Study on Deep Well Dewatering Optimization Design in Deep Foundation Pit and Engineering Application

Based on analyses of the theories of groundwater unsteady flow in deep well dewatering in the deep foundation pit, Theis equations are chosen to calculate and analyze the relationship between water level drawdown of confined aquifer and dewatering duration. In order to reduce engineering cost and diminish detrimental effect on ambient surrounding, optimization design target function based on the control of confined water drawdown and four restriction requisitions based on the control of safe water level, resistance to throwing up from the bottom of foundation pit, avoiding excessively great subsidence and unequal surface subsidence are proposed. A deep well dewatering project in the deep foundation pit is optimally designed. The calculated results including confined water level drawdown and surface subsidence are in close agreement with the measured results, and the optimization design can effectively control both surface subsidence outside foundation pit and unequal subsidence as a result of dewatering.

Optimization of dewatering schemes for a deep foundation pit near the Yangtze River, China

A deep foundation pit constructed for an underground transportation hub was excavated near the Yangtze River. Among the strata, there are two confined aquifers, between which lies an aquiclude that is partially missing. To guarantee the safety of pit excavation, the piezometric head of the upper confined aquifer, where the pit bottom is located, should be 1 m below the pit bottom, while that of the lower confined aquifer should be dewatered down to a safe water level to avoid uplift problem. The Yangtze River levee is notably close to the pit, and its deformation caused by dewatering should be controlled. A pumping test was performed to obtain the hydraulic conductivity of the upper confined aquifer. The average value of the hydraulic conductivity obtained from analytical calculation is 20.45 m/d, which is larger than the values from numerical simulation（horizontal hydraulic conductivity K_H = 16 m/d and vertical hydraulic conductivity K_V = S m/d）. The difference between K_H and K_V indicates the anisotropy of the aquifer. Two dewatering schemes were designed for the construction and simulated by the numerical models for comparison purposes. The results show that though the first scheme could meet the dewatering requirements, the largest accumulated settlement and differential settlement would be94.64 mm and 3.3‰, respectively, greatly exceeding the limited values. Meanwhile, the second scheme,in which the bottoms of the waterproof curtains in ramp B and the river side of ramp A are installed at a deeper elevation of-28 m above sea level, and 27 recharge wells are set along the levee, can control the deformation of the levee significantly.

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.

Foundation pit dewatering optimization design based on GMW-2005 and LGR technique

The slope instability and uneven settlement caused by the foundation pit dewatering pose a great threat to the project and surrounding environment.Thus,it is necessary to carry out optimization design of the foundation pit dewatering project.To solve this problem,this paper introduces the simulation-optimization method and establishes the groundwater model based on MODFLOW-2005.The local grid refinement(LGR)technique is employed to achieve the refinement of dewatering area.Under the premise of construction safety,the minimum cost of the project is set as the objective function to solve the optimization problem of foundation pit engineering with GWM-2005.The results show that the optimization with GWM-2005 will be more accurate combined with the LGR model,but the improvement of the optimization is not obvious.It is necessary to choose a suitable local refinement model considering the engineering requirement.

FINITE ELEMENT NUMERICAL SIMULATION OF THREE-DIMENSIONAL SEEPAGE CONTROL FOR DEEP FOUNDATION PIT DEWATERING

For deep foundation pit dewatering in the Yangtze River Delta, it is easy to make a dramatic decrease of the underground water level surrounding the dewatering area and cause land subsidence and geologic disasters. In this work, a three-dimensional finite element simulation method was applied in the forth subway of Dongjiadu tunnel repair foundation pit dewatering in Shanghai. In order to control the decrease of the underground water level around the foundation pit, the foundation pit dewatering method was used to design the optimization project of dewatering ,which was simulated under these conditions that the aquifers deposited layer by layer, the bottom of the aquifers went deep to 144.45 m, the retaining wall of foundation pit shield went deep to 65 m, the filters of the extraction wells were located between 44 m to 59 m, the water level in the deep foundation pit was decreased by 34 m, and the maximum decrease of water level outside the foundation pit was 3 m. It is shown that the optimization project and the practical case are consistent with each other. Accordingly, the three-dimensional finite element numerical simulation is the basic theory of optimization design of engineering structures of dewatering in deep foundation pit in such areas.

邻近深埋地铁车站水-土阻隔效应及其对基坑抽水致沉的影响

既有地下结构会阻隔地下水渗流(阻水效应)和其周边土体运动(阻土效应),使得该条件下的基坑抽水致沉规律与无邻近地下阻隔时不同。以实际基坑抽水试验为工程依托,以基坑外既有地铁车站结构的水−土阻隔效应为例,通过建立不同车站位置、车站埋深及基坑抽水强度的系列三维数值仿真模型来研究不同水−土阻隔效应条件下的基坑抽水致沉规律。研究表明:当基坑外存在车站结构时,基坑抽水沉陷被加剧或被限制,这取决于阻水和阻土效应孰强孰弱,而车站与基坑间距D的远近将决定阻水和阻土效应的相对强弱,进而决定基坑抽水沉陷是被加剧还是被限制;车站埋深H只会单一影响阻水或阻土效应的发挥强度,不会决定阻水和阻土效应谁起主导作用。工程中应针对基坑外邻近车站的具体位置和埋深考虑其水−土阻隔效应的影响,从而更准确地评估该条件下的抽水致沉规律。

天津地铁某车站基坑开挖前降水试验研究

为了验证基坑围护结构的隔水能力,预判降水施工对周边环境的影响,通过天津地铁某车站基坑开挖之前的降水试验,明确造成潜水层和第一承压水层水位下降的主要原因;对该车站基坑地下连续墙水平位移、地表沉降、附近建筑物沉降情况进行了分析,认为提高地下连续墙施工质量和避免一次性降水深度过大能有效控制降水对周边环境的影响。

基于长春某工程在基坑降水引发地面沉降方面的影响分析

本文以长春某基坑实际工程为背景,通过Midas GTS NX软件对基坑内的降水状况进行了模拟,得出施工的降水阶段对周边地表环境及支护结构的变形影响规律。结果表明,降水过程对周边地表造成的沉降作用以降水井为中心,呈降水漏斗状曲线,沉降最大值出现在降水井附近,对支护结构产生变形作用的最大值出现在桩顶,随桩体深度的下降呈下降曲线,分层降水对控制地表沉降可以起到良好的作用。

天津某地铁车站深基坑突涌事故分析

基坑施工过程中渗漏、突涌水灾害时有发生,地下水处理不当是引发基坑事故的重要影响因素之一。突涌事故往往发生于土方开挖等施工阶段,具有突发性、难以可视化的特点。基于分析天津市某地铁车站基坑工程的突涌水事故的现场实测数据、处理措施等内容,回溯事故发生原因,主要是开挖基底下的承压层减压降水效果不明显,通过围堰反压、注浆、混凝土封堵等措施,配合启动基坑外承压层降水井进行减压降水,有效控制了突涌水事故。基坑开挖施工期间,须严格监测基坑内外水位,发现异常须及时排查原因解决,现场人员须加倍重视。当基坑采用悬挂式止水帷幕且坑内没有条件进行减压井施工时,可利用基坑外观测井进行降水减压,但应避免引发周边环境的地层大范围沉降、土层不均匀沉降等,必要时采取相应措施加以保护。

基坑免降水开挖的组合式冻结法支护工艺研究

近年来,随着地下水资源保护力度的不断增大,对基坑工程进行免降水开挖的需求愈加紧迫。为解决富水地层基坑工程施工中的免降水问题,提出一种新的基坑支护方法:传统悬挂式止水帷幕与水平冻土止水帷幕相结合的“组合式冻结法”。根据以往市政工程中的冻结法施工经验,总结组合式冻结法的施工流程,提出泄压孔、温度限位孔和局部冻结技术相结合的冻胀控制措施,以及利用冻结管嵌入地连墙的界面区强化措施。通过模型试验对组合式冻结法的可行性进行验证,试验结果表明,组合式冻结法可以有效实现基底的支护,在冻结薄弱环节临时竖井、地连墙附近冻结壁承载能力满足设计要求;观测发现地层冻结对地连墙强度和变形无明显影响。

不规则深基坑降水开挖稳定性及其对紧邻地下综合管廊的影响研究

针对北京市通州区某紧邻两条地下综合管廊不规则深基坑工程,建立了基坑及地下综合管廊的三维有限元分析模型,模拟分析了基坑降水开挖的稳定性及其对地下综合管廊受力变形的影响。结果表明:鉴于基坑形状极不规则,沉降峰值主要出现在基坑北侧“尖”的两侧,西侧和南侧侧移值较大;基坑边缘周边地表沉降基本呈现先增大后减小的趋势。不同断面支护结构的受力变形也不同。非管廊侧锚杆轴力约为管廊侧的70%,护坡桩在不同断面处发生不同程度的水平位移,靠近管廊侧桩顶平均水平位移较大;基坑降水开挖引起的两条紧邻综合管廊的最大竖向和水平位移发生的区域基本相同。此外,两管廊交会处最大剪应力、最大主应力和最小主应力值都相对较大。

基于有效控制抽水量和地面沉降的长江Ⅰ级阶地深基坑深井降水参数取值优化

针对目前深基坑深井降水研究中仅关注目标降深及相应地面沉降,未考虑满足目标降深条件下的有效控制抽水量问题,以武汉长江隧道明挖段深基坑深井降水工程为例,通过抽水试验分析了井管直径、进水段的相对位置b/M以及井的不完整程度l/M对基坑涌水量的影响,确定了止水帷幕长度、井管直径、过滤管有效长度和合理位置等降水参数,根据试验数据拟合出该地区非完整井涌水量经验公式;建立了控制抽水量和地面沉降的深基坑深井降水参数优化设计模型,求解Maxf(l)得出的过滤管长度,与抽水试验结果相近。结果表明:止水帷幕进入强透水层厚度2/3处时,其对基坑坑内抽水量和坑内外水位有控制作用;选用Φ250井管、过滤管设置在强透水层上部(b=0)、l/M值较小及附加阻力系数差(ξ_(1)-ξ_(2))最大时,基坑涌水量较小;q_(0)-l、Δq_(0)/Δl-l曲线中曲率最大点对应的过滤管长度范围为14.0~16.0 m,其对应的基坑涌水量q0最小;考虑井群干涉的影响,干涉井涌水量在基坑中心处最小、角点处最大。

基坑开挖前坑内预降水对基坑变形的影响

为研究基坑开挖前坑内预降水对基坑变形及对水位变化情况影响,依托某地铁站基坑工程,采用室内模型试验方法,研究了抽水速度和降水深度对坑外水位变化、围护结构变形、地表沉降、孔隙水压以及土压力的影响。运用MIDAS GTS NX有限元软件对实际工程进行数值模拟,将试验数据与数值计算结果进行对比。结果表明:坑外水位随着与围护结构距离的增加,水位下降越小且降水曲线斜率越小,与试验得到的“先凹后凸”形坑外降水曲线结果一致;围护结构侧移随着降水深度的增大而增大,增加幅度逐渐减小,整体呈现“悬臂”形,底部由于受到渗流力的作用产生了一个指向坑内的“踢脚变形”,最大围护结构侧移出现在围护结构顶部;坑外地表沉降呈现“对勾”形分布,最大坑外地表沉降的位置并不会随降水深度改变,而是始终出现在距围护结构大约10 m左右;围护结构侧移和地表沉降均随降水深度或抽水速度的增加而增大;基坑降水后孔隙水压无论坑内还是坑外均减小,坑内孔隙水压减小幅度明显大于坑外,且两者在变化规律上是不同的,坑内孔隙水压呈现先减后增,而坑外孔隙水压则是一直减小;坑内外侧向土压力随着降水深度的增加均显著减小;坑内土压力随着抽水速度的增大变化不明显,而坑外侧向土压力随着抽水速度增加明显减小。

复杂地质条件下水利工程高水位深基坑降排水施工方法研究

为了解决进口引江济淮工程施工中高水位深基坑降排水问题,对复杂地质条件下水利工程高水位深基坑降排水施工方法开展了研究。采用混凝土防渗墙技术对导向槽进行优化设计,提高防渗墙的防渗性能。利用真空预压地下水位测量方法为降水井的参数设计与位置设置提供水位参考。通过人工开挖探孔方式确定最佳降排水方案,并计算降水漏斗曲线的影响值。利用有限元软件模拟试验发现:该降排水方法在防渗墙高程为-36 m时能够达到最佳降排水效果,说明使用该方法能够阻隔基坑外水体渗入,且降水井能够将深基坑中的积水排出,有效地解决了进口引江济淮工程施工中高水位深基坑降排水难题,具有较好的实用性和推广价值。

降水条件下基坑宽长比对邻近既有隧道变形的影响

针对基坑降水施工时对邻近既有隧道变形的影响已有学者进行研究,但降水条件下基坑宽度与长度的比值对隧道最大竖向位移的影响尚有空白。为探讨基坑降水时基坑宽长比对邻近隧道的变形影响,首先借助模型试验研究邻近隧道在基坑宽度和降水深度改变时产生的受力变形规律,再利用数值软件进一步分析基坑宽长比的影响,最后针对隧道竖向位移进行归一化处理,分析了不同降水深度影响下基坑宽长比对邻近既有隧道变形的影响规律。结果表明:通过模型试验可以发现,降水会使坑外出现明显的降水漏斗,并且随着基坑宽度和降水深度的增加,出水量会逐渐增加,坑内水位不断降低,既有隧道的竖向位移和弯矩产生明显变化,坑外隧道呈现压扁式的“横鸭蛋”变形。通过数值分析可以发现,当基坑宽长比较小时,应将宽长比和降水深度综合考虑,而当基坑宽长比较大时,应格外避免超降现象。另外,通过归一化发现,各降水深度下的位移曲线近似满足同一函数关系,当基坑宽长比小于0.4时,基坑宽度越大隧道位移越大;当基坑宽长比大于0.5时,隧道的竖向位移不会随着基坑宽度的增加而增大,但隧道位移的范围会增加4至5倍。

基坑管涌处理工艺设计及施工

为解决建筑深基坑施工过程中常发生的管涌、流砂问题,通过滤沙降水的设计思路,将流砂中的水与泥沙过滤、分离,将水排出基坑,泥沙留在基坑,保证基坑不会因泥沙流失而失稳,降低基坑因泥沙流失而坍塌的风险,从根本上解决了管涌治理难题。实践结果表明,滤沙降水工艺在建筑深基坑管涌、流砂治理中有很高的经济性、实用性,解决了深基坑工程管涌流砂治理困难、治理成本高等问题。

邻近基坑预降水影响下既有隧道受力变形分析

为探讨基坑降水引起的既有隧道受力变形特性,依托实际工程建立三维数值模型,以隧道的最大竖向位移为评价指标,使用正交表筛选最优组合和主要影响因素,而后对主要影响因素进行单变量分析,研究隧道受力变形规律。研究表明:通过极差分析和方差分析发现,连续墙插入深度是影响坑外隧道最大竖向位移的主要因素,故邻近既有隧道的新建基坑工程有降水需求时,可先选取该因素进行隧道变形控制;随着连续墙插入深度的增加,隧道弯矩和变形均会随之减少,当连续墙插入深度到达透水性较差的土层后隧道变形显著减小,此时隧道变形能得到有效控制,而继续增加插入深度对于控制隧道变形意义不大;降水使坑外邻近隧道产生朝向坑内斜向下的“横鸭蛋”沉降变形,施工过程中应重点关注隧道拱顶和邻近基坑侧拱腰的变形情况。

复杂地质条件下基坑降水方案设计研究

随着城市化进程的推进,对于空间的开发利用逐渐转向地下,设计科学恰当的基坑降水方案对基坑工程的安全与成本控制至关重要。以某工程项目基坑为研究对象,设计了有悬挂式止水帷幕的半封闭式基坑降水方案,通过模拟软件对降水效果和降水后对周围环境的影响进行模拟分析,得出了该方案设计的半封闭式基坑降水方案降水效果良好,可供相关工程参考。

临近建筑物深基坑降水施工技术要点分析

城市发展日渐成熟导致建筑不断增多,在城市内、临近已有建筑物的条件下,建筑工程施工的主要难点体现在降水专项施工中,该文将根据某一工程实例,探讨临近建筑物条件下基坑降水施工的技术要点,通过数值模拟的手段验证降水施工方案降水井布置数目及位置的合理性,根据现场验证试验确认地下水流通情况。在数值模拟和验证试验的基础上,得出以下结论,该工程在拟定的降水井布置条件下可以按预定要求完成降水工作,同时可以通过设置止水帷幕的方式有效减少降水工作对周边建筑物的影响;现场验证试验结果显示水位恢复率达到57.2%,认为在已有的地质条件下基坑内外土体存在水力联系。

邻近既有高速铁路深基坑降水问题探讨

针对既有高速铁路附近深基坑降水问题,以某高速铁路盾构井为研究对象,开展现场水文试验以分层获取岩土层渗透系数,探讨渗透系数对基坑降水浸润线的影响,并采用不同方法预测基坑涌水量。首先介绍了深基坑降水设计所需水文地质资料来源,然后绘制既有高速铁路桥墩的水文地质断面图,以检算基坑降水对既有高速铁路的影响是否符合规定限值,为设计合理的降排水措施提供依据。本文的研究成果可为类似工程提供参考。