渗流条件下基坑水-土压力不同计算方法的比较

地下水渗流一直是基坑工程研究中的难点和热点。近年来,一些学者基于自己的研究成果分别提出了渗流条件下基坑水-土压力的计算方法（统称传统方法）。另外,岩土有限元软件PLAXIS则通过渗流-应变耦合来计算渗流条件下的水-土压力。结合工程算例,比较了当前基坑工程中传统水-土压力计算方法与PLAXIS程序的计算结果,发现由于部分计算假定与实际情况不符,导致传统水-土压力计算方法存在一定误差;提出了传统水-土压力计算方法的适用条件,并指出真实的水力条件下,PLAXIS程序考虑渗流-应变耦合得到的水-土压力计算结果是合理可信的。     

基于浅层地下水回灌的基坑工程沉降防治分析与计算

采用下负荷面剑桥模型,联合渗流方程,建立土-水完全耦合平衡方程计算模型,探讨浅层地下水人工回灌在基坑工程降水过程中抑制基坑周边地面沉降的作用,并以邻近上海地铁一号线的淮海中路3号地块基坑降水为例,对基坑工程实测沉降和回弹数据进行数值拟合,理论结果与实际数据吻合。本方法对基坑工程地下水回灌防止地面沉降有一定参考指导作用。     

悬挂式止水帷幕涌水量计算数值模拟及验证

在基坑工程中,为合理保护地下水资源和节约工程成本,悬挂式止水帷幕在工程中的应用越来越广泛。为更好地研究悬挂式止水帷幕基坑涌水量及影响半径的影响范围,依托北京市东城区第一人民医院基坑项目,进行了悬挂式止水帷幕基坑涌水量的数值模拟,并现场进行数据监测,从而对数值计算结果进行验证。结果表明:(1)利用有限元数值模拟软件分析的基坑内涌水量与实际监测的数值趋势相同,日涌水量可以达到1970 m3;(2)设置悬挂式止水帷幕的基坑,原基坑降水影响半径不再适用,降水影响半径比原来有所减小;(3)随着距离止水帷幕距离的增加,基坑外水头值越来越大;随着距离止水帷幕距离的越来越大,基坑外水头值增加的趋势越来越小。     

南通市南山湖综合楼基坑工程降水数值模拟分析

基坑降水的成功与否,关系到整个基坑的安全。南通市南山湖综合楼基坑开挖过程中虽建立了止水帷幕,但仍需对基坑降水的方案进行安全评估。采用MIDAS/GTS数值分析软件建立三维渗流模型,通过对现场单井抽水试验进行模拟分析,综合地勘报告中室内试验渗透系数和抽水试验的渗透系数,反向推演出符合工程实际的渗流边界函数和渗透系数;然后利用反推得到的计算参数,建立三维渗流模型,模拟群井抽水状态下水位降深与时间的变化关系,对群井降水效果进行分析,验证降水井设计是否合理,指导基坑土方的开挖。     

软土深基坑支护内力和变形的三维有限元分析

针对天津经济技术开发区某软土深基坑工程,采用大型有限元分析软件ABAQUS对基坑支护体系的内力、变形进行了全三维非线性有限元分析：应用单元生死功能精确地模拟实际开挖过程;为减小不同类型单元自由度耦合引起的计算误差,对土体及支护体系均采用实体单元模拟;通过数值分析结果与实际测量数据的比较,验证了分析方法的精度,计算方 法及结论有益于今后对类似基坑工程进行更可靠的分析设计。     

深基坑开挖卸荷对下卧既有地铁隧道的影响分析

长沙田汉大剧院地下空间商业项目一期基坑工程位于地铁1号线某区段正上方,坑底距隧道顶的最小距离约为6.2m。基坑坑底位于圆砾层中,地下水较丰富,考虑到降水对周边环境的影响,基坑工程采用封闭式止水帷幕并设置抗浮锚索。基坑开挖、降水引起的土体卸荷、地下水渗流,以及施工的抗浮锚索,共同影响坑底土体回弹,从而对下卧地铁隧道产生影响,如何分析与计算其影响成为该项工程的重点和难点。为此先通过两种常用的回弹变形估算方法计算坑底回弹量和隧道隆起位移;然后利用MIDAS GTS有限元软件建立三维数值分析模型,分别进行3种工况的模拟,包括不考虑地下水渗流影响并不采用抗浮锚索工况,考虑应力渗流耦合但不采用抗浮锚索工况,考虑应力渗流耦合并采用抗浮锚索工况。对比分析表明:帷幕渗透率较低时,考虑应力渗流耦合与不考虑渗流影响的坑底回弹和隧道隆起位移模拟结果基本一致;规范推荐估算方法在合理修正其卸荷应力,并确定合理的卸荷影响深度后,其计算结果与数值模拟结果相近。所得成果可为优化设计和施工提供有益的参考,并为类似工程提供借鉴。     

Processing Modflow在基坑疏干抽排水中的应用

以天津某基坑为研究对象,采用Processing Modflow数值模拟软件建立了地下水三维渗流与地面沉降耦合模型。模拟了基坑疏干抽排水在完成支护、止水帷幕施工条件下其周边地下水位和地面沉降的实时变化:即由于疏干抽排水引起的基坑周边约80 m内地下水水位降幅在0.90~82.15 cm之间,约80 m内地面沉降量在0.001~26.21 mm之间。模拟结果与后期监测数据吻合较好,能够较为真实的反映降水过程对周边环境的影响。     

悬挂式止水帷幕条件下深基坑开挖变形特性研究

降水条件对基坑开挖的变形特性具有重要影响。为了研究悬挂式止水帷幕结合承压非完整井组成的墙井系统条件下基坑开挖过程中的变形问题,以某悬挂式止水帷幕深基坑为例,通过定义降水井和地表渗流边界条件建立了考虑分级降水和基坑开挖实际工况的三维流固耦合有限元数值分析模型,使用现场监测数据与数值模拟结果互相验证的方法研究了悬挂式止水帷幕情况下基坑开挖过程中地下连续墙变形和地表沉降的变化特征,对比分析了悬挂式止水帷幕和落底式止水帷幕条件下的地表沉降。结果表明：在不同分级降水情况下,降水深度初次达到场地第一承压水含水层降水期间产生的地下连续墙水平位移增量最大,地表沉降也主要在这一期间产生;悬挂式止水帷幕情况下的地表沉降最大值约为落底式止水帷幕的2.7倍,最大值位置距地下连续墙边缘的距离比落底式止水帷幕大0.85 m;地下连续墙水平位移峰值处,降水期间产生的位移占28%,地表沉降峰值处,降水期间产生的沉降占49%;使用悬挂式止水帷幕时,距地下连续墙边缘12倍开挖深度处,地表沉降与地表沉降峰值的比值为0.1、该距离比落底式止水帷幕大13 m左右。研究成果对确定深基坑降水方案、保证深基坑开挖施工安全具有一定的参...     

基于改进阻力系数法的悬挂式帷幕基坑渗流计算

帷幕结合基坑内降水的方案是深基坑工程中地下水控制的主要趋势,由于基坑渗流场的复杂性和尺度效应,业内尚无成熟且精确的解析计算方法。在分析悬挂式帷幕基坑与闸坝渗流场特点异同的基础上,对闸坝改进阻力系数法进行修正,提出了承压水条件下悬挂式帷幕基坑渗流段的分段方式,推导了新的考虑基坑特殊段"水平汇流段"的等效阻力系数计算公式,并将之拓展至含水层各向异性条件下,实现了悬挂式帷幕基坑涌水量和坑外水位降深的定量计算;最后,结合典型基坑案例,通过数值模拟验证了该方法的精确性。研究表明:经过合理分段、等效和修正处理后,改进阻力系数法在悬挂式帷幕基坑承压水渗流计算中,与数值法计算结果的相对误差能控制在5%以内,精度较高,且具有分段灵活、计算简单的优点,能适用于不同场地条件下基坑降水设计的计算。     

基坑开挖中渗流-应力耦合模拟

根据基坑工程施工实际情况,基于地下水渗流的基本规律以及岩土体受力的本构模型,建立了在应力场和渗流场耦合作用下基坑受力的力学模型,通过有限元方法模拟了基坑开挖过程中周围应力场及渗流场的变化规律。计算结果表明：在设置挡土墙并及时排水情况下,随着开挖的进行,基坑周围的土体变形增大,挡土墙前的土体发生沉降,墙体最终发生变形,基坑底部土体向上隆起,在基坑角落处水头梯度较大,发生应力集中,容易导致工程事故,必须谨慎处理。另外,基坑在降水时内外水头差过大将会引起墙体承受过大压力,基坑周围沉降会更大,这样不利于墙体和周围建筑安全。     

落底式止水帷幕条件下基坑涌水量计算研究

涌漏点的存在使得落底式止水帷幕条件下基坑涌水量的计算成为一个有待解决的技术难题。在承压含水层中开挖落底式深基坑时,针对落底式止水帷幕渗漏的不确定性,建立承压含水层中落底式基坑非完整井降水水文地质模型。基于Theis非完整井非稳定流理论,结合映射原理,推导等效渗透系数计算公式,并提出涌漏系数法用以评价落底式止水帷幕隔渗效果以及计算基坑涌水量。以武汉某深基坑降水工程为例,采用该方法评价止水帷幕的隔渗效果以及对其基坑涌水量进行计算,并与实测结果和传统方法计算结果分别进行对比验证。结果显示涌漏系数法计算误差较小,且计算过程简便,兼具安全性和经济性。研究结果可应用于指导工程设计,具有一定实际应用价值。     

Numerical simulation and land subsidence control for deep foundation pit dewatering of Longyang Road Station on Shanghai Metro Line 18

In terms of controlling groundwater in deep foundation pit projects, the usual methods include increasing the curtain depth, reducing the amount of pumped groundwater, and implementing integrated control, in order to reduce the drawdown and land subsidence outside pits. In dewatering design for confined water, factors including drawdown requirements, the thickness of aquifers, the depth of dewatering wells and the depth of cutoff curtains have to be considered comprehensively and numerical simulations are generally conducted for calculation and analysis. Longyang Road Station on Shanghai Metro Line 18 is taken as the case study subject in this paper, a groundwater seepage model is developed according to the on-site engineering geological conditions and hydrogeological conditions, the excavation depth of the foundation pit as well as the design depth of the enclosure, hydrogeological parameters are determined via the pumping test, and the foundation pit dewatering is simulated by means of the three-dimensional finite difference method, which produces numerical results that consistent with real monitoring data as to the groundwater table. Besides, the drawdown and the land subsidence both inside and outside the pit caused by foundation pit dewatering are calculated and analyzed for various curtain depths. This study reveals that the drawdown and the land subsidence change faster near the curtain with the increase in the curtain depth, and the gradient of drawdown and land subsidence changes dwindles beyond certain depths. In this project, the curtain depth of 47/49 m is adopted, and a drawdown-land subsidence verification test is completed given hanging curtains before the excavation. The result turns out that the real measurements basically match the calculation results from the numerical simulation, and by increasing the depth of curtains, the land subsidence resulting from dewatering is effectively controlled.     

Numerical simulation and land subsidence control for deep foundation pit dewatering of Longyang Road Station on Shanghai Metro Line 18

In terms of controlling groundwater in deep foundation pit projects, the usual methods include increasing the curtain depth, reducing the amount of pumped groundwater, and implementing integrated control, in order to reduce the drawdown and land subsidence outside pits. In dewatering design for confined water, factors including drawdown requirements, the thickness of aquifers, the depth of dewatering wells and the depth of cutoff curtains have to be considered comprehensively and numerical simulations are generally conducted for calculation and analysis. Longyang Road Station on Shanghai Metro Line 18 is taken as the case study subject in this paper, a groundwater seepage model is developed according to the on-site engineering geological conditions and hydrogeological conditions, the excavation depth of the foundation pit as well as the design depth of the enclosure, hydrogeological parameters are determined via the pumping test, and the foundation pit dewatering is simulated by means of the three-dimensional finite difference method, which produces numerical results that consistent with real monitoring data as to the groundwater table. Besides, the drawdown and the land subsidence both inside and outside the pit caused by foundation pit dewatering are calculated and analyzed for various curtain depths. This study reveals that the drawdown and the land subsidence change faster near the curtain with the increase in the curtain depth, and the gradient of drawdown and land subsidence changes dwindles beyond certain depths. In this project, the curtain depth of 47/49 m is adopted, and a drawdown-land subsidence verification test is completed given hanging curtains before the excavation. The result turns out that the real measurements basically match the calculation results from the numerical simulation, and by increasing the depth of curtains, the land subsidence resulting from dewatering is effectively controlled.     

Dewatering induced subsidence during excavation in a Shanghai soft deposit

Foundation dewatering has become a major cause of land subsidence in Shanghai. The burial depth of foundations in relation to geotechnical construction works is less than 75 m, and the corresponding groundwater includes phreatic, low-pressure artesian, and the first confined aquifers. Based on the geological and hydrogeological conditions beneath Shanghai, methods of dewatering may be divided into three modes and further five patterns according to the insertion depth of the dewatering-retaining system. The most common dewatering mode aims to reduce the water pressure in the confined aquifer by setting the dewatering wells inside the pit, whilst the retaining walls are buried in the confined aquifer and partially cut off the confined aquifer layer. To predict the settlement due to foundation dewatering, numerical models are generally adopted, which are similar to those used to predict land subsidence induced by regional groundwater withdrawal; however, since foundation dewatering is conducted along with the setting of retaining walls and foundation pit excavation, which differs from regional groundwater withdrawal, interactions between the retaining wall-dewatering well, the dewatering-excavation, and dewatering-recharge are important factors affecting the analytical model. Since the grading of the shallow soil layers is different, stratified settlement characteristics of the shallow soil strata and seepage erosion, which results in additional deformation, need to be given particular consideration.     

Physical model test of transparent soil on coupling effect of cut-off wall and pumping wells during foundation pit dewatering

Water level is decreased during foundation pit excavation to avoid water inrush under confined water pressure. Cut-off wall is often used as waterproof curtain to partially cut off the dewatered aquifer. When a foundation pit is located in a built-up area and the underlying confined aquifer is not cut off, the drawdown must be minimized outside the pit to avoid land subsidence in buildings and pipelines. The coupling effect of the cut-off wall and pumping well is used to control the drawdown outside the foundation pit. However, the coupling mechanism is not intuitively well understood because of the limitations of existing experimental methods. In this study, transparent soil was introduced to model the coupling mechanism in the physical model test. High-purity fused silica and mixed paraffin oil were used as skeleton and fluid to simulate the confined aquifer and groundwater. Industrial solid dye and paraffin oil were used as tracers. A camera was used to collect flow information. Tests were performed for the combinations of cut-off wall and partially penetrating pumping wells. The insertion depth ratio of the cut-off wall most effectively influenced the drawdown. The layout of the pumping wells in horizontal direction influenced water level distribution and flow rate. The optimal depth of the pumping wells was 1–5 m above the bottom of the cut-off wall, and the optimal horizontal distance between the cut-off wall and the pumping wells was 25% of the pit width. Non-Darcy flow was observed within the range of 0–10 m around the bottom of the cut-off wall. These results were significant in understanding the cut-off wall and pumping well coupling effect on foundation pit dewatering.

     

大型锚碇基础围护工程冻结帷幕力学性态研究

冻结围护是利用人工制冷技术,使地层中的水结冰,将天然土层变成冻土作为围护挡土止水的新技术。依据冻结基本原理,在大型锚碇深基础中采用排桩内支撑结构并结合冻结帷幕止水是一种新的基坑围护形式,含水土层冻结后形成的冻结帷幕具有良好的封水性能,冻土墙刚度和强度较原土层有很大的提高且质量易于控制。相对于常规围护工程,冻结围护结构的冻胀力学特性复杂且研究资料贫乏,因此,其力学性状的研究将进一步完善冻结围护技术理论,保障基础工程顺利安全进行。通过对锚碇基础围护工程冻结帷幕温度场-应力场耦合的三维非线性数值模拟分析研究,得到冻结围护工程中冻结帷幕的冻胀力学性状特征,结合大型锚碇基础冻结围护工程实例的现场测试成果分析,验证数值模拟分析方法结果的合理性以及冻结围护技术的可靠性,研究成果将为今后冻结围护技术的工程推广应用提供理论基础。     

桩基后压浆对深基坑降水渗流场的影响

钻孔灌注桩后压浆后,与坑外含水层相比,基坑范围内含水层的渗透系数减小,这对基坑降水渗流场具有比较明显的影响。某广场式建筑由五幢塔楼和地下车库组成,采用桩侧后压浆钻孔灌注桩,基坑开挖深度21m,承压含水层组厚10m,透水性较强,采用敞开式完整井降水,井位主要沿基坑周边布置。在对水文地质条件进行概化后,建立了地下水三维非稳定流数值模型,进行了非均质渗流场模拟,并与均质渗流场作比较。结果表明,坑内后压浆区域的渗透系数减小后,坑内水力坡度变陡,单井涌水量减小,井数应相应增加;若模型计算域全部按注浆后的渗透系数考虑,则会严重降低补给强度、低估基坑总涌水量;在相同的降深要求下,坑内后压浆后渗透系数减小、但坑外渗透系数不变,基坑总补给量基本没有变化。现场监测与模拟结果较为一致。研究成果对类似工程地下水控制有一定的借鉴意义。     

悬挂式帷幕基坑底侧突涌的坑内水头抬升研究

基坑采用悬挂式帷幕减压降水时,因隔水帷幕局部存在缺陷,而导致坑内承压含水层水头抬升所诱发的基坑底侧突涌事件常有发生。为研究这种现象,以某地铁线风井基坑为范例,建立了三维非稳定流的地下水数值模型,重点考虑了渗透系数的各向异性,对隔水帷幕无缺陷,28~33m、33~38m、38~43m深度的缺陷,加大流量抽取等7种情况,进行了模拟和比较分析。研究结果表明,悬挂式帷幕降水帷幕无缺陷时,帷幕内外的水头差在含水层顶最大,并沿帷幕向下递减;在帷幕缺陷形成的渗流通道附近,渗流场的等势线非常密集,补给的地下水抬升了坑内水头,并加剧了坑外的水头下降;经缺陷处的渗流通道流入坑内的地下水,渗流速度矢量以水平向为主;帷幕缺陷位于渗透系数较大的含水层时,相应补给量也较大;加大流量抽取,可以减小水头抬升的平面范围,但缺陷附近过大的流速也会诱发地层流土;缺陷位置与帷幕底竖向距离越远,坑内水头的抬升率越大,缺陷位置与帷幕底竖向距离越近,抬升率越小。研究成果对分析承压水底侧突涌的致灾机制、预测预警以及基坑动态风险评估等有一定的参考意义。     

基坑降水—回灌共同作用下地下水浸润曲线求解研究

基坑工程中,通常采用地下水回灌措施降低降水对周边地质环境产生的不良影响,然而目前基坑降水—回灌的相关设计理论仍处于探索阶段。本文通过引入平面二维流势函数理论和叠加原理,分别求解得到了无止水帷幕工况下潜水完整井和承压完整井在降水—回灌共同作用下的地下水浸润曲线方程;此外,本文通过空间汇点原理和镜像原理分别求得基坑内降水和基坑外回灌对基坑外地下水位的影响,并运用叠加原理得到了有止水帷幕工况下,深基坑降水—回灌作用下的地下水浸润曲线解析式。本文利用得到的解析式探讨了在具有止水帷幕条件下回灌井距基坑围护结构的距离、渗透系数等主要因素对浸润曲线的影响,为基坑降水—回灌设计提供了参考依据。