to choose overburden thickness when overlaying soil layer is quite thin or bedrock is basically exposed. Overburden thickness determined by norwegian subsea tunnel construction experience is rather conservative, and this method does not consider the impact of excavation area. Reasonable Overburden thickness is related with not only the overlying water depth, Rock properties and excavation area, but also the situations of surrounding layer. The calculational value gained from the minimal distance method regression formula can be used as the lower limit.for overburden thickness.8. The method system for selecting rational overburden thickness and determining lognitudinal section line is put forward.