Tunnel Roof Research Articles

Study of the fracture behavior of mode I and mixed mode I/II cracks in tunnel under impact loads

Blast excavation may induce radial cracks around a tunnel, and these cracks may initiate and propagate under nearby mining activities. In order to study the dynamic fracture behavior of mode I and mixed mode I/II cracks in a tunnel under impact loads, a new large sample, i.e. a tunnel with a radial crack (TWSRC) was proposed. Impact tests by using the TWSRC samples and a drop-weight impact testing device were also implemented. The advantage by using the TWSRC samples is that the corresponding results could be directly employed to tunnel engineering practice, and could lead tunnel designers to improve tunnel dynamic stability and to prevent tunnel hazards. The pre-crack was designed in a tunnel roof and was parallel to the symmetrical axis of the tunnel. The ratio α of the distance between the crack and the symmetrical axis to the radius of tunnel roof was in the range between 0 and 1. The crack initiation time and the crack propagation characteristic were measured by strain gauges and crack propagation gauges. A finite difference commercial code AUTODYN was used in the simulation of crack propagation paths and the fracture mechanism was analyzed. A finite element commercial code ABAQUS was employed to calculate crack dynamic stress intensity factors (DSIFs), and the experimental-numerical method was used to measure crack initiation toughness. The results showed that for mode I cracks, the cracks propagate along the pre- exiting crack direction, whereas the mixed mode I/II cracks, they propagate with a certain angle to form a wing crack at the early stage, and finally they propagate along the major principal stress direction. For mixed mode I/II cracks, the critical mode I SIF increases slightly with the distance to the symmetrical axis, whereas the critical mode II SIF increases largely as compared to the mode I SIF.

Roof filling control technology and application to mine roadway damage in small pit goaf

To recover coal resources that have been damaged by traditional mining methods and ensure stability of the lower roadway in a small pit goaf, the goaf area must be filled and reinforced. In this research, the 1202 working face of the Hanzui mine is considered as an example for classifying the roof of the mining tunnel under the small kiln destruction zone, the effect of the goaf on the roadway is determined based on the radio tunnel penetration method, a mechanical model to determine the roof filling control mechanism was established, and the duct foaming system and roof filling process were designed. The results show that the scope and degree of influence of the goaf on the mining lane are large, but safe tunneling can be ensured through the use of a steel shed and advanced grouting techniques. When the roof conditions are not similar, materials with different filling heights and filling strengths can be used to control the roof filling of the roadway. By combining field experience and laboratory tests, it was determined that a high-foaming material with a water-cement ratio of 1:0.6, a suitable high-foaming additive, and a water volume ratio of 1:30 is cost-efficient for filling and meets the filling strength requirements. Finally, the reliability of the proposed technology was verified by field experiments, which provide a reference for filling operations in similar mines.

Displacement-Based Estimation of the Best Time for Secondary Lining Construction Using Grey Model GM (1,1)

Large deformation or collapse often occurs in tunnels due to the high ground stress and rock rheology. When it cannot be effectively curbed by primary support, the secondary lining construction at a reasonable time comes to be particularly important in maintaining the stability of surrounding rock. In this paper, an efficient approach to estimating the best time for secondary lining construction in circular tunnels is presented based on Grey Prediction Model GM (1,1). Taking the tunnel in Hunan Province as an example, according to the classic rock-support interaction model, the limit displacement of tunnel roof is determined by investigating the stress–strain relationship of the surrounding rock in the framework of elastic–plastic theory. Subsequently, the GM (1,1) is employed and modified by optimizing the weight coefficient of background value μ and boundary condition D to predict the time sequence of the displacement of tunnel roof. Accordingly the best time for secondary lining construction is reasonably predicted. The results show that the greater μ is, the higher the model precision will be. The dispersion of the predicted results can be significantly reduced with the optimized D. The predicted displacement of tunnel roof reaches 55.840 mm in the 23rd observation period while the theoretical limit displacement of surrounding rock is 55.33 mm. Hereby, it is suggested that the secondary lining construction should be completed just before the 23rd observation period. The biggest advantage of the proposed approach is that it can be efficiently performed with resort to less parameters, and proves to be practical and feasible in tunnel design and construction.

A probabilistic method to analyze collapse failure of shallow rectangular tunnels

It is difficult to forecast the collapse failure of rectangular tunnel roof because mechanical properties of surrounding rocks or soils are strongly affected by uncertainties. A uniform design (UD)-based response surface method (RSM) is proposed to handle uncertainties and to forecast the stability of rectangular tunnel roof with the help of Tunnel Seismic Prediction 203 plus (TSP203PLUS). Firstly, the influence factors of shallow rectangular tunnels are studied by the excavation half-width (Lu), which is derived using Mohr envelope. Since TSP203PLUS is an essential approach to predict the geohazard in tunnels, and a series of parameters such as the density (ρ), P-wave seismic velocity (V) and dynamic Young's Modulus (E) of geomaterials can be read from TSPwin (the software of TSP203PLUS). Then, the relationship among Lu and ρ, V and E are established, so that ρ, V and E read from TSPwin are treated as input variables in the UD-based RSM. Finally, a tunnel located in China is illustrated to demonstrate the efficiency of this approach and the result obtained from the proposed method is validated by Monte Carlo simulations (MCS). The proposed method provides a quick and effective method to study the collapse failure of rectangular tunnel roofs.

Practical shotcrete rock reinforcement for hard rock openings

Conventional cast in-situ concrete linings are a costly, time consuming activity and environmentally unfriendly solution for supporting hard rock excavations because concrete linings are unable to utilise the inherent strength of the rock. In evidence, by the application of confining pressure via pattern bolts suffices to improve the strength of the rockmass at the underground opening (i.e. cavern or tunnel). In the implementation of this measure, the rock arch formed by the tunnel roof and walls is considered to be a supporting arch capable of sustaining a thrust at the arch ends. However, a potential rock wedge failure, of comparatively small size, is highly probable and may occur between rock bolts. To deal with this issue, shotcrete linings will then be introduced to act as thin “protective skin” liners to support the rock surface with performance similar to a reinforced concrete slab. A detailed discussion of these combination measures, so called “shotcrete rock reinforcement” (SRR), including analysis of the strengthened rock arch and the design of shotcrete linings’ structural capacity according to Eurocodes for hard rock underground openings are presented in the paper together with numerical modelling to check against the boundary deformation of openings. Key issues of the constructability of SRR are also discussed.

Risk management during construction of a new road tunnel crossing under a 150 year old railway tunnel

AbstractIn Schaffhausen, Switzerland, a new road tunnel (Galgenbuck tunnel) has been built crossing under a 150 year old railway tunnel of the German railways. The distance between the roof of the road tunnel and the floor of the railway tunnel is only 6 m. The intense rail traffic was not to be disturbed and the railway tunnel should not suffer any damage. Both tunnels lie in jurassic limestone rock with karst. To deal with the risk of cavities, clay pockets or other karst phenomena, that are difficult to locate in advance, the road tunnel design was done very carefully. Different measurements were taken to control risks: measurement of all blast shocks, geodetic monitoring of the tracks, geotechnical measurements like distometers, horizontal inclinometers, extensometers. Risk management procedures were implemented, and – following them ‐ the tunnel has been built without interrupting the rail traffic. Good cooperation between the client, the railway company, the tunnel specialists and the people on the site was essential.

Experimental Study on Rockfall Impact Force Applied to Frame Shed Tunnels

A model experiment is performed to test the rockfall impact force applied to frame shed tunnels under the protection of different types of cushions embedded on tunnel roof slabs. The changing rule of impact force induced by rockfall is analyzed according to different impact energies and types of cushions embedded on shed tunnels. An enlargement coefficient reflecting the relationship between average impact force and maximum impact force is presented using experimental data and theoretical analysis. The improved method for calculating rockfall impact force can consider the many factors that affect this phenomenon. For a particular rockfall impact energy, the rockfall impact force with different types of cushion properties is affected by the cushion property function f∼(h,E,μ,γ). A large f value equates to the minimal discreteness of the expansion coefficient of rockfall impact force under different types of cushion, and vice versa.

Dynamic Responses of Bridge–Tunnel Overlapping Structure for High-Speed Railway Under Different Seismic Excitations

There are many bridges and tunnels been built overlapping or close to each other during railway constructions in west or south China recent years. For bridge–tunnel overlapping structures on high-speed railway, serious damage or even complete destruction could be caused when powerful earthquake occurred in complex topography or poor geological areas. However, there is still little research yet on this kind of complex system taking interactions of bridge–tunnel–rock into account, especially for seismic damage or dynamic response mechanism. For purposes of dynamic response analysis and reasonable anti-seismic measures for complex bridge–tunnel overlapping structures, large scale shaking table model tests have been carried out. In shaking table model test, EL-Centro wave (EL) and the Wenchuan wave (WC) were adopted as loading wave in X, Y, Z-direction (perpendicular to the tunnel axis, horizontal along the tunnel axis, and vertical). Test results demonstrated significant differences existing on dynamic responses or variation laws especially in different seismic excitation cases, and fateful consequences could be caused. It also indicated that several sensitive and fragile parts (e.g., the arch vault and arch foot at enlarged section of the tunnel, at the end of the beam and the top of the abutment inside the tunnel portal, and the soil above the tunnel roof etc.) should be strengthened or required special treatment for structural security or overall stability. Simultaneously, seismic excitation combination analysis is important for such overlapping structures in weak surrounding rocks or soils. And the influence caused by the tunnel longitudinal direction should not be ignored for seismic response analysis for such structures. Bidirectional or multi-directional seismic excitation method is proposed for the rationality and accuracy on seismic tests or anti-seismic analysis for same or similar structures.

EXPERIMENTAL STUDY ON THE STABILITY OF SURROUNDING ROCK IN TUNNEL BLASTING CONSTRUCTION

In this study,criteria and blast ing technologies are introduced in order to control the stability ofsurrounding rock of tunnel built using drill and blast safety . The paper is composed of three parts,namely, a blast vibration propagation law in roof surrounding rock in close proximity to tunnel face,two formulae to calculate particle critical vibration velocity of shotcrete and key structural element atthe roof of tunnel, and innovative technologies of tunnel blasting . The blast vibration propagationlaw is the base to control the sta bility of surrounding rock during tunnel blast ing Based onMorhr Coulomb criterion and the dynamic analys is, two formulae to calculate the critical particlevibration velocity are proposed Based on a series of trial blasts using electronic detonators, tw oinnovative blasting technologies are derived. One is the blast holes detonated one by one by usingelectronic detonator , and another is the blast holes detonated by combining initiation system ofelectronic detonators and nonel detonators T he use of ele ctronic detonators in tunnel blasting notonly leads to a smaller blast vibration but also to a smaller extent of the EDZ (excavation damagedzone)

Instability Mode Analysis of Surrounding Rocks in Tunnel Blasting Construction with Thin Bedrock Roofs

The thin bedrock roof tunnel is widely distributed in China, and its influence of blasting construction on surrounding rock is obviously different from that of ordinary homogeneous surrounding rock tunnel. In order to control the safety of tunnel construction under the condition of thin bedrock roof effectively, the method of the numerical simulation is adopted to establish 6 models with the bedrock roof thickness of 5, 4, 3, 2, 1 and 0.5 m respectively, and research the vibration distribution characteristics of tunnel surrounding rock caused by blasting construction. The stability of tunnel surrounding rocks with different bedrock roof thickness is analyzed according to the influence partition of blasting vibration, and the corresponding instability mode is proposed. The results show that: when the roof thickness is 5 m or more, the surrounding rock is stable; when the roof thickness decrease to 4 m, the whole bedrock roof on the most unstable transverse section (1 m behind the tunnel face) produces slight tensile spallation; when the roof thickness decrease to 3 m, part of bedrock roof on the most unstable transverse section produces serious stretching cracks and radial cracks, the possible instability mode is over excavation; when the roof thickness decrease to 2 m and below, the stability of surrounding rock is poor, the possible instability mode is collapse or falling within 1 m behind the tunnel face.

Stability of Tunnel Roof in Nonhomogeneous Soils

AbstractThe stability of tunnel roof is investigated in nonhomogeneous soils using the theory of plasticity. Based on the nonlinear failure criterion, the expressions of tunnel roof in nonhomogeneo...

Dynamic Pressures on Tunnel Roofs due to Vehicle Passages

Pressure and proximity measurements made in a tunnel indicate that a typical vehicle passage produced on the tunnel roof an initial pressure increase of small magnitude, followed by a sharp and more substantial drop in pressure below atmospheric. The magnitude of the pressure drop was found to increase with smaller clearances between the vehicle top and the tunnel roof, consistent with the Bernoulli relation and the vehicle speed. The dynamic pressures potentially may have significant effects on the vibration and noise environments on the lower floors of “air rights construction” buildings that span highways.

Factors Influencing Ground Settlements on Different Geomorphology Units Based on Principal Component Analysis

Based on the case Nanjing Metro Line 4, the method Principal Component Analysis (PCA) was used to study the influence of ground settlement by shield tunnel construction on different geomorphology units. Correlation analysis and weighted least square method (WLS) were applied for variables selection and to obtain their relationship with settlement. 5-7 principal components could be used to present the initial 19- 21 variables after decreasing the dimensions of data. For the floodplain of Yangtze River, variable parameters that highly linearly dependent on settlement were depth of tunnel, distance between the roof of tunnel and the bottom of soft soil layer, thickness of soft soil, compression modulus of soil that tunnel passed through and speed of the cutter head. For the Qinhuai ancient channel, variable parameters were Poisson's ratio, porosities, moisture content, unit weight, cohesion, internal friction angle, compression modulus of soil that tunnel passed through, advancing speed, earth chamber pressure. For the terrace of Yangtze River, variable parameters were cohesion, porosities, moisture content, Poisson's ratio, compression modulus and unit weight of soil. In addition, for the geomorphology unit with col landform, variable parameters were different. Residuals of regression formula are small, which will have certain reference value in practical engineering.

Catastrophe analysis of active-passive mechanisms for shallow tunnels with settlement

In the note a comprehensive and optimal passive-active mode for describing the limit failure of circular shallow tunnel with settlement is put forward to predict the catastrophic stability during the geotechnical construction. Since the surrounding soil mass around tunnel roof is not homogeneous, with tools of variation calculus, several different curve functions which depict several failure shapes in different soil layers are obtained using virtual work formulae. By making reference to the simple-form of Power-law failure criteria based on numerous experiments, a numerical procedure with consideration of combination of upper bound theorem and stochastic medium theory is applied to the optimal analysis of shallow-buried tunnel failure. With help of functional catastrophe theory, this work presented a more accurate and optimal failure profile compared with previous work. Lastly the note discusses different effects of parameters in new yield rule and soil mechanical coefficients on failure mechanisms. The scope of failure block becomes smaller with increase of the parameter A and the range of failure soil mass tends to decrease with decrease of unit weight of the soil and tunnel radius, which verifies the geomechanics and practical case in engineering.

Roof failure of shallow tunnel based on simplified stochastic medium theory

The failure mechanism of tunnel roof is investigated with upper bound theorem of limit analysis. The stochastic settlement and nonlinear failure criterion are considered in the present analysis. For the collapse of tunnel roof, the surface settlement is estimated by the simplified stochastic medium theory. The failure curve expressions of collapse blocks in homogeneous and in layered soils are derived, and the effects of material parameters on the potential range of failure mechanisms are discussed. The results show that the material parameters of initial cohesion, nonlinear coefficient and unit weight have significant influences on the potential range of collapse block in homogeneous media. The proportion of collapse block increases as the initial cohesion increases, while decreases as the nonlinear coefficient and the unit weight increase. The ground surface settlement increases with the tunnel radius increasing, while the possible collapse proportion decreases with increase of the tunnel radius. In layered stratum, the study is investigated to analyze the effects of material parameters of different layered media on the proportion of possible collapse block.

Reliability analysis of tunnel roof in layered Hoek-Brown rock masses

This paper develops a two-layer model and upper bound solutions for the shape of collapse block in rectangular tunnel subjected to seepage pressure, and then the reliability-based analysis is performed. According to the layered Hoek-Brown rock masses with different characteristics, a continuous failure mechanism of roof collapse in the deep cavities is derived by using upper bound solutions. The performance function of roof stability is proposed and is adopted for reliability analysis and design. Parameters involved in the performance function are random or deterministic variables. For comparison, improved response surface method (RSM) and Monte-Carlo simulation are utilized to calculate the Hasofer-Lind reliability index and the failure probability. It is found that the improved RSM is a reliable method in probabilistic calculation and the supporting pressure has a significant influence on the reliability index. The normal or lognormal distribution of variables has no effect on the failure probability, while the coefficient of variation (COV) of random variables would greatly influence the failure probability. A reliability-based design is performed to determine the probabilistic tunnel pressure for a target reliability index when different COVs of random variables are considered.

The effect of topography on stability of shallow tunnels case study: The diversion and conveyance tunnels of Safa Dam

Abstract Due to ignoring the role of real topography in the numerical modeling carried out for back analysis and considering that in the tunnel longitudinal profile the distance of tunnel roof to the ground surface is the maximum, some results inaccuracy may occur. This issue is especially critical for cases when two tunnels or underground structures are constructed next to each other. The purpose of the present research is to study the role of real topography on the obtained results of back analysis and determine the accurate geomechanical parameters of the rock mass using back analysis based on convergencemeter data. For this aim, 8 instrumentation sections were modeled using FLAC3D 5.0 and the real topography was determined. Next, another numerical model was built to study the role of real topography and the obtained modeling results were compared for cases with and without real topography; where a significant difference was obtained between the results. In the case of ignoring real topography, the error function obtained from monitoring and numerical method results are between 1.5 and 10 times more than the case of considering real topography, suggesting the significant role of real topography. Furthermore, stability analysis and optimization of tunnels support system performed in this study using the real topography data. The results showed that removing the designed rock bolts the tunnel was still stable through other designed supports.

PREDICTION OF ROCK MASS PROPERTIES, TUNNEL STABILITY AND SUPPORT PRESSURE BY GEOLOGICAL STRENGTH INDEX (GSI) IN CROCKER FORMATION: A CASE STUDY

This study was conducted along a tunnel in Crocker Formation. The objectives of this study are to determine the value of Geological Strength Index (GSI) and to predict rock mass properties, very unfavourable discontinuities combination and tunnel support pressure for rock bolts or shotcrete for the tunnel. Engineering geological mapping, rock sampling and estimation of GSI values and the disturbance factor were conducted along the tunnel faces. Laboratory analysis includes Point load and dry density test and data analysis consists of kinematic analysis and limit equilibrium analysis. The rock mass was characterised by 94.88 MPa UCS, 0.024 MN/m3 unit weight, widely space and high persistency of discontinuities. The GSI value is 50 with 0.8 disturbance factor. The cohesion, tensile strength and friction angle are 3.671 MPa, 0.056 MPa and 25.20°, respectively. There are eight possibilities of discontinuities combinations on tunnel roof that have factor of safety (F.O.S) lower than 2 and combination of joints 2, 4 and 6 has the highest maximum wedge volume of 28.37 m3. The maximum support pressure of rock bolts or shotcrete for F.O.S of 2 is 0.04 MN. The individual discontinuity plane has been identified to overestimate friction angle and cohesion values, then the GSI system should be applied in homogeneous or isotropic and not in structurally controlled rock masses.

2D and 3D stability analysis of tunnel roof collapse in stratified rock: A kinematic approach

This paper aims to predict the potential roof collapse of deep tunnels subjected to water table level changes and deterioration of rock overlying the tunnel crown. The effect of partly weathered rock masses is represented with lower mechanical parameters. Meanwhile, the pore water pressure is taken as external loads acting on the rock bank below the water table without consideration of capillary effect. Under the combined effects of the above two conditions, a novel and comprehensive failure mechanism is postulated for analyses of roof stability of tunnels. Both two-dimensional (2D) and three-dimensional (3D) collapse mechanisms formed by multi-surfaces are proposed in kinematic analyses where upper bound solutions of the failure mechanism are derived based on variational principle and stepwise optimization technique. Validation of the proposed approach is carried out which showed consistencies of failure mechanisms with analytical and numerical results presented in published literatures. Parametric studies with rectangular and circular tunnel cross-sections are also conducted in 2D and 3D analyses so as to depict the effect of weathered parameters and pore water on incipient falling blocks. The output from these parametric studies could potentially provide useful reference for engineers in the design and construction of deep-buried tunnels.

Limit analysis of failure mechanism of tunnel roof collapse considering variable detaching velocity along yield surface

Limit analysis of failure mechanism of tunnel roof collapse considering variable detaching velocity along yield surface