Abstract
The process of crack propagation and tunnel failure is affected by the cross-sectional geometry of an underground tunnel. In order to quantify the effect of section shape on the process of crack propagation in deep tunnels under high ground stress conditions, a total of four physical models with two cross-sectional shapes and twelve stress levels were designed and several large-scale physical model tests were conducted. The results indicated that, when the vertical stress is 4.94 MPa, the length and depth of the cracks generated in the rock surrounding the horseshoe tunnel are about eight times that around a circular tunnel. The position where the circumferential displacement of the horseshoe tunnel begins to be stable is about two, to two and a half, times that around a circular tunnel. After the deep chamber was excavated, continuous spalling was found to occur at the foot of the horseshoe tunnel and microcracks in the surrounding rock were initially generated from the foot of the side wall and then developed upwards to form a conjugate sliding shape to the foot of the arch roof, where the cracks finally coalesced. Discontinuous spalling occurred at the midheight of the side wall of the circular tunnel after excavation, and microcracks in the surrounding rock were initially generated from the midheight of the side wall and then extended concentrically to greater depth in the rock mass surrounding the tunnel. Tensile failure mainly occurred on the surface of the side wall: shear failure mainly appeared in the surrounding rock.
Highlights
Academic Editor: Constantin Chalioris e process of crack propagation and tunnel failure is affected by the cross-sectional geometry of an underground tunnel
In order to quantify the effect of section shape on the process of crack propagation in deep tunnels under high ground stress conditions, a total of four physical models with two cross-sectional shapes and twelve stress levels were designed and several large-scale physical model tests were conducted. e results indicated that, when the vertical stress is 4.94 MPa, the length and depth of the cracks generated in the rock surrounding the horseshoe tunnel are about eight times that around a circular tunnel
E size of the three-dimensional numerical model of the Horseshoe tunnel is shown in Figure 7. e size of the threedimensional numerical model of the circular tunnel is demonstrated in Figure 8. e methods of mesh generation for the fine division near the excavation tunnel and coarser division at the boundary are adopted. ere are 45,143 zones and 38,509 grid points in the horseshoe tunnel mode. ere are 32,246 zones and 33,801 grid points in the circular tunnel mode. e Mohr–Coulomb constitutive model with a tension cutoff was employed with a finite difference approach
Summary
Academic Editor: Constantin Chalioris e process of crack propagation and tunnel failure is affected by the cross-sectional geometry of an underground tunnel. Based on FLAC3D, Meng et al [4] simulated the excavation of roadways with different cross sections, including rectangular, trapezoidal, straight wall arch, horseshoe, oval, and circular shapes, and the deformation characteristics of surrounding rock and the distribution of plastic zone of surrounding rock were explored. Based on the UDEC program, Ma et al [6] carried out a series of numerical simulations for roadways with five different cross sections to analyze the energy release characteristics and distribution of stress and plastic zones in the surrounding rock and reveal the influence of horizontal tectonic stress. Xu et al [7] proposed a rock burst energy release rate (RBERR) criterion, and the evolution of rock damage and failure of advancing excavation faces with different cross sections were simulated: most of these studies into the influence of section shape on tunnel stability are qualitative, and there is a lack of quantitative analysis of the effects of section shape
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