Tunnel Face Research Articles

Monitoring and Analysis of Deformation Refinement Characteristics of a Loess Tunnel Based on 3D Laser Scanning Technology

Loess tunnels often undergo large-scale deformation with complex spatial and temporal distribution. Mastering the characteristics of spatial and temporal deformation is conducive to precise policy implementation and the control of large deformation of the tunnel. In this study, relying on the Yulinzi Tunnel in Gansu Province, China, based on 3D laser scanning technology, the tunnel was monitored for a short period of 24 h and a long period of 36 days. The refined characteristics of the temporal and spatial deformation of the representative points of the interrupted surface, the tunnel face, and the excavation mileage during the excavation process of the three-bench and seven-step method of the tunnel were analyzed. The results show that the tunnel’s arch has large deformation, and there is twisting deformation. The distribution of the overall deformation of the tunnel is related to the excavation sequence, showing a stepped deformation law. With the construction of the following excavation process, the deformation rate of the tunnel always indicates the characteristics of significant in the early stage and small in the later stage, and the overall deformation changes with time in accordance with the distribution law of the exponential function. The research results provide a reference for predicting the deformation development trend of loess tunnels and providing reasonable deformation control methods.

Tunnel Concealed Karst Cave Joint Detection by Tunnel Seismic and Transient Electromagnetic

Abstract The water-rich giant karst cave is the risk source of tunnel construction in the karst area. To reduce the risk of tunnel construction, it is necessary to accurately explore the spatial distribution range of the karst cave in the direction of advance and both sides of the tunnel. Reflection seismic and transient electromagnetic (TEM) are the primary geophysical tools for tunnel advanced prediction; they have strengths and weaknesses. The tunnel seismic can only describe the boundary interface between cave bodies and surrounding rock in the direction of advance. Although the TEM method can detect the spatial distribution range of cave bodies, the inversion results depend on the initial resistivity model. The single method is insufficient to describe the spatial distribution of cave bodies. To overcome the shortcoming of a single method, we developed tunnel seismic and TEM joint detection of karst cavern and established a complete data processing flow. The tunnel seismic migration profiles can describe the interface between the karst cave and surrounding rock in the direction of advance. We put forward a reasonable initial model: (1) the layer’s thicknesses are determined by the impedance interface from the tunnel migration data; (2) the initial resistivity values are determined from the pilot hole and prior geological data. Comparison of the inversion results of different initial models of tunnel face horizontal line data proves that the proposed initial model method can reduce multisolution, save calculation time, and improve inversion accuracy. The multidirectional TEM inversion profiles can describe the spatial distribution of the giant cave. Combined with the results of tunnel seismic and TEM, the interpretation errors can be reduced and the extension of the karst cave can be delineated. Later excavation results also verify the accuracy of the prediction results.

Face stability analysis of large diameter shield tunnel in soft clay considering high water pressure seepage

The aim of this paper is to study the influence of high water pressure seepage on the stability of large diameter shield tunnel face in soft clay. Based on allowable displacement rate field of the established double logarithmic spiral collapse body, the theoretical solution of limit support force of large diameter shield tunnel face under seepage was derived, the theoretical solutions of limit support force under different water levels were verified. And the influence of water levels and internal friction angles on the stability of large diameter shield tunnel face under seepage was also analyzed. The results show that the seepage force is the main part of the total limit support force. The influence of seepage on the stability of large diameter shield tunnel face in theoretical analysis can be regarded as seepage force on tunnel face and pore water seepage force in collapse body. The seepage effect in collapse body can be considered as the downward seepage in soil layer above the tunnel face, and the horizontal seepage in soil layer where the tunnel face is located. The influence of water levels and soil internal friction angles on the face stability is obvious when considering seepage effect.

Analytical Prediction of Strip Foundation Building Response to Shallow Tunneling Considering the Tunneling Process

This paper presents an analytical method to predict the response of a strip foundation building to shallow tunneling based on the two-stage method. The existing building is simplified as a Euler–Bernoulli beam resting on the Pasternak model. The tunneling process and different relative positions between the tunnel and the existing building can be considered in the proposed method. The accuracy of the proposed method is verified through comparisons with results from the finite element and finite difference methods. The results indicate that the differential settlement of the building reaches a maximum and the rotation angles are symmetric with respect to the building centerline when the tunnel face arrives at the middle of the building. The maximum bending moments occur at the middle of the building, while the maximum shear forces occur at about one-fifth and four-fifths of the building length when the tunnel face is located at the two ends of the building. According to the parametric analysis, the alignment angle, elastic modulus and Poisson’s ratio of the soil, bending stiffness, and gap parameter greatly affect the building response.

Laboratory model test of slurry shield tunnelling in saturated sandy soil

Experimental tests were carried out using a laboratory-scale model of a slurry shield machine to investigate the mechanical behaviours of soil in response to excavation in saturated sandy ground. The experimental model includes a miniature slurry shield machine that can approximate real tunnel construction, replicating the in situ methods of cutterhead excavation, temporary support with pressurised slurry at the face, advancement of the shield machine and a slurry circulation system. The effects of slurry characteristics, soil permeability and advance rate of the shield machine on the ground displacement, slurry infiltration and excess pore water pressure were analysed and these data were compared with predictions in previous studies. The results confirm that the model tests can effectively simulate the principal operations of a real slurry shield machine. Compared to the test results presented, previously proposed models tend to overestimate the increase rate of pore water pressure in front of the tunnel face, especially in cases of soil with lower permeability, higher concentrations of bentonite mixed slurry and lower advance rates. The slurry-infiltrated zone in front of the shield machine is non-uniform across the tunnel face and extends radially to a larger area than the tunnel diameter. The shape of the slurry-infiltrated zone can be categorised as one of two typical types: (a) cone; (b) bulb.

Seismic Stability of 3D Tunnel Face considering Tensile Strength Cut-Off

Earthquakes can have a very adverse effect on tunnel excavation. The Mohr-Coulomb failure criterion is modified by introducing tensile strength cut-off, which allows nonlinear strength reduction of soil under tensile conditions, and kinematic analysis of tunnel face subjected to the earthquake is carried out. The pseudo-dynamic method is adopted to simulate the earthquake with the acceleration varying with time and space, improving the accuracy of the simulation. The external power and internal energy dissipation rate are equal to obtain the expression of tunnel face support pressure under the limit analysis framework. And the most unfavorable tunnel face support pressure is optimized by using the method of exhaustion. Compared with other research results, the validity of this method is proved. The influence of soil parameters and pseudo-dynamic parameters on the critical support pressure is revealed by further parameter analysis. Finally, this study provides an effective reference for seismic design of tunnel engineering construction.

Three-dimensional tunnel face stability considering slurry pressure transfer mechanisms

In slurry-shield tunneling, the bentonite slurry tends to infiltrate into the soil skeleton ahead of the tunnel face, causing a forward seepage flow associated with excess pore pressures in the ground. Both the slurry infiltration and the forward seepage flow significantly impact the tunnel face stability. In the present paper, a kinematic approach of limit analysis with the classical three-dimensional discretized rotational failure mechanism is employed to assess the face stability of a slurry-shield-driven tunnel. Three situations of slurry infiltrations that influence the face pressure transfer mechanisms were considered: a full filter cake, a penetration zone, and pure groundwater flow without a filter cake or a penetration zone. The pore pressure distributions surrounding the tunnel face were firstly obtained using numerical calculations. Afterward, the incorporation of numerically obtained pore pressure distributions into the work rate calculations allows considering a three-dimensional forward seepage flow ahead of the tunnel face, which provides a rigorous upper-bound estimation to the tunnel face safety factor determined by the strength reduction technique. The proposed method is validated by comparing the calculated results with those provided by previously published literature. The effects of excess slurry pressures, hydraulic conditions, pressure drop coefficients, and soil shear strength were investigated. Several design charts are given for practical application.

Parametric Study of Water Inrush in a Tunnel Crossing a Fault Based on the “Three Zones” Fault Structure

As tunnelling progresses into the complex geological environment such as fault zones, water inrush has become one of the main geological hazards during tunnel construction. Consequently, understanding the evolution of pore pressure and flow velocity when a tunnel is excavated in a fault zone is crucial to ensure safe working conditions and reduce construction risks. In this work, based on the concept of “Three Zones” fault structure, we simulate the nonlinear water inrush process by solving the Darcy-Brinkman flow equation for the host rock and the fault zone. We examine the impacts of 1) the angle between the tunnelling direction and the fault and 2) the relative position from the tunnel face to the fault on the evolution of pore pressure and flow velocity near the tunnel face. The results show that within 5 m to 20 m ahead of the working face, pore pressure, flow velocity, and water inrush rate are the smallest when the angle is 90°. As the angle decreases, both pore pressure and flow velocity ahead of the working face increase. The pore pressure is larger when the tunnel has not reached the fault zone than when the tunnel has crossed the fault zone. Flow velocity also exhibits similar behaviour as pore pressure. With different relative positions from the tunnel working face to the fault, the closer the tunnel face to the fault, the lower the pore pressure and the larger the flow velocity ahead of the tunnel face. The largest water inrush rate occurs when the tunnel face is excavated to the center of the fault core, and the water inrush rate declines as the distance away from the fault increases. The simulation results provided a new method for simulating water inrush when a tunnel crosses a fault and could provide valuable references for the prediction of water inrush for underground projects.

Predicting earth pressure balance (EPB) shield tunneling-induced ground settlement in compound strata using random forest

The prediction and control of the ground settlement induced by shield tunneling in compound strata is a crucial challenge for tunnel excavation. In this regard, a finite element model for the analyses of ground surface settlement induced by earth pressure balance (EPB) tunneling was established and validated by the field test. The influence of single influence factors, such as rock proportion in the face, support pressure, and tunneling angle, on the surface settlement above the tunnel face was systematically studied. To investigate the joint influence of different factors on ground deformation mechanism, the multi-factor analysis based on field data and the multiple regression (MR) method was conducted. However, the prediction accuracy of the MR method was limited. Therefore, a prediction approach of shield tunneling-induced settlement in compound strata based on a random forest (RF) was developed. Tunnel geometry, geological features, and operation parameters were investigated as input variables in the RF model to achieve this goal. The applicability effect of the RF-based method was proved by utilizing two data sets (i.e., the settlement ahead of tunnel face and final settlement). The results indicated that the prediction accuracy of shield tunneling-induced settlement in compound strata by the RF-based model is higher than the MR method. The relative importance analysis of variables indicates that advance rate, the cover depth of the tunnel ring, and pressure in the chamber were the most influential features for the settlement ahead of the tunnel face, while the thrust of the cutter head and the grout quantity were the most significant variables for the settlement after the segment assembling is completed. Overall, the conducted study could provide a helpful reference for the efficient excavation of EPB tunnels across compound strata.

Failure analysis and control measures for tunnel faces in water-rich sandy dolomite formations

Revealing the mechanism of sand gushing triggered by failure of tunnel face in water-rich sandy dolomite strata contributes to the safe construction of tunnels. In this paper, we find that sandy dolomite and the water environment are the key factors causing sand gushing through geophysical prospecting at the tunnel face where 12,000 m3 sand surge occurred. Through the flow-solid coupling model, the extrusion deformation at the tunnel face is the main deformation form in the tunnel of water-rich sandy dolomite strata. The influence of different factors on the deformation value of the tunnel face is from strong to weak: sandification degree, head height, and tunnel depth. Combined with the study of pressure arch characteristics of the model, the limit equilibrium theory considering seepage effects and pressure arch characteristics is proposed. We get that sand gushing will occur in the tunnel in the intensively sandy dolomite strata where the head exceeds 80 m. For the intensively sandy dolomite strata with head below 80 m, the stability of the tunnel face is controlled by the degree of seepage damage. According to the reason of sand gushing, the comprehensive control measures consisting of risk identification by over-detection, double-layer close-packed pipe shed, grouting sealing technology and dewatering technology, and Milling and blasting combined construction method are proposed, and the effective control of tunnel working face is realized in practical application.

Theoretical analysis of the shield tunnel face stability in dry sandy strata

Predicting reasonable tunnel face support pressure is crucial for maintain the tunnel face stability. The silo-wedge model is widely used in calculating face support pressure in dry sandy strata. As one of the key assumptions in the silo-wedge model, the influence of area equivalent method on the calculations of limit face support pressure is investigated in this study. To better describe the geometry of failure zone in three-dimensional (3 D) perspective, an improved limit equilibrium model which consist of the semi-cylindrical silo, the semi-circular truncated cone, and the local sphere model is proposed based on the experimental results of previous research. The formula of limit face support pressure can be derived without the assumption of area equivalent, which is assessed via a series of existing theoretical models and experimental results.

Upper-Bound face stability analysis of rectangular shield-driven tunnels in undrained clays

The face stability of rectangular shield-driven tunnels in undrained clays where the undrained shear strength increases linearly with depth is investigated numerically and analytically in this paper. In the framework of the kinematic approach of limit analysis, two new failure mechanisms based on the translational movement of rigid blocks and continuous deformation of soil mass are proposed for collapse and blowout cases. The analytical solutions of the failure mechanisms were compared with those of finite element simulations and the existing approaches in the literature to evaluate their validity and productivity. The critical support pressures obtained from the analytical models are relatively consistent with the results of the numerical simulations for square and wide rectangular tunnels. Simple design formulas were provided for analytical models to compute the limit support pressure for practical use. Finally, parametric studies were performed to explore the impact of different geometrical and geotechnical parameters pertaining to the stability of the tunnel face.

Face stability of shield tunnels considering a kinematically admissible velocity field of soil arching

Existing mechanism of simulating soil movement at tunnel face is generally based on the translational or rotational velocity field, which is, to some extent, different from the real soil movement in the arching zone. Numerical simulations are carried out first to investigate the characteristics of the velocity distribution at tunnel face and above tunnel vault. Then a new kinematically admissible velocity field is proposed to improve the description of the soil movement according to the results of the numerical simulation. Based on the proposed velocity field, an improved failure mechanism is constructed adopting the spatial discretization technique, which takes into account soil arching effect and plastic deformation within soil mass. Finally, the critical face pressure and the proposed mechanism are compared with the results of the numerical simulation, existing analytical studies and experimental tests to verify the accuracy and improvement of the presented method. The proposed mechanism can serve as an alternative approach for the face stability analysis. • A proper velocity field is proposed to agree with soil movement in soil arch zone. • An improved failure mechanism is constructed based on spatial discretization. • Proposed mechanism provides a better upper bound solution of critical face pressure. • Proposed mechanism corresponds well with numerical simulation and experiment test.

Three-dimensional tunnel face stability considering the steady-state seepage in saturated and unsaturated regions with changing water levels

For tunnels excavated in the unsaturated soils with changing water levels, there may be steady-state unsaturated flow above the water table and saturated flow below the water table toward the tunnel face due to the excavation. Changing water levels affect the adverse effect of the underwater saturated flow and the contribution of the unsaturated flow to the shear strength. To figure out the regularity concerning the region and magnitude of unsaturated and saturated flow effects, an analytical framework is developed within the kinematic approach of limit analysis combined with the numerical simulation. A suction stress-based equation is employed to express the contribution of unsaturated flow to shear strength. The development of the saturated flow below the water table is numerically simulated, and the resulting pore pressure distribution is extracted and interpolated on a 3D discrete rotational failure mechanism. The work rate balance equation incorporating the apparent cohesion due to unsaturated flow and the pore pressure affected by the saturated flow is established to optimize the critical face pressure. The proposed analytical framework is verified by comparisons with previous literature and numerical simulations of hydromechanical analysis. A parametric analysis is conducted to investigate the effect of groundwater levels, flow types, unsaturated properties, and hydraulic hysteresis on the tunnel face stability. The results show that it is only necessary to consider the effect of unsaturated flow and the differences caused by various unsaturated characteristics when the unsaturated zone is large enough. This work can provide a good reference for the design of face supports in the initial stage for tunnels excavated in unsaturated soils with changing groundwater levels.

A Case Study of Deep Shaft and Open Face Tunnelling Induced Excessive Ground Settlement in Water Rich Strata in Guangzhou, China

During a shaft and associated sprayed concrete lining (SCL) tunnel construction in a new line of Guangzhou Metro, site monitoring recorded ground surface settlement was much larger than that predicted in the design. This raised the alarm to the safety of an adjacent high-pressure gas pipe. Close-form and analytical calculations and non-coupled and coupled numerical analyses were carried out to back analyze the settlement. It was found the primary contributing factor to the excessive ground settlement is water loss-induced ground consolidation, which is commonly encountered during underground construction in south China where complex strata, such as granite residual soil and fully weathered granite, are present. This paper details the back analysis process and discusses mitigation measures that should be adopted for construction in similar ground in the future.

Stability analysis and support system design of Angouran underground mine batching room

Purpose. The aim of this paper is to develop a numerical model to investigate the stability of Angouran underground mine batching room, in which appropriate support systems are selected, designed, and compared to choose the most optimal case. Methods. Stability analysis is carried out by critical shear strain criterion using finite element method (FEM). The effect of support system on the stability of batching room has been determined for two types of composite support systems: one, incorporating rock bolt and shotcrete, and the other, comprising steel frame with one-meter distancing and shotcrete, based on comparing the displacements around stopes with the amount of critical and allowable strains resulted from Sakurai criterion. Findings. The results of the analysis show that the excavation of the batching room without a support system causes its instability and failure, so it is imperative to design a suitable support system according to the geomechanical and stress conditions of the area. Comparison between the two selected support systems also shows that there is not much difference between these two support systems and in both cases, batching room stability can be provided. Originality. A wide range of parameters involved in the calculation were explored. Results suggest that the failure strain seems to be a promising tool when used correctly. Considerable time saving can be achieved without losing accuracy. Practical implications. Numerical methods can be utilized to analyze tunnels with non-circular cross-sections, considering heterometric ground conditions around the tunnel, often heterogeneous in situ stresses, rock mass behavior and tunnel face effects. By selecting suitable rock failure criteria, it is possible to identify weaknesses around the excavations via numerical modeling and design an appropriate support system based on it.

Active failure characteristics and earth pressure distribution around deep buried shield tunnel in dry sand stratum

The stability of the tunnel face is a key issue in shield tunnel engineering. In this paper, a series of 1 g model tests were conducted in dry sand for various buried depth ratios H/D = 2, 3, 4, 5, and 6 to study the failure characteristics, earth pressure distribution and the soil arching effect. Each test provided a measurement of the earth pressure with the face withdrawal displacement. And the soil deformation and the failure characteristics were investigated by Particle Image Velocimetry (PIV) system. The development of the failure characteristics was discussed for the buried depth ratios H/D = 3. The relationships between the failure characteristic and buried depth ratios were investigated. The variation of limit support pressure, soil stress distribution at different depths and earth pressure around the tunnel are analyzed. The soil arching effect around the tunnel at deep burial conditions is revealed. With the withdrawal displacement increasing, the tunnel face is in close contact with the support panel at first, and then gradually separated from the support panel. The larger the H/D is, the smaller the increment of horizontal soil stress is. It could be concluded that the increase of tunnel depth directly leads to the soil arching effect.

Investigation and application of a high performance grouting material in water-rich silty fine sand stratum

Grouting reinforcement of water-rich silty fine sand stratum is a key and difficult problem in the construction of tunnel and underground engineering. In this paper, a kind of high-performance grouting material (DS grout) for water-rich silty fine sand stratum was prepared by using sodium silicate as the base material and diacid ester as the curing agent. The results show that the DS grout has adjustable setting time, high strength and high durability. The reaction mechanism of sodium silicate and curing agent was studied by infrared spectroscopy (IR) and scanning electron microscope (SEM). The acid generated from the hydrolysis of diacid ester reacted with sodium silicate solution and polymerized to generate silica gel. Through the grouting model experiment, the grouting effect of DS grout and phosphate-sodium silicate grouting material was compared. In addition, DS grout was used to reinforce the face of tunnel in water-rich silty fine sand stratum. The results show that DS grout can effectively penetrate into water-rich silty fine sand stratum and form high strength sand solidification body.

Blasting Vibration Law and Prediction in the Near-Field of Tunnel

The stability of surrounding rock near the tunnel face is very important to the safety of constructors and the tunnel itself. The vibration law of surrounding rock obtained from the vibration data in the far-field of tunnel blasting is not suitable for the near-field. Therefore, in order to guarantee the safety of tunnel drilling and blasting, it is important to study the vibration law of surrounding rock near the tunnel face. Taking Zhaitang Tunnel, the New Line Expressway of China National Highway 109, as the engineering background, acceleration sensors were installed in the surrounding rock near the tunnel face to test the blasting vibration acceleration of the surrounding rock at the arch waist of the tunnel, to study the blasting vibration attenuation law near the large section tunnel, to modify traditional blasting vibration formula, and to improve the vibration prediction accuracy. The research will bring some guidance for the construction of similar projects.

Seepage effect on failure mechanisms of the underwater tunnel face via CFD–DEM coupling

Seepage plays a significant role affecting the tunnel face failure process. However, its effect on the stability of the underwater tunnel face remains unclear. To study the seepage effect on the failure mechanism of the underwater tunnel face, this study models the failure process by the Computational Fluid Dynamics-Discrete Element Method (CFD–DEM) approach, in which the seepage force is simulated by fluid–particle interaction between CFD and DEM. The contact parameters of the sand particles in water were obtained by calibrating the results of underwater sand column collapse tests. In this study, three hydraulic conditions including dry, undrained, and seepage are considered for the tunnel face. The simulation results indicate that seepage affects the failure mechanism and the support pressure significantly. Then, a comparison between CFD–DEM results and solutions in previous literature was conducted, which suggests that the tunnel face permeability is the key factor that causes the difference in the support pressure under different hydraulic conditions. Additionally, failure mechanisms with different water depths were analyzed by displacement field, water pressure field, flow field, and support pressure. The results show that under the seepage conditions the failure zone is enlarged as the water depth increases. Besides, the microscopic contact information under the seepage condition in different regions was presented. This work provides evidence for evaluating the support pressure of a stable tunnel face when considering the seepage.