Prediction of the Collapse Region Induced by a Concealed Karst Cave above a Deep Highway Tunnel

The presence of a concealed karst cave above a deep highway tunnel may cause the collapse of the rock mass between the karst cave and tunnel during excavation. Rock mass collapse threatens the safety of tunnel construction personnel. A prediction method of the collapse region induced by a concealed karst cave above a deep highway tunnel is proposed on the basis of the upper bound theorem of limit analysis. An analytical expression of the collapse surface is derived from the variational principle. Using the analytical expression of the collapse surface, the shapes of the collapse surfaces are plotted for different rock mass parameters. Moreover, the minimum safe distance between the karst cave and tunnel is defined, and the computational equation of the minimum safe distance is derived. The proposed method is applied in a highway tunnel excavated in a karst terrain as a case study. Based on geological survey report parameters, the shape of the collapse surface and the minimum safe distance between the karst cave and tunnel are obtained. Finally, the collapse surface of the rock mass provided by the proposed approach is compared with that provided by numerical simulation, and the favorable result comparison shows that the proposed method is valid.

Stability Analysis of Water-Resistant Strata in Karst Tunnel Based on Releasable Elastic Strain Energy

In this paper, the energy instability criterion of water-resistant strata and rock mass failure index (RMFI) are proposed, respectively, based on releasable elastic strain energy Ue. RMFI is employed to represent the damage extent of water-resistant strata. When RMFI<1.0, rock mass is stable. When RMFI=1.0, rock mass is in the critical instability state. When RMFI>1.0, rock mass is unstable. The releasable elastic strain energy Ue and RMFI program is performed by FISH programming language of Flac3D software. Then, the authors apply Flac3D software to analyze the distribution law of releasable elastic strain energy Ue and failure zone under different width of concealed karst cave. Finally, combined with the numerical analysis, a case study is carried out to illustrate the rationality, effectiveness, and feasibility through using RMFI to predict safe thickness of water-resistant strata.

Study of Safe Distance Criterion between Concealed Karst Cave and Tunnel

On the basis of analysis of the stability criterion of surrounding rock and instability mechanism of the rock stratum between concealed karst cave and tunnel, the rock stratum energy catastrophe and safety coefficient catastrophe as safe distance criterion between concealed karst cave and tunnel were put forward, then the comparison and analysis of numerical simulation results based on the run-through of plastic zone criterion, energy catastrophe criterion and safety coefficient catastrophe criterion were carried out with engineering examples, the results showed that energy catastrophe criterion was more reasonable.