地下水水位预测在深基坑工程中的应用

通过地下水水位预测,能够确定从降水井合笼到基坑开挖时的日期、开挖速度,可调整降水井的布局,确定最佳降水方案。结合工程实例,阐述地下水水位预测在深基坑降水方案设计中的应用,具有较好的实用价值。     

倾斜平原区地下水的开发决策与管理——以济宁地区为例

在多年监测资料的基础上，根据倾斜平原区的水文地质特征，建立地下水水位降速与大气降水、地下水开采量的回归方程，对不同大气降水、地下水开采量条件下的地下水水位降速进行了预测，在考虑了大气降水概率、保证率及不同降水时地下水开采量概率的条件下，对地下水水位降速进行了随机综合决策。依据不确定决策法中的乐观系数法，获得了不同大气降水前提下，地下水开采量的最佳对策，并提出了地下水开发管理的原则。     

喷锚支护与管井降水技术在基坑支护中的应用

南阳市文化小区商住楼B座距白河较近,地下水含量非常丰富,水位较高,对基坑开挖的安全性影响很大,施工的主要难点是降水问题。通过制定严格的基坑支护及降水方案,保证了施工的安全。介绍了喷锚支护及管井降水设计和施工工艺在该工程基坑支护中的应用。     

武汉一级阶地基坑降水引起土层水位变化及压缩变形研究

武汉一级阶地地层具有典型的二元结构特征,从浅层至深层土体渗透性逐渐增加,在基坑工程开挖降水过程中,随着疏干含水层水位的下降,各层土体水位变化特点各不相同,水位的变化影响着土体固结的压缩变形。采用现场群井抽水试验的方法,通过在抽水试验期间对不同深度含水层水位的观测及不同深度土体沉降的监测,结合各层土体性质及相关理论对武汉一级阶地基坑降水引起的土层水位变化及压缩变形规律进行研究。结果表明,武汉一级阶地基坑降水水位变化存在着明显的时空效应,沉降与水位变化密切相关,且沉降过程中土层中出现架空效应。     

焦作煤矿区岩溶水水位统计模型研究

针对研究区地下水系统结构等资料不完备,本文基于质量守恒原理和微积分原理,建立了地下水水位统计预测模型和水位延迟模型,并在焦作矿区得到验证。焦作矿区岩溶地下水对于降水响应结果表明,当延迟时间为10个月时,岩溶地下水水位与降水的相关系数最大,为0.83,而延时2个月和5个月时,相关系数分别只有0.71和0.78;模型观测孔水位预测结果表明,相关系数超过0.9的观测孔达60 %,相关系数超过0.8的观测孔则高达95 %;模型水位影响因素分析结果显示,在降雨、矿坑排水、地下水蒸发3个影响因素中,煤矿区岩溶地下水水位动态对降雨量响应最敏感,说明降雨量,尤其是长期稳定的补给量是地下水水位最主要的影响因素。该类模型适用性强,简易方便,有广阔的应用前景。      

天津中心城区基坑降水对地面沉降的影响范围研究

依托天津地区5个典型工程案例,对基坑降水引起的地面沉降规律进行了基本分析。由于基坑降水引起地面沉降的范围较远,往往能达到墙后5~10倍基坑开挖深度的距离,而实际基坑工程坑外沉降的测点往往布置在墙后1~4倍基坑开挖深度的距离,因此难以全面的获得不同类型基坑(如基坑深度不一)降水对地面沉降的影响范围。本文利用有限差分软件Modflow建立三维地下水渗流模型,并利用文化中心站的工程实测数据对该模型进行验证,最后利用该模型研究不同开挖深度的基坑(5~25m)降水对地面沉降的影响范围,并探讨5种不同止水帷幕截断方式的工况下坑内降水后坑外水位及地面沉降随时间发展关系。     

呼和浩特市地下水水位动态变化及趋势预测研究

利用多年地下水水位资料,较系统地分析了地下水水位动态变化规律,建立了基于BP网络的地下水水位预测模型,预测结果表明地下水水位将持续下降,承压水下降速率远大于潜水的下降速率。     

雄安新区地下水水位与降水及北太平洋指数的小波分析

基于1991—2016年雄安新区4个地下水监测孔的长时间序列水位数据、降水数据及北太平洋指数（North.Pacific.Index,NPI）,采用连续小波、交叉小波变换等方法,分析了三者的周期性变化及其之间的相互关系。结果表明:（1）雄安新区地下水水位、降水与NPI的主波动周期及各序列间共振周期均为1.a。（2）丰水年的地下水水位时滞小于全时段的地下水水位时滞,高降水量对潜水-承压水水位时滞的影响大于对承压水的影响。（3）地下水水位对NPI的时滞大于对降水的时滞;在丰水年,地下水水位对NPI的响应更快;地下水对降水和NPI的响应速度,明确反映了研究区地下水水位动态变化的主要气候影响因素是降水。     

某深基坑开挖对周围环境影响的监测

介绍了南京某超高层建筑深层坑开挖的周围环境监测结果，分析了该深基坑开挖所引起的环境问题，包括地下水水位下降；建筑物和土层变形和裂缝，得到了一些有益的结论。认为基坑地质条件较好且开挖满足支护系统安全稳定的条件时，仍可能对周围环境造成较大的影响。     

开挖前降水引发基坑变形机制模型试验研究

现有基坑相关研究主要关注土方开挖过程引起的变形,认为围护结构变形起点是土方第1次开挖。然而,一些工程实测表明,基坑开挖前降水阶段即可引起围护结构及周边地层发生厘米级的变形。显然,未考虑开挖前变形的基坑监测数据将低估基坑施工的环境效应。为了研究基坑开挖前降水引发基坑变形的机制,开展了室内模型试验,对基坑开挖前降水过程进行了缩尺精细化模拟。通过微型降水井的设置与调控,模型试验真实再现了实际基坑降水过程中井流效应对围护结构受力变形的影响。试验过程中发现,随着降水的进行,坑外降水漏斗不断扩展,围护结构悬臂式侧移及坑外拱肩式地面沉降也随之产生。另外,降水导致墙前水压力明显减小,并诱发墙前侧向总压力重分布（以减小为主）,围护结构为此发生指向坑内的悬臂式运动以寻求新的受力平衡,并通过墙后土体损失诱发坑外地层变形。     

复杂地质条件下某深大基坑降水方案优化设计

某深大基坑位于长江下游岸边,场地地层为典型的二元结构,基坑开挖涉及的两层承压含水层间的弱透水层局部缺失,之间水力联系密切,基坑场地地质条件极为复杂,基坑施工降水直接关系到基坑工程的安全。这里将基坑分3个区,分别进行降水设计。具体降水方案:Ⅰ区两层承压含水层和Ⅱ区第一承压含水层被围护结构隔断内外水力联系,采用疏干井降水;Ⅱ区第二承压含水层未被围护结构隔断,Ⅲ区两层承压含水层水力联系密切,针对这两个区域设计了两套降水方案,方案一在Ⅱ区和Ⅲ区均布置有降水井,在Ⅱ区对第二承压含水层降水,在Ⅲ区对第一承压含水层降水;方案二仅在Ⅲ区布置降水井对第一承压含水层降水。为了更好地对降水方案进行对比分析,验证方案的可行性,对此进行了数值模拟分析,计算结果显示,两种方案均能满足降水要求,不过方案二布井数量少,基坑外水位降深较小,对周边环境影响更小,优于方案一。     

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.     

深基坑降水三维变参数非稳定渗流与地面沉降耦合模型

为了精确模拟预测松散沉积层中深基坑降水引起的地下水渗流场和地面沉降的变化特征,考虑土体孔隙度、渗透系数、储水率随地下水位下降发生的动态变化,建立了深基坑降水三维变参数非稳定渗流与太沙基一维固结理论的地面沉降耦合模型,并采用有限元数值分析方法对模型进行求解。以南京地铁三号线浦珠路站深基坑降水为例进行模拟计算。结果表明：采用15口坑内抽水井,抽水井过滤器埋深为22.0～37.0 m,基坑围护连续墙底部埋深至41.5 m为最优降水方案;不仅使基坑内地下水位满足开挖要求,又使基坑外地面沉降在控制范围内。经验证,所建立的模型合理,计算结果可靠,研究理论用于模拟预测此类地区深基坑降水引起的地下水流场变化具有较高的可信度。     

长江漫滩复杂地层条件下超大超深基坑降水设计

长江漫滩地区第四系松散层是一个巨厚的复杂含水体,地下水丰富,对深基坑施工影响极大。详细分析了南京梅子洲过江通道及青奥轴线地下交通系统工程中B2-J1区基坑降水工程的特征,分区分段进行降水设计,并根据各区段的地层特征,选用合适的降水井结构,利用全孔填滤料的方式对开挖面以上含水层进行疏导,取得良好的降水效果;针对坑内、外水位差大的区段设置坑外备用井,防止因围护结构缺陷造成基坑管涌,为基坑安全增加保障,并有效地控制了降水对周边环境破坏。     

沈阳市阳光港湾建筑群基坑降水施工研究

沈阳市阳光港湾建筑群高层建筑采用筏板基础,多层建筑采用人工挖孔灌注桩基础,在基础施工时,均需进行相应的降水工作。采用了分期降水的施工方法,首期降水施工是在分析场地水文地质条件的基础上,利用稳定流"大井法"预算基坑的涌水量,布置了9口井,使基坑周边的1#、2#、3#、4#观测孔水位均下降至5.11m,基本上达到了首期的降水的目的;第二期降水是利用首期降水观测的水位资料,采用稳定流干扰井群计算公式对场区地下水位进行了预报,重新设计了3口井。12口井全部投放使用后,测得稳定排水量为43 000m3/d,并形成了以基坑为中心的地下水降落漏斗,降水效果明显。此经验值得同类工程借鉴。     

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.     

扬州深大基坑降水方案优化设计

扬州东部综合客运枢纽地下配套工程深大基坑采用大放坡+地下连续墙+内支撑围护,场地地层为深厚强透水地层,降水井布置在坡底还是坡间平台,将直接影响降水的动态控制。本文提出了方案一（坡底布井）和方案二（坡间平台布井）2套降水方案,并利用数值模拟方法进行了比选。对比结果：虽然方案一井数和最终总涌水量略少于方案二,但是需提前降水,会造成大量水资源的浪费;而方案二可以根据基坑开挖深度的增加,逐步增加抽水井,体现了按需降水的原则。因此,认为方案二更优。类似基坑进行降水方案设计时,不应仅考虑总井数和最终总涌水量,还应考虑到不同开挖阶段,动态控制、按需降水,才能从源头上实现基坑工程的绿色施工。