Construction Technology and Safety Risk Control Measures of Deep Foundation Pit Excavation

Deep foundation pit excavation is a basic and key step involved in modern building construction.In order to ensure the construction quality and safety of deep foundation pits,this paper takes a project as an example to analyze deep foundation pit excavation technology,including the nature of this construction project,the main technical measures in the construction of deep foundation pit,and the analysis of the safety risk prevention and control measures.The purpose of this analysis is to provide scientific reference for the construction quality and safety of deep foundation pits.

Deep Foundation Pit Excavations Adjacent to Disconnected Piled Rafts: A Review on Risk Control Practice

Foundation pit excavation engineering is an old subject full of decision making. Yet, it still deserves further research due to the associated high failure cost and the complexity of the geological conditions and/or the surrounding existing infrastructure around it. This article overviews the risk control practice of foundation pit excavation projects in close proximity to existing disconnected piled raft. More focus is given to geotechnical aspects. The review begins with achievements to ensure excavation performance requirements, and follows to discuss the complex soil structure interaction involved among the fundamental components: the retaining wall, mat, piles, cushion, and the soil. After bringing consensus points to practicing engineers and decision makers, it then suggests possible future research directions.

Deformation Control of Deep Excavation Pit and Numerical Simulation with Finite Element Method

The authors firstly introduce deformation control of deep excavation pit indetail, and then put forward new conceptions such as: effective coefficient of excavation pit,effective area, ineffective area and critical line, and also put forward the referential criteria ofdeformation control. The System of Optimization Design with Deformation Control of Deep ExcavationPit and Numerical Simulation with Finite Element Method (SDCDEFEM) is also briefly introduced.Factors influencing deformation of excavation pit are analyzed by the system. The measured andsimulated data of maximum deformations (settlement, displacement and upheaval) and their positionsare analyzed and discussed. The statistic formula estimating maximum deformations and theirpositions was gained, and economical-effective measures of deformation control were brought forward.

Research of deformation prediction method of soft soil deep foundation pit

In view of the characteristics of soft soil deep foundation pit for the construction and geotechnical characteristics of the special medium,it is difficult to calculate theoreti- cally accurately structural deformation of the foundation pit,so in the course of excavation on the construction of the information is particularly important.The analysis and compari- son of several popular non-linear forecasting methods,combined with the actual projects, set up a grey theoretical prediction model,time series forecasting model,improved neural network model to predict deformation of the foundation pit.The results show that the use of neural network to predict with high accuracy solution,it is the foundation deformation prediction effective way in underground works with good prospects.

Deformation analysis and safety assessment of existing metro tunnels affected by excavation of a foundation pit

The excavation of a foundation pit considerably affects the adjacent structures and underground pipelines owing to the change in the stress state of the surrounding soil,resulting in deformation.The study of an actual engineering case was conducted to examine the influence of excavation on the deformation of adjacent subway tunnels.The finite element analysis software PLAXIS 3D was used to simulate the entire excavation process.The structural design of the foundation pit was optimized based on the simulation results to ensure the stability of the foundation pit and the safety of the existing subway tunnel structure.Finally,the safety evaluation of the excavation of the foundation pit that caused the deformation of the adjacent subway tunnel was performed.The influence of the excavation and unloading of the foundation pit on the subway tunnel is closely related to the distance between the subway and the foundation pit,the amount of earthwork excavated at one time,and the engineering geological conditions.The results of this paper can provide useful reference for the design optimization and safety assessment of similar projects.

Intelligent feedback analysis on a deep excavation for the gravity anchorage foundation of a super suspension bridge

In order to ensure the construction safety of the 38.5 m deep excavation for the gravity anchorage foundation of Fuma Yangtze River Bridge, an intelligent feedback analysis was applied to this excavation project. First, a three-dimensional numerical model that simulating the construction process of the excavation was built,and the deformations of the supporting structures were calculated by the finite difference program FLAC3 D. Then,the non-linear mapping relationship between the geomechanical parameters and the excavation-induced displacements was established by the back-propagation neural network(BPNN). Last,the geomechanical parameters were optimized intelligently by the genetic algorithm(GA) based on the developed BPNN model and the measured displacements,and the deformations during the subsequent excavation stages were predicted based on the back-calculated parameters. The research results showed that:the back-calculated values of E1,μ1,c1,and φ1 of the completely weathered stratum,and E2 of the heavily weathered stratum were greater than the initial values,while the inversion value of E3 of the moderately weathered stratum was smaller than the initial value. The magnitudes and the variation tendencies of the predicted displacements were in good accordance with the measured displacements. At the end of the excavation,the retaining piles and the top beams had a maximum displacement of 15–20 mm,exhibiting a quite small magnitude as comparing with other case histories. Local concentration of shear stress mainly occurred at the soil-pile interface and at the toe of the excavation slope,and the plastic zones mainly appeared in the completely weathered stratum. After the completion of the excavation,there were no yielding elements in the model,and the convergence of the numerical computation was achieved,indicating the excavation was in a stable state. This study lays the basis for the subsequent construction and operation of the bridge,and offers a significant reference for the feedback analysis of similar anchorage excavation projects.

Influencing factors and control measures of excavation on adjacent bridge foundation based on analytic hierarchy process and finite element method

Many uncertain factors in the excavation process may lead to excessive lateral displacement or overlimited internal force of the piles,as well as inordinate settlement of soil surrounding the existing bridge foundation.Safety control is pivotal to ensuring the safety of adjacent structures.In this paper,an innovative method is proposed that combines an analytic hierarchy process(AHP)with a finite element method(FEM)to reveal the potential impact risk of uncertain factors on the surrounding environment.The AHP was adopted to determine key influencing factors based on the weight of each influencing factor.The FEM was used to quantify the impact of the key influencing factors on the surrounding environment.In terms of the AHP,the index system of uncertain factors was established based on an engineering investigation.A matrix comparing the lower index layer to the upper index layer,and the weight of each influencing factor,were calculated.It was found that the excavation depth and the distance between the foundation pit and the bridge foundation were fundamental factors.For the FEM,the FE baseline model was calibrated based on the case of no bridge surrounding the foundation pit.The consistency between the monitoring data and the numerical simulation data for a ground settlement was analyzed.FE simulations were then conducted to quantitatively analyze the degree of influence of the key influencing factors on the bridge foundation.Furthermore,the lateral displacement of the bridge pile foundation,the internal force of the piles,and the settlement of the soil surrounding the pile foundation were emphatically analyzed.The most hazardous construction condition was also determined.Finally,two safety control measures for increasing the numbers of support levels and the rooted depths of the enclosure structure were suggested.A novel method for combining AHP with FEM can be used to determine the key influencing aspects among many uncertain factors during a construction,which can provide some beneficial references for engineering design and construction.

Shanghai center project excavation induced ground surface movements and deformations

Empirical data on deep urban excavations can provide designers a significant reference basis for assessing potential deformations of the deep excavations and their impact on adjacent structures. The construction of the Shanghai Center involved excavations in excess of 33-m-deep using the top-down method at a site underlain by thick deposits of marine soft clay. A retaining system was achieved by 50-m-deep diaphragm walls with six levels of struts. During construction, a comprehensive instrumentation program lasting 14 months was conducted to monitor the behaviors of this deep circular excavation. The following main items related to ground surface movements and deformations were collected： （1） walls and circumferential soils lateral movements; （2） peripheral soil deflection in layers and ground settlements; and （3） pit basal heave. The results from the field instrumentation showed that deflections of the site were strictly controlled and had no large movements that might lead to damage to the stability of the foundation pit. The field performance of another 21 cylindrical excavations in top-down method were collected to compare with this case through statistical analysis. In addition, numerical analyses were conducted to compare with the observed data. The extensively monitored data are characterized and analyzed in this paper.

开挖顺序及内支撑早期刚度对软土基坑稳定性影响规律

深基坑开挖变形控制是软土基坑工程的难题,工程中常采用密集的混凝土内支撑来提高支撑刚度,然而实际工程中未能合理考虑软土深基坑的开挖顺序和混凝土内支撑早期刚度影响,往往导致基坑实际变形远大于设计值。结合珠三角水资源配置工程的软土深基坑,利用验证后的数值模拟方法研究了不同开挖顺序及混凝土内支撑早期刚度对基坑受力和变形特性的影响规律,得出了分层分区非对称开挖和对称开挖对地连墙水平位移和地表沉降的影响规律。结果表明:尽管非对称开挖会造成基坑两侧的地连墙位移和地表沉降差异,但是密集内支撑条件下非对称开挖比对称开挖对于基坑周围地表沉降和地连墙水平位移影响更小并能够大量节约工期;受混凝土内支撑早期刚度影响,施作内支撑后越早进入下一步开挖,地连墙水平位移和周围地表沉降越大;考虑内支撑早期刚度影响时基坑上部的内支撑轴力会增大,而下部的内支撑轴力会减小,早期刚度越小,这种趋势越明显;现场施工时难以实时动态监测混凝土的弹性模量,极易造成内支撑轴力的早期监测数据失真,甚至给工程带来错误决策。

软黏土地区深大基坑工程桩-连续墙体系空间变形效应

土体开挖过程中深大基坑存在明显的空间变形效应,对其主体结构力学行为产生影响。以新建苏州南站枢纽深大基坑为工程背景,考虑地连墙厚度及工程桩的影响,基于ABAQUS建立三维空间有限元模型,采用修正剑桥模型考虑软黏土的应变硬化特征,从地连墙侧向位移、墙后地表沉降、工程桩和坑底土体隆起3个方面,研究软黏土地区深大基坑工程桩-连续墙体系的变形特征及空间效应。研究结果表明:深大基坑地连墙的侧向位移和地表沉降均呈现明显的角部效应,基坑A区(地连墙厚度1 m、桩径1 m)地连墙侧向位移角部效应的影响范围为1.05~1.71倍开挖深度,基坑B区(地连墙厚度0.8 m、桩径1.5 m)地连墙侧向位移角部效应的影响范围为0.91~1.48倍开挖深度;基坑A区、B区地表沉降角部效应的影响范围分别为1.24~2.10和1.77~2.47倍开挖深度;深大基坑空间变形角部效应与工程桩桩径及地连墙厚度有关,并且对基坑长边的影响效果大于短边。工程桩可有效增大基坑开挖面以下土体的刚度,减小地连墙的侧向位移;增大地连墙厚度可有效降低基坑的整体变形。基坑工程桩与坑底土体隆起总体呈现中间大、周边小的特征,工程桩周边土体受其约束,隆起较小;在一定范围内,工程桩对土体隆起抑制作用与其桩径呈正相关,与其桩间距呈负相关。软黏土地区深大基坑设计计算中需关注工程桩-连续墙结构体系的空间变形效应。

基于Midas GTS的某近海深基坑开挖三维有限元数值模拟分析

为研究近海深基坑在桩锚支护体系下土体开挖变形规律,考虑水流渗透作用,运用Midas GTS对某深基坑开挖过程进行模拟,并结合已有资料进行分析。结果表明:该深基坑开挖后的第一步为最危险施工步,桩顶土体最大位移为13.97 mm,越往下开挖支护体系越完善,位移变化越小,实际监测最大位移值为15.36 mm;整个等效地连墙在钢腰梁与锚索位置处向基坑内部变形收缩程度最大,为6.6 mm,符合实际变形规律。Midas GTS能有针对性地指导现实施工阶段,可为基坑监测布置设置点提供参考价值。

软土地层临近地铁基坑施工影响控制研究

随着城市地铁沿线地下空间的持续开发,基坑施工对临近地铁变形影响控制越来越受到关注。以上海某邻近地铁的基坑工程为例,通过实测分析和数值计算的方法研究了基坑开挖和拆撑对临近隧道变形的影响,并对伺服系统控制临近隧道变形进行定量分析。结果表明:覆存于软土地层中的隧道对临近施工扰动非常敏感,尽管大区基坑与隧道间距超过2.5倍开挖深度,隧道变形仍超过了监护要求的控制值;大区地下结构施工时拆撑对隧道收敛变形影响超过开挖影响的50%;应力伺服支撑系统对控制临近隧道变形比较有效,适当的“负位移”可以对隧道变形产生有利效果。

成都膨胀土地区深基坑降雨稳定性分析与变形预测

成都膨胀土分布广泛,降雨是引发该地区深基坑工程事故的一个重要因素。基于工程实例的调研,分析了降雨入渗作用下排桩支护的成都膨胀土地区深基坑的典型变形失稳过程;总结了基坑常见破坏类型;基于工程实测数据,将基坑水平变形曲线概括为3种类型:陡变型、渐变型和稳定型。根据该地区深基的变形失稳的发展过程以及降雨初期3种不同类型实测水平变形曲线的发展规律,指出成都膨胀土地区深基坑工程的早期的风险防范可以以围护结构的水平变形为依据。基于小波分析、人工神经网络和Copula随机变量相关性分析方法,建立了考虑降雨影响的基坑水平变形预测模型,并对实际深基坑的变形进行了预测,基于预测的变形提前给出了风险预警。变形的预测结果与实测结果具有较好的一致性,初步证实了所提出变形预测模型的有效性。在采用相同的变形预警指标情况下,根据预测变形进行风险预警能够大幅提前预警时间,为优化处理方案提供依据。

基于离心机和数值模拟的深基坑开挖支护结构受力和变形研究

依据实际基坑开挖和支护结构工程特性进行模型概化,得出适合的离心模型试验方案,采用离心模型试验分析超深基坑开挖过程支护结构工程的受力和变形分布规律特征。同时建立三维有限元模型对圆形基坑开挖过程中支护结构的变形进行数值模拟,并将试验和数值计算结果进行对比分析。结果表明:支护结构水平位移模式为两头小,中间大的“涨肚型变形”;并且随着基坑深度的加深,地下连续墙水平位移最大值点逐渐下移;地表沉降呈凹槽形沉降形式,随着开挖的进行,沉降槽底部向着远离基坑的方向发展;地连墙背后土压力变化值呈非线性,开挖初期,土压力沿深度变化很小,但随着开挖的进行,土压力变化量逐渐增大;数值分析与离心模型试验的开挖支护结构变形特征结果较相近。研究成果可为基坑开挖设计切实有效的支护结构提供科学依据。

盾构机始发井深基坑力学和变形特性离心模型试验研究

为探究盾构机始发井深基坑开挖过程中周边土体的应力、变形和土压力分布规律,通过对工程原型进行概化,在典型土层条件下,采用停机-开挖-支护的模拟方式开展了方形基坑和圆形基坑两种方案共2组离心模型试验,从地连墙的弯矩和其后土体的水平位移、基坑外侧土压力分布以及地表沉降等角度,对比分析了两种盾构机始发井深基坑开挖过程的工程特性。结果表明:地面沉降随基坑开挖深度增大逐渐增加,形成沉降槽状;地连墙土压力变化呈非线性,随着基坑开挖不断深入的,离地表较近处的土压力逐渐变大,而深处的土压力则逐渐减小;在第5步开挖时2组模型地下连续墙水平位移均达到最大值,圆形基坑是方形基坑的1.4倍,同时地连墙的弯矩也达到最大值,圆形基坑比方形基坑小320 kN·m。研究成果可为深埋隧道深基坑的优化设计和开挖提供指导。

盖挖法地铁基坑施工对围护结构及其与立柱差异沉降影响

本文依托武汉地铁12号线中一路站基坑工程,采用Plaxis3D有限元软件建立数值模型,研究不同施工阶段地连墙水平位移、地表沉降和地连墙与立柱差异沉降的影响及变化规律;与明挖法相比,采用主体结构多次分段浇筑并架设内支撑方法对原方案进行优化分析。结果表明:随着基坑的开挖,地连墙水平位移和地表沉降逐渐增大,而地连墙与立柱的差异沉降呈现波动变化;与明挖法进行对比,盖挖法可有效控制基坑变形和降低对周边环境的影响;随着分段浇筑的层数和支撑道数的增加,基坑变形有减小的趋势,地连墙与立柱的差异沉降依旧呈现波动变化,当每个楼板间分3层浇筑且中间架设两道撑时,地连墙最大水平位移可减少22.47%左右,最大地表沉降可减少19.01%左右,本文得到的结论可为类似工程的设计和施工提供参考。

旁侧深大基坑开挖对邻近地铁隧道变形影响研究

为探究深基坑开挖对邻近地铁隧道的影响,文中以杭州某邻近地铁隧道的深基坑工程为背景,建立坑底在隧道正侧方的三维模型,分析基坑开挖过程中邻近隧道的变形规律,并结合研究结果提出相应的监测方法。结果表明基坑开挖时要重视隧道顶部的裂缝监测;隧道变形以隆起为主,并产生远离基坑的水平位移及扭转;基坑与隧道水平距离超过25m后基坑开挖对隧道基本没有影响;基坑底板的施工能有效控制隧道变形,研究结果可为类似工程施工提供参考。

西安湿陷性黄土地区狭长深基坑变形分析

为了探究西安地铁车站深基坑开挖过程中桩体侧移、地表沉降等时空演变规律,以西安地区某地铁车站深基坑为工程背景,同时收集了该地区2#、3#、4#线20个狭长基坑的实测数据,并对其进行统计分析。结果表明:地铁基坑围护结构最大侧移δhm=0.04%H~0.10%H(H为开挖深度),平均值为0.06%H,最大侧移位置深度为0.7H;地表最大沉降值δvm=0.036%H~0.116%H,主要影响区在1.4H范围以内;高斯函数模型可以预测西安地区的地表沉降,其影响范围为3.5H;在特定范围内,提高支护桩的刚度和插入比可以有效控制基坑变形。研究结果可为西安市其他地铁车站基坑支护结构的设计与施工提供借鉴。

基于SSA-BP的深基坑地表变形预测研究

文中采用麻雀搜索算法优化BP神经网络,对深圳市某在建地铁车站深基坑周围地表沉降监测点进行变形预测。通过对基坑地表变形监测点DBC16-4的118期监测数据进行训练学习,并与粒子群算法优化BP神经网络、遗传算法优化BP神经网络和标准BP神经网络横向对比,验证了训练效果。结果表明,麻雀搜索算法对BP神经网络权重寻优速度较快,收敛精度更高,麻雀搜索算法优化BP神经网络模型预测平均相对误差仅为1.72%,拟合精度较其他算法更高,预测效果良好。

基于模态分解方法的深基坑支护桩水平变形预测

为了预测深基坑支护桩水平变形的长期发展规律,在卷积神经网络(convolutional neural network,简称CNN)数据空间特征提取基础上,结合长短时记忆神经网络(long and short term memory,简称LSTM)分析数据的时序性和注意力机制(attention mechanism,简称AM)的划分特征权重,构建了能够预测支护桩变形的AM-CNN-LSTM模型。以北京地区某深基坑工程为背景,基于灰色关联方法明确了影响支护桩最大变形的因素,通过构建的模型分析支护桩的单点变形规律,并与反向传播神经网络(back propagation neural network,简称BPNN)、CNN和传统CNN-LSTM模型的预测所得结果进行比较分析。研究结果表明:支护桩最大变形值与深基坑开挖深度、临空天数、支撑内力、土壤性质、桩的尺寸和嵌固深度等因素关联度较高;AM机制显著提升了初始数据信息挖掘深度和变形预测精度,通过梯度下降法不断更新直至满足误差要求;与BPNN、CNN及CNN-LSTM模型相比,AM-CNN-LSTM模型的应用对于支护桩的长期变形预测稳定性较好;通过与实测数据对比,AM-CNN-LSTM模型的预测精度误差在5%~10%以内。