砂土地层土压盾构隧道施工掌子面稳定性研究

土压平衡盾构在自稳性较差的砂土地层施工时若仓内支护压力过小可能诱发掌子面失稳,应引起高度重视。采用三维离散元方法分析了砂土地层土压盾构掘进与停机状态下的掌子面稳定性。研究建立了较为精细的土压盾构机模型并引入盾构动态施工过程,充分考虑了刀盘旋转切削土体与面板支撑对掌子面的影响,探讨了刀盘型式、隧道埋深以及刀盘转速等因素对掌子面极限支护压力与失稳区分布的影响规律,并从细观角度解释了砂土地层土压盾构隧道掌子面失稳机理。与既有研究相比,本文考虑了土压盾构动态施工对掌子面稳定性的影响,更加接近工程实际,研究成果可为确保砂土地层土压盾构隧道施工掌子面稳定提供参考。     

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.     

Earth pressure balance tunnelling in rock masses: Laboratory feasibility study of the conditioning process

Earth pressure balance (EPB) shield tunnelling is considered to be an effective tunnelling method when surface settlements must be avoided by controlling face stability and underground water inflow. EPB technology is applied increasingly to the conditioning of rock masses in which the presence of polluting material is foreseen and must be controlled, or when explosive gases from the rock mass must be prevented from entering in the machine. The use of an EPB machine in ‘closed mode’ can prevent the dispersion of dangerous materials in the underground environment. This study was performed to evaluate the feasibility of using EPB technology in rock masses and the effectiveness of this tunnelling process in terms of soil conditioning, conditioned soil permeability, pressure transmission on the tunnel face, and the ability to extract the material with a screw conveyor. The research was developed with a set of laboratory tests carried out on three different types of rock masses where tunnelling with EPB machines are planned to be constructed in Italy but the results can be easily applied to similar rock mass types. The tests have been carried out both with slump tests and with a specially constructed screw conveyor extraction device. In the paper the most important results are presented and discussed.     

Artificial neural networks for predicting the maximum surface settlement caused by EPB shield tunneling

Numerous empirical and analytical relations exist between shield tunnel characteristics and surface and subsurface deformation. Also, 2-D and 3-D numerical analyses have been applied to such tunneling problems. Similar but substantially fewer approaches have been developed for earth pressure balance (EPB) tunneling. In the Bangkok MRTA project, data on ground deformation and shield operation were collected. The tunnel sizes are practically identical and the subsurface conditions over long distances are comparable, which allow one to establish relationships between ground characteristics and EPB – operation on the one hand, and surface deformations on the other hand. After using the information to identify which ground- and EPB-characteristic have the greatest influence on ground movements, an approach based on artificial neural networks (ANN) was used to develop predictive relations. Since the method has the ability to map input to output patterns, ANN enable one to map all influencing parameters to surface settlements. Combining the extensive computerized database and the knowledge of what influences the surface settlements, ANN can become a useful predictive method. This paper attempts to evaluate the potential as well as the limitations of ANN for predicting surface settlements caused by EPB shield tunneling and to develop optimal neural network models for this objective.     

地铁隧道盾构法施工过程中地层变位的三维有限元模拟

在全面分析土压平衡式盾构施工过程中影响周围土体变形各主要因素的基础上，提出一种能够综合考虑各种因素的盾构施工三维非线性有限元模拟方法，通过对某地铁隧道盾构施工过程的模拟，分析了盾构推进过程中隧道周围及地表处土体的位移和变形以及横断面不同深度上的沉降分布规律，计算得到的隧道纵向地面沉降分布曲线与实测数据非常接近，计算结果表明所提出的盾构施工模拟方法是有效可行的。     

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。     

土压平衡盾构施工中泡沫改良砂土的试验研究

土压平衡盾构在隧道施工中,特别在地铁区间隧道施工中已经得到广泛应用。土压平衡盾构主要适用于粘性土地层开挖,其在砂土地层的掘进过程中通常会遇到掌子面土压平衡难以形成、螺旋出土器无法正常排土、刀盘扭矩过大及刀具磨损过快等一些问题。最有效的解决方法是使用泡沫等添加剂对土体进行改良,但是目前工程施工现场添加剂的使用尚存在一定的盲目性。文章首先通过自制的试验室发泡装置生产泡沫,并对泡沫的性能与发泡剂溶液浓度的关系进行研究,为有效使用泡沫提供依据,并通过一系列试验研究泡沫对砂土的改良效果,得到适合土压平衡盾构施工的优化泡沫掺入比。     

砂土地层盾构隧道施工对地层扰动的室内掘进试验研究

土压平衡式盾构机在穿越流塑性差、渗透系数大的砂土地层时容易对隧道周围土体产生扰动,导致地表沉降不易控制和作用在衬砌结构上的土压力发生改变。针对地铁盾构隧道穿越砂土地层引起的地层扰动,采用一种能完全反映盾构隧道动态施工全过程的分析方法尤为重要。以城市地铁盾构区间隧道施工采用的土压平衡式盾构机为原型,研制 800 mm土压平衡式模型盾构机,该机主要包括推进机构、掘削机构和出土机构,能实现盾构始发、刀盘切削、螺旋出土、管片拼装等主要功能,以此开展砂土地层中盾构施工的室内掘进试验。试验过程中对盾构掘进引起的地层沉降及衬砌结构上的土压力进行量测,分析地层沉降形态和衬砌结构上土压力的分布形态,同时将试验结果同理论计算、数值分析及现场资料进行对比。研究结果表明,土体性状和盾尾注浆对地层沉降具有重要影响,地层损失是地层发生沉降的主要原因。未注浆情况下盾尾脱环引起的地表沉降值占总沉降值的60%以上,且由于未注浆而增大的地表沉降所占比例为20%～30%,沉降时程曲线具有阶段性和时效性。地表沉降槽宽度系数i与现场测试数据具有一致性。衬砌结构上的土压力分布类似于上下端为长半轴、左右端为短半轴的椭圆形,数值上试验实测值较理论计算值要小。     

砂卵石地层土压力平衡盾构施工开挖面稳定及邻近建筑物影响模型试验研究

盾构技术在砂卵石地层中应用越来越多,砂卵石地层具有很强的不确定性特征,盾构施工的关键问题之一是保持开挖面稳定性及减小地面沉降。利用土压平衡盾构模型,研究北京砂卵石地层中不同埋深时邻近建筑物影响下的开挖面稳定性及地表沉降规律。试验中,分析柔性基础邻近建筑物及埋深对开挖面极限支护力和地表沉降的影响,揭示开挖面稳定性、土拱效应与极限支护力及地表沉降的关系。在邻近建筑物影响下,砂卵石地层中的支护压力呈非对称分布,且砂卵石地层中盾构推进引起的沉降值大于基于Peck公式的计算值,研究成果对砂卵石地层中盾构施工有重要的指导意义。     

基于聚焦频域激电法的盾构施工地质预报系统

针对盾构法施工时开挖仓封闭,地质预报实施难的问题,设计一种基于聚焦电流法和频域激电法的超前地质预报方法,可实现低功率下探测电流的发射和有效接收。为了提高测量精度和稳定性,引入电流测量线圈实现探测电流的有效聚焦,解决了聚焦电流法中易产生的欠聚焦和过聚焦问题。本文介绍了聚焦电法和频域激电法的原理以及二者结合后的优势,建立了6 m级土压平衡盾构机的二维轴对称模型,并计算理论探测深度。依据本文所研究的地质预报方法,对大连地铁5号线08标(后盐站—后关村站)使用的土压平衡盾构机进行针对性设计,实现聚焦频域电法在盾构机上的搭载以及施工过程中的实时预报应用。通过对预报结果的分析以及施工过程中的不断验证,该方法基本达到预期目标,多次实现对掌子面前方不良地质的准确预报,具有很大的推广和应用价值。     

土压平衡盾构土舱压力建立机制及设定准则研究

 土压平衡盾构掘进施工中,土舱压力的设定对于控制盾构施工对周围环境的影响意义重大。结合超大直径土压平衡盾构施工的上海市迎宾三路隧道工程,利用理论分析和实测数据分析的方法对土压平衡盾构的土舱压力建立机制和设定准则进行研究。基于进出土平衡理念研究土舱压力建立过程中各施工参数间的关系,提出开挖面土体挤压量这一概念以揭示土舱压力的建立机制。建立盾构刀盘的细化模型,分析刀盘挤压作用下盾构开挖面上土体的挤压情况。基于控制开挖面土体不发生二次扰动的理念,提出土压平衡盾构土舱压力的设定准则,并对该土舱压力设定准则进行工程应用。现场监测数据显示,盾构掘进施工引起的地表隆沉得到有效的控制。     

On the influence of face pressure, grouting pressure and TBM design in soft ground tunnelling

A three-dimensional finite element simulation model, which includes all relevant shield tunnelling components and allows for the modelling of the step-by-step construction process of the tunnel advance is used to analyse the influence of TBM operation parameters and design parameters for a shallow tunnel advance in homogeneous, soft, cohesive soil below the ground water table. The numerical sensitivity studies presented in this paper focus on the face support pressure, the grouting pressure, the trailer weight and the length, weight and taper of the shield machine. The simulation results are evaluated with respect to the settlements of the ground surface, the shield movement and the loading of the tunnel lining. The evaluation of the sensitivity analyses helps to obtain a more detailed insight into the influence of selected parameters relevant for the design and steering of TBM tunnel advances.     

Probabilistic approach to assessing and monitoring settlements caused by tunneling

Tunnel construction commonly causes deformations of the surrounding ground, which can endanger buildings and other structures located in the vicinity of the tunnel. The prediction of these deformations and damages to buildings is difficult, due to limited knowledge of geotechnical conditions and due to uncertainty in predicting the response of the structures to the settlements. This motivates the development of a probabilistic model for the prediction of tunneling-induced damage to buildings. We propose such a model, based on the classical Gaussian profiles for the approximation of the subsidence trough and the equivalent beam method for modeling the response of the building walls. In practice, settlements are commonly monitored through deformation measurements. To account for this, we present a Bayesian method for updating the predicted settlements when measurements are available. Finally, we show how maximum allowable settlements, which are used as threshold values for monitoring of the construction process, can be determined based on reliability-based criteria in combination with measurements. The proposed methodology is applied to a group of masonry buildings affected by the construction of the L9 metro line tunnel in Barcelona.     

Effect of jet-grouting on surface settlements above the Aeschertunnel, Switzerland

The Aeschertunnel is a large shallow non-circular tunnel that was excavated in glacial moraine and Molasse bedrock. In the sections, where the excavation was in the glacial moraine, the construction procedure included the use of a jet-grout arch ahead of the tunnel face for excavation support. Measurements taken during the jet-grouting process and the excavation of the tunnel heading suggested a settlement trough much narrower than that observed using conventional NATM tunneling methods. The volume losses were limited to 0.35%, approximately the volume losses achieved with slurry shield and EPB TBM tunneling. A numerical back-analysis was performed on the surface settlements where jet-grouting was used. The back-analysis suggests that the narrow settlement trough results from localized shear bands that were induced by high pressures associated with the jet-grouting.     

矩形顶管隧道掌子面支护压力及稳定性分析

为了研究繁华城区大断面顶管隧道施工过程中掌子面的支护压力及稳定性规律,本文采用有限元数值模拟,计算分析了不同地层下隧道掌子面变形发展规律,对比分析了两种工况下极限支护压力比λ,同时计算分析了隧道埋深和土体强度因素对极限支护压力的影响规律。研究成果能够为工程施工中掌子面稳定压力的确定提供数据支持。     

Laboratory model tests and field investigations of EPB shield machine tunnelling in soft ground in Shanghai

In the last decades, many studies have been presented by different scholars on the environmental problems induced by the shield tunnelling in soft ground. But it mainly concentrated on ground surface settlement, tunnel surface stability and the influence to existing structures. Among them, little attention was paid to soil disturbance caused by the mismatch of machine’s operation parameters. In order to reveal this inherent relation, a series of laboratory model tests were carried out in the 1/16 scale for a field tunnel in practice where the tunnel had 6.4 m diameter. The tests to simulate earth pressure balance (EPB) shield tunnelling in soft ground were conducted with a microshield machine (0.4 m diameter). Measurements were carried out simultaneously in each test for total jack thrust force, cutting torque, chamber pressure, mucking ratio, ground surface displacement, and earth pressure. Based on the analysis of test results under different overburden ratio, cutterhead aperture ratio and screw rotation speed, the relationships between machine’s operation parameters themselves and its influence on disturbance to surrounding soil mass were discovered. Such discoveries were also verified by the field investigations. These results are useful for engineers and technicians to select suitable and serviceable machine operation parameters and reduce environmental influence at all stages of tunnel construction.     

软土地层土压平衡盾构法施工的模型试验研究

我国沿海地区以及许多内陆城市的地下广泛分布着很深的软黏土沉积层,目前该区域的许多城市都已兴建或正在筹建地铁。为确保在这种软弱地层中能采取与之相适应的盾构工法、减少对周围环境的影响以及降低工程造价,以上海地铁M8线某区间隧道工程为研究背景,采用室内模型试验的方法,进行了土压平衡盾构针对软土地层的适应性试验研究。为此,首先根据相似理论和模型试验方法建立了土体–盾构机器之间的相似系统;随后,针对上海地区特定的软土地层,进行了人工模型土壤的配制和模型试验方法的设计,并利用直径400 mm的模型盾构机模拟直径6340 mm的原型盾构机;最后,进行盾构不同工作参数组合的掘削试验,并记录下试验过程中机器工作参数及土体内应力和变形变化的有关试验结果,通过对试验数据的整理与分析,得出了软土地层中土压平衡盾构法施工的一些有用的规律。     

Face failure in cobble-rich soil: Numerical and experimental approaches on 1 g EPB reduced scale model

Recent years have witnessed several accidents associated with tunnel face failure in cobble-rich soil in the city of Chengdu, China. Due to its lack of cohesion, cobble-rich soil can be easily disturbed by shield tunneling. Based on the general conditions of the Chengdu Metro Line 1 project, the mechanisms of face failure of tunnels in cobble-rich soil driven with earth pressure balance (EPB) machines are studied. Specifically, we present results of tests carried out using a laboratory reduced-scale model of EPB tunneling operations in cobble-rich soil. The failure kinematics and limit face pressures are presented and analyzed. Then a three-dimensional (3D) discrete-element method (DEM) model, which is able to simulate the main EPB excavation processes is employed to gain further insight into the mechanisms of face failure in cobble-rich soil. Comparisons of these results with the observations based on previous studies are discussed. The results reveal a fundamentally different tunnel-face failure mechanism in cobble-rich soil in contrast with that in clayey or sandy soils. It shows that the ground movement during face failure is sudden in cobble-rich soil, which is different from the progressive mechanism in frictional–cohesive materials. The observed sinkhole at surface takes the shape of an oval, and the failure zone behind tunnel face extends almost as far as that ahead of the face, which is different from the observations in previous studies. The failure zone is found to be wider than that of sandy soils in both the transverse and longitudinal directions.     

Modelling of the EPB TBM shield tunnelling advance as a tool for geological characterization

One of the main causes of problems during the construction of tunnels with tunnel boring machines (TBMs) is a lack of characterization of the soil. Both geological and hydrogeological characterizations are essential to avoid unexpected events. The advance of TBMs produces groundwater oscillations due to hydraulic and mechanical effects. The magnitude of these oscillations depends on the characteristics of the soil and on the “parameters” of the TBM (e.g., earth pressures, penetration). Given that the impact caused in the groundwater could be estimated numerically, this paper proposes to use hydrogeological models based on the parameters of the TBM to validate or improve the previous geological characterization. This procedure was tested by modelling the advance of a TBM-type earth pressure balance (EPB) at a real site. This study arose during the construction of the tunnel for the high speed train in Barcelona. The previous geological characterization revealed a vertical fault whose exact position was unknown. The advance of the EPB was modelled to validate the previous characterization and to locate the fault. The numerical model included a detailed geology and hydrogeology of the study site and the parameters of the EPB. Note that the parameters of the EPB used in the model were more related to the groundwater response. These were determined statistically from all of the measures taken by the machine. Given the results obtained, hydrogeological modelling of EPBs was revealed to be a useful tool to validate previous characterizations, both the geological and the hydrogeological, and to determine the position of some geological structures, such as faults.     

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.