Research On Surrounding Rock Pressure Calculating Method And Dynamic Construction Mechanical Behavior Of Multi-arch Tunnel

Multi-arch tunnels have been designed and constructed for more than thirty years. So far, several hundreds of multi-arch tunnels have been built successfully. However, due to its multi-arch structure, the load on structure are frequently changed in several steps of excavation and support. Variation of surrounding rocks stress and relations between surrounding rock and the structure are complicate, so there still remain some problems, including: (l)Differences are often found between surrounding rock classification provided by designer and that of in-situ construction. What is more, it is difficult to be modified in time. (2) So far, there is not ground pressure theory which meets multi-arch tunnel features. Particularly, estimating error in rock load is still large under shallow and unsymmetrical pressure condition. (3)Safety or quality problems are often found such as invalidation of support, crack of lining and leakage.In this paper, study on determination of tunnel width parameter of multi-arch tunnel and calculation model of center pillar provide the basis to characterization of multi-arch tunnels. Secondly, critical covering thickness of unsymmetrical pressure terrain is studied and used for bound between unsymmetrical pressure and symmetrical pressure tunnel is established. On this basis, determination methods of ground pressure under unsymmetrical pressure and under symmetrical pressure are studied. In order to make parameter consistent with construction practice, method of rapid evaluation for surrounding rock and physics-mechanical indexes determination method of surrounding rock are researched. Finally, numerical simulation mechanical behavior of dynamic construction of multi-arch tunnel is studied and checked in actual project.Aiming at multi-arch tunnel, surrounding rock pressure calculating method and dynamic construction mechanical behavior are studied. The main research fruits are the following:1. Determination of tunnel width parameter of multi-arch tunnel and calculation model of center pillarCenter pillar and construction program have little effect on the surrounding rock elastic zone, but final excavating span has more for multi-arch tunnel. when load is calculated, the worst condition should be considered. When the wholly multi-arch tunnel width is from the gallery width, it is reasonable and safe.2. Critical covering thickness of unsymmetrical pressure terrain Pressure arch curve equation is got by using rock pillar theory which is also called Bierbaumer theory. Draw a tangent outside the tunnel with cartography method parallel with the tangent of pressure arch curve equation in term of the topography gradient. The distance of the two tangents is the critical covering thickness. Based on this way, unsymmetrical pressure terrain critical covering thickness of multi-arch tunnels is gained under condition of large span, different slopes and rock classification, which provide reliable basis for design of unsymmetrical pressure multi-arch tunnels.3. Determination method of uniform surrounding rock pressure of multi-arch tunnelCross-sectional area of multi-arch tunnel is much larger than that of single-track tunnel. Through theoretical research and systematical analysis of current ground pressure theory and formula, present the following viewpoints: load of large span double-arch tunnel is calculated with theory of soil columns and Xie Jiaxiu under condition of quite shallow depth and shallow depth; modified rock pillar theoretical calculation formula of Bierbaumer for surrounding rock pressure is presented for vertical formation Pressure, which well meets double-arch and big cross-section tunnel feature.4. Calculation method of surrounding rock pressure and structure of multi-arch tunnel under unsymmetrical pressureFor shallow multi-arch tunnel under unsymmetrical pressure, because not only active formation pressure on the tunnel structure is uneven, but also passive load on the tunnel structure needs to be adjusted, determining method of active unsymmetrical load and passive uneven load for shallow unsymmetrical pressure multi-arch tunnels and reasonable calculating method of support structure of tunnels under the condition of unsymmetrical pressure terrain are presented. Reasonable foundation elasticity resisting coefficient of shallow buried soil under the condition of different slopes and rock classification is also get, which provides reference for moderate unsymmetrical pressure multi-arch tunnels with load-structure model.5. Rapid evaluation method for surrounding rock in construction and the conjecture for rock parameter by Q indexBased on the experiences of surrounding rock classification at home and abroad, aiming at surrounding rock discrimination characteristics and demand in construction, establish a method of rapid evaluation of surrounding rock, combining quantitative and qualitative indexes, in-situ observing, measuring and rapidly evaluating. Due to the difficulty of obtaining the practical ground mechanical indexes, propose a conjecture method for rock strength and elasticity modulus with Q designation and in-situ point load strength.6. Numerical simulation for dynamic construction mechanical behavior of multi-arch tunnelFor shallow depth multi-arch tunnel under unsymmetrical pressure, deep side advancing drift should be first excavated, while shallow side main tunnel should be first excavated. For multi-arch tunnel under symmetrical pressure, main tunnel can be excavated with benching tunneling method under the good surrounding rock condition and the reasonable working face spacing of main tunnel excavation is 2.0D~3.0D(D is length of single span). The center pilot is partly backfilled after it being completed to facilitate the initial support directly acting on the center pillar, which can not only effectively improve integral stiffness of supporting, but make the center pillar more reasonably stressed and thus improve its state of behavior.Via test of construction and in-situ monitoring of Fuxi multi-arch tunnel under unsymmetrical pressure, conclusions presented in this paper are correct and reasonable, including: Determination of gallery width parameter of multi-arch tunnel, bound between deep and shallow depth of unsymmetrical multi-arch tunnel, calculation of passive and active ground pressure, rapid in-situ evaluation of rock classification and construction method and technology. All these not only supply technical support for successful construction, but also offer experiences for similar projects.