Study On The Excavation Stability Of High-Density Loess Tunnel Construction

Numerous loess tunnels are being built in China's western region with the continuous improvement of transportation infrastructure in this region. The section steel support and steel grille support are two primary support forms in the construction of loess tunnels which are excavated by "New Austrian Tunneling Method" in current China. Grille support has been chosen as the ideal primary support because of its low cost, light weight, easy transportation and installation, and that it combines tightly with sprayed concrete and thus no shrinkage cracks appear on its surface in later stages. In the construction process of loess tunnel, steel arch support has been chosen as the primary support for security and ease of management. However, the body of deep-lying tunnel always lies in high-density loess stratum, and the surrounding rock in the stratum often contains calcareous cementation layers, and it is always with joints, without bedding and rupture. And thus the stratum has good continuity and high stability. In this situation, it is necessary to study whether the grille support can be chosen as the primary support of the steel arch form. This thesis discusses the application of grille support in the high-density loess tunnels excavated with the method of three stairs and seven steps. And it discusses as follows:1. Taking the 1 tunnel of Luze valley which belong to the railway passage of Shanxi central south as an example, and based on the sufficient survey and analysis of the geological formation of the wall rock, this thesis put the use of grille support in the construction of high-density and deep-lying loess tunnels as a hypothesis, and proposes a concrete design in applying grille support and the wall rock displacement index, structure intensity index and construction monitoring speed index in distinguishing the stability of tunnels with grille support.2. Taking the #1 tunnel of Luze valley which belong to the railway passage of Shanxi central south as the research object, applying the method of finite element numerical simulation, and contrasting with construction with section steel support, this thesis compares the tunnel stability differences between the two kinds of support, and finds that when applying the excavation method by three stairs and seven steps, the horizontal convergence value of side wall is greater than the vault sedimentation. And under the condition of the grille support, the two values increase respectively, and the convergence value changes greater. Compared with the condition of under section steel support, when under the grille support, the final axial force decreases, bending moment and the cross-section stress increases, the system anchoring bolt and locking tremie increases, but within the material's ultimate strength. Therefore, it is estimated that the application of the grille support with shotcrete as the primary support of high-density loess tunnel is feasible.3. When analyzing the field wall rock displacement monitoring index with grille support, it finds that all the displacement changing speeds are smaller than the designed ones, except the first day. And the total displacement doesn't exceed the designed index, which shows the feasibility of grille support as the primary support of high-density loess tunnels. Comparing the testing data with the calculating result, it finds that there is almost no deviation in the calculating data, and proves that the calculating result is reliable and that the method in primary estimation is correct.4. Using the three-dimensional finite element model, it studies the influence of 1m and 2m cyclical footage to the stability of tunnels. The comparison shows that in the condition of 2m cyclical footage, the vault sedimentation rarely changes, while the convergence value of the side wall significantly reduces, the axial force of the grille arch increases, the stress increases and the whole section is pressed, the whole section of shotcrete is pressed, and the stress of inside and outside changes differently, the maximum stress decreased, the axial forces of System anchoring bolt and locking tremie decrease, all the structures' stress are within the ultimate strength. The feature point stress of the wall rock and the depth of plastic zone have almost no changes. Consequently speaking, when excavating the loess tunnels with the method by three stairs and seven steps,2 m cyclical footage is better than that of lm.5. Using three-dimensional finite element model, it analyzes, under the condition of grid support, the impact of the core soil with the maximum and minimum length prescribed in "Guide to the railway section tunnel operation with the excavation method of three stairs and seven steps" to the tunnel stability. The study finds that under the condition of minimum-length steps, the tunnel's vault sedimentation and horizontal convergence value of the wall side significantly decreases, with the latter even more apparent. The bending moment and shearing force of the grille arch decreases, the axial force increases, the total stress increases and the whole section is pressed, while the stress doesn't exceed the steel yield strength; the whole section of shotcrete is pressed, the inside and outside circumferential stress increases, and the maximum stress doesn't exceed the Compressive Strength of shotcrete; the axial force of system anchoring bolt and locking tremie decreases; the feature point stress of the wall rock increases, the maximum equivalent plastic strain reduces, and the depth of plastic zone decreases. The vertical displacement of the tunnel face midline increases, and the touch point of the tunnel face and the core soil increases most. With point safety factor method commonly used in slope stability analysis, and with the sliding surface referred as the most dangerous section in the thesis, it analyzes the stability of the core soil under the two construction conditions—long and short steps. The studies show that, the safety factors under the two conditions are both greater than 1. It proves that the core soil is stable under either condition. It finally proves that when applying the excavation method of three stairs and seven steps on construction of the high-density loess tunnels, short steps is better than long steps, and the core soil and surrounding rock are both stable.