Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from-16.6 to-43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.

Optimized functional linked neural network for predicting diaphragm wall deflection induced by braced excavations in clays

Deep excavation during the construction of underground systems can cause movement on the ground,especially in soft clay layers.At high levels,excessive ground movements can lead to severe damage to adjacent structures.In this study,finite element analyses(FEM)and the hardening small strain(HSS)model were performed to investigate the deflection of the diaphragm wall in the soft clay layer induced by braced excavations.Different geometric and mechanical properties of the wall were investigated to study the deflection behavior of the wall in soft clays.Accordingly,1090 hypothetical cases were surveyed and simulated based on the HSS model and FEM to evaluate the wall deflection behavior.The results were then used to develop an intelligent model for predicting wall deflection using the functional linked neural network(FLNN)with different functional expansions and activation functions.Although the FLNN is a novel approach to predict wall deflection;however,in order to improve the accuracy of the FLNN model in predicting wall deflection,three swarm-based optimization algorithms,such as artificial bee colony(ABC),Harris’s hawk’s optimization(HHO),and hunger games search(HGS),were hybridized to the FLNN model to generate three novel intelligent models,namely ABC-FLNN,HHO-FLNN,HGS-FLNN.The results of the hybrid models were then compared with the basic FLNN and MLP models.They revealed that FLNN is a good solution for predicting wall deflection,and the application of different functional expansions and activation functions has a significant effect on the outcome predictions of the wall deflection.It is remarkably interesting that the performance of the FLNN model was better than the MLP model with a mean absolute error(MAE)of 19.971,root-mean-squared error(RMSE)of 24.574,and determination coefficient(R^(2))of 0.878.Meanwhile,the performance of the MLP model only obtained an MAE of 20.321,RMSE of 27.091,and R^(2)of 0.851.Furthermore,the results also indicated that the proposed hybrid models,i.e.,ABC-FLNN,HHO-FLNN,HGS-FLNN,yielded more superior performances than those of the FLNN and MLP models in terms of the prediction of deflection behavior of diaphragm walls with an MAE in the range of 11.877 to 12.239,RMSE in the range of 15.821 to 16.045,and R^(2)in the range of 0.949 to 0.951.They can be used as an alternative tool to simulate diaphragm wall deflections under different conditions with a high degree of accuracy.

Numerical analysis of effect of friction between diaphragm wall and soil on braced excavation

A plane strain finite element model was established to investigate the effect of friction between diaphragm wall and soil on braced excavation. The behavior of interface between diaphragm wall and soil was simulated with the interface model of ABAQUS. Parametric studies were conducted with different diaphragm wall external friction angles δ. The results show that deflection of diaphragm wall and ground surface settlement decrease with the decrease of δ. However, the reduction effect on diaphragm wall deflection is the most significant at the depth where the maximum wall deflection occurs and can be neglected at the wall base. The ratio between wall deep inward component and wall cantilever component reaches its peak value 2.7 when δ=5°. The ratio of the maximum ground surface settlement to the maximum wall lateral deflection decreases at a reduced rate with the increase of δ. For excavation with braced diaphragm wall, the effect of friction between diaphragm and soil on the deflection of diaphragm wall and ground settlement, especially the distribution of ground surface settlement behind diaphragm, should be taken into account.

Test and Analysis of an Existing Tunnel Vibration Induced by Blasting Construction of Diaphragm Wall

According to the blasting construction of the diaphragm wall of Puxi approaching section of East Fuxing Road river-crossing tunnel, the monitoring project of the vibration of the existing tun-nel induced by the blasting construction is put forward, which includes the sensors’ location, moni-tor method and the vibration monitoring system. Based on the monitoring data of the explosion vibration, the vibration wave forms, velocities, acceleration responses, main frequencies and fields of measure points are analyzed under the conditions of three locations and different charge quanti-ties. According to the safety-judging standard of explosion vibration, the conclusion that the exist- ing tunnel is safe under the explosion vibration is then drawn. Furthermore, the spectrum character-istics of three explosion vibrations and the spectrum changing properties of explosion vi-bration wave transmitting in different directions are concluded, which can provide reference to similar projects.

Construction of an ultra-deep diaphragm wall:a case study

This paper presents a case study on an ultra-deep diaphragm wall with a depth of 110 m constructed in Ningbo City. The in-situ application shows that using Bauer BC40 cutter machine in conjunction with cutter wheels specified for different strata would be qualified for constructing the 110 m diaphragm wall with high efficiency and precision given that the quality of slurry and poured concrete can be guaranteed. The ground settlement can be effectively controlled by using the overlapping construction method. Sliding failure as a whole characterized by pronounced lateral deformation is likely to occur in the upper muddy clay layer due to its high compressibility and sensitivity. In contrast, local collapse of trench walls tends to happen in the sandy silt strata. Furthermore, careful attention should be paid to sandy silt during the entire construction period as the vertical displacement of the sandy silt continues to develop even atter concrete pouring.

北京地铁薄壁地下连续墙基坑变形规律统计分析

[目的]地下连续墙(以下简称“地连墙”)厚度对基坑变形量与变形模式的影响较为明显,尤其是薄壁地连墙对于基坑开挖和坑边荷载十分敏感。因此,有必要对北京地铁不同壁厚的地连墙基坑变形规律进行研究。[方法]以北京地铁6座薄壁地连墙车站基坑工程为例,使用统计学方法对其地面竖向变形、地连墙墙顶竖向变形、地连墙水平位移及钢支撑轴力的监测数据进行统计分析。[结果及结论]薄壁地连墙基坑的地面变形可分为隆起-沉降型和沉降型两种,且地面变形中隆起变形的占比较高。土方开挖阶段,地连墙呈快速隆起趋势,开挖结束后墙顶隆起值产生小幅回落,并最终达到稳定值。根据地连墙变形特征,将其分为正常抛物线型、荷载抛物线A型、荷载抛物线B型3种类型。地连墙最大变形位置一般处于0.50H~0.85H(H为基坑深度),累计变形量集中于10~20 mm。第1道钢支撑轴力变化特征较为明显,其曲线可分为快速上升、快速下降、缓慢下降、快速上升4个阶段。

装配式地连墙接缝变形特性研究

为了研究接缝对装配式地连墙变形特性的影响并采取合理的改进措施,采用现场试验的方法将局部现浇地连墙替换为装配式地连墙,并对其实测的变形数据进行了对比分析;在实测数据分析的基础上,通过PLAXIS2D有限元软件模拟分析了不同深度横向接缝对装配式地连墙变形特性的影响规律;通过PLAXIS3D有限元软件对比了现浇墙与装配式地连墙水平方向及竖直方向变形形态,分析了竖向接缝对装配式地连墙变形特性的影响规律;最后,针对实测数据显示出的变形偏大问题,提出了改变冠梁刚度、围檩刚度、墙幅宽度、支撑位置4种改进措施,并通过PLAXIS3D有限元软件进行了验证。结果表明:装配式地连墙的接缝对墙体的刚度存在弱化作用,增大了其变形程度;位于坑底以上的横向接头失效时,墙体水平位移会显著增大,位于坑底以下的横向接头失效时,不会引起墙体水平位移的显著增大;竖向接头会显著减小装配式墙体的整体刚度,使其协同变形能力变差,变形更加难以控制;适当调节支撑位置可有效缓解墙幅间的错动和减小墙体变形。

南京仙新路长江大桥主桥结构设计

南京仙新路长江大桥主桥为跨径1760 m的单跨钢箱梁悬索桥,主缆垂跨比1/9,边跨跨径580 m,边中跨比0.33。该桥上、下游各设1根主缆,单根主缆由169股127∅5.4 mm镀锌铝高强钢丝索股组成,采用PPWS法施工,钢丝标准抗拉强度2100 MPa。吊索与索夹采用销接式结构,跨中设置柔性中央扣索,短吊索设置关节轴承。主索鞍采用宽鞍槽单纵肋铸焊结合构造,散索鞍采用底座式全铸结构。加劲梁采用扁平流线型封闭整体钢箱梁,总宽31.5 m,梁高4 m,顶板与U肋之间采用双面埋弧全熔透焊接。桥塔采用门形混凝土结构,总高277.3 m,其上横梁为预应力混凝土结构,外包N字造型钢结构;桥塔基础采用直径2.8 m钻孔灌注桩。南锚碇采用外径65 m圆形地下连续墙基础;北锚碇采用沉井基础,平面尺寸70 m×50 m,高50 m。对结构进行静力分析及抗风性能理论和试验研究,结果表明:结构强度、刚度均满足规范要求;在加劲梁上设置0.67 m高中央稳定板、两侧风嘴处设置1 m宽水平稳定板后,大桥的颤振、涡振等抗风性能均满足要求,且具备一定的阻尼储备。

开洞刚性墙后土体破坏模式运动单元上限分析

超载作用下开洞地下刚性连续墙后土体稳定性问题具有广泛的工程应用背景。考虑墙体矩形开洞宽度足够大,且刚性墙与土体完全光滑接触等条件,参考构造刚性多块体上限法思路,预设墙后土体极简破坏机构并转化为初始三角网格,提出交替循环式新增节点、单元缩减、对角互换及成层加密等网格更新措施,同步开展基于刚体平动运动单元上限方法的刚性墙后土体破坏分析,通过多层级非线性规划运算获得高精度临界超载系数σ_(s)/c上限解,界定墙后土体滑移线网破坏模式及其定量范围与扩展边界。进一步依前述流程计算获得多参数影响下的临界超载系数σ_(s)/c上限解图表,结果表明:1)刚性墙后土体典型破坏特征表现为土体上方和开洞处的整体下沉刚性区域,开洞前方核心区域以密集滑移线网区隔的刚性滑块群,以及外围主要滑动面;2)临界超载系数σ_(s)/c随土体内摩擦角ϕ增大而增大,且后续增幅显著增加;3)σ_(s)/c随开洞深度比C/D的增大呈现出近似线性增大的趋势;4)相较于ϕ和C/D,土体重度参数γD/c的变化对于σ_(s)/c的影响不甚明显,σ_(s)/c随γD/c增大呈现近似线性减小的趋势;5)σ_(s)/c受剪胀角参数ψ/ϕ的影响与ϕ的增大程度密切相关。本研究结果可为开洞地下刚性连续墙后土体失稳破坏模式定量分析、破坏机理研究及工程措施制定等工作提供有力的数据支持。

饱和黏性土地基中邻近圆形基坑相互影响分析

间距很小的圆形基坑后方土压力分布复杂,很难判断考虑圆拱效应的竖向弹性地基梁法和三维弹性地基板法的适用性。采用三维连续介质有限元法对位于饱和黏性土中2个间距很小的由地下连续墙组成的圆形事故应急池基坑进行整体建模分析。结果表明,对于1号池开挖到底后和2号池开挖到底后2种工况,地下连续墙的最大水平位移和最大弯矩的差异都不大。基坑位移和竖向弯矩的实测结果与计算值也较为接近。2个相邻圆形基坑在间距很小的情况下,其相互影响也是非常有限的。圆形地下连续墙支护结构对周围土体刚度参数的变化不敏感。采用三维连续介质有限元法对圆形地下连续墙支护结构进行计算分析,得到的结构内力远小于考虑圆拱效应的竖向弹性地基梁法和三维弹性地基板法,在经济性上有明显优势。

基于粒子图像测速技术的节状地下连续墙变形特性与破坏模式研究

节状地下连续墙(简称节状墙)是一种新型的地基基础形式,具备良好的工程特性,相对于传统地下连续墙而言,由于节部的存在,其抗拔承载力得到了有效地提升。目前,节状墙的应用及研究尚处于起步阶段,其变形特性与破坏模式亟待摸清。通过室内模型试验辅以粒子图像测速(particle image velocimetry,简称PIV)技术对节状墙基础竖向受拉下的位移和破坏形态开展了分析,研究结果表明:端部与中部节的设置扩大了深部与浅部土体的影响范围,多部节的设置相对于单部节有利于调动更广范围的土体。节状墙的破坏模式包括垂直滑移面、倒金字塔状或正切曲线和花瓶状曲线(即曲线滑移面)相连接的滑移面。总体而言,与抗拔桩相比,节状墙的抗拔破坏面受到节部数量和位置的影响而表现为复合型,且部分滑移面的走向与土体内摩擦角有关。

地下连续墙施工过程引起的地层和既有隧道变形响应分析

为探究地连墙施工对既有隧道变形影响,以武汉某基坑工程为例,分析地下连续墙施工过程中周边土体变形规律和既有结构变形响应特征,以为优化地连墙施工过程控制隧道变形提供指导。采用PLAXIS 2D软件,建立二维数值模型,研究地连墙开挖成槽和混凝土浇筑2个阶段引起的土体变形和隧道变形特征,分析不同泥浆重度下地层变形和既有隧道变形演化规律。结果表明,地下连续墙施工引起的地层变形受泥浆重度影响显著,泥浆重度较大时泥浆压力大于土体水平压力,引起周边土体向槽外变形。地下连续墙施工完成后引起的地层水平位移随泥浆重度增大而增大,既有隧道拱顶沉降随泥浆重度增加而显著减小。当泥浆重度从12 kN/m3增至14 kN/m3时,施工引起土体的最大水平位移从3.48 mm增长至22.04 mm,左线隧道拱顶沉降由5.54 mm减少至0.90 mm,右线隧道拱顶沉降由5.33 mm减少至0.84 mm。

地下连续墙新型刚性接头设计与研究

城市建设的快速发展促使对地下空间的需求日益增加,地下连续墙作为深大基坑的支护或结构外墙的应用也越来越多。针对“两墙合一”的地下连续墙,提出了一种新型刚性接头、防绕流接头板及施工工艺,能有效预防混凝土绕流问题,进而提高地下连续墙的整体刚度、强度以及接头的止水能力。通过数值模拟计算了刚性接头的承载力与延性,并与普通工字钢接头进行了对比。结果表明,采用新型刚性接头的地下连续墙,具有较好的刚度和受力性能,施工可行性高,具有较好的应用和推广前景。总结的经验可供今后类似工程借鉴。

渠式切割水泥土连续墙的应用和发展

作为超深、等厚、连续、可靠的截水挡土帷幕,渠式切割水泥土连续墙在国内有广泛应用。本文概括了渠式切割水泥土连续墙在国内的引进、应用和发展历程。结合对国内200多个既有应用案例的统计分析,研究了渠式切割水泥土连续墙的施工设备保有情况、成墙深度范围、常用墙体厚度和主要应用场景;罗列了渠式切割水泥土连续墙在复杂地层施工的辅助措施以及典型案例;指出了渠式切割水泥土连续墙在敏感环境微扰动施工的控制要点;介绍了渠式切割预制装配式混凝土连续墙的设计要求和用途,尤其是内插预制板材的截面设计和制造工艺;最后用工程实例表明渠式切割预制装配式混凝土连续墙同时继承了渠式切割微扰动和地下连续墙支护刚度好的特点,无论是切割成墙期间还是土方开挖期间,施工对周边环境的影响都较小。

T型超深地连墙钢筋笼制作控制要点分析

在超深异型地连墙施工中,钢筋笼制作对钢筋笼吊装及混凝土浇注等后续工艺有着重要影响,是整个超深异型地连墙施工控制的重中之重。基于天津某市政工程T型超深地连墙钢筋笼制作的施工经验和工程案例,从钢筋笼断面形式、钢筋笼分段位置和钢筋笼吊点加强区三方面提出T型超深地连墙钢筋笼制作控制要点,降低钢筋笼卡槽、撕裂以及槽壁坍塌、混凝土沉渣过厚等施工风险,确保后续混凝土浇注灌注和地连墙整体施工质量,为类似超深异型地连墙钢筋笼制作提供了施工经验和依据。

黏性土中条壁式地连墙基础竖向承载特性研究

条壁式地连墙基础长宽比较大,承载性能优越,施工速度快捷,具有较好工程应用前景。但其承载机理尚不明晰,通过建立条壁式地连墙基础数值模型,研究了条壁式地连墙基础竖向承载特性及墙-土相互作用机理,分析了墙体参数、土体参数等因素对条壁式地连墙基础竖向承载力的影响。计算结果表明,单幅地连墙每增加10 m埋深可使承载力提高34%~60%。每增加12 m墙长度可使承载力提高14%~89%。地连墙承载力随土体黏聚力和内摩擦角增加而基本呈线性增长,随土体弹性模量增加而呈对数增长。当墙顶荷载接近极限时,墙端附近形成一个锥形的潜在滑移面。地连墙长度越长,地连墙对周围土体影响范围越来越大,墙幅间对周围土体影响存在叠加效应。

上海超深基坑地下连续墙的空间变形特性实测分析

基于大量现场实测数据,对上海城区某大型通道工程超深基坑在开挖和降水耦合作用下地下连续墙的空间变形特性及规律进行了详细分析与研究。结果表明:①当基坑长深比、长宽比较小时,受坑角效应影响,地下连续墙变形呈现显著的三维效应,且局部出现较大横向挠曲,其中靠近坑角两侧的局部挠跨比大于中部,靠近坑角两侧的横向挠跨比平均值约为(0.32~0.56)δ_(h)/H_(e),跨中位置横向挠跨比平均值约为(0.15~0.23)δ_(h)/H_(e);②地下连续墙最大侧移δhm随着开挖深度H的增加而呈非线性变化,特别地当开挖深度H超过12m之后,其变化率明显增加;③大幅度坑内超前预降水的卸荷效应会造成地下连续墙先期变形显著增加,并导致后续累计变形的大幅增加,受此影响,基坑开挖期间地下连续墙最大侧移及竖向变形分别为0.7%He,0.1%He(回弹),坑中立柱回弹变形最大为0.2%He,约为地下墙回弹的两倍,各变形量均显著大于上海地铁标准车站基坑(深度16~20 m)的实测统计数据。因此,在超深基坑施工过程中应尽量采用分步降水方式,避免一次性预降水。

能源连续墙对室内侧壁面热湿耦合传递行为的影响

目前国内外针对能源连续墙的研究主要集热交换性能,鲜有研究关注其对邻近地下建筑热湿环境的影响。本文通过建立热湿耦合传递三维数值模型,研究了能源连续墙对相邻地下室墙体壁面热湿状态和与室内环境热湿交互通量的影响。研究结果表明,能源连续墙的运行明显改变了室内侧壁面的热湿流方向和热湿流密度大小;与纯传热结果相比,虽然传湿过程对壁面温度的影响小于0.2℃,但是这部分差异会造成显热热流密度预测偏差超过10%;不计潜热热流会造成对地下室内热负荷的低估超过2.4%;绝缘层的隔热防潮作用能够阻碍流向室内的热量和湿汽。

地下连续墙受力变形光纤监测与数值模拟研究

地下连续墙是一种常见的深基坑支护方式,具有刚度大、防渗性能好、适应范围广等优点,但其受力变形规律较为复杂。基于超弱光纤光栅感测技术,对某地铁车站基坑地下连续墙水平位移进行实时监测。将监测数据与数值模拟结果进行对比,验证了数值分析模型选取的合理性,并基于该数值分析模型研究了不同厚度软土层中,地下连续墙水平位移的变化规律,通过墙体位移梯度(D_(w))和软土位移梯度(D_(r))进行临界厚度比计算。研究结果显示:水平位移随软土厚度的变化呈“凸肚”状,沿深度方向划分为相对稳定区、线性增长区、最大变形区和线性下降区。利用D_(w)和D_(r)标定出临界厚度比为0.24;土压力变化在基坑前四步开挖过程中较为稳定,而在第五步开挖与施工后40 d变化显著。上述研究结果为类似工程的设计、施工和监测方案的制定提供了重要参考。

“两墙合一”深基坑工程设计与技术创新

北京CBD核心区某项目基坑深度36m,因受场地空间限制,采用“两墙合一”+内支撑支护体系,在实现基坑挡土、止水功能的同时完成了地下结构外墙施工。为提高工程质量和施工效率,针对地连墙与主体结构的连接、槽段接头处理方式、有限空间导墙结合近接建筑物预换撑等方面进行了技术创新,取得了较好的工程效果,可供类似工程参考。