| With much domestic investment being used for transport infrastructure, groups of loess tunnels with large cross section will soon appear in the west of China. The railway passenger delicated line from Zhengzhou to Xi'an, which has taken the lead in building, includes a lots of loess tunnels. These tunnels have a cross section of more than 170 m~2 that broke the history record in loess region. Loess ground is so special and weak that this building project will have to encounter huge challenge. At present, there is quite a lack in relative construction & design experience and necessary guidance of criterion for large loess tunnel, for that reason the research of constructing technology and support theory seems very urgent. This dissertation firstly carries out a study on the mechanical behavior of large loess tunnel by multiform methods.1. Through the research of special loess property in passenger delicated line, it is pointed out that loess's structural strength could be concluded into bilinear yielding rule, and that vertical joint could be treated as ubiquitous joint. Both of them have different growth environment.2. According to loess's special property, bilinear constitutive model should be adopted for tunnel's mechanical analysis. The results reveal that structural strength, with a little effect on displacement form, affects mainly plastic zone and the magnitude of displacement, and that vertical joint affect not only displacement mode but also breakage form of wall rock.3. By three-dimensional numerical method and monitoring data from experimental tunnel, constructing schemes that include method of double side-wall, CRD and arch lead hole, has been studied in dynamic mechanics process. It comes to the conclusion that either shallow burial or deep burial is not enough secure. Some typical characters, such as big displacement magnitude and release ratio, durative deformation and high early deformation, great support stress, etc, will occur synchronously in large loess tunnel. Monitoring method suggests support deformation should be controlled in shallow burial condition, and double control of support stress and deformation should be done in deep burial condition.4. Based on mechanical character of three methods above, countermeasure or optimization is proposed to make the tunnel construction safe. Numerical analysis validates the amendatory effect.5. To obtain cohesion strength between rockbolt and loess ground, shear-lag theory is modified to evaluate rockbolt pull-out test. On the basis of midpoint principle and data fit of numerical analysis, interaction mechanism of rockbolt-ground is studied, and finally finds that deformation mode occurring at different rock wall location will bring a critical effect to rockbolt axial force.6. Through numerical analysis and 1:1 antetype test, anchor effect of systemic rockbolt in large loess tunnel is investigated. The result shows its' weak in the tunnel crown and work relatively well in the sidewall.7. In term of loess' structural strength and hardening of shotcrete, plane method is created for analyzing the combined effect of steel rib and shorcrete. The outcome indicates that under fast hardening lattice girder and profiled bar have the same support effect.8. By three-dimensional numerical method, it is pointed out that the difference between lattice girder and profiled bar is in the resistance capability to horizontal displacement, not in the resistance to arch settlement.9. 1:1 antetype support experiment is used to compare lattice girder with profiled bar, detecting that under fast hardening they have a little distinction. All of them may be applied in large loess tunnel.
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