Research On Spatial Deformation Characteristics And Supporting Mechanism Of Shallow Loess Tunnels With Large Section In High-Speed Railway

The tunnel design and construction theory developed rapidly with the development of railway tunnel in China. The newly constructed Baoji-Lanzhou high-speed railway is an important part of the national railway network, with the main line length of 402.6km, among which there exists 357.1 km in Gansu Province and the bridge and tunnel proportion is 92%. Tianshui-Lanzhou section, mainly distributing the sandy loess, lies in the collapsible loess region, with large curve radius, low self-supporting capacity, numerous shallow zones, and the settlement is difficult to control, leading to a big challenge to the construction. This paper studied the spatial deformation characteristics and supporting mechanism of shallow loess tunnels with large section in high-speed railway based on the results of literature survey, field testing, centrifuge testing and numerical simulation, and the following conclusions can be drawn:(1)The spatial deformation characteristics of shallow loess tunnels with large section are studied in the shallow section of Sujia Chuan tunnel by measuring the surface subsidence, stratigraphic layer settlement, vault settlement, horizontal displacement of the tunnel face, vault settlement inside the hole and level convergence. Results show that the stratum exhibits sinking trend in the vertical direction, increasing slightly with the depth. The surface subsidence area is small and attenuates very slowly, consistent with the fitting results by adopting the Peck equation. The horizontal displacement is small. The deformation measured is mainly the vertical settlement and the one in the early stage cannot be measured.(2) Take a research on the time effect and spatial effect during the loess tunnel excavation, Flac3d numerical software is adopted to analyze the influence of invert closing time and distance on the deformation for the typical sections in Sujia Chuan tunnel and Xipo tunnel with the same invert closing time or distance. The numerical results are consistent with the field testing data. By comparing the time effect and spatial effect during the loess tunnel excavation, the spatial effect is more significant and the appropriate supporting measures are proposed.(3) The force mechanism of anchors in the loess tunnel with large sections is analyzed. Combine the Flac3d numerical results, the Boltzmann function is used to fit the surrounding rock displacement and then the internal force calculation formula for loess tunnel arch portion and wall is derived. Based on the centrifuge test results with various length of systematic anchor, primary support and ground deformation shows a slight decrease with the increase of the anchor length, because the systematic anchor applied will increase sharply at the time of completing the primary support, comprehensive analysis to increase system anchoring bolt length of the effect is not obvious, inferring that the anchor can be abolished.(4) In order to study the mechanism of steel and sprayed concrete of large section loess tunnel, the field test is conducted in Xipo tunnel to measure the surrounding rock pressure, early strength of sprayed concrete, stress of primary support steel, stress of the sprayed concrete and the contact pressure between primary support and lining, then the results are analyzed. The surrounding rock characteristic curve is adopted based on the elastoplastic solution of tunnel excavation deformation and the surrounding rock parameters. The supporting curve and safety factor of three different thicknesses of combination between sprayed concrete and steel are given, then the reasonable supporting parameters are provided.(5) The technical features of Double wall pilot tunnel method, CRD method as well as the step method are analyzed. By comparing with the advance prebuilt archon excavation, the pre-cutting has significant effect on the surrounding rock deformation controlling. From the centrifuge test results, constraint arch deformation shows a good controlling effect on the ground deformation, prebuilt arch structure deformation as well as the stress, and the pre-cutting method is proposed to apply in the large section loess tunnel, then the arch supporting technique is provided.