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.     

Non-linear description of ground settlement over twin tunnels in soil

Tunneling inevitably leads to ground settlement. The ground settlement trough over a single tunnel is well described by the generally accepted Peck’s formula. A new model of settlement trough is proposed by extending Peck’s formula to the case of horizontally aligned twin tunnels, which is a widely used tunnel configuration in urban metro projects. The feasibility of the new model is demonstrated through exploration of a large amount of ground settlement data accumulated from a metro tunnel project in China. Two numerical methods are implemented in the data exploration process to solve the problem of non-linear curve-fitting and estimation of model parameters. The Levenberg–Marquardt method is shown to be more suitable than the Nelder–Mead method. Based on the new model of settlement trough, a new method for calculating ground loss over twin tunnels is also proposed. The concluded empirical value of the ratio of ground loss is considered to provide an excellent reference for similar urban railway projects in the future.     

Interaction of twin tunnels and shallow foundation at Zand underpass, Shiraz metro, Iran

Tunnel construction in urban areas may inevitably require crossing close to adjacent structures. In this paper, the driving of Shiraz metro twin tunnels beneath Zand underpass of a major municipal artery built 15 years ago has been analyzed. The earth pressure balance (EPB) operation parameters consisting of face pressure, grout pressure and thrust force, which are effective on the ground movement and tunnels-underpass interaction, were investigated. For precise analysis, the effects of twin tunnels construction on modification of normal forces and bending moments of the underpass structural members and first tunnel lining were also studied. The results showed that, face pressure as compared to grout pressure and thrust force had a more significant impact on the underpass settlement. The rate of displacement differences because of increasing those EPB operation parameters at tunnel crown was greater than the surface and piles toe of the underpass. The changes in normal forces of the underpass were greater than bending moments, which could be related to high bending moment of the underpass members. The normal forces showed remarkable variations in the upper concrete plate of the underpass. The effects of second tunnel construction on the first one, regarding to the changes in normal forces, bending moments and displacements showed less than 10% increment.     

盾构隧道衬砌结构的壳-接头模型研究

提出了一种新型的盾构隧道衬砌结构计算模型——壳-接头模型。该模型中采用壳单元模拟衬砌管片,管片和管片环之间通过接头联接单元相连。基于对盾构隧道衬砌结构接头力学特性的分析,建立了接头联接单元的刚度矩阵和计算流程,定义了弯曲、剪切和拉压刚度的计算方法,使之能准确模拟接头的力学行为。最后利用壳-接头模型模拟盾构隧道接头原型试验,计算结果与试验较为吻合,验证了壳-接头模型模拟盾构隧道衬砌结构力学行为的准确性和适用性。     

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.     

小净距隧道群下穿既有运营隧道离心模型试验研究

以深圳地铁7号线、9号线四条小净距隧道近距离下穿既有地铁1号线工程为研究背景,通过离心模型试验方法,分析了小净距隧道群施工对周围土体应力影响规律,揭示了盾构多次近距离穿越施工引起既有线变形及受力变化机理。研究表明:(1)小净距四线隧道开挖具有明显的"群洞效应",隧道群的形成会导致松动区扩大、土拱向上扩展,从而引起新建隧道承受竖向土压力增加;(2)既有隧道沉降随穿越次数的增多而增大,最终沉降为4次穿越叠加的结果,峰值位置基本位于4条隧道中心线正上方,盾构隧道每次穿越引起沉降增幅为13%~48%,穿越区域横向影响范围可达60 m以上;(3)新建隧道的开挖对既有隧道环向弯矩的影响不大,弯矩变化不超过10%,而对既有隧道纵向弯矩影响较大,随着穿越次数增加纵向弯矩明显增大,这也是隧道下穿施工引起纵向裂缝和渗漏水的主要原因。结合数值模拟计算进行对比分析,得到规律与试验相一致,进一步验证了试验结果对实际工程的可靠性。     

Effects of building three-dimensional modeling type on twin tunneling-induced ground settlement

Buildings are usually modeled as elastic beams in plane strain finite element (FE) simulation of tunnel-building interaction. However, neither tunnel nor surface buildings behave in a plane strain manner. In order to assess the effect of building modeling type on twin tunneling-induced ground settlement, some full three-dimensional FE models were analyzed and the effect of building modeling type was reviewed. Two dimensional (2D) plane strain interaction and green-field situation models were also analyzed to compare the results with the three dimensional (3D) models and assess the influence of modeling type. Outcomes of the studies showed that three-dimensional modeling has great influence on the results and equivalent surface beam, which is used in common practice of two-dimensional plane strain models, over-estimates the stiffness of building and leads to less settlement prediction in comparison with 3D simulation.     

Interaction between surface structures and tunnelling in sand: Centrifuge and computational modelling

Tunnelling in urban areas requires a careful estimation of the consequence of soil settlements on existing buildings. In this paper the interaction between the excavation of a tunnel in sand and surface structures is investigated. A two dimensional finite element model is presented and validated through comparison with centrifuge test results, both with and without structures. The model is then used to perform a sensitivity study on the effect of building weight on soil movements and structural deformations. The results of the validation indicate that assuming a no-tension interface between the soil and the structure is essential to capture the soil–structure interaction that was experimentally observed. The parametric analyses show that the relation between the building stiffness and the tunnelling-induced deformations depends on the building weight.     

软土地区公路地基沉降的分析预测

交通运输的客观要求,时常需要在软土地区修筑公路,而路基沉降的预测是一个亟待解决的关键问题。文章概述了一些工程上常用的预测方法,指出了一些不足之处。在此基础上,建议在推算地基沉降时,应当选取荷载稳定后的沉降资料采用曲线拟合法进行预测,而且需要根据施工进程的不同,分段进行。     

Tunnelling-induced response of a jointed pipeline and its equivalence to a continuous structure

The response of a jointed pipeline to tunnelling-induced ground settlement relies largely on the soil-pipe interaction. An analytical solution incorporating the Pasternak model is formulated here to estimate a jointed pipeline's deflection and bending moment. Subsequently, a procedure is proposed to transform the jointed pipeline to an equivalent continuous structure for a deflection evaluation. Case studies are conducted to verify the analytical solution, followed by parametric studies to assess the effects of the Pasternak model's parameters and the pipe joints. The analytical solution is validated by the experimental results in the literature. The parametric studies show that the number and distribution of the joints have a mild effect on the pipe deflection, whereas their influence on the pipe bending moment is significant. Similarly, the pipe bending moment is more sensitive than the pipe deflection to subgrade shear stiffness.     

高速公路隧道施工对既有铁路隧道的影响分析

以重庆绕城高速公路施家梁隧道工程为例,通过模拟隧道的三维空间数值计算,分析了新建公路隧道施工对既有铁路隧道的影响,确定了铁路隧道的影响范围,并对影响范围内的结构进行了内力检算和安全度评估,结果表明：隧道结构安全系数满足规范要求。     

新建隧道施工对既有隧道影响的试验研究

通过新宝塔山隧道施工过程对既有宝塔山隧道的影响进行了数值模拟,并实测了新隧道开挖过程中既有隧道衬砌结构的变形和衬砌混凝土的应变变化情况。数值模拟结果和现场实测数据表明,既有隧道衬砌结构的变形和衬砌混凝土的应变变化较小,新隧道施工对既有隧道影响不明显,对后续工程施工有指导意义。     

既有地下结构受下穿施工影响的力学响应与安全控制研究

北京地区下穿工程中,新建隧道断面及施工方法种类众多、地层条件复杂、既有结构形式多样,有极其复杂的组合关系,目前尚无对既有结构力学响应的系统分析。通过北京地区13个下穿工程案例,总结了新建隧道结构形式及施工措施,明确既有地下结构变形特点,采用两阶段法分析及预测了各因素影响下既有地下结构的力学响应。研究表明:(1)新建隧道包括市政管道、地铁区间及车站,常见的施工方法有多导洞法、台阶法,洞桩托换法和中洞法,新建隧道开挖面积与施工方法有较明确的对应关系。(2)既有地下结构实测最大沉降概率分布符合数学期望4.89,方差16.4的正态分布。其中,新建市政管道、地铁区间和地铁车站下穿施工引起的既有地下结构平均最大沉降分别为2.56,3.82,11.07 mm。(3)根据新旧隧道空间位置关系的不同,穿越工程可分为7种组合,其中,既有地下结构力学响应有V,U,W 3种模式。(4)严格控制新建隧道开挖面积,不留或少留间隔土,尽可能选择W型穿越模式,以此减小对既有地下结构的扰动。     

下穿既有地铁车站方案设计

以北京地铁四号线宣武门车站下穿既有地铁车站工程为例 ,介绍了其具体工程概况 ,提出了相应的设计方案。经采用该方案托起既有地铁车站 ,确保了既有线运营和施工安全。     

Effects of above-crossing tunnelling on the existing shield tunnels

Tunnelling in the dense urban areas frequently results in over-crossing or bypassing the existing tunnels. It is obvious that the over-crossing tunnelling will adversely affect and even damage the existing tunnels if the induced deformation exceeds the capacity of tunnel structures. Increasing concerns have been raised about the interactions between the over-crossing tunnelling and underlying tunnels. In order to obtain a better mechanical understanding of the effects of the over-crossing tunnelling on the existing tunnels and provide a quick but low cost assessment alternative method for evaluating the behavior of underlying tunnels prior to construction, a simplified analytical method is proposed in this study. In this simplified method, the tunnel is simply considered as a continuous Euler-Bernoulli beam with a certain equivalent bending stiffness. The unloading stress at the tunnel location caused by the over-crossing tunnelling is computed through Mindlin’s solution, ignoring the presence of the existing tunnel. Then, the tunnel-soil interaction due to the relief stress is analyzed based on the commonly-accepted Winkler foundation model. The applicability of the presented method is validated by three well-documented case histories. Results of these case studies show a reasonable agreement between the predictions and observations. Finally, a parametric analysis is also preformed to investigate the influences of the different factors on the behavior of the existing tunnels, including clearance distance, advancing distance and multiple tunnels construction.     

Use of artificial neural networks to predict 3-D elastic settlement of foundations on soils with inclined bedrock

The application of the theory of elasticity for the calculation of foundation settlements has yielded equations that are well-established and consolidated in geotechnical standards and/or that are recommended for use. These equations are corrected by an influence factor in order to increase their precision and to encompass the existing complex geotechnical casuistry. The study presented herein utilizes neural networks to improve the determination of the influence factor (I_α), which considers the effect of a finite elastic half-space limited by the inclined bedrock under a foundation. The results obtained through the utilization of artificial neural networks (ANNs) demonstrate a notable improvement in the predicted values for the influence factor in comparison with those of existing analytical equations.     

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.     

杭州某市政隧道过既有铁路暗挖施工技术

重点介绍了富水砂质粉土地层浅埋暗挖法隧道修建技术,通过采用夯管超前预支护、全断面注浆、CRD工法等辅助施工措施穿越既有运营铁路干线,保证了近距离双洞大跨隧道安全施工。     

既有地下室增层开挖引起的桩基侧摩阻力损失分析

既有高层建筑地下增层开挖,不可避免地引起地基应力场改变,导致桩土接触面和桩端土层应力变化,从而降低原基础底板下桩基承载力。由于实际工程情况限制,增层开挖后桩基的承载力无法通过现场试验获得,因而采用理论分析和有限元模拟增层开挖后桩基承载力就显得尤为重要。采用经典桩侧摩阻力计算公式,简单地考虑上覆土压变化以及应用Mindlin应力解考虑开挖引起的附加应力,分别计算桩侧极限摩阻力,然后利用有限元法计算桩侧摩阻力并和前述计算结果相对比,最后分析了增层开挖条件下桩侧极限摩阻力损失比随不同增层开挖宽度和深度的变化规律,结果表明：损失比随增层开挖宽度的增大而增大,开始呈线性增大然后趋于稳定;损失比随增层开挖深度的增大而增大,大致呈线性趋势;损失比沿桩身由上往下逐渐减小。