| In recent years,China’s urban underground space has been moving towards largescale,networked,deep and intelligent.Great progress has been made in modern tunnel construction technology and intelligent equipment research and development technology,due to the changes in China’s population structure,construction efficiency,cost control and other factors,it has increasingly advocated "replacing people with machinery",so as to achieve less and even unmanned operations.At the same time,more and more attention is paid to the standardization,industrialization and assembly of the support structure.The on-site mechanized assembly method is adopted to achieve rapid support and strictly control the deformation of surrounding rock.In the construction of urban underground rail transit,many engineering cases of large section and super large section,close to or under the existing structure have emerged.Due to the untimely support or the insufficient performance of the support structure,engineering disasters such as excessive deformation and even collapse of the underground tunnel sometimes occurred,which has become an urgent scientific problem affecting the safe construction of urban underground engineering.In order to rationally develop and utilize underground space,improve construction safety and city quality,this article relies on the National Key Research and Development Program subject "Assembly support technologies for urban underground space construction"(2018YFC0808705).Based on the technical problems of tunnel construction in complex urban environment,the new concept,new structure and new technology to solve the support problem of large cross-section tunnel were sought by means of theoretical analysis,numerical simulation,indoor model test,and field test.Drawing on the concept of "timely support and strong support",the control idea of forming strong support in time and bearing the load of surrounding rock in real time was proposed,and high-strength prefabricated spatial reticulated grid support structure and joint connection modes were creatively developed.Taking full advantage of the performance advantages of spatial reticulated grid structure and the mechanized construction technology of underground engineering,the spatial reticulated grid structure was used for the primary support for undercutting subway tunnel for the first time,focusing on the structural design of the spatial reticulated grid,bearing capacity of joint components,the bearing characteristics of the spatial reticulated grid,the coupling mechanism between the spatial reticulated grid and the concrete,the mechanized rapid construction technology and the engineering application effect.A real-time strong support system suitable for urban complex environment was constructed.A tunnel rapid assembly support system based on on-site assembly and construction mechanization was constructed,and the mechanized rapid construction technology of prefabricated spatial reticulated grid was formed,so as to provide a reference for mechanized construction and assembly support.The main research work and innovative achievements are as follows:(1)In view of the problems of high construction risk and strict control of stratum deformation under the complex urban environment,the timely high-strength support concept of "timely formation of strong support and real-time bearing of surrounding rock load" was put forward.The structure and joint connection types of the prefabricated spatial reticulated grid structure were creatively designed,and the rapid assembly support system for large cross-section tunnel was established.The prefabricated spatial reticulated grid was optimized design from the aspects of structural section form,material selection,component section,and joint connection form,and the corresponding design method was formed.(2)The indoor mechanical performance tests and numerical simulations of bolted endplate joint components,combined sleeve joint components,and jointless components under different loading modes were carried out,and the effects of different joint types and structural parameters on the basic mechanical properties of the components were compared and analyzed.The deformation characteristics,failure forms and stress-strain evolution laws of each component under different loads were revealed.The difference of bearing performance between circular arch members and short straight members was obtained.By introducing the performance index of the joint,the difference and influence degree of the bearing performance of the combined sleeve joint under different factors were analyzed.(3)The indoor mechanical performance tests of bolted endplate concrete joint component,combined sleeve concrete joint component,and jointless concrete component under the pure bending were carried out.The evolution laws of surface cracks,surface concrete stress and strain,mid-span deflection and joint angle of each component in the whole loading process were systematically analyzed.Combined with the numerical simulation method,the influence laws of concrete spraying layer strength,spatial reticulated grid layout spacing,and longitudinal connection distribution spacing on the bearing performance of the initial support system were further analyzed,and the stable bearing mechanism of spatial reticulated grid concrete composite structure was defined.(4)The numerical simulation method was used to analyze the stability bearing characteristics and instability failure characteristics of single spatial reticulated grid and composite spatial reticulated grid in and out of the plane.The relationship between the displacement and stress release coefficient of surrounding rock at different levels was discussed,the longitudinal deformation law of surrounding rock under the influence of spatial effect of excavation face was obtained,and the reasonable support time was determined.Based on the principle of stiffness equivalence and superposition,the support characteristic curve of spatial reticulated grid + shotcrete + system bolt was solved.Combined with the underground tunnel project of Guangzhou Metro,the interaction curve of surrounding rock and spatial reticulated grid bolted shotcrete support was given,the stability coefficient was obtained,and the rationality of the support scheme was verified.(5)In order to further clarify the mechanical state of the prefabricated spatial reticulated grid support and the surrounding rock control effect,and give full play to the advantages of mechanized supporting construction,the deformation and stress simulation analysis of the prefabricated spatial reticulated grid during the hoisting process of was carried out,the assembly process test of the spatial reticulated grid was conducted.The smoothness and structural stability in the assembly process were tested,and the construction process of the prefabricated spatial reticulated grid was introduced.Relying on the underground tunnel project of Guangzhou Metro,the field comparison tests of new spatial reticulated grid and grid arch was implemented for the first time,which achieved good engineering application results,and solved the problems of deformation control,efficient support and rapid construction of large section tunnels in high density urban areas,which could provide a reference for similar project construction in the future. |
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