超大直径泥水盾构在砂土中的开挖面稳定性分析

超大直径泥水盾构近年来在国内外得到越来越多的应用,其开挖面的稳定性受到高度关注,而泥水支护压力控制则是保证开挖面稳定的关键。对超大直径盾构在砂土地层中的开挖面稳定性进行三维数值分析,重点研究在不同摩擦角下开挖面的失稳模式和极限支护压力,揭示了由泥水容重导致的支护压力不均匀分布对于预测结果的重要性。同时,还将三维数值模拟结果与基于极限平衡理论的楔形体模型进行对比,指出楔形体模型对于分析超大直径盾构开挖面稳定性的局限性。本项研究可为超大直径盾构在砂土中掘进时合理确定泥水支护压力和保持开挖面稳定提供参考。     

砂土地层盾构隧道开挖面被动破坏极限支护力研究

 盾构掘进时推力过大极易造成地表隆起,导致开挖面前方土体被动破坏。基于莫尔–库仑屈服准则,采用大型有限元软件ABAQUS对砂土中盾构隧道开挖面的被动破坏进行模拟,得到开挖面的被动破坏支护力并揭示开挖面的被动破坏模式。进而,将传统的三维楔形体极限平衡模型中的棱柱体修正为具有一定倾角的倒棱台,使其破坏区域更为接近真实的土体隆起区域。推导开挖面被动极限支护力关于楔形体倾角的表达式,并通过试算,找出最小的支护力值即为被动破坏支护力。采用修正后的极限平衡模型得到的被动支护力与数值计算结果吻合良好,并且都较好的落在E. Leca等计算的上下限范围内。该模型为盾构隧道施工中确定支护力的上限值提供参考。     

浅覆盾构隧道开挖面挤出刚性锥体破坏模式极限分析

浅覆隧道盾构施工过程中可能引起开挖面挤出破坏。为研究开挖面破坏规律,借助已有刚性锥体破坏模式,推导隧道开挖面三维临界挤出压力表达式,建立极限分析上限法非线性规划模型并编程求解。利用该程序,研究了隧道埋深、地层参数等因素与开挖面临界挤出压力曲线关系及破坏模式的演化特征。结果表明,隧道埋深和土体内摩擦角对开挖面临界挤出压力及挤出破坏模式影响较大。发生挤出破坏时,开挖面附近和上方地表范围为主要破坏区域。最后,利用上限法和有限差分法对长沙地铁2号线长沙大道—体育公园站区间隧道某浅埋段进行开挖面稳定性分析,上限法与有限差分法所得结果相互印证,但上限法计算更为简便。     

高水压条件下泥水盾构开挖面稳定离心模型试验研究

开挖面稳定是越江跨海盾构隧道工程安全的关键,尤其是高水压条件下,开挖卸荷导致开挖面稳定控制更加困难。以越江跨海盾构隧道为背景,研制了一套包含材料和设备的高水压泥水支护形式的开挖面稳定模拟试验装置,通过大型离心模型试验研究了高水压下开挖面坍塌失稳破坏模式和土、水应力变化规律。研究结果表明:①高水压条件下开挖面失稳具有突发性,土体呈现由局部–整体形式急速发展破坏,极小的泥水压力变化幅度即可导致土体迅速发展为整体破坏并传至地表,失稳过程中可观测到滑移倾角减小、破坏范围扩张;②随着泥浆压力的降低,开挖面前方土压力呈现先减小后增大最终趋于稳定值,开挖面失稳可以划分为微观变形、局部破坏、土拱形成、整体失稳四个阶段;③开挖面发生主动破坏时,孔隙水压会发生突然降低现象,这是由于高应力条件下密砂具有剪胀效应,从而引起负孔压导致孔隙水压力急剧下降。这种孔压波动会对开挖面失稳带来不利影响,加速开挖面失稳进程、导致失稳区域的扩大。研究成果对越江海水下隧道工程具有指导意义。     

A tunnel face failure mechanism for layered ground,considering the possibility of partial collapse

A recent rotational face collapse mechanism is extended to compute critical pressures, in the context of the upper-bound limit analysis, for tunnels in layered (or stratified) ground. The mechanism can consider both (i) cases in which contacts between layers intersect the tunnel face; and (ii) cases in which they are located above the tunnel crown. In addition, the mechanism is extended to address partial collapse in a two-layered ground (with a softer-top and a stronger-bottom), without the need to scale mechanisms that were derived for global collapse. A 3D numerical model is employed to validate the predictions of the limit analysis mechanism, demonstrating that it provides, with a significantly reduced computational effort, good predictions of critical pressure, of the type of collapse (global or partial), and of its geometry. The mechanism is then employed to conduct parametric studies of the influence of several geometrical and mechanical parameters on face instability, considering the possibility of partial collapse, of tunnels in layered soils. Results show that a weak material in the upper section of the tunnel face can lead to a significant increase of the collapse pressure; it also makes a partial collapse possible. They also show that, when partial collapse is not critical and a constant face pressure is considered, the lower layer has a stronger influence on the computed values of collapse pressure. Finally, the results of the proposed mechanism are compared to a recent limit analysis solution for layered soils.     

PSO-based Elman neural network model for predictive control of air chamber pressure in slurry shield tunneling under Yangtze River

The excavation face stability is crucial for safety and risk management in slurry shield tunneling, especially for the river-crossing tunnel. To avoid face collapse or blow-out, shield operators need to keep air chamber pressure balanced using their own experience, which would be difficult, discontinuous and less reliable in the process of construction. Considering the disadvantage of the manual control process, this paper presents a predictive control system for air chamber pressure in slurry shield tunneling using Elman neural network (ENN) model. It mainly contains a theoretical model, an ENN predictor and an ENN controller to set optimal control parameters automatically tracking the desired air chamber pressure. Moreover, to improve the learning capability of ENN model, a particle swarm optimization (PSO) algorithm is implemented. This system has been tested with collected data of slurry shield operation parameters in the Yangtze riverbed metro tunnel project in Wuhan, China. Analysis revealed that the predictive control system using PSO-based Elman neural network model in this paper has the potential for enhancing face stability in slurry shield tunneling.     

非均质黏土地基隧道环向开挖面稳定上限分析

基于多块体极限分析上限法,推导了不排水条件下饱和黏性土地基中隧道环向开挖面稳定支护压力的计算公式。编制相应的计算程序,优化得到了隧道环向开挖稳定的最优上限解答。通过与已有极限分析上限解答的对比,验证了多块体极限分析上限法在隧道开挖稳定性分析的适用性。对照隧道开挖稳定的最优上限解的变化规律及相应破坏面的形状,详细分析了隧道埋深比、土体重度及强度非均质性对开挖面极限支护压力和因隧道开挖产生的滑动面范围和位置的影响。在此基础上,进一步基于隧道失稳的简化破坏模式推导了黏性土地基隧道开挖稳定的极限支护压力的简化上限解。通过与已有离心模型试验的对比验证,指出本文上限解可直接用于工程中初步确定开挖面支护压力,为隧道工程设计提供可靠的理论依据。     

双平行圆形隧道稳定的塑性极限分析上限解

从塑性极限分析上限法的基本原理出发，通过分析单圆形隧道4种类型垮落机制所获得的稳定率上限解，构建黏土层中双平行圆形隧道的垮落机制，阐述双平行圆形隧道稳定与垮落间的临界稳定分析过程，导出浅土层中双平行圆形隧道稳定率的上限方程。该方程根据隧道间不同距离，综合两隧道重叠（单隧道）、两隧道相接触、相互影响的双隧道以及互不影响的两单隧道的稳定特征。通过该方程进一步讨论土重对双隧道稳定率上限解的影响。最后，由离心模型试验结果证实其上限解的正确性。     

基于非线性M–C准则的深埋土质隧道三维塌落破坏上限分析

围岩条件较差时,深埋土质隧道在隧道开挖过程中容易发生塌方,准确预测深埋土质隧道塌方土体的范围极其重要,目前能预测深埋土质隧道塌落范围的理论研究不够成熟。为了提前预测土质隧道围岩顶部塌落体的范围,基于非线性Mohr–Coulomb准则和极限分析上限法,推导出深埋土质隧道在三维破坏机制下塌落体的上限表达式,得到了深埋土质隧道塌落体范围的精确解。通过数值软件Matlab绘制出了塌落体的三维形状,研究了各参数对深埋隧道塌落体形状的影响,并与既有研究进行对比分析,研究结果表明:土体中各参数、隧道顶部圆弧的半径和支护力对深埋土质隧道塌落体的范围影响比较大;基于非线性Mohr–Coulomb准则下深埋土质隧道塌落体的上限分析可以求解出有、无支护力条件下塌落体的高度和宽度,求解合理、可靠,并能给出防止深埋土质隧道塌方发生的支护力大小,可为隧道工程设计提供理论依据。     

地下水渗流作用下的盾构隧道开挖面安全系数上限分析

利用数值模拟技术对地下水渗流情况下盾构隧道开挖面的渗流场进行模拟,获得了隧道开挖面周边各节点的孔隙水压力。基于极限分析上限定理,利用各节点孔隙水压力计算出隧道开挖面上限破坏机制中的孔隙水压力功率,并将其视为一个外力功率代入虚功率方程中,构建出考虑渗流影响的开挖面安全系数目标函数。通过非线性序列二次规划法对该目标函数进行优化计算,得到开挖面安全系数上限解。利用强度折减法验证了该方法的有效性,并将其用于考虑地下水渗流作用的盾构隧道工程实例分析。研究表明:在考虑地下水渗流的情况下,开挖面安全系数随土体黏聚力、摩擦角、开挖面支护力的增大而增大,随地下水位的升高而减小;开挖面的破坏范围随摩擦角的增大而显著减小,但地下水位线的位置对开挖面的破坏范围影响较小。     

黏土地层隧道开挖面三维稳定性上限分析

对不排水均质黏土地基隧道开挖失稳三维破坏模式进行改进,弥补了目前隧道开挖稳定性问题中三维极限分析上限法的不足。采用截椭圆柱体构造多块体平动破坏模式,克服了现有三维多块体平动破坏模式与隧道开挖面不完全接触的缺陷;进一步针对三维多块体平动破坏模式存在的问题,提出了多块体转动–剪流组合破坏模式,得到了均质黏土地层中隧道开挖稳定的上限解,明显改善了现有的极限分析上限解,并与三维弹塑性有限单元法结果及文献中的离心试验结果进行对比,验证了上限分析方法的有效性。     

泥水盾构支护压力设定范围及其影响因素分析

与土压平衡盾构对掘进面的被动支护不同,泥水盾构是依靠液态介质实现对掘进面的主动支护。泥水支护的关键是选择合适的泥水和支护压力从而形成并维持泥膜的完整性。基于泥水劈裂(渗透破坏)和仓筒理论给出了泥水支护压力的上下限。结合静水压力、土体特性、盾构直径和覆土厚度等因素研究了支护压力区间特性(可设定范围)。研究表明:支护压力下限主要受静水压力和土体摩擦角的影响,其中静水压力起决定性作用。一般情况下,泥水压力设定可以取为静水压力+20 kPa;支护压力上限为泥水劈裂(渗透破坏)压力,主要受静水压力和覆土厚度的影响。增加覆土厚度可以提高地层的泥水劈裂(渗透破坏)抗力,从而改善地层的泥水支护特性,增大泥水支护压力区间长度。然而,增加静水压力只可以平移泥水支护压力区间,而不能使其增大。泥水支护压力区间长度还受土体摩擦角的影响,而其它因素影响较小。考虑泥水支护区间长度的影响,实施带压换刀的隧道覆径比不宜小于0.8～1.0。     

The study of seepage forces acting on the tunnel lining and tunnel face in shallow tunnels

In this paper, the seepage force problems arising from the flow of groundwater into a tunnel were studied. Firstly, the effect of seepage forces acting on the tunnel lining was studied for the case of shallow drainage-type tunnels and these results were compared with the lining stresses developed for waterproof-type tunnels. This model was then reviewed through a comparison with an actual case study of the Seoul Subway Line No. 5. Secondly, the effect of seepage forces on the tunnel face stability was studied. In this study, two factors were considered simultaneously. The first factor considered was the effective stresses acting on the tunnel face, calculated from the upper bound solution of limit analysis and the other factor was the seepage forces, calculated from a numerical analysis under a steady-state of groundwater flow conditions. Consequently, reasonable design concepts applicable to the design of tunnel lining and to the evaluation of the support pressure required for maintaining the stability of the tunnel face were suggested for underwater tunnels.     

考虑局部失稳的盾构隧道开挖面#br# 挤出破坏数值模拟与理论分析

通过数值模拟和理论分析,研究了盾构隧道开挖面挤出破坏机理。首先,利用有限元软件进行了数值模拟,揭示了隧道开挖面挤出破坏的局部失稳模式,得到了盾构隧道开挖面挤出破坏的局部失稳比和极限支护压力。此外,还进行了摩擦角和相对埋深对局部失稳比率和极限支护压力的参数敏感性分析。其次,基于极限分析方法,提出了一种考虑局部失稳模式的隧道开挖面挤出破坏新机制。新的被动破坏机理由五个圆锥台和作用在圆锥台上的分布力组成。通过对定义的角度和隧道开挖面局部失稳直径进行数值优化,计算得到挤出破坏模型的上限解,进而研究了摩擦角和相对埋深对局部失稳比的影响。比较了数值模拟和理论分析得出的局部失稳比结果。最后,将获得的被动极限支护压力与现有方法进行了比较,表明新的挤出破坏机理为极限支护压力提供了较为满意的预测结果。     

Upper-bound stability analysis of a plane strain heading in non-homogeneous clay

This paper investigates the undrained stability of a plane strain tunnel heading in cohesive soil, whose undrained shear strength is assumed to increase linearly with depth. Upper bound stability solutions for a practical range of parameters of geometries and soil conditions are found using the multi-rigid-block upper bound method. The upper bound solutions obtained from the multi-rigid-block mechanisms significantly improve the classical solutions and have good agreement with those of the finite element limit analysis when C/D is small. An improved simple collapse mechanism which intermix inhomogeneous deforming region and rigid blocks translation together is proposed based on the multi-rigid-block upper-bound analysis. An upper bound analytical solution is then obtained in view of the numerical results of both the multi-rigid-block collapse mechanism and the improved simple collapse mechanism. And it predicts the plane strain heading stability relatively accurate for shallow tunnels.     

Collapse mechanism of shallow tunnel based on nonlinear Hoek–Brown failure criterion

Collapse shape of shallow circular tunnel is derived using a new curved failure mechanism within the framework of upper bound theorem. Nonlinear Hoek–Brown failure criterion is adopted in the present analysis. With the consideration of supporting pressure, the energy dissipations of the new failure mechanism are calculated by employing integral technique. Equating the rate of energy dissipation to the external rate of work, the objective function is formulated, and is optimized with the variational approach. Numerical results are presented, and collapse mechanisms of shallow tunnel taking into account supporting pressure are obtained. The present results are compared with the previous solutions, and the agreement shows that the present method is valid. The effects of different parameters on the failure mechanisms are discussed, and a critical depth expression is proposed for classifying shallow and deep tunnels.     

泥水盾构泥膜渗透性及其对开挖面稳定性影响

泥膜是维持泥水盾构开挖面稳定的关键因素。通过渗透柱试验研究了加压泥浆向砂土渗透、并在砂土表面形成泥膜的行为,获得了泥膜渗透性与时间的关系。结果显示泥水盾构在开挖时,由于刀盘不断切削泥膜,开挖面上只能形成微透水的泥膜;当拼装管片时,刀盘停止转动,开挖面上则会形成难透水的泥膜。建立了在开挖面上设置微透水的泥膜的二维数值模型,通过瞬态渗流分析获得土体孔压的最大值。根据地层孔压计算了失稳区域内的渗透力,获得了不同泥浆压力下在泥膜表面和地层中的分配比例。当有效泥浆压力越大,孔压在泥膜和失稳区以外地层的下降幅度就越大,对维持开挖面稳定的贡献越小。基于极限平衡法,提出了泥水盾构开挖面极限泥浆压力的计算方法,结果显示目前工程中采用的方法高估了开挖面极限泥浆压力。     

黏土中隧道垮落的塑性分析和数值模拟

~~黏土中隧道垮落的塑性分析和数值模拟@谢骏$伦敦南岸大学!伦敦~~     

The stability of shallow circular tunnels in soil considering variations in cohesion with depth

This paper presents an upper bound investigation of the three dimensional stability of a tunnel face in a deposit of soil whose strength varies with depth. The upper bound theorem of limit analysis incorporating the linear variation of the soil cohesion with depth was used to calculate the pressure at the tunnel face of a closed face excavation. For an open face excavation, the factor of safety against the tunnel face instability was calculated using the strength reduction technique and the upper bound theorem. The results, in terms of the minimum required face pressure, were then compared with other solutions available from the literature for verification, and the numerical results in the form of dimensionless design charts are also presented. In addition, a comparative study between the simplified approaches adopting a singular soil cohesion parameter representing the whole layer instead of considering its actual variation with depth is presented. It was concluded that adopting the mean soil cohesion that does not vary with depth would lead to a conservative design, that is, a higher minimum face pressure being required during construction and a lower factor of safety against face instability. However, adopting the local cohesion obtained from the tunnel face may result in underestimating the required face pressure and may lead to an unsafe design.     

Face stability analysis of shallow circular tunnels in cohesive–frictional soils

The face stability of a circular tunnel in cohesive–frictional soils was numerically and theoretically investigated. Three-dimensional (3D) numerical simulations were first performed to analyze the face stability of a circular tunnel with a series of tunnel diameter-to-depth ratios and soil properties. The limit support pressure on the tunnel face and the failure zone in front of the tunnel face were both obtained from the numerical simulations. A simple and feasible criterion was suggested to outline the boundary strip of the failure zone at collapse in displacement clouds under different conditions. Based on the numerical simulation results, a new 3D failure mechanism was proposed using the kinematic approach of limit analysis theory to determine the limit support pressure of the tunnel face. The new 3D failure mechanism was composed of four truncated cones on which a distributed force acts. Finally, the limit support pressures and failure zones obtained from the new failure mechanism and the numerical simulations were compared. In addition, comparisons between the results of this work and those of existing approaches were performed. Overall, the new failure mechanism is substantially more consistent with the shapes of the failure zones observed in numerical simulations and experimental tests than the existing multi-block failure mechanisms. The new failure mechanism is more effective and reasonable.