Stability of long unsupported twin circular tunnels in soils

Advances in Civil Engineering

Wang

Niu

et al. 2020

In the construction of mountain highway, in order to avoid complicated geology and adapt to the requirements of the terrain of the large height difference and the large slope, the double-line spiral tunnels are gradually applied. The purpose of this paper is to analyze the mechanical behavior of the double-line spiral tunnel and its surrounding rock under different line curvatures, and to obtain the influence of the curvature of the spiral tunnel on the stability of the double-line tunnel construction. The analysis of this paper is based on the engineering background of double-line spiral tunnels in China’s Yunnan province. The elastoplastic three-dimensional rock strata-tunnel by means of finite difference FLAC3D software was established to simulate the construction process. The model was verified by comparing the calculation results and the actual monitoring data of tunnel vault settlement. The small curvature radius spiral makes the mechanical behavior of the double-line tunnel uneven and the surrounding rock deformed unevenly. A quantitative analysis and qualitative evaluation of the influence of curvature radius were established by the systematic evaluation index of (1) ratio of compressive stress on both sides of the tunnel, (2) stress ratio of double-line tunnel, (3) convergent deformation of the cross section of the tunnel, and (4) deformation of the surrounding rock on the top of the tunnel. The results show that the small curvature radius (less than 200 m) will make the inner pressure of the inner tunnel significantly greater than the external pressure stress, showing obvious asymmetry, and the inner tunnel vault tensile stress is greater than the outer tunnel. With the increase of the curvature radius (about more than 400 m), the ratio of the compressive stress on the inside and outside of the tunnel tends to be constant, and the bias condition is weakened and stabilized. Meanwhile, the smaller curvature radius makes the convergent deformation of the cross section of the tunnel appear asymmetrical, and the compression quantity inside the tunnel center line is larger. It provides a reference basis for the stability control of the construction of the double-line spiral tunnels in the mountainous area.

Section: Comparison With the Analytical Solutionmentioning

confidence: 99%

Effect of the Double‐Line Spiral Tunnel Curvature on the Tunnel Construction Stability

Advances in Civil Engineering

Wang

Niu

et al. 2020

“…Because circular tunnels can provide good stability and are convenient to construct, they are commonly used in practical engineering applications. Circular tunnel stability has been widely analyzed [32][33][34][35][36]. Compared to circular tunnels, rectangular tunnels have a higher degree of utilization of underground space and a lower volume of earthwork excavation; thus, a rectangular tunnel is often a valid option due to the development of advanced tunneling machines.…”

Section: Introductionmentioning

confidence: 99%

Assessment of the Stability of an Unlined Rectangular Tunnel with an Overload on the Ground Surface

Advances in Civil Engineering

Hang

Feng

et al. 2020

City tunnels are often constructed at shallow depths, and tunnel failure may be initiated by overloads resulting from surrounding buildings, structures, heavy-haul trailers, and other installations. Although several works have been reported on tunnel stability, stability numbers have mainly been obtained for cases with fully cohesive soils. Moreover, little information has been presented about the influence of overloads on the failure patterns for unlined rectangular tunnels. This paper uses upper-bound finite element methods to assess the stability of an unlined rectangular tunnel in cohesive-frictional soils with an overload acting on the ground surface. A complete set of dimensionless parameters covering the tunnel size and shallow tunnel depth and Mohr-Coulomb material parameters are determined to obtain the dimensionless overload. In addition, failure modes that are similar to slip line fields are acquired. A failure mechanism that may cause base heave is proposed in this paper to improve the accuracy of the results. These failure patterns are more complex for cases with larger dimensionless depth, larger internal friction angle, and smaller dimensionless unit weight. Compared with the rigid-block mechanisms from the upper-bound rigid-block analysis method, these computed failure mechanisms are better suited for rectangular tunnel stability analysis.

“…Conventionally stability of tunnel structures can be analyzed with, eg, finite element method and discrete element method . And the stability of unsupported (unlined) tunnel model has been studied for long such as in literature . With the great amount of parameter studies presented in these literatures, the stability of a specific unsupported tunnel could be quantitatively assessed with a factor of safety, whereas the stability of a shotcrete supported crown/tunnel is not well studied yet.…”

Section: Introductionmentioning

confidence: 99%

Stability analysis of shotcrete supported crown of NATM tunnels with discontinuity layout optimization

Num Anal Meth Geomechanics

Zhuang

Lackner

2018

Summary For tunnel constructed by New Austrian Tunnelling Method, the crown is the upper part of tunnel section, constructed during excavation process and supported by shotcrete. The stability of the crown has great influence on the safety of tunnel itself and the buildings above, which correlates, among others, with geometrical setup of tunnel and material properties of shotcrete and soil/rock. In this paper, aiming at analyzing the stability of shotcrete supported crown, a recently presented numerical method discontinuity layout optimization is adopted, which introduces a great amount of potential discontinuities cross over one another and provides a wide search space for efficient upper limit analysis. In the analysis, a well‐established hydration model of cementitious material is implemented for accounting the hydration of shotcrete. Then assumptions based on convergence‐confinement method are used for accounting the 3‐dimensional effect in 2‐dimensional analysis, finally providing time‐space–dependent assessments of stability of shotcrete supported crown.