Intelligent model selection with updating parameters during staged excavation using optimization method

Various constitutive models have been proposed, and previous studies focused on identifying parameters of specified models. To develop the smart construction, this paper proposes a novel optimization-based intelligent model selection procedure in which parameter identification is also performed during staged excavation. To conduct the model selection, a database of seven constitutive models accounting for isotropic or anisotropic yield surface, isotropic or anisotropic elasticity, or small strain stiffness for clayey soils is established, with each model numbered and deemed as one additional parameter for optimization. A newly developed real-coded genetic algorithm is adopted to evaluate the performance of simulation against field measurement. As the process of optimization goes on, the soil model exhibiting good performance during simulation survives from the database and model parameters are also optimized. For each excavation stage, with the selected model and optimized parameters, wall deflection and ground surface settlement of the subsequent unexcavated stage are predicted. The proposed procedure is repeated until the entire excavation is finished. This proposed procedure is applied to a real staged excavation with field data, which demonstrates its effectiveness and efficiency in engineering practice with highlighting the importance of anisotropic elasticity and small strain stiffness in simulating excavation. All results demonstrate that the current study has both academic significance and practical significance in providing an efficient and effective approach of adaptive optimization-based model selection with parameters updating in engineering applications.

     

软弱土层复合土钉支护试验研究

复合土钉支护是从土钉支护基础上发展起来的、应用范围更广的一项基坑支护新技术。笔者通过模型试验，研究了复合土钉支护的作用原理，为其设计和施工提供了参考依据，促进了这项技术快速发展。基坑模型采用“相似模型的长度与变形的时间成反比”的相似法则设计，相似比为1:10。试验模拟现场分步开挖与支护的过程，测试内容包括土体位移、地表沉降、基底隆起、地面裂缝及超挖引起的破坏形态。试验结果表明：复合土钉支护能够充分调动周围土体共同作用，有效地控制基坑变形；复合土钉支护中止水帷幕的插入深度和强度对控制边坡变形与失稳有较大作用；复合土钉支护效果明显优于一般的土钉支护。     

钢管预加固措施对隧洞围岩变形的影响分析

隧道洞口段埋深浅,围岩松散,自稳能力较差,开挖后洞内及地表均出现多条裂缝。经方案比选,采用钢花管对洞顶及两侧部分的地表和隧道地基与拱脚部位进行注浆加固。在注浆加固施工的过程中,隧洞内也同时开挖,几种工序的施工扰动产生叠加,势必对隧道的稳定及安全产生影响。结合工程实践,对洞内开挖及地表注浆工况并结合隧道监测数据进行分析,得出一些初步结论。     

Horizontal Displacement Monitoring of Soil Sidewalls of Beijing Subway

The horizontal displacements are monitored during excavation procedure in the sidewalls of Fuxingmen Station, a returning tunnel of Beijing Subway buried in soil media. Based on these results with those obtained from other engineering projects, the characteristics of horizontal displacement are analyzed in shallow soil tunnels during grouting and excavation procedure of the tunnels. The usage of inclinometer for construction monitoring is studied primarily, and a new underground construction method in soil media, i.e., grouting prior to NATM construction is assessed.     

北京地铁复兴门折返站工程隧洞侧壁水平位移观测

本文根据北京地铁复兴门折返站工程开挖全过程中隧洞侧壁土体水平位移观测结果,结合其它工程的观测资料,分析了浅埋隧洞注浆开挖全过程中土体水平位移的一些规律。初步探讨了如何运用测斜仪在施工过程中进行安全观测的问题,并从水平位移这个角度评价一种先在土体中注浆然后采用新奥法施工的新施工方法。     

工字钢水泥土搅拌墙基坑支护的力学性能研究

河南某基坑最大开挖深度为5.8 m,场地以饱和淤泥质粉质黏土为主,与周边既有建筑最近距离为1.2 m,采用工字钢水泥土搅拌墙和预应力扩大头锚杆进行支护。运用PLAXIS有限元软件对该基坑支护结构进行数值模拟,得到了不同工况的土体位移、工字钢水泥土搅拌墙轴力和弯矩、预应力锚杆的锚固力和各开挖阶段的总乘子 ,结果表明,数值计算的土体水平位移与实际监测数据比较吻合,验证了工字钢水泥土搅拌墙建模的合理性;PLAXIS软件能较好地模拟基坑开挖过程中土层及结构的变形特点,验证了PLAXIS有限元软件在基坑工程的适用性;数值计算的土体水平位移、锚杆轴力、采用强度折减法计算的各开挖阶段的总乘子 均满足基坑设计要求,验证了工字钢水泥土搅拌墙在基坑支护的可行性,为类似基坑设计提供了理论依据。     

An efficient probabilistic back-analysis method for braced excavations using wall deflection data at multiple points

This paper presents an efficient Bayesian back-analysis procedure for braced excavations using wall deflection data at multiple points. Response surfaces obtained from finite element analyses are adopted to efficiently evaluate the wall responses. Deflection data for 49 wall sections from 11 case histories are collected to characterize the model error of the finite element method for evaluating the deflections at various points. A braced excavation project in Hang Zhou, China is chosen to illustrate the effectiveness of the proposed procedure. The results indicate that the soil parameters could be updated more significantly for the updating that uses the deflection data at multiple points than that only uses the maximum deflection data. The predicted deflections from the updated parameters agree fairly well with the field observations. The main significance of the proposed procedure is that it improves the updating efficiency of the soil parameters without adding monitoring effort compared with the traditional method that uses the maximum deflection data.     

Soil-structure interaction in shield tunnelling in soft soil

The development and extension of large cities creates a need for multiple shallow tunnels in the soft ground of building areas. Prediction of the ground settlement caused by the tunnel excavation is a major engineering challenge. A numerical simulation using a finite element method was implemented in the aim of developing a procedure to predict the movement induced by shield tunnelling in soft soil. This study describes a two-dimensional modelling and compares two procedures. The first procedure is done in a simple way (called “deconfinement modelling”) simulating the excavation using a stress decrease vector exerted on the excavation boundary (inside the tunnel) described by a stress release scalar parameter λ (named the “deconfinement factor”). The second procedure is composed of a complete stage of modelling (called “phase modelling”) taking into account different phases which simulate the different kinds of interactions between the tunnel and the soil (deconfinement, lining installation, pore pressure applied on the lining, and weight of the lining). Using a shallow lined tunnel with homogeneous soil conditions, the two procedures are analysed and compared. Then, the second modelling procedure is applied to the case of the metro of Lyon where field data have been obtained. Observations of the results and comparison with the experimental data demonstrate that the proposed modelling is adequate for the analyses of settlement induced by tunnelling in soft soil.     

双基坑大直径双环梁支护体系监测与分析

双基坑大直径双环梁支护体系是一种新型的基坑支护结构形式。通过对天津铜锣湾广场基坑工程中的环梁支撑内力和坑周土体水平位移的现场施工监测和数据结果分析,讨论了基坑开挖过程中环梁结构受力的特点以及对周围环境的影响,得到了基坑周边土体水平位移的变化规律,为考虑施工因素的环梁结构设计提供了依据。     

Time-dependent ground movements induced by shield tunneling in soft clay: a parametric study

In this work, the effects of coupled hydromechanical (consolidation) processes associated with shield tunneling excavation in soft clays are investigated with particular attention to the prediction of ground movements at the ground surface. A series of 2d FE analyses have been carried out in parametric form in order to investigate the effects of tunnel excavation velocity relative to the soil consolidation rate and the hydraulic boundary conditions at the tunnel boundary. The shield advancement process has been simulated with a simplified procedure incorporating both volume loss and ovalization of the tunnel section. In order to investigate the relative importance of soil consolidation during the excavation process, different characteristic times for the tunnel face advancement and for the consolidation process around the tunnel have been considered, for the two limiting conditions of fully permeable liner and impervious liner. The potential damage induced by the tunnel excavation on existing structures, based on computed ground surface distortions and horizontal deformations, has been found to vary significantly with time during the consolidation process. The results of the simulations allowed to obtain useful information on the minimum tunnel face advancement speed for which the assumption of fully undrained conditions for the soil during the excavations is acceptable, as well as on the speed range for which solving the fully coupled hydromechanical problem is necessary.     

南京某深基坑边坡支护设计与施工

南京某深基坑周边环境较为复杂,地下水丰富,基坑开挖较深,最深处达13m。根据周边环境的情况和开挖的深度,采用了钻孔灌注桩和土钉锚杆相结合的支护形式。分析了该基坑施工的技术要点,通过监测结果,说明该支护方案是可行的。     

考虑基坑开挖影响的群桩基础竖向承载性状数值分析

对土体采用Mohr-Coulomb弹塑性本构模型,用接触面单元模拟桩-土相互作用,利用ABAQUS建立桩筏基础--地基--基坑开挖三维有限元分析模型。对基坑开挖影响下的群桩基础竖向承载性状进行了分析,讨论了桩顶反力分布、桩身轴力、桩侧摩阻力以及开挖引起的桩身水平位移及其弯矩的变化规律,并进行了考虑基坑开挖与不考虑基坑开挖的群桩基础竖向承载性状的对比分析。通过研究,取得了基坑开挖对高层建筑桩筏基础影响的基本认识,这些认识对于改进桩筏基础设计理论有一定的参考意义。     

考虑K0固结的软土基坑工程三维非线性有限元分析

为考虑基坑工程的空间效应,土体分别采用K0固结和正常固结试样固结不排水(CU)试验得到的Duncan-Chang非线性弹性模型和Mohr-Coulomb理想弹塑性模型,运用ABAQUS软件按照三维实体单元、壳单元、梁单元考虑接触相互作用的耦合有限元法,建立了福州市一典型软土基坑工程整体三维有限元分析模型,对基坑施工的各工况下整个体系的响应进行了数值模拟"目标试验",并与实测结果和二维数值模拟结果进行了比较,结果表明,采用K0固结试样CU试验参数的Duncan-Chang模型对该基坑进行的三维非线性数值模拟分析方法是较为可靠的,较之Mohr-Coulomb理想弹塑性模型和二维有限元分析,其结果的优势是明显的,是值得推崇和具有较好应用价值的方法。     

深基坑开挖过程及空间效应影响的数值模拟

结合工程实例,利用FLAC3D有限差分法分析软件对某双排桩深基坑的开挖和支护过程进行模拟,分析了开挖过程中土体位移场的变化规律。研究表明,支护结构和土体的空间受力性状、土与支护结构之间的相互作用以及施工开挖过程等均会对支护结构的受力状态和变形特性产生显著影响。此外,还讨论了双排桩之间土体刚度的变化对位移和沉降的影响,表明适当地增加两排桩之间土体的刚度能有效地减小基坑外侧土体的位移。     

考虑土体硬化的基坑开挖性状及隆起稳定性分析

基坑开挖过程中,土体应力路径、卸载回弹再压缩特性与简单加载或卸载不同,采用常规的理想弹塑性模型模拟基坑开挖,得到的围护墙位移、坑内土体回弹以及坑外沉降较大。分析了基坑开挖不同区域土体的性状,采用土体硬化模型模拟基坑开挖的卸载与土体硬化行为,结合工程算例,对比土体硬化模型和理想弹塑性模拟以及实测的围护结构土压力、围护墙水平位移和坑外土体沉降,并利用强度折减法分析基坑的稳定性。计算结果表明,考虑土体硬化的HS模型有限元方法能体现土体卸载再加载与开挖的特性,所得土压力、围护结构水平位移以及基坑抗隆起稳定性符合软土地区基坑工程的实践。     

香港五号干线前期工程大直径钻孔桩塌孔事故的处理

主要介绍在香港五号干线前期工程石围角至柴湾角段的灌注桩桩墙施工中的塌孔事故的处理。     

天津某深基坑工程施工监测及数值模拟分析

本文介绍了天津铜锣湾广场深基坑工程开挖实例。通过对开挖过程中的支护结构内力、坑周土体水平位移等的现场监测和数值模拟分析,讨论了基坑开挖过程中支护结构受力的特点及其对周围环境的影响,得到了基坑周边土体水平位移的变化规律,为考虑施工因素的支护结构设计提供了依据。     

土钉支护施工阶段土钉轴力的计算分析

土钉支护的形成过程对其受力和变形有直接影响。结合土钉支护边开挖、边支护且开挖在前的施工特点,定义开挖影响面以反映土体开挖时边坡滑动趋势的变化,与开挖影响面相交处的土钉在土体开挖时轴力增加最多。开挖土压力表示以开挖影响面为边界的土块体产生的土压力,由相邻两步开挖土压力之差求得挖土引起的不平衡力。增加的土钉轴力在平衡此不平衡力的同时,维持着边坡稳定。据此,以开挖影响面及其土钉轴力增量为主要研究对象,提出了施工阶段土钉轴力的分析方法。该方法在求得土钉轴力最大值及其作用位置的同时,还给出了土钉轴力沿钉长方向的分布。应用此方法对法国CLOUTERRE项目1号墙足尺试验进行了计算,结果表明：计算与测试结果吻合良好。     

Three-Dimensional Back-Analysis of an Instrumented Shallow Tunnel Excavated by a Conventional Method

The excavation of a shallow tunnel induces deformations of the soil volume in the vicinity and above the tunnel and consequently on the nearby buildings. The range of these deformations depends among other on the geological conditions, the geometry of the tunnel, and the excavation method. In this context, this research focuses on the 3D numerical modeling of a shallow tunnel instrumented during its construction, located on the Toulouse (France) subway line B for which the excavation has been carried out in a conventional manner in an over consolidated molassic geological context. The objective of this analysis is to estimate the tunnel behavior in terms of vertical and horizontal movements of the surrounding soil and the deformations of the existing buildings. The explicit finite differences numerical code FLAC^3D is used to model the various implementation phases of the work where the fluid–soil interaction is taken into account through an undrained coupled analysis. The results of this 3D model are compared to those of the in situ measurements in order to validate the geotechnical characteristics of the molasses. The latter are a useful basis for the back-analysis of the different monitoring sections implemented in areas where the tunnel excavation is made by TBM with pressurized front.     

开挖卸荷对既有群桩竖向承载性状的影响分析

既有建筑下增层开挖对已有桩基础的影响不同于基坑开挖对坑内桩基的影响。基于工程实例验证的有限元参数,用硬化土弹塑性模型模拟土体,用接触面单元模拟桩土相互作用,建立了桩筏基础-地基-增层开挖三维有限元模型,对增层开挖后群桩基础的竖向承载性状进行研究。分析了桩顶刚度、桩身轴力、桩侧摩阻力、桩端阻比以及土体回弹的变化规律,并研究了不同桩端土体刚度和增层开挖深度对这些参数的影响。结果表明,增层开挖后群桩中不同基桩表现出不同的承载性状,增大桩端土体刚度可明显提高单桩承载力和端阻比临界值;随着增层开挖深度的增加,侧阻和端阻的发挥程度也随之提高。研究结果有望为地下室增层开挖施工中的结构托换变形控制和补桩设计提供依据。