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.

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.

Construction Technology of Deep Foundation Pit Support in Municipal Civil Engineering Foundation Construction

Municipal civil engineering is the key content of municipal construction,and the construction scale is usually large.The quality of the project plays an important role in the development of urban economy.Due to the rapid increase of high-rise buildings,skyscrapers and underground buildings,the construction technology of deep foundation pit support has gradually become an indispensable construction technology.Therefore,the selection of foundation pit support construction technology is crucial in ensuring that whether the foundation is firm and stable,and whether the subsequent construction activities can be carried out smoothly.In view of this,the article discusses the application of deep foundation pit support construction technology in municipal civil engineering,aiming to provide reference for subsequent projects.

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.

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.

Co-Evolution Optimization of Anchored Row Piles for Deep Foundation Pit

The thinking of co evolution is applied to the optimization of retaining and protecting structure for deep foundation excavation, and the system of optimization of anchored row piles for deep foundation pit has been already developed successfully. For the co evolution algorithm providing an evolutionary mechanism to simulate ever changing problem space, it is an optimization algorithm that has high performance, especially applying to the optimization of complicated system of retaining and protecting for deep foundation pit. It is shown by many engineering practices that the co evolution algorithm has obvious optimization effect, so it can be an important method of optimization of retaining and protecting for deep foundation pit. Here the authors discuss the co evolution model, object function, all kinds of constraint conditions and their disposal methods, and several key techniques of system realization.

Accident Analysis and Emergency Response Effect Research of the Deep Foundation Pit in Taiyuan Metro

Taking the deep foundation pit accident occurring at a station of metro Line 2 in Taiyuan as an example,the influence of the seepage and inrush of the foundation pit on the retaining structure and surrounding environment were studied under the geological conditions of the confined aquifer on the east coast of Fenhe River.The causes of deep foundation pit accident were also analyzed systematically based on the monitoring data,and various emergency measures were proposed to control the occurrence of secondary accident for deep foundation pit.The results showed that the occurrence of inrush for foundation pit was mainly caused by the insufficient dewatering.The development of the accident was effectively controlled by the adding of the dewatering wells,local grouting of retaining structure to stop seepage,surface grouting to reinforcement and uplift soil.The successful experience can provide some guidance to the construction of similar projects in the proposal.

Elastic-plastic analysis for pile-anchor supporting system of deep foundation pit

By using numerical analysis methods to simulate the deep excavation,a lot of analyses are established on the basis of two-dimensional plane strain,ignoring the fact that foundation pit possesses three dimensions. For soil constitutive relation,people always take linear and nonlinear model,without considering the plastic behavior of soil. Using plastic-elastic hardening model to simulate constitutive relation of soil characteristics,the authors carried out mechanical analysis for pit excavation and support. The results show that the analysis for the stress state of pile anchor system is an effective way which provides theoretical basis for calculation of soil displacement.

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.

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.

Numerical Simulation and Field Monitoring Analysis for Deep Foundation Pit Construction of Subway Station

To investigate the effect of deep foundation pit excavation on the stability of retaining structure, a subway stationin the city of Jinan was selected as a project, and a FLAC3D-based three dimensional model was developed fornumerical simulation. The horizontal displacement of the retaining structure, the axial force of the support, andthe vertical displacement of the column were studied and compared to the collected data from the field. The findingsindicate that when the foundation pit is excavated, the maximum deformation of the retaining structure progressivelydecreases from the top, the distortion of the retaining structure gradually rises, and the final maximumdeformation is around 17 meters deep. In each layer of support, the largest axial force support is located in thefirst reinforced concrete support;the uplift of the pit bottom caused by soil unloading plays a primary role in thevertical displacement of the column, and the column exhibits an upward trend under all construction conditions.When compared to the measured data, the generated findings are comparable and the fluctuation trend is extremelyconsistent. The findings of this article may give technical direction for the development of subway stationswith a comparable engineering basis.

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.

Numerical Evaluation of Land Subsidence Induced by Dewatering in Deep Foundation Pit

To predict the behavior of land subsidence due to groundwater withdrawal, a 3D numerical model is established. The model takes the confined aquifer and soft deposit of Ningbo into account. According to the pumping test data, changing regulations of groundwater level, and land subsidence during dewatering are analyzed. Comparison between the calculated value and measured value shows that the 3D model simulates the measured value fairly well. The future behavior of land subsidence during dewatering period provides the scientific basis for the construction of deep foundation.

基于某复杂地层深基坑降水开挖数值模拟分析

本文以长春某深基坑项目为例,利用Midas GTS NX软件对施工过程进行模拟。研究降水开挖对地表沉降、坑底隆起、围护结构的变形影响以及分析不同桩径、降水对基坑开挖的影响。结果表明,围护结构最大变形与最大地表沉降随开挖深度不断增大,最大变形位置与开挖深度之比分别为0.72和0.63;富水砂层地区基坑降水开挖后地表沉降、围护结构变形降水关系占比较大;适当改变桩径可以减小围护结构的水平变形,此次建模也验证模型的准确性。

基于VMD-Self-attention-LSTM的水闸深基坑变形智能预测方法

针对水闸深基坑变形监测数据具有非稳定性的特点,提出了基于VMD-Self-attention-LSTM的水闸深基坑变形预测方法。该方法分为三个主要模块,第一模块采用VMD算法自适应调整变形分解模数,将原始变形数据分解为若干具有明显周期规律的分量,有效解决变形数据的非稳定性,为提升变形预测精度奠定初步基础;第二模块,在传统LSTM算法基础上进一步发展Self-attention-LSTM方法,提升模型对基坑变形样本的时序关系挖掘能力,进而提升基坑变形预测精度;第三模块,将各分量对应的变形预测结果重构得到最终预测值。实例分析可知,所提方法有效解决了非稳定特性给变形预测精度带来的不利影响,与VMD-LSTM、Self-attention-LSTM、LSTM等深度学习方法相比,VMD-Self-attention-LSTM的预测精度最大提升41.49%,与BP、ELM等传统机器学习算法相比,预测精度最大提升50.43%,为水闸深基坑安全监控模型的构建提供了新思路。

某档案馆建设对邻近既有供水隧洞安全影响分析

为分析某档案馆建设及运行对邻近既有供水隧洞安全的影响,采用MIDAS GTS构建三维有限元模型进行分析,模拟了档案馆从施工到运行的全过程对邻近既有供水隧洞安全的影响。结果表明,项目施工及运行过程中,对既有供水隧洞在各方向上的位移量均小于规范允许值,满足安全要求。

非平衡建筑超载下软弱地层地铁深基坑变形特性及加固方案研究

软弱地层中某地铁深基坑外侧作用有建筑物偏压超载,对深基坑开挖存在较为不利影响。为研究外部建筑物偏压超载对软弱地层中基坑支护结构的变形影响,确定合理的隔离桩及基底加固方案,采用有限元方法进行了不同加固方案下基坑开挖模拟计算,并与实测结果对比分析。结果表明:临近深基坑的外部建筑物超载将对基坑支护结构产生明显的偏压效应,偏压超载对坑底软弱地层范围内支护结构影响最为明显;基坑外侧设置隔离桩加固,可有效减小地下连续墙水平位移,隔离桩变形在软弱地层范围最大,增加桩长可使桩底产生支点约束效应,但不能有效减小支护结构位移及支撑轴力;同样,改变隔离桩刚度对支护结构位移及支撑轴力影响较小;仅采用基底加固方案,可减小坑底范围内支护结构位移,但对上部支护结构变形影响较小,采用隔离桩+基底加固组合方案可有效减小支护结构变形及支撑轴力,降低了偏压荷载对基坑的不利影响,具有较好的加固效果。

基于三维地质模型的深基坑土方开挖管理研究

建设项目弃土的资源化利用可有效降低施工企业的成本,同时按照弃土的工程特性进行预处理并分类填筑施工,可有效降低弃土边坡的稳定风险。基于岩土工程勘察成果,建立工程场地范围内的三维地质模型,通过模型分析及计算,对拟开挖的土石方按照岩性及工程位置进行工程量计算及统计,结合工程位置和土体的工程特点,合理组织施工,提高施工效率;加强开挖过程中土质的分类和管理,可有效控制弃土的综合成本,同时为合理组织弃土的预处理与回填施工提供依据。