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.     

Displacement characteristics of high-speed railway tunnel construction in loess ground by using multi-step excavation method

The sequential excavation method (SEM), in which the tunnel face is partitioned into several temporary drifts, can promote face stability and reduce ground displacement. Three-bench seven-step excavation method (TSEM) as a kind of the SEM, which can stabilize the tunnel face but does not require any additional support, has been widely used in loess tunnels with large cross-sections for high-speed railway (HSR) in China. This study focuses on the effects of TSEM on the displacement characteristics for HSR tunnels in loess ground. A series of in-situ tests and numerical simulations were conducted to reveal displacement characteristics and obtain optimal construction approaches for large-span loess tunnels in China. The strata around the loess tunnels have undergone significant settlement either at the arch part or at the ground surface due to loess’s metastable structure especially in the new loess ground. Upper strata above loess tunnels subsided as a whole, and the subsidence developed suddenly and devastatingly. Large residual ground surface settlements still occurred after the ring closure of the primary support even after the application of the upper lining. The best construction approach of TSEM was proposed for its potential to limit surface and subsurface ground displacements, as revealed both by numerical simulation and field monitoring.     

浅埋暗挖风道进主体施工技术优化分析

地铁开挖过程中引起的地表沉降对周边建筑及道路有很大的影响,浅埋暗挖风道与主体交叉段的施工是工程中的重难点工序,为控制施工过程中地表沉降量,并对施工进行指导,做出以下研究:以长春地铁解放大路换乘工程为依托,采用FLAC-3D程序数值模拟的方法对CRD风道转入PBA工法车站主体进洞施工进行优化分析;通过对比双拱挑高进洞方案、加强环梁进洞方案与直接进洞方案在风道和主体开挖阶段地表沉降量,发现在风道开挖阶段,双拱挑高进洞方案沉降量较小,而主体开挖阶段,加强环梁进洞方案沉降量较小,得到如下结论:(1)双拱挑高进洞方案在风道开挖阶段对地表沉降控制较好;(2)加强环梁进洞方案在主体开挖阶段对地表沉降控制较好。     

地铁隧道施工拱顶下沉值的分析与预测

由于变形监测的滞后，在城市地铁隧道施工中所测到的拱顶下沉通常仅为总下沉量的一部分，并且其所占的比例在不同地层条件下具有较大差异。而隧道拱顶的下沉过程及其最终下沉量则是隧道支护设计及地层环境控制的重要基础。通过对深圳地铁一期工程部分区间隧道拱项下沉的回归分析认为，隧道拱项下沉过程遵循指数函数关系，由此可对未量测部分的拱顶下沉量进行预测。通过数值模拟验证了该分析结果的可靠性，并且开挖面前方的超前拱顶下沉量较大。结合隧道开挖应力释放及地下水流失造成的地层失水效应，分析了不同地层条件下拱顶下沉与地表沉降的关系，较好地解释了深圳地铁一期工程建设中拱顶下沉与地表沉降较大而且两者之间呈线性关系的特点，这是由于现场监测滞后以及地层中富水含细砂所致。该成果对城市地铁隧道暗挖施工、支护以及地层环境的控制设计都具有重要的参考价值。     

遮拦叠交效应下地铁盾构掘进引起地层沉降分析

 地铁隧道施工诱发的土体沉降以及临近地下构筑物变形是我国城市轨道交通施工安全控制和风险评估中较为关心的一类施工问题。目前,针对该领域地层沉降的简化理论研究还仅仅针对自由位移场,没有考虑临近既有构筑物的遮拦效应影响。依托上海在建地铁施工工程实践,采用简化理论方法、三维有限元数值模拟方法以及现场监测方法,分析考虑运营隧道遮拦效应影响的土压平衡盾构施工引起的周围土体沉降规律,并与自由位移场条件下盾构施工引起的地层变形进行对比分析;在此基础上,给出地铁盾构复杂叠交穿越引起的临近地铁隧道的变形规律。研究表明,本文提出的简化理论方法和三维有限元数值模拟方法可以较好地模拟遮拦叠交效应下地铁盾构掘进引起的地层沉降变形;临近既有建(构)筑物施工,盾构施工引起的周围土体沉降较大程度地受到遮拦效应影响,与自由位移场条件下的计算结果对比存在较大差别。最后,结合盾构施工监测数据,提出复杂遮拦叠交效应下的盾构叠交施工变形控制技术措施。成果可为合理制定施工场地存在复杂建(构)筑物工况条件的地铁隧道开挖对周围环境保护措施提供一定的理论依据。     

Ground surface settlements due to construction of closely-spaced twin tunnels with different geometric arrangements

This paper examines the ground surface settlement profiles due to the construction of closely-spaced twin tunnels using the shallow tunnelling method. In the concerned zone, where the twin-tunnelling was performed in stacked and offset arrangements, the ground surface settlements of in total 18 cross-sections were continually recorded during construction. To cater for different conditions of the twin tunnels in the concerned zone, partial face, full face and forepoling reinforcement schemes were adopted. The recorded surface settlements and settlement troughs of three typical sections are reported and illustrated. The surface settlement troughs induced by each of the twin tunnels are fitted by the Gaussian function. The parameters that characterize the surface settlement troughs induced by each of the twin tunnels, such as the maximum settlement, percentage of ground loss, trough width and empirical trough width parameter are presented and compared.     

Effect of pipe characteristics in umbrella arch method on controlling tunneling-induced settlements in soft grounds

Recent developments in tunneling have stimulated design practitioners to more effectively utilize the underground spaces. However, tunneling at shallow depth in soft grounds gives rise to concerns associated with tunnel instability. Umbrella arch method (UAM), as a pre-reinforcement approach of tunnels in complex geological conditions, is widely used to maintain the tunnel stability. Quantitative assessment of the impacts of the entire approach and forepoling pipe features on tunnel stability remains challenging due to the complex nature of the UAM application. This study aimed to assess the effect of pipe design parameters on reinforcing the tunnels excavated in soft grounds. This practical investigation considered the actual field conditions attributed to the tunneling procedure and UAM deployment. Then, the tunneling process was modeled and the tunnel excavation-induced settlements were calculated. The post-processed results confirmed that deploying the UAM substantially reduced the tunnel crown and ground surface settlements by 76% and 42%, respectively. Investigation of various design parameters of pipes underscored the significance of incorporating the optimum value for each individual parameter into design schemes to more effectively control the settlements. Additionally, contrasting the settlement reduction rates (SRRs) for pipe design variables showed that the tunnel stability is more sensitive to the changes in the values of diameter and length, compared to values of the installation angle and center-to-center distance of the pipes.     

重叠隧道的施工力学研究

由于地下空间的不断利用，出现了重叠隧道。这种形式布局的隧道在第1孔隧道完成之后，引起了地层内初始应力的变化，而且由于地层中已有一个相邻的已衬砌隧道，地层的刚度条件不再与单孔隧道的垂直轴对称，第2孔隧道的修建又要对已建的第1孔隧道发生影响。应用ANSYS程序对深圳地铁1期工程中的3种典型的重叠断面的计算表明：重叠隧道地表和拱顶下沉及两隧道间土体的稳定难于控制，上面第1孔隧道易发生结构整体下沉。     

Finite difference analysis of an advance core pre-reinforcement system for Toulon's south tube

The stability of shallow tunnels excavated in full face has been a major challenge to the scientific community for a long time.In recent years,new techniques based on the installation of a prereinforcement system ahead of the tunnel face were developed to control the deformations and surface settlements induced by the excavation and to ensure the sustainability of the tunnel in the long term.In this paper,a finite difference numerical simulation was conducted to study the behaviors and effects of two pre-reinforcement systems,i.e.the face bolting and the umbrella arch system installed in a section of southern Toulon tunnel in France.For this purpose,two approaches were taken and compared:a two-dimensional(2D) approach based on the convergence-confinement method,and a three-dimensional(3D) approach taking into account the complete modeling of the tunnel.A 2D numerical back-analysis was performed to identify the geomechanical parameters that offer satisfactory agreement with the measurement results.The limit of this method lies in the exact choice of the stress relaxation ratio a.To overcome this uncertainty,a 3D model was developed,which permitted to study the influence of different pre-support systems on the reaction of ground mass.Both 2D and 3D numerical approaches have been fitted to measurements recorded in a section of the Toulon tunnel and the very satisfactory correspondence has allowed validating the simulations.The results show that the 3D numerical analysis with a full discretization of the inclusions seems unquestionably the most reliable approach.     

Effects of twin tunnels construction beneath existing shield-driven twin tunnels

This paper presents a case of closely spaced twin tunnels excavated beneath other closely spaced existing twin tunnels in Beijing, China. The existing twin tunnels were previously built by the shield method while the new twin tunnels were excavated by the shallow tunnelling method. The settlements of the existing tunnels and the ground surfaces associated with the new tunnels construction were systematically monitored. A superposition method is adopted to describe the settlement profiles of both the existing tunnels and the ground surfaces under the influence of the new twin tunnels construction below. A satisfactory match between the proposed fitting curves and the measured settlement data of both the existing tunnels and the ground surfaces is obtained. As shown in a particular monitoring cross-section, the settlement profile shapes for the existing tunnel and the ground surface are different. The settlement profile of the existing structure displays a “W” shape while the ground surface settlement profile displays a “U” shape. It is also found that due to the flexibility of the segmental lining, the ground losses obtained from the existing tunnel level and the ground surface level in the same monitoring cross-section are nearly the same.     

地下隧道施工对邻近建筑物影响的研究现状

地下隧道的开挖必须以地表建筑物的正常使用为前提,过大的地表沉降会引起建筑物的开裂、沉降和倾斜等问题,对建筑物安全使用造成严重的影响。在总结前人研究的基础上,对盾构隧道和暗挖隧道施工对邻近建筑物影响的研究方法和成果进行了回顾和总结,提出了需要进一步研究的课题和研究思路。     

基于数值模拟的北京地区地下道路立交隧道围岩变形分析

城市地下道路立交隧道施工过程中围岩变形分析是工程建设中的核心问题。依托北京城市地下道路工程,采用城区地质条件分层概化方法、三维数值模拟方法分析了临时仰拱台阶法（台阶法）、中隔壁法（CD法）、交叉中隔壁法（CRD法）和双侧壁导坑法（双侧壁法）施工影响下的立交隧道地表沉降、洞周变形和塑性区分布特征,明确了立交区域上下层隧道施工的影响规律。研究发现,立交区域同时受到了上下层隧道施工的影响,在地表形成椭球形的沉降盆,在洞周出现了较大的差异沉降|在地表地层和洞周拱脚、拱肩截面的“X”形区域出现了较为集中的破坏区|CRD法和双侧壁法在控制立交隧道地层变形和破坏方面具有明显优势。研究成果可为北京城市地下道路立交隧道设计、施工决策制定提供参考。     

软土地区地铁盾构区间隧道近接桩基数值分析

拟建的上海市轨道交通11号线区间盾构隧道近距离下穿交大海洋重点工程实验室群桩基础。通过采用莫尔-库仑弹塑性屈服准则,建立有限元数值模型。依据数值模拟结果,先施工左侧隧道与先施工右侧隧道对于桩基础的沉降影响不大;在右侧隧道施工后,隧道埋深处桩体的竖直应力变化最大,最大值为367 kPa,桩体水平应力在与隧道同一深度处变化较大,最大值为9.6 kPa;单桩的最大差异沉降为6.8 mm,按照桩基设计规范,不需要采取加固措施减便可确保建筑物基础的安全可靠;地表的最大沉降值为18.6 mm;由于双线隧道的运营导致的地面附加沉降为1.5 mm。     

地铁竖井施工对周边建筑影响及加固措施

竖井施工不可避免扰动地层,引起地铁隧道周边建筑物沉降,影响其安全和正常使用。结合某地铁区间隧道,分析竖井施工引起的周边建筑物的变形特点、原因及规律。以邻近某建筑为例,提出开挖掌子面进行全断面封闭措施;隧道内超前深孔帷幕注浆;隧道台阶法开挖及时进行支护,并向初期支护背后注浆回填、径向注浆以及地面注浆加固技术措施,对建筑物...     

Ground response of closely spaced twin tunnels constructed in heavily overconsolidated soils

As part of the City of Edmonton’s light rail transit expansion, twin 6.5 m diameter oval shaped tunnels were constructed using conventional tunnelling methods. The geology of the site consists predominately of a hard, fissured cohesive till unit. The tunnel face construction was sequenced as top header, bench and invert excavations. At its narrowest, the pillar separating the twin tunnels was less than 1.5 m across or 0.23 tunnel diameters. Typically, the minimum pillar spacing required to reduce the interaction between twin, side by side tunnels is one tunnel diameter. Because the tunnel construction was within an urban environment, there was considerable concern with ground loss and excessive surface settlements. This study demonstrates that within similar materials, a pillar width of 0.5 tunnel diameters sufficiently reduces the tunnel interactions and minimize the risk of damage to nearby structures.     

Control of surface settlements with umbrella arch method in second stage excavations of Istanbul Metro

In metro tunnel excavations, it is important to control surface settlements observed before and after excavation, causing damages to the surface structures. Otherwise, metro tunnel cannot perform the task expected and the advantages of metro tunnel are lost. For this purpose, second stage excavation of Istanbul Metro between Unkapani and Yenikapi was studied. Geology in this section is composed of clay, claystone, sand and marl. New Austrian tunnelling method (NATM) was used in tunnels having cross-section of 36 m². In this study surface settlements are compared for NATM and umbrella arch method (UAM) in sensitive regions. As a result of this study, it was found out that surface deformations especially in clay bearing formations could be controlled efficiently by using the UAM.     

地下工程施工对房屋影响的损害评价

随着城市地铁、公路隧道等地下工程数量越来越多,浅埋隧道近距离穿越建筑物的现象明显增多。如何评价地表房屋的受影响程度是隧道安全穿过房屋的首要前提之一。首先根据房屋的实际情况,确定房屋在隧道施工前的损坏情况;然后参考房屋损坏评价标准,确定房屋的控制损坏等级和极限变形值,将房屋的极限变形值与房屋损坏等级所对应的变形值之差即得房屋的容许变形值;最后采用三阶段分析法对隧道施工诱发房屋的损害进行评价。如果该分析评价法的各阶段计算结果均超出房屋的容许变形值,则需对房屋采取相应的加固措施,从而保证隧道顺利通过房屋。同时,该评价方法对同类工程具有参考价值。     

城市隧道施工引起建筑物变形的损坏评估

近年来随着城市地铁建设的快速发展,隧道施工过程中引起地面建筑物或其他结构变形而产生的损坏问题屡见不鲜。地铁建设中需要对大量的建筑物开展变形损坏风险评估工作,而科学的建筑物变形及其损坏的评估方法方面的研究尚很不成熟。针对量大面广的多层砖混结构,研究隧道施工可能造成的建筑损坏问题。利用北京市在20世纪70年代对建筑裂缝控制的研究基础和工程应用经验、特别是扎实可靠的数据得到的最小曲率控制参数,基于对建筑物在隧道施工影响下变形特征的研究和沉降预测的修正刚度法,避开不容易准确估计的建筑材料模量、截面惯性矩等参数,直接根据建筑物的沉降曲线形态,得到用拉应变控制的评估方法,发展了用最小曲率参数来控制隧道施工引起沉降的有效方法。这种评估方法与用拉应变控制的方法取得很好的一致性。     

盾构隧道60°斜穿地裂缝的变形破坏机制试验研究

 从西安地铁盾构隧道工程背景和西安地裂缝地质环境出发,根据相似理论设计盾构隧道管片衬砌结构60°斜穿地裂缝的物理模型试验。管片混凝土应变、纵向和环向螺栓应变、结构接触土压力和结构外围土压力、结构内部收敛位移、模型顶表面土体变形以及宏观变形破坏现象表明,盾构隧道管片衬砌结构60°斜穿地裂缝的变形破坏模式以剪切变形为主,局部有扭转和弯曲变形;结构破坏范围为上盘0.75D(D为管片环外径1.20 m),下盘0.50D;管片混凝土破坏主要发生在螺栓孔附近,地裂缝处纵向螺栓发生较强的剪切、扭转和拉伸变形破坏;管片衬砌结构变形破坏不对称,管片环向处于偏压状态;环缝拱顶错位量大于拱底和拱腰,拱顶最大错位量达30 mm(0.025D),模型难以适用地裂缝错动变形20 cm(0.166 7D),盾构管片衬砌结构不适用于地裂缝活动强烈的地质环境。     

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。