Behavior of Ground and Response of Existing Foundation Due to Tunneling

An apparatus has been developed to model the excavation of a tunnel in the laboratory. With this apparatus, 2D model tests are carried out to investigate the surface settlement and the earth pressure brought about by the tunneling. Finite element analyses using an elastoplastic subloading t_ij model are also conducted. The influence of volume loss on the surface settlement and the earth pressure, due to the shallow tunnelling, is illustrated based on the model tests and the corresponding numerical analyses. It is revealed that the surface settlement troughs and the earth pressure distributions around shallow tunnels depend on both the volume loss and the crown drift of the tunnel. The effect of the interaction between the tunneling and existing nearby foundations is also demonstrated in this paper. For existing foundations, the building loads control the surface settlements and the zone of deformation during the tunnel excavation. The behavior of the foundations depends on the deformation mechanism of the ground during the tunnel excavation. The induced axial force and bending moments in the piles of a piled raft are investigated numerically, and it is shown that the axial force changes due to the stress relaxation of the ground. Bending moments are induced in the piles at a lower value of soil cover due to the differential settlement of the piled raft.     

Influences of 3D Effects,Wall Deflection Process and Wall Deflection Mode in Retaining Wall Problems

Two-dimensional (2D) and three-dimensional (3D) model tests and the corresponding elastoplastic finite element analyses were carried out to investigate the influences of 3D effects, wall deflection process and wall deflection mode on the earth pressures and the ground movements in retaining wall problems. Aluminum rod mass were used in 2D model tests, and alumina balls were used in 3D model tests. The stress-strain behavior of these materials is similar to that of dense sands with positive dilatancy. In the finite element analyses, a recently developed elastoplastic constitutive model, named subloading t_ij model, were used. This model can describe typical stress deformation and strength characteristics of soils such as the influence of intermediate principal stress, the stress path dependency of plastic flow and the influence of density and/or confining pressure properly. The test results show that the earth pressures on the retaining wall in 3D condition are much smaller than those in 2D condition, and these distributions of earth pressure are more influenced by the wall deflection process than by the wall deflection mode. Observed surface settlements at the backfill just behind the wall in 3D condition are larger than those in 2D condition, but 3D surface settlements occur more locally. These 2D and 3D surface settlements are influenced by the wall deflection process more than the wall deflection mode. However, at the same wall deflection process and the same wall deflection mode, there is not much difference in the shapes of the earth pressure distributions and the surface settlement troughs between 2D and 3D conditions. These differences and similarities of the earth pressures and the surface settlements are simulated not only qualitatively but also quantitatively in the analyses using the above constitutive model.     

Ground Behavior Due to Tunnel Excavation with Existing Foundation

Two-dimensional (2D) and three-dimensional (3D) model tests of tunnel excavation with nearby existing foundation are carried out to investigate the influence of the existing foundation due to the interaction between ground and the existing structures. Three types of foundations: flat foundation, group-pile foundation and piled raft are considered. 2D and 3D finite element analyses using subloading t_ij model are also conducted. The deformation mechanism and distribution of earth pressure during tunnel excavation in the ground with nearby foundation are found to be different from those of green field condition. Surface settlement trough due to tunnel excavation in the ground with existing foundation does not follow the usual pattern of a Gaussian distributive curve, which can be observed in the case of green field. Especially, in the case of pile foundation, D_p, the distance between pile tip and tunnel is an important factor for the ground deformation and surface settlement. For a short distance D_p, although the length of pile is long, the ground deformation is concentrated at a place near the front pile and the rotation of foundation becomes larger. The maximum surface settlement in the case of existing foundation is also larger than those in the case of green field. Due to the existing foundation, unsymmetrical distributions of earth pressure occurred at the bottom of the ground due to tunnel excavation, both in model tests and numerical analyses. The earth pressure at the crown of tunnel in the case of existing foundation is almost the same as those in the case of green field. The arching at the shoulder of tunnel in the case of existing foundation, however, is much larger than those in the case of green field due to the dead load exerted on the foundation. The numerical results agree well with the results of the model tests.     

基坑开挖对旁侧隧道影响及隔断墙作用离心模型试验研究

开展了干砂地层中基坑开挖对旁侧隧道影响及隔断墙保护作用的三维离心模型试验和数值分析,获得了隧道上浮、隧道内力、隧道周围土压力、地表沉降等变化规律以及隧道空间位置和基坑开挖深度的影响。试验结果表明,基坑回弹量与采用Boussinesq解计算的回弹量比较接近;地表沉降量与文献报道的试验结果相近,而明显小于现场实测沉降;靠近基坑一侧的隧道周围土压力有所减小,而远离基坑一侧的隧道周围土压力则有所增加。隔断墙的设置可以一定程度上减小地表沉降、隧道外土压力变化、围护墙水平位移以及隧道上浮和弯矩。数值计算结果表明,隧道上浮量和水平位移随着隧道埋深及其与围护墙距离的增大而减小,而随着基坑开挖深度的增大而增大。     

连拱隧道围岩压力的释放率分析

连拱隧道作为公路隧道新的结构型式,理论上还不成熟.以云南省元磨高速公路的两座连拱隧道作为工程背景,按弹性阶段相似原则进行两组连拱隧道室内模型试验,包括隧道处于常规应力状态下和偏压状态下的模型试验.采用压力盒量测施工过程中隧道围岩压力,并提出了围岩压力释放率的概念,对连拱隧道开挖过程中围岩压力的变化特征进行了分析,同时对两种应力状态下的试验结果进行了对比分析.得出的结论对连拱隧道的设计和施工有积极的指导作用.     

Simulation of Conventional and Inverted Braced Excavations Using Subloading tij Model

Different construction sequences may be used in braced excavations. In the conventional or “ordinary method”, temporary retaining walls are initially placed and struts are used to brace them as the excavation proceeds downwards. In an “alternative method”, sometimes called “inverted excavation”, the tunnel walls are used to retain the surrounding earth, while the roof slab helps to brace the excavation. Laboratory model tests were devised to simulate these construction procedures. The tests were carried out under two-dimensional conditions, using aluminum rods to represent the soil mass. The walls were constructed with aluminum plates fully instrumented with strain gauges in both sides to measure the bending strains of the wall. Cylindrical bars with and without springs were used to brace the excavation in the ordinary method; while an instrumented aluminum block was used as the top slab in the alternative method. As excavation proceeded, all data relative to surface settlements, wall deflections and bending moments, as well as axial strut loads was carefully recorded. Later the laboratory tests were simulated with finite element analyses using the recently proposed subloading t_ij model (Nakai and Hinokio, 2004). The numerical results were compared with the experimental data obtained during the models tests for both construction methods. The overall recorded behavior in terms of displacements, deflection, bending moment and axial load could be reproduced with striking accuracy both qualitatively and quantitatively. This shows the capability of the model to represent the complex behavior of granular materials even under the low stress range used in the model tests. The results of model tests and numerical analyses show that the alternative method is viable and effective in controlling induced surface settlements, provided that the tunnel walls are constructed with an appropriate thickness.     

软弱围岩隧道施工三维有限元分析

对软弱围岩隧道施工中的力学性态进行了计算机模拟与分析.以杭新景高速公 路白炭坞隧道为实体建模,采用有限元程序对其施工全过程进行三维弹塑性分析;重点分析不同的施工方法和支护方式对控制围岩变形的作用.计算结果表明:隧道的支护方式对减少由开挖引起的扰动起着重要作用,掌子面距离支护段距离越短,引起的沉降越小;对于软弱围岩,采用台阶法开挖时,台阶的长度不宜过长,一般应在0.5倍的洞径左右.     

平行盾构开挖离心机模拟试验研究

通过离心模型试验模拟平行盾构隧道近接开挖施工,研究了盾构隧道近接开挖对既有隧道结构内力、管片变形和地表沉降的变化规律。结果表明:1隧道开挖引起地表沉降的大小与开挖的步骤有关,而沉降槽的范围基本不变;2既有隧道靠近新建隧道一侧受拉,这一侧弯矩出现负增量,侧向土压力也有一定的减小,且既有隧道直径水平向变大,而垂向直径基本不受影响;3由于土拱效应,新建隧道已完成开挖部分管片拱顶的土压力随开挖进程先减小后增大;4采用地层结构法可以准确模拟隧道开挖过程的隧道结构力学特性与变形规律。     

深长隧道突水地质灾害三维模型试验系统研制及其应用

 深长隧道突水地质灾害中的最小安全隔水岩层厚度研究对于工程设计施工具有重要指导意义。基于相似模型试验的方法,研制出一套深长隧道突水地质灾害三维模型试验系统。该系统由三维模型试验主装置、试样成型装置以及模拟隧洞开挖卸荷掘进装置组成,能够满足模型对所受地应力场、水压等初始环境的模拟。试验系统具有荷载稳定、可精确模拟隧洞开挖、试验方便简洁、周期短并能开展大样本统计试验等特点。将该试验系统应用于成都—兰州(成兰)铁路线龙门山隧道的突水三维模型试验中,试验围压、轴压为0.6 MPa,水压为0.2 MPa时,获得模型最小安全隔水岩层厚度为47 mm,试验结果可有效指导工程的设计与施工。     

基槽开挖引起沉管隧道竖井变型的模型试验研究

根据拟建中的长江沉管隧道竖井场址的地质情况及其设计参数,推导了大型竖井一侧进行深开挖时竖井稳定性的模型试验相似关系.通过模型试验,获得了不同基础埋深和水位变化时,竖井倾斜、位移、沉降、土压力等关系数据.分析研究了竖井的不同基础埋深对稳定性影响,试验成果解决了大型深埋竖井一侧要进行深开挖时的有关设计难题,亦可为地下深埋结构一侧大开挖时的设计和稳定性分析提供参考.     

挡土结构侧土压力与水平位移关系的试验研究

目前挡土结构侧土压力计算方法中,一般未考虑挡土结构位移对土压力的影响。而控制挡土结构位移,使土体不能达互极限平衡状态,正是实际基坑工程中的常见情况。本文通过一定数量的室内模型试验研究,对不同挡土结构位移下,侧土压力由静止土压力逐步向主动土压力发展过程进行了测定,得出挡土结构位移和侧土压力强度值间的关系,提出了一种考虑挡土结构侧向位移的侧土压力简化计算方法。     

板桩结构土压力理论的创新发展

对于板桩码头,其主要的荷载为作用于码头前墙上的土压力,该荷载一方面是由于港池开挖引起前墙两侧土压力的不平衡产生,另一方面是由于码头表面荷载作用于地基土,从而增加了前墙陆侧的土压力。板桩码头深水化的关键要求必须解决港池挖深导致的前墙土压力急剧增大问题,“遮帘”和“卸荷”是减少前墙土压力的有效途径,由于设置了遮帘桩和卸承台,使得板桩结构的受力情况更加复杂,涉及的关键科学技术问题是土和结构的相互作用。针对遮帘式和分离卸荷式板桩码头新结构开发过程中的土压力问题,先后研究了土体密度与粒径对静止土压力系数的影响、遮帘式板桩结构的土压力“桶仓压力效应”和“遮帘效应”,以及分离卸荷式板桩结构的土压力“卸荷效应”,为板桩码头新结构的发展奠定了理论基础。     

地表超载作用下盾构隧道劣化机理与特性

对某软土地区地铁盾构隧道进行了调研与分析,发现盾构隧道在现有计算理论所允许的地表超载作用下极易发生横向变形超限,并引发管片纵缝接头破损与渗漏水,对此展开了模型试验、数值仿真及理论分析。研究表明:地表均布超载导致的隧道附加竖向土压力并不是均匀分布,且在隧道中心正上方一定范围内要大于地表均布超载;隧道的穿越土层越软弱,地表超载导致的隧道周围附加土压力对隧道结构抵抗横向变形越不利;隧道发生横椭圆变形过程中,管片纵缝接头是管片环中的最薄弱部位。最后提出了软土地区盾构隧道采用"刚性衬砌"的设计理念,并给出了加大管片纵缝接头强度与刚度的建议。     

土压平衡盾构土仓压力设定与控制方法探讨

利用现有的土力学理论,对土压平衡盾构推进与停机两种状态下开挖面前方土压力与土仓内土压力类型进行了分析。通过理论计算和分析,提出了根据开挖面前方地层的地质条件和沉降要求等进行土仓压力设定的方法,指出了当土仓内土体与开挖面前方土体基本一致时,土仓压力很难与开挖面前方土压力基本相同。通过理论分析,提出了土仓压力控制的方法,以及采取土仓内土体改良、注入泡沫或高压气体形成气压、严格控制出土量等控制土仓压力的技术措施。     

A hybrid model for estimation of ground movements due to mechanized tunnel excavation

To provide realistic predictions of mechanized tunnel excavation‐induced ground movements, this research develops an innovative simulation technique called hybrid modeling that combines a detailed process‐oriented finite element (FE) simulation (submodel) with the computational efficiency of metamodel (or surrogate model). This hybrid modeling approach has three levels. In Level 1, a small scale submodel is cut out from the global model and the continuous simulations are conducted in this submodel. Level 2 deals with identification of uncertain soil parameters based on the measurements (e.g., surface settlements) during tunnel excavation. In Level 3, the tunneling process parameters (e.g., grouting pressure) can be optimized to control tunneling‐induced ground movements or building deformations according to the design criterion. The proposed hybrid modeling approach is validated via a 3D numerical simulation of mechanized tunnel excavation. The results show the capability of the proposed approach to provide reliable model responses in the near field around the tunnel with reduced computational costs.     

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.     

连拱隧道动态施工模型试验与三维数值仿真模拟研究

连拱隧道作为公路隧道新的结构型式，理论上还不成熟。以云南省元磨高速公路的一座连拱隧道作为工程背景，按弹性阶段相似原则进行连拱隧道室内模型试验，模拟连拱隧道的施工工况，采用压力盒、数码摄像、沉降板等仪器量测施工过程中隧道围岩应力和位移分布，采用三维连续介质快速拉格朗日元模拟连拱隧道的施工工况，得出施工过程中隧道围岩位移、应力和塑性区分布规律，其结果与模型试验所得结果基本一致。     

基于不完全拱效应的隧道预处理机制与计算方法

 隧道开挖中,掌子面–超前核心土的变形同样会使其周围产生拱效应,但该种拱效应不同于后方已开挖区域周围的拱效应,称之为不完全拱效应。不完全拱效应的发挥与预收敛变形和挤出变形有很大的关系,基于不完全拱效应提出了超前核心土周围围岩压力的计算公式,此公式与太沙基公式的不同在于考虑了预收敛变形对围岩压力的影响,并在黏性土的推导中考虑了非垂直滑移面效应,假定破裂面与垂直方向存在夹角 。算例分析显示,随着预收敛变形的增加拱顶土压力的变化可分为近线性快速下降、缓慢下降及稳定阶段,计算土压力随内摩擦角、埋深比及黏聚力的增大而减小,且在黏性土中随破裂面倾角的增大而增大。在此基础上,推导了计算基于新意法的超前核心土加固参数的理论计算公式,结合算例验证了理论公式的适用性,该公式计算简单,便于理解,可用于新意法的初步设计阶段。     

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.