Study On Construction Mechanics Effect Of Shallow-buried Loess Double-arch Tunnel Under Unsymmetrical Pressure

As an innovative tunneling structure, double-arch tunnel has many advantages,such as high flexibility to different landform condition, smooth line shape,less occupation of space, large span, structural integrity, and relatively low integrated cost. Along with small-distance tunnel, double-arch tunnel is one of the feasible solutions for tunnel construction in China at present. However, because of its own charaeteristies, such as shallow depth of burial, unsymmetrical pressure, small height-to-span ratio, complex geological conditions and structural stress, as long as special engineering properties of loess and lack of experience in construction of biasing double-arch tunnel in loess geological condition, the application and study of biasing loess double-arch tunnel are still at a preliminarily stage, and this is especially true for the systematic study on the structural design and the mechanism of the mechanical response during construction, which cause the lack of guideline for design and construction in engineering practice.. More and more hightway tunnels and railway tunnels will get through loess area with the development of the western region and project of 5 vertical & 7 horizontal national lines which will be completed in 10 years..Therefore, it is very important to analyze physical and mechanical properties of loess combining with the pratical engineering project in order to consummate the design and construction of unsymmetrically loaded loess double-arch tunnel.The thesis depends on Lishi double-arch which is the first domestic losess double-arch tunnel , through doing laboratory test to get the physical and mechanical properties of intact loess Q2 of Lishi double-arch tunnel, also use numerical simulation software to make numerical analysis on construction method and distance between tunnel faces of Lishi loess-arch tunnel. Get conclusions as follows.①Summarize the design and construction technology of unsymmetrically loaded loess double-arch tunnel as well as make detailed analysis on some key technologies on the basis of collecting research data of design and construction technology of domestic unsymmetrically loaded loess double-arch tunnel.②According to Q2 loess conventional triaxial compression test curves in different stress path, it can draw the relations of shear stress q and shear strainεs, relations of body strainεv vs shear strainεs. Besides, the change rules of Elastic constants against confining pressure, peak strength and remain strength against confining pressure are obtained. Then provide basis for selection of constitutive model and parameters of surrounding rock in numerical analysis.③Use the numerical analysis software FLAC3D to do comparative analysis on three-pilot drift method and central pilot drift method for Lishi double-arch tunnel, compaired the satability of surrounding rock and support in different construciton method, then get the conclusion that three-pilot drift method in better than central pilot drift method for Lishi double-arch tunnel.④Use the numerical analysis software FLAC3D to analyze the spatial effect of tunnel face, interaction of left and right cavity, as well as stability of mid-partition during construction. Then get the conclusion that three times the diameter is the most resonable distance bwtween left and right excavation faces during construction for Lishi double-arch tunnel.