Study on Mechanism of Water Inrush of Karst Tunnels

Water inrush,which is one of the challenging issues and hot topics in the tunneling industry,is very easy to occur during the construction of karst tunnels.The mechanism of water inrush of karst tunnels is discussed and analyzed in the paper:the water inrush of karst tunnels is generally divided into three steps,i.e.,the forming of the hazard source,the forming of the water inrush passage and the failure of the anti-inrush rock mass.The failure of the anti-inrush rock mass of karst tunnels are classified into 5 types,i.e.,the integral tensile-shear failure,the hydraulic fracturing,the infiltration induced sliding of the filling medium,the loss of key blocks and the comprehensive water inrush mode.The failure mechanism is studied on basis of typical cases and by means of numerical simulation or theoretical analysis.Conclusion is drawn that most of the water inrushes in actual tunneling are comprehensive water inrushes,which are the comprehensive results of the interrelation and interaction of various water inrush types,and that different types of water inrushes have related continuity and progressive evolution relationships under certain conditions.

Energy analysis of rock plug thickness in karst tunnels based on non-associated flow rule and nonlinear failure criterion

The geological hazards, such as water inrush and mud outburst, are easily induced by the high water pressure caverns ahead of a karst tunnel face. Therefore, it is a pivotal issue to determine the reserved thickness of rock plug during the construction of tunnels. The limit analysis principle is employed to analyze the safe thickness from the point of energy dissipation, and the nonlinear and non-associated characteristics of geotechnical materials are both considered. On the basis of a plane failure pattern of rock plug, the expressions of detaching curve and rock plug thickness are derived. The effect of each parameter on the safe thickness of rock plug is discussed in detail, which interprets the corresponding failure scope of rock plug. The obtained results indicate that the thickness of rock plug is highly influenced by the nonlinear dilatancy coefficient and the nonlinear coefficient. The proposed method is validated by a comparison of the calculated results with those of the engineering project of the "526 karst cavern" of Yunwushan tunnel. This proposed method can provide reference basis for the design and excavation of karst tunnels in the future.

A Comparative Study on Hydrodynamics and Hydrochemistry Coupled Simulations of Drainage Pipe Crystallization Blockage in Karst Tunnels

Drainage pipe system is the requisite component of the traffic tunnels in Karst area.To reveal the dynamic process of crystallization blockage in drainage pipes,a novel hydrodynamics and hydrochemistry coupled simulation model was developed for calculating the deposition rate of CaCO_(3) fouling in pipeline surface.Sediments adhering to the pipe walls involve a deformable domain with moving geometric boundaries,and moving mesh and level set methods are proposed for simulation of for tunnel turbulence and crystallization fouling process.The simulation results are compared with the experimental results showing similar trend.The effects of temperature,flow velocity,and solution concentration on crystallization blockage were analyzed by comparative simulation studies.The simulation results show that:(1)the moving mesh method simulated nozzle shrinkage caused by crystalline deposition,without accounting for geometric topology shape changes.However,the level set method tracked the moving topology and thus can simulate the process of complete blockage;(2)the flow velocity in the longitudinal pipe generally exceeded that in the transverse pipe,and the CaCO_(3) crystal concentration in the transverse pipe eclipsed that in the longitudinal pipe,which meant crystallization blockages primarily occurred in the transverse pipe;(3)the temperature and concentration correlated positively with the crystallization rate,while the crystal precipitation value decreases with the increasing of inlet flow velocity increases.This study advances a hydrodynamics and hydrochemistry coupled crystallization blockage model to provide technical support for the early identification of crystallizationinduced pipe blockage in the drainage system in karst tunnel sites.

Minimum safe thickness of rock plug in karst tunnel according to upper bound theorem

High pressure and water-bearing caverns ahead of a karst tunnel face tend to cause geological disasters, such as water and mud bursts. So, the determination of safe thickness of the reserved rock plug is a key technical problem to be solved for karst tunnel construction. Based on the Hoek-Brown nonlinear failure criterion, the minimum safe thickness of rock plug was investigated in the light of the limit analysis theory. On the basis of the proposed failure mode, the expression of the minimum thickness for rock plug was obtained by means of upper bound theorem in combination with variational principle. The calculation results show the influence of each parameter on safe thickness and reveal the damage range of rock plug. The proposed method is verified by comparing the results with those of the drain cavern of Maluqing Tunnel. The research shows that with the increase of compressive strength and tensile strength as well as constant A of Hoek-Brown criterion, the safe thickness decreases, whereas with the increase of cavern pressure, tunnel diameter, and constant B from Hoek-Brown criterion, the safe thickness increases. Besides, the tensile strength, or constants A and B affect the shear failure angle of rock plug structure, but other parameters do not. In conclusion, the proposed method can predict the minimum safe thickness of rock plug, and is useful for water burst study and prevention measures of tunnels constructed in high-risk karst regions.

An attribute recognition model for safe thickness assessment between concealed karst cave and tunnel

An attribute recognition model for safe thickness assessment between a concealed karst cave and a tunnel is established based on the attribute mathematic theory.The model can be applied to carrying out risk classification of the safe thickness between a concealed karst cave and a tunnel and to guarantee construction’s safety in tunnel engineering.Firstly,the assessment indicators and classification standard of safe thickness between a concealed karst cave and a tunnel are studied based on the perturbation method.Then some attribute measurement functions are constructed to compute the attribute measurement of each single index and synthetic attribute measurement.Finally,the identification and classification of risk assessment of safe thickness between a concealed karst cave and a tunnel are recognized by the confidence criterion.The results of two engineering application show that the evaluation results agree well with the site situations in construction.The results provide a good guidance for the tunnel construction.

Detection and treatment of water inflow in karst tunnel：A case study in Daba tunnel

In a karst tunnel, fissures or cracks that are filled with weathered materials are a type of potential water outlet as they are easily triggered and converted into groundwater outlets under the influence of high groundwater pressure. A terrible water inrush caused by potential water outlets can seriously hinder the project construction. Potential water outlets and water sources that surrounding the tunnel must be detected before water inflow can be treated. This paper provides a successful case of the detection and treatment of water inflow in a karst tunnel and proposes a potential water outlet detection(PWOD) method in which heavy rainfall(>50 mm/d) is considered a trigger for a potential water outlet. The Daba tunnel located in Hunan province, China, has been constructed in a karst stratum where the rock mass has been weathered intensely by the influence of two faults. Heavy rain triggered some potential water outlets, causing a serious water inrush. The PWOD method was applied in this project for the treatment of water inflow, and six potential water outlets in total were identified through three heavy rains. Meanwhile, a geophysical prospecting technique was also used to detect water sources. The connections between water outlets and water sources were identified with a 3-D graphic that included all of them. According to the distribution of water outlets and water sources, the detection area was divided into three sections and separately treated by curtain grouting.

Sensitivity analyses of random cave groups on karst tunnel stability based on water–rock interaction using a novel contact dynamic method

This paper concentrates on the sensitivity and dynamic simulation of randomly distributed karst cave groups on tunnel stability and connectivity extended ratio based on water–rock interaction using a novel contact dynamic method(CDM).The concept of karst cave group connectivity extended ratio during tunneling and water inrush is proposed.The effects of cave shape and spatial distribution on Qiyueshan tunnel are investigated.Tunnel deformation and damage index,and connectivity extended ratio with uniform random karst cave groups are evaluated.The results demonstrate that the connectivity extended ratio is verified as a crucial judgment in predicting the safe distance and assessing the stability of the tunnel with the karst cave group.CDM model captures the fracture propagation and contact behavior of rock mass,surface flow,as well as the bidirectional water–rock interaction during the water inrush of Qiyueshan tunnel with multiple caves.A larger cave radius and smaller minimum distance between the cave and tunnel increase the deformation and damage index of the surrounding rock.When the cave radius and cave area ratio increase,the failure pattern shifts from overall to local failure.These findings potentially have broad applications in various surface and subsurface scenarios involving water–rock interactions.

Large-scale model test study on the water pressure resistance of construction joints of karst tunnel linings

Model tests and numerical calculations were adopted based on the New Yuanliangshan tunnel project to investigate the water pressure resistance of lining construction joints in high-pressure and water-rich karst tunnels.A large-scale model test was designed and conducted,innovatively transforming the external water pressure of the lining construction joint into internal water pressure.The effects of the embedded position and waterstop type on the water pressure resistance of the construction joint were analyzed,and the reliability of the model test was verified via numerical calculations.The results show that using waterstops can significantly improve the water pressure resistance of lining construction joints.The water pressure resistance of the lining construction joint is positively correlated with the lining thickness and embedded depth of the waterstop.In addition,the type of waterstop significantly influences the water pressure resistance of lining construction joints.The test results show that the water pressure resistance of the embedded transverse reinforced waterstop is similar to that of the steel plate waterstop,and both have more advantages than the rubber waterstop.The water pressure resistance of the construction joint determined via numerical calculations is similar to the model test results,indicating that the model test results have high accuracy and reliability.This study provides a reference for similar projects and has wide applications.