Theoretical Study on Relaxed Surrounding Rock Pressure on Shallow Bias Neighborhood Tunnels under Seismic Load

To study the distribution of relaxed surrounding rock pressure on the shallow bias neighborhood tunnels under the combined action of horizontal and vertical earthquake force, finite element software was used for failure mode analysis. Moreover, with the pseudo-static method, the calculation formula for the relaxed pressure on the shallow bias neighborhood tunnels was derived and used to analyze the variation of the rupture angle of these tunnels under the action of the seismic force. The study shows that: shallow bias neighborhood tunnels basically follow a “W” failure pattern under the combined action of horizontal and vertical seismic force, and the failure scope of the surrounding rock is controlled by four rupture angles. Rupture angles β2 and β3 between the deep and shallow tunnels of the shallow bias neighborhood tunnels are not affected by the surface slope. For tunnels with the same grade of the surrounding rock, the greater the seismic intensity, the smaller the value of β2, and the greater the value of β3. While at the same seismic intensity, the higher the grade of the surrounding rock, the smaller the β2 and β3. Ruptures angles β1 and β4 are influenced by the surface slope, seismic intensity and surrounding rock grades. A steeper surface slope leads to a smaller β1 and a greater β4; β1 increase and β4 decrease with increasing seismic intensity; while, β1 and β4 both show a decreasing trend with an increasing rock grade.

Stability of a Rock Tunnel Passing through Talus-Like Formations: A Case Study in Southwestern China

Tayi tunnel is one of the component tunnels in the Jian-Ge-Yuan Highway Project located in Yunnan Province, southeast of China. It mainly passes through talus-like formations comprised of rock blocks of diverse sizes and weak interlayers with clayey soils with different fractions. Such a special composition leads to the loose and fractured structure of talus-like formations, which is highly sensitive to the excavation perturbation. Therefore, Tayi tunnel has become the controlled pot of the whole highway project as the construction speed has to be slowed down to reduce the deformation of surrounding talus-like rock mass. To better understand the tunnel-induced ground response and the interaction between the surrounding rock mass and tunnel lining, a comprehensive in situ monitoring program was set up. The in situ monitoring contents included the surrounding rock pressure on the primary lining, the primary lining deformation, and the stress of steel arches. Based on the monitoring data, the temporal and the long-term spatial characteristics of mechanical behavior of surrounding rock mass and lining structure due to the excavation process were analyzed and discussed. It is found that the excavation of lower benches released the surrounding rock pressure around upper benches, resulting in the decrease of the surrounding rock pressure on the primary lining and the stress of steel arches. In addition, the monitoring data revealed that the primary lining sustained bias pressure from the surrounding rock mass, which thereby caused unsymmetrical deformation of the primary lining, in accordance with the monitored displacement data. A dynamically adaptive support system was implemented to strengthen the bearing capacity of the lining system especially in the region of an extremely weak rock mass. After such treatment, the deformation of the primary lining has been well controlled and the construction speed has been considerably enhanced.

Temporal and Spatial Effect of Surrounding Rock and Supporting Construction of a Large Soil Tunnel

Based on the Zaosheng No. 3 tunnel of the Yinchuan-Xi’an high-speed railway, the surrounding rock pressure, contact pressure of the primary support, and secondary lining and internal force of the secondary lining concrete are systematically tested using a vibrating wire sensor, and the correlation between the advance construction distance and the surrounding rock release rate is studied with finite element software. The results show that the pressure on the surrounding rock is low when the deeply buried soil tunnel is excavated and can be divided into three stages: rapid growth, slow growth, and flattening with time. It is more reasonable to calculate the surrounding rock pressure by using tunnel planning calculations. For the contact pressure, although the value of each measuring point in the inverted arch changes a little, the arch pressure obviously has the characteristics of rapid growth and a sharp rebound. Most of the test points of the second lining concrete show a compression state, which is far less than the ultimate compressive strength. At the same time, the initial support of the tunnel bears a large load, while the secondary lining bears a relatively small force, and the load sharing ratio of the two ranges between 0.1 and 0.7; with the progress of the excavation section, the surrounding rock deformation (deformation release rate) increases gradually. When the excavation face is close to the monitoring section, the deformation (deformation release rate) is the most severe. With the increase in the distance between the excavation section and the monitoring section, the deformation (deformation release rate) tends to be flat.

Calculation Model and Influencing Factors of Surrounding Rock Loosening Pressure for Tunnel in Fold Zone

This research aims to study the surrounding rock loosening pressure variation law of tunnel in the fold area. Based on the calculation method of surrounding rock loosening pressure for shallow tunnel, a new calculation model of the surrounding rock pressure was proposed for tunnel in the fold area; through this calculation model, the effects of tectonic stress (F), the angle ( φ 1 ) between tectonic stress and horizontal plane, tunnel buried depth (h), friction angle ( θ ), the multiple (k) between tectonic stress and rock mass gravity in the upper part of the tunnel, lateral pressure coefficient ( λ ), and tunnel midline offset (t) on tunnel surrounding rock loosening pressure in fold area are studied, respectively. Results show that in the anticline area, when φ 1 increases, the vertical loosening pressure (q) decreases; when q > 0, the surrounding rock is in the elastic deformation stage, and q decreases monotonously as F increases; when q < 0, the rock mass is in the initial stage of failure, and as F continues to increase, the number of internal cracks increases, the rock mass reaches its ultimate bearing capacity and then fails completely, and q increases linearly in this process; q decreases with the increase of θ and k; the greater k is, the easier it is to reach its bearing limit; the horizontal loosening pressure (e) increased monotonously with the increase of h and λ . The research process of surrounding rock loosening pressure of tunnel in the syncline area is similar to that of tunnel in the anticline area; q decreases with the increase of θ and λ ; q monotonically increases with F increasing.

The Expression of Seismic Surrounding Rock Pressure for a Shallow Tunnel Is Derived Using the Pseudostatic Method

Terzaghi developed a generalized expression of the vertical surrounding rock pressures of shallow tunnels by considering the limit equilibrium of soil masses. In this paper, based on the Terzaghi failure mode, the pseudostatic method is used to derive this expression under seismic loading conditions. The surrounding rock in the fractured zone of the tunnel side wall is analyzed as an isolated body using the limit equilibrium method to obtain the explicit calculation expressions of the horizontal surrounding rock pressures of a shallow tunnel under seismic loading. Case analysis indicates that the proposed method is feasible. In addition, the influence of the seismic acceleration coefficient on surrounding rock pressures is further discussed. The results show that the horizontal surrounding rock pressure decreases with the increase of seismic acceleration coefficients. The vertical surrounding rock pressure increases as the horizontal seismic acceleration coefficient increases, and it decreases with the increase of the vertical seismic acceleration coefficient, and the effect of the seismic acceleration coefficient on surrounding rock pressure is significant. The study results can provide reference for the seismic safety evaluation and structural seismic design of shallow tunnels.