Investigation of response of metro tunnels due to adjacent large excavation and protective measures in soft soils

The inevitable influence of large excavation in soft soils on nearby tunnels is of great concern in practice. In this paper, the influence of a nearby large excavation on existing metro tunnels of the Ningbo Metro Line 1 in sensitive soft soils is investigated and presented. Considerable displacement in the left tunnel closer to the excavation induced by the nearby excavation was revealed by field monitoring. Visible cracks and leakages were observed in left tunnel linings. Three dimensional numerical simulations are conducted to investigate the responses of the ground and left tunnel due to the adjacent excavation. The development of bending moment and displacement of the left tunnel during different construction stages of the nearby excavation is obtained. Then the interaction mechanism between the nearby excavation, surrounding soils and existing twin tunnels is investigated, which is of significance to the interpretation of the influence of the nearby excavation on the existing twin tunnels. Several protective measures for alleviating the influence of adjacent excavation on left tunnel are studied, including divided excavation, soil improvement and a cut-off wall. It is found that the left tunnel is influenced to varying degrees during different construction stages and the time effect is distinct for this large excavation in soft soils, which would be suggestive to engineers to pay more attention to the protection of adjacent tunnel during the crucial construction stages. The bending moment and displacement of the left tunnel is strongly related to the unloading effects and displacement of surrounding soils, which can be alleviated by means of proper improvement of excavation sequence. Comparatively, longitudinally divided excavation is more effective in protecting the left tunnel than soil improvement or a cut-off wall. This study is of certain reference value for protecting metro tunnels adjacent excavation in soft soils.     

Evaluation of deformation response for adjacent tunnels due to soil unloading in excavation engineering

A major challenge in the design and construction of soil excavation and foundation pit engineering in urban areas is the protection of adjacent underground structures, such as existing tunnels. Excavation-induced soil unloading can adversely affect and even damage the tunnels in the vicinity. A simplified analytical approach is presented to analyze the deformation response for adjacent tunnels due to excavation-induced soil unloading in excavation engineering. Firstly, the green soil unloading stress due to adjacent excavation is estimated at the existing tunnel location. Secondly, the deformation response of the tunnel subjected to green soil unloading stress is calculated by the Galerkin’s method, which can be used to obtain the finite element equation converted from the differential equation. The accuracy of the proposed method is verified by comparisons with 3D finite element numerical simulation, centrifuge model tests provide by Kusakabe et al. (1985) and measured data in situ. Finally, the parametric analysis for deformation influence factors of the existing tunnel, including the tunnel buried depth, the distance from the excavation site, the soil geo-characters and the outer diameter of the tunnel, is presented to demonstrate the performance of the proposed method. This proposed method may provide certain basis to make protective measures of existing tunnels influenced by excavation engineering and enables a quick estimate of the deformation behavior of excavation-induced adjacent tunnels, resulting in savings in time and costs.     

砂土中隧道施工引起近接隧道位移的试验研究

新建隧道施工对近接隧道的影响,一直是隧道设计和施工中必须考虑的问题,但现阶段对此还没有一致的认识。为了进一步研究新建隧道施工对近接隧道影响机理,设计了能够模拟隧道分步开挖的室内模型试验装置,并用该试验装置进行了砂土地层中新建隧道施工引起近接水平平行隧道位移的试验研究。试验重点研究了两平行隧道间净距、埋深对既有隧道位移的影响。通过对试验结果分析,得到了既有隧道拱顶和靠近新建隧道一侧拱腰部位的位移发展规律。试验结果分析表明,新建隧道开挖会对既有平行隧道结构产生侧向挤压效应,隧道埋深和净距对既有隧道的位移都有影响,但隧道间净距变化对位移量的影响更大。当隧道间净距大于2倍隧道直径时,新建隧道开挖对既有水平平行隧道位移的影响非常小。研究成果可为砂土地层中近距离平行隧道设计和施工提供参考。     

既有隧道在上覆基坑卸荷下的形变响应简化算法

基坑开挖将打破土体原有应力场,对下卧隧道产生不利影响。鉴于此,提出一计算既有隧道在上覆基坑卸荷影响下的形变响应简化计算法。首先,利用搁置于Pasternak地基上的Euler-Bernoulli梁来模拟既有隧道,分析中,采用可考虑隧道埋深影响的地基反力系数;其次,借助于Mindlin解给出基坑开挖所引起的隧道轴线位置处的土体自由位移;然后,再结合隧道–地基土的位移耦合条件建立起隧道的变形控制方程;最后,通过有限差分法给出基坑开挖引起的下卧既有隧道的纵向响应解答。通过与三维有限元和既有理论解的对比验证了所提方法的准确性和适用性。接着对不同因素对隧道形变响应的影响做了参数分析,包括:隧道埋深、基坑–隧道相对空间位置、基坑开挖深度、基坑几何形状、隧道–地基土的相对刚度等。在此基础之上,进一步提出下卧隧道在上覆基坑开挖卸荷影响下的最大竖向位移简化预测公式。上述研究结论可为类似工程提供一定理论支持。     

基坑施工邻域隧道围土应力路径演变规律研究

为研究基坑开挖过程中邻域既有隧道周围土体应力路径演变规律,采用Mindlin经典理论,求解基坑底部和四周侧壁卸荷效应在隧道围土各点引起的附加应力场,并与初始应力场相叠加可得各点的现有应力场与应力路径,选用Mohr-Coulomb强度准则作为土体破坏控制标准以判断隧道围土各方向土体安全度。研究表明 :(1)随基坑开挖深度增加,隧道围土各点应力路径呈现靠近主应力破坏线K_f的演变趋势,其中隧道顶部与靠近基坑侧土体处于易破坏状态,隧道底部与远离基坑侧土体处于较安全状态。(2)沿隧道轴向,基坑开挖对邻域既有隧道围土应力场影响以一倍、二倍基坑开挖宽度为界线。一倍基坑宽度内为强影响区,一倍至二倍基坑开挖宽度内为过渡区,二倍基坑宽度外为弱影响区。(3)基坑与隧道净距越大,基坑开挖对隧道顶部和靠近基坑侧土体应力路径影响越小,该方向计算点处土体应力路径演变规律越趋于相似和稳定。     

堆土加卸载与基坑开挖叠加对既有隧道变形的影响研究

针对堆土加卸载与基坑开挖叠加效应导致既有地铁隧道变形较大的问题,建立考虑加卸载叠加效应影响的三维空间分析模型,研究不同堆土加卸载叠加基坑开挖卸载模式对邻近地铁隧道变形规律的影响,探讨隧道在堆土加载、移土卸载再叠加基坑开挖下的变形规律。结果表明:正上方堆土加卸载对隧道的竖向位移影响较大,是侧向堆土加卸载的3倍～5倍; 在经历堆土加卸载后,隧道会残留不可忽视的变形,其残留竖向位移约为加载后位移的62%; 堆土加卸载叠加侧方基坑开挖时,隧道变形受基坑开挖深度的影响较大,大于隧道埋深的开挖阶段会加剧隧道变形; 4种叠加模式中,正上方堆土加卸载-侧方基坑开挖卸载隧道最终竖向位移最大,约17 mm,侧方堆土加卸载-异侧基坑开挖卸载隧道最终水平位移最大,约8 mm,邻近隧道施工时应充分考虑叠加效应的影响,尽量避免这两种情况。     

Finite element analysis of tunnel–soil–pile interaction using displacement controlled model

Significant additional loads could be induced in pile foundations adjacent to new tunnels. Accurate prediction of magnitude and shape of the ground displacements, which define curvature changes, is crucial for the computation of tunnelling induced bending and axial stresses in pile foundations. The finite element simulation of tunnelling by removing forces corresponding to initial stress-state, tend to predict incorrect shape of ground displacement profiles, hence incorrect forces in pile foundations adjacent to tunnels. To overcome this difficulty, this paper describes the development and application of a simple and useful displacement controlled model (DCM) to predict the effects of tunnel excavation on adjacent pile foundations. The DCM simulates tunnelling by applying displacements to the tunnel boundary. A method to determine magnitude and direction of tunnel boundary displacements, based on convergence patterns observed in field and centrifuge test results, is proposed. Back analyses of numerous greenfield tunnel case histories using the DCM indicate good agreement between computed displacement profiles and field/test data. The suitability of the DCM in modelling tunnel–soil–pile interaction problems is demonstrated through back analysis of a centrifuge test and a field case study.     

基坑开挖对邻近隧道纵向位移影响的计算方法

基坑开挖卸荷会对邻近隧道产生影响,因此有必要对隧道的变形进行预测,确保隧道正常运行.针对目前计算模型的分析方法未考虑基坑壁应力卸荷对隧道位移的影响,以及有限元分析过程较为复杂繁琐,提出采用Mindlin解计算基坑壁与坑底卸荷的附加应力.然后将隧道结构视为弹性地基无限长梁,将开挖引起的附加应力施加于隧道结构上,建立隧道结构纵向变形方程,从而得到隧道位移及内力的计算公式.最后,将计算方法与数值模拟算例、工程实测进行对比分析,计算结果与其较为吻合.     

地铁隧道开挖过程三维物理模型动态试验研究

据相似材料大比尺三维概念物理模型理论,高仿真度制作了成型盾构隧道地铁站厅隧道开挖试验物理模型。模拟了双盾构隧道之间开挖站厅隧道的全过程,研究了站厅隧道、横通道开挖过程中模型地表沉降,土体的变形,盾构隧道及站厅隧道洞周特征点的位移。模拟并研究了混凝土加固桩对土体位移的影响,总结出土体的动态力学特征和变形规律,得出一些有益的认识和结论。对指导地铁施工、研究在已建成双隧道间拓展开挖站厅空间具有十分重要的意义。     

基坑开挖引起邻近盾构隧道转动与错台变形计算

基坑开挖会对邻近盾构隧道的变形产生显著影响。考虑基坑开挖时卸载在隧道轴线处产生的附加应力,建立了综合考虑剪切错台变形和刚体转动变形的隧道变形计算模型,结合最小势能原理推导出隧道的纵向变形量、环间错台量、环间转角和环间剪切力的计算公式。选取两组工程实例进行分析,将计算结果与实测数据进行比较。研究结果表明:邻近并平行于盾构隧道的基坑侧壁的卸荷效应对旁边隧道的影响最大;隧道水平位移最大值附近的管片基本不发生错台变形和刚体转动变形,环间剪切力值也很小;而在隧道水平位移曲线的反弯点处,隧道的剪切错台量、环间转角和环间剪切力值达到最大;在基坑开挖工况下,邻近隧道的水平位移变形模式以剪切错台为主,刚体转动为辅。     

基于HSS模型的基坑开挖对近邻地铁影响数值分析

随着城市轨道交通网络的不断扩大,城市深基坑工程大多邻近地铁区域.基坑开挖将导致周围土体产生附加位移,当土体位移过大时,就会对邻近结构物产生不利影响,因此开展基坑开挖对邻近地铁隧道区域和车站安全性影响的分析研究具有重要的现实意义.以天津某地铁车站明挖基坑工程为例,运用有限元软件Plaxis 3D分析基坑开挖对邻近隧道区间...     

下穿隧道对邻近桩基承载力与沉降的影响

总结了下穿隧道开挖对邻近桩基产生的影响和小孔扩张-收缩理论的相关研究,分析了两阶段分析法、整体分析法和室内模型试验3种常见研究隧道-桩基相互作用方法的优缺点,同时在总结岩土介质小孔扩张-收缩理论研究及其在桩基工程和隧道工程中应用的基础上,指出了现有小孔扩张-收缩理论在研究浅埋隧道开挖中的不足。提出一种关于浅埋隧道-桩基相互作用的求解思路,即考虑地表效应的影响,采用有限介质小孔扩张-收缩理论进行求解。给出了该方法的初始计算模型和计算流程图,并将理论计算所得各阶段土体的位移场与离心模型试验结果进行对比,验证了方法的可行性,最后基于此计算模型给出了各阶段土体的应力场及变化量。结果表明:小孔扩张-收缩理论可以考虑三维隧道开挖对邻近桩基的影响,为解决隧道-桩基相互作用提供了新的解决思路。     

深基坑开挖与邻近隧道相互影响的分析

采用三维快速拉格朗日方法(FLAC3D)模拟软土地区深基坑的分步开挖和支护过程。根据隧道与基坑不同位置关系,分为"紧贴型"和"浅埋型"两类、共7种工况,分析基坑分步开挖对邻近地铁隧道变形的影响以及隧道对基坑连续墙变形和墙后地表位移的影响。数值分析土体采用修正剑桥模型,考虑了连续墙和土体的接触滑移作用。数值分析结果发现隧道变形大小与基坑距离关系并不完全单调,隧道的存在对基坑墙后土体有明显的"加筋效应"。     

Ground movement induced by parallel EPB tunnels in silty soils

When constructing tunnels with poor geotechnical conditions in densely populated urban areas, there are many challenges including intolerable ground movement, face failure, and potential damage to adjacent structures (i.e., tunnels, piles, and pipelines). Earth pressure balanced (EPB) shields have been widely used to solve these problems. However, tunnel excavation causes release of in situ soil stress, which results in the soil movement. This paper focuses on field measurements of parallel tunnels using EPB shields in silty soils. Specifications on the ground profile, construction procedure, and field monitoring of pore pressure in the soils, ground subsidence, subsurface settlement, and horizontal displacement are reported. During shield advancement, the pore pressures in the soils showed the zigzag-shape distribution along the distance. The settlements indicated upheaval-subsiding behavior in the longitudinal direction. The soil settlement decreased from the crown of the excavation face to the ground surface and to the invert of the excavation face in the transverse direction. Outward horizontal displacements of soils adjacent to the tunnels and inward horizontal displacements of the soils near the ground surface were also observed before the tail injection. The second tunnel excavated rendered a slight squeezing effect on the first tunnel. These satisfactory measurements indicate the effectiveness of the EPB technique in reducing potential damage to adjacent structures.     

近距巷道平行开挖合理错距研究

矿井下运输巷道是矿业生产的重要通道。文中针对极松软地层近距双水平巷道掘进问题,以内蒙古上海庙矿区为工程背景,依据工程区地应力场的实测结果及岩石力学实验参数,运用FLAC^3D三维数值分析计算软件对开拓巷道的合理错距进行了数值模拟研究。通过分析巷道周边应力分布、塑性区变化、位移速率矢量场方向变化以及监测位移数据变化,研究了巷道在分步开挖过程中,不同的开挖错距对已开拓巷道监测端面的影响。结果表明在极松软地质条件下,巷道围岩周边高应力区分布范围较大,新开拓的巷道会对相邻已开拓巷道周边的应力、塑性破坏区及位移速率矢量场产生影响,双近距水平巷道合理开拓错距宜保持在50 m以上。     

软土地区紧邻地铁隧道深基坑支护设计与实践

以上海竹园2-16-1地块项目深基坑工程为背景,介绍了邻近地铁的软土深基坑变形控制方法及其效果。根据基坑工程的特点,设计时采取了多种地铁保护专项技术措施,包括基坑分区实施方案、支护体系、钢支撑轴力补偿系统、坑内被动区加固、承压水控制措施等。结果表明:基坑各分区地下连续墙最大侧向位移小于上海软土地区基坑地下连续墙最大侧移的统计平均值0.42%H(H为基坑最大开挖深度),特别是靠近地铁侧的地下连续墙最大侧向位移接近上海软土地区基坑地下墙最大侧移的统计下限值0.1%H; 地铁侧坑外承压水位总体保持在比较平稳的水平,最大水位变化仅为0.72 m; 邻近的地铁隧道上行线和下行线的累计最大沉降量分别为8.2 mm和5.1 mm,均小于地铁下沉量允许值(20 mm),且隧道曲率半径满足控制值要求; 本基坑采用的系统变形控制措施有效地保障了邻近地铁的安全,其设计和施工方法可以为软土地区同类基坑工程设计提供参考。     

Three-dimensional numerical simulation of a mechanized twin tunnels in soft ground

The increase in transportation in large cities makes it necessary to construct of twin tunnels at shallow depths. Thus, the prediction of the influence of a new tunnel construction on an already existing one plays a key role in the optimal design and construction of close parallel shield tunnels in order to avoid any damage to the existing tunnel during and after excavation of the new tunnel.Most of the reported cases in the literature on parallel mechanized excavation of twin tunnels have focused on the effects of the ground condition, tunnel size, tunnel depth, surface loads, and relative position between the two tunnels on tunnel behaviour. The numerical investigation performed in this study, using the FLAC^3D finite difference element programme, has made it possible to include the influence of the construction process between the two tunnels. The structural forces induced in both tunnels and the development of the displacement field in the surrounding ground have been highlighted.The results of the numerical analysis have indicated a great impact of a new tunnel construction on an existing tunnel. The influence of the lagged distance between the two tunnels faces has also been highlighted. Generally, the simultaneous excavation of twin tunnels causes smaller structural forces and lining displacements than those induced in the case of twin tunnels excavated at a large lagged distance. However, the simultaneous excavation of twin tunnels could result in a higher settlement above the two tunnels.     

考虑隧道剪切效应的基坑开挖对邻近隧道纵向变形分析

 基坑施工将打破周围地层已平衡的应力场,引起应力释放,对既有下卧地铁隧道产生不利的影响。针对既有研究的不足,提出考虑隧道剪切效应的基坑开挖对下卧隧道影响的解析解。既有隧道简化为搁置于Winkler地基上的Timoshenko梁。通过两阶段分析法,考虑基坑卸荷作用下已建隧道的响应。首先基于Timoshenko梁理论,建立考虑隧道剪切效应的隧道纵向变形微分方程,然后将由Mindlin弹性解计算得到附加荷载施加于既有隧道上,最后通过有限差分法得到在附加荷载作用下隧道的纵向变形解答。收集3个已发表工程实例的实测数据,并与本文方法及Euler-Bernoulli梁法的计算结果进行对比分析,发现实测结果与2种方法得计算结果有较好的一致性。然而,在内力分析上,相对比于本文方法,Euler-Bernoulli梁法明显高估基坑卸荷引起的隧道弯矩与剪力。由于本文方法可有效模拟既有隧道剪切效应,因而可进一步给出1在卸荷作用下隧道管片间错台量。研究成果可为合理预测邻近基坑施工对既有隧道的影响提供一定的理论支持。     

曲线隧道盾构引起地表沉降分析

以某市地铁工程双曲线隧道区间为研究背景,建立了三维有限差分数值模型.通过数值分析得出盾构引起地表沉降的范围、地层水平移动规律以及双线隧道之间的相互影响.曲线隧道施工不仅在地表内侧引起的沉降范围较大,还影响了隧道周围土体的水平移动趋势.随着曲线半径的减小,隧道两侧沉降范围的差别逐渐增大,外侧土体的水平移动趋势向内侧扩展;随着隧道埋深的增加,沉降范围也逐渐增大,沉降值却不断减小.     

软土地区盾构上穿越既有隧道的离心模拟研究

随着地铁网络不断完善,新建盾构隧道近距离穿越既有隧道的现象越来越多。盾构近距离穿越既有隧道的影响问题,比常规盾构施工的研究更为复杂。结合上海外滩通道盾构上穿越地铁 2 号线工程,采用离心模型试验与现场实测相结合的方法对盾构上穿越对周围地层及既有隧道的影响进行了研究。文中选用排液法在离心场中模拟盾构施工,在国内首次实现了在不停机状态下模拟隧道开挖卸载、地层损失和注浆过程,并分析了盾构上穿越施工引起的地层、新建隧道与既有隧道的纵向位移变化规律。通过现场实测数据分析了既有隧道在盾构上穿越过程中纵向变形与时程曲线的变化规律。