•A large-scale physical model for the stability shield tunnel face was designed.•Response of excavation face under two types of transparent soil was studied.•Failure model of excavation face ...instability was obtained by 3D reconstruction.•Instability mechanism of excavation face in clay and clay-gravel layer was revealed.
The soil layers that are traversed by urban tunnel are usually complex. The failure mode of the tunnel face under complex soil conditions with different layers plays a key role for the construction and design of the tunnel. In order to investigate the response of the excavation face under different soil conditions and buried depth of the tunnels, two different types of transparent soils are used in the present paper to carry out six large-scale physical model tests by self-designed model device. Two high-precision motoring linear control platforms are utilized to simultaneously control the optical laser and a Charge Coupled Device (CCD) high speed camera aiming to perform “CT scan”. The failure model of the excavation face is obtained through a three-dimensional program created by the team of this research. Based on the physical tests, it can be observed that the displacement and support pressure curve of the excavation face with respect to the withdrawal of the rigid plate can be divided into two stages in the clay stratum and three stages in the clay-gravel composite stratum. By conducting image analysis on the pictures captured by the CCD high speed camera, the two-dimensional failure zone of tunnel face is obtained. In the clay stratum, the failure zone forms a funnel that reaches the surface for small buried depths. Nevertheless, for larger depths, it consists of a sliding triangle and an overlying loosen rectangle without reaching the surface. In the clay-gravel stratum, the failure zone consists of a triangle and a funnel for small depths, while for larger depths, it consists of a triangle and a rectangle. However, the three-dimensional reconstruction of the failure mode provides more detailed information about its spatial distribution, constituting a useful guideline for the design and construction of tunnels.
The upper bound limit analysis method is one of the main approaches to check the basal stability of foundation pits against upheaval. However, existing studies have often failed to consider the ...effects of external supporting structures, including isolation piles and others, on basal stability against upheaval. This study derives a formula for the coefficient of basal stability against upheaval under the action of isolation piles by simplifying the pile-soil relationship and systematically examines the impact of isolation pile parameters on basal stability against upheaval using the theory of continuous velocity fields and the upper bound limit analysis method. A comparison of simulation results indicates that this technique can accurately identify the variation trend of basal stability against upheaval under the influence of isolation piles and achieve high calculation precision under the operating conditions of wide foundation pits and short isolation piles. Accordingly, a moderate increase in isolation pile parameters produces a significant supporting effect for narrowed foundation pits. Whereas for wide foundation pits, the supporting capacity of isolation piles can be maximized when pile length equals excavation depth.
The chief objective of the article is to learn the spatial characteristics of stress distribution around a shallow buried spherical Karst cave containing fill materials in limestone strata. First, ...considering the effect of external load, stress field in the Earth’s crust, internal filling and Karst geomorphology characteristics in China, a spatial axisymmetrical model was established. Concurrently, combing available work and the concept of elasticity, the boundary conditions are determined. Subsequently, the Love displacement method was introduced, and stress component expressions were obtained. The diagram characteristics of each stress component are summarized, which are affected by various influencing factors. Finally, to prove the rationality of the general solution, a numerical simulation was carried out based on practical engineering, and the maximum error is less than 5%. Thus, the analytical solution could represent the spatial characteristics of stress distribution around a shallow buried spherical karst cave containing fill materials in limestone strata.
In order to solve the problem of the safe thickness between concealed karst caves and tunnels under the influence of compressive faults, a simplified calculation model of the safe thickness under ...three working conditions is constructed. Based on the cusp catastrophe theory, the analytical formulas of the safe thickness between concealed karst cave and tunnel are derived. On this basis, based on the finite difference method, the evolution law of the maximum displacement of rock slabs under different cave water pressures is revealed, the criterion of “when the maximum displacement of the rock slab exceeds the thickness of the rock slab, the water inrush caused by the strength failure of the rock slab occurs in the tunnel” has been established, and the rationality and effectiveness of this criterion have been verified. When the compressive fault is located in the middle of the rock slab, the difference between the theoretical thickness of the rock slab and the maximum displacement of the rock slab is only 0.26 m, the simplified model can accurately determine the thickness of the anti-outburst rock slab. When the compressive fault is located in a quarter of the length of the rock slab, the simplified model also has strong applicability. Research has shown that when the cave water pressure is small, the rock slab displacement increases approximately linearly with the increase of cave water pressure; When the water pressure increases to a certain extent, the cave water pressure and rock slab displacement show a nonlinear correlation, and the rock slab deformation enters the plastic stage; The spatial location of faults has a significant impact on the applicability of analytical formulas, when the fault is located in the middle of the rock slab, the influence of the deformation of the fault itself on the stability of the rock slab should be considered; For the working condition where the karst cave above the tunnel, the theoretical thickness of the rock slab has a certain linear relationship with the length of the rock beam, the corresponding theoretical thickness of the rock slab when the fault is located at other locations can be determined according to the linear relationship; When the location of the fault moves from the middle of the anti-outburst rock slab to the root of the anti-outburst rock slab, for working condition three, the theoretical safety thickness required for the fault located in the upper part of the rock slab is slightly greater than that required for the symmetric position. The research methods and results of this paper have certain reference and application value for similar research and engineering problems.
•Two centrifuge model tests and numerical simulations are carried out.•Sand behavior is modeled by an advanced hypoplasticity constitutive model.•Test results are compared with the results obtained ...from an analytic solution.•The major influence zone on pipeline induced by tunnelling is within ±1.2D.•It should be caution to adopt the superposition principle.
The tunnel-pipeline-soil interaction has gotten momentum recently, but most of the previous studies were focused on the responses of pipeline to single tunnelling and simplified the interaction as a plane strain problem. Up to date, there is still a lack of research on the responses of a pipeline to twin tunnelling at different depths. This study presents a series of three-dimensional centrifuge model tests which were performed to investigate the effects of side-by-side twin tunnelling at varying depths on an existing buried pipeline in dry sand. In the centrifuge model tests, both the volume loss and the weight loss were simulated in-flight using a novel technique. To gain better understanding of the tunnel-pipeline interaction, the centrifuge tests were back-analysed numerically based on an advanced hypoplastic model, which accounts for strain- and path-dependent soil stiffness at small strains. Based on the physical and numerical investigations, it was revealed that the distribution of pipeline maximum shear force is greatly affected by the second tunnelling. The shifted distance of the pipeline maximum shear force induced by the second tunnelling in the case of cover-to-diameter ratio (C/D) of 2 was 80% larger than that in the case of C/D of 4. The major influence zone on the pipeline induced by tunnelling is within 1.2D (tunnel diameter) ahead of and after the tunnel face. When the pipeline is located within the major influence zone due to tunnelling, there was a sharp increase in the relative pipeline-soil stiffness and the additional bending strain in the pipeline. The tests results were compared with the results from a dimensionless chart and an analytic solution in the literature. Although these two approaches can reasonably predict the bending strain in the pipeline due to the first tunnelling, they fail to accurately capture the incremental bending strain solely caused by the second tunnelling. This implies that one should be cautious in using the superposition principle to predict the bending strain in a pipeline subjected to twin tunnelling. Because the deviatoric strain of the soil around the pipeline was increased by the second tunnelling, leading to further degradation of the soil stiffness around the pipeline.
To study the instability and failure mechanism of tunnel face in composite stratum and the evolution law of supporting pressure in the areas with spring, this paper used two different types of ...transparent soil and a self-designed 3D model test system. Six large transparent soil model tests were carried out by considering different confined water heads of spring and tunnel burial depth. The optical laser and high-speed camera were controlled to move on a high-precision linear platform. CT scanning was performed to obtain the failure model under different conditions. The finite element method considering a two-way fluid–structure coupling was used to validate the model test. The research results indicate under spring, the support pressure curves can be divided into three stages: rapid decline, rebound-rise, and constant. There is no rebound-rise stage under the condition of no spring. With increased confined water heads or reduced tunnel burial depth, limit support pressure shows an increasing trend. 2D and 3D damage models for different working conditions were obtained by PIV technology and 3D reconstruction technology. If there is a spring, the maximum displacement moves to the top of the tunnel with the increase of water head and the failure mode is a combination of “silo shape” and “inverted prism”. When there is no spring, the maximum displacement appears at the interface of the soil layer, and the failure mode is a combination of “silo shape” and “wedge shape”. The presence or absence of springs and the change of the confined water head have no significant effect on the height of the loosening area. With the tunnel burial depth ratio of 0.5 to 2.0, the height of the loose area increases from 0.17 to 0.83 D, and the soil arch area develops outward.
•Soil deformation during tunnel excavation is conducted in heavy rainy weather.•Modified Green-Ampt model with assumption of soil stratification is adopted.•Complex variable approach is introduced to ...evaluate tunnel structural construction.•Analytical solutions for ground responses are compared with observed data.•Parametric analysis are performed for influence of sensitive parameters of rainfall.
The evaluation of influence of heavy rainfall on tunnel structural construction is a problem being faced by engineers around the world. Current analytical solutions on the tunnelling-induced ground movements are generally based on the normal sunny weather and provide little attention on the rainfall intensity and duration risk. In this paper, a time-dependent complex variable approach is proposed to estimate the deformation and stress of surrounding soils caused by tunnelling considering the rainfall infiltration mechanisms. The modified Green-Ampt model under the assumption of stratification is used to simulate the rainfall infiltration process and the complex function theory is employed to calculate the subregion mapping for the saturated, transitional, and natural layers. The non-uniform convergence deformation boundary is adopted at the tunnel structure opening so as to consider the displacement controlled excavation effects due to tunnelling. The analytical presented solution is then verified by comparisons with a total of three engineering case records. It is revealed that the presented complex variable solution considering the rainfall infiltration mechanism can provide a prediction of surface settlement, soil subsidence and horizontal displacement in good agreements with the field measurements. Finally, the parameters that affect the ground deformation and stress induced by tunneling in rainy day, including the rainfall intensity, saturated permeability, matric suction, initial water content, are studied. The paper contributes to provides a relatively quick and easy way of accurately evaluating the time-dependent feedback in tunnel-soil interaction system involving heavy rainfall as a potential risk in the preliminary design of tunnel structures considering the adverse weather condition.
The stability of excavation face in shield tunneling plays a key role for construction safety. The ignorance of soil anisotropy in most previous studies would induce inaccurate stability assessment. ...This paper studies the failure of shield tunnel face in cross-anisotropic granular media by physical model tests and discrete element simulation. Model tests were carried out on the tunnel face stability in anisotropic granular media, and initial anisotropy was generated by controlling the long axis of non-spherical particles. By conducting image analysis on the picture taken by HD camera, the failure mode of tunnel face was obtained. It consists of a sliding wedge and an overlying loosen area, and the inclination angle of sliding wedge varies with the bedding plane. The variation in limit support pressure with the intersection angle of the shield tunneling direction and the soil bedding plane was obtained. Discrete element simulation was further employed to study the tunnel face stability in cross-anisotropic granular media; the microscopic parameters were calibrated by fitting against the particle drop test and repose test. Clump particle consisted of three identical ball was used in the simulations, and its long and short axes were in accordance with rice particles. The obtained varying characteristic of limit support pressure with intersection angle from simulation is consistent with the test results, and the obtained failure mode is also similar to that of physical model test. The principal stress distribution at failure state was analyzed in the discrete element simulation, and the change of major principal stress direction from vertical to nearly horizontal in the loosen area clearly shows the formation of soil arches.
In the case of sudden surcharge loading, shallow shield tunnels in areas with soft soil experience substantial deformation responses. It is very important to understand the different loading modes ...and control measures above the shallow shield tunnels in soft soil for improving the safety of tunnel structure and reducing the influence of deformation. In this study, a three-dimensional numerical model of shallow shield tunnels in soft soil is established with FLAC. Ground and tunnel deformations are analyzed under different loading modes, and the effects of different deformation control measures are also studied. The numerical simulations in this paper show that the surface and tunnel deformation responses vary when induced by different loading modes above shallow shield tunnels in soft soil. After surface hardening with a 20-cm-thick layer of C20 concrete, the surface settlement is effectively controlled, and the uneven longitudinal settlement of the tunnel vault is improved. However, controlling the height of the surcharge is the most direct deformation control method. When the height of the surcharge is reduced from 6 m to 4 m and 2 m, the maximum ground settlement is reduced by 37.8% and 69.4%, respectively, and the maximum longitudinal settlement of the tunnel vault is reduced by 35.3% and 65.2%, respectively. During the operation of shallow shield tunnel in soft soil area, sudden surcharge loading should not be allowed. In the inevitable case, the surcharge loading on one side of the tunnel should be prevented and the surcharge loading height should be strictly limited.
Rock squeezing has a large influence on tunnel construction safety; thus, when designing and constructing tunnels it is highly important to use a reliable method for predicting tunnel squeezing from ...incomplete data. In this study, a combination SVM-BP (support vector machine-back-propagation) model is proposed to classify the deformation caused by surrounding rock squeezing. We design different characteristic parameters and three types of classifiers (a SVM model, a BP model, and the proposed SVM-BP model) for the tunnel-squeezing prediction experiments and analyse the accuracy of predictions by different models and the influences of characteristic parameters on the prediction results. In contrast to other prediction methods, the proposed SVM-BP model is verified to be reliable. The results show that four characteristics: tunnel diameter (
D
), tunnel buried depth (
H
), rock quality index (
Q
) and support stiffness (
K
) reflect the effect of rock squeezing sufficiently for classification. The SVM-BP model combines the advantages of both an SVM and a BP neural network. It possesses flexible nonlinear modelling ability and the ability to perform parallel processing of large-scale information. Therefore, the SVM-BP model achieves better classification performance than do the SVM or BP models separately. Moreover, coupling
D
,
H
, and
K
has a significant impact on the predicted results of tunnel squeezing.
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