Limit Analysis Method Of Surrounding Rock Pressure And Excavation Progress Optimization Of Unsymmetrical Pressure And Shallow Tunnel

With the rapid development of traffic infrastructure of our country, it makes great efforts to prompt widely use of tunnel engineering, it is inevitable to emerge more and more shallow-buried and unsymmetrical pressure tunnels because of limit of geology and line alignment. Differ from conventional tunnels, the structural stress of unsymmetrical pressure tunnels is more complicated, construction difficulty is bigger. How to determine the surrounding rock pressure and the failure mechanism accurately are key problem of construction and safety of long-term operation. However, at present, most study is focus on static derivation based on limit equilibrium and numerical analysis methods based on the finite element, which suffers from too many assumptions, low computing efficiency and other defects, the calculation results of it are often far from satisfactory. Therefore, relying on science and technology project of Shanxi Provincial Department "study om design and construction methods of bias tunnels", problems of rock pressure of low-buried and unsymmetrical pressure tunnels and the excavation footage optimization were studied concretely and systematicly by numerical simulation, model tests and theoretical derivation.(1)Based on finite element theory, numerical models were established, plastic zone of surrounding rock, vibration internal force of lining, development law and distributions under factors of different partial pressed angle, depth, surrounding rock grades, structure types and other influencing factors were studied, the effect degree of unsymmetrical pressure and other factors on the stability of tunnels and the change law of the structure stress were proved, with buried depth is lower, increase of partial pressed angle, cave surrounding rock will produce evident tension stress zone or plastic zone, the stability degree gradually weakened; structure force of lining presents significantly asymmetric distribution, stress concentration region increases, the structure produces local failure and then leads to the failure of the whole supporting system.(2) Through model test indoor of typical terrain unsymmetrical pressure tunnels, under conditions of different unsymmetrical pressure, surrounding rock pressure of construction stage of single tunnels and neighbourhood tunnels were particularly studied as well as the dynamic distribute form and change law of internal force of lining, the failure mode of shallow-buried bias single tunnels and neighbourhood tunnels were obtained respectively. The results show that development of surrounding rock pressure is greatly influenced by construction excavation footage,it consists evident time-space charicteristic, the distribution form of it is more unfavorable than the calculation results of the standard method, the asymmetry of it is more significant. Meanwhile, for the failure mode of cave surrounding rock, it is a single tunnel inverted cone conductor when it is small clear distance(3) Combined with the result of model tests indoor, the physical failure mode of shallow-buried bias single tunnels and neighbourhood tunnels were established, relied on sliding block limit analysis theory and associated flow rule, the corresponding kinematically admissible velocity fields were established, and velocity recurrence formula of each sliding block and sliding block discontinuity, finally, based on Principle of Virtual Work, through derivation, surrounding rock pressure upper bound method calculation formula of single tunnels and low-buried and unsymmetrical pressure neighborhood tunnels were obtained.(4) Basing on Janssen Silo theory and Mohr-Coulomb strength criterion, the failure limit analysis model of the face was constructed, introducing excavation footage variables, through further derivation, the mathematical expressions of low-buried and unsymmetrical pressure tunnels'excavation footage optimization were obtained,moreover, according to optimization theory, the corresponding mathematical model and Optimization Algorithm based on "Dichotomy" were constructed.(5) According to the research results, using FORTRAN language and Delphi7platform based on Obect Pascal language, the calculation program and interface were established, efficient solution for surrounding rock pressure and excavation footage optimization of shallow-buried bias single tunnels and small-spacing tunnels were realized and the corresponding engineering example calculation analysis was carried out. The results show that the calculated value of the limit upper bound method derived in this article is well consistent with the existing research results and meets the engineering practice better, the reliability of the software is proved.