Estimation of Land Subsidence Based on Groundwater Flow Model

This article presents an approach for estimating land subsidence due to withdrawal of groundwater. The proposed method calculates the groundwater seepage in 3-D-condition and calculates the land subsidence one-dimensionally. The governing equation on groundwater seepage is based on the three-dimensional mass conservation law and the principle of effective stress. The land subsidence calculation method is derived based on the following assumptions: (1) displacements occur only in the vertical direction, and (2) in vertical direction the total stresses do not change. The governing equation is solved by numerical method, i.e., finite element method (FEM) in spatial discretization and finite difference method (FDM) in time series discretization. In FEM Galerkin method is adopted and in FDM, lumped matrix method is employed. The proposed method is calibrated via analyzing 1-D consolidation problem and the results are compared with those from Terzaghi's one-dimensional consolidation theory and oedometer test. The proposed method is employed to analyze the consolidation of a soft layer due to withdrawal of groundwater from an aquifer under it. Moreover, this method is also applied to a field case of land subsidence due to groundwater pumping in a gas production field in Japan. The analytical results are compared with the field observed data. The results show that this approach simulates the field case well.     

黄河三角洲地区地面沉降驱动因素研究

黄河三角洲地区地势低平、生态脆弱, 地面沉降使得海水入侵和风暴潮灾害加剧。弄清该区域地面沉降驱动因素, 对油田安全生产和湿地生态保护都有积极的意义。以20 m 地层为界, 将地面沉降驱动因素分为浅地层和深地层因素。分析了地表荷载增加、地下水和油气开采、沉积物固结压实、新构造运动等对该区地面沉降的驱动作用。此外, 探讨了海平面上升和地震灾害对该区地面沉降的影响。结果表明: 该区地面沉降驱动因素主要为沉积物固结和地下水开采, 但控制范围存在区域性差异;1969 年地震使该区产生明显地面沉降, 海平面上升使该区地面沉降形势更加严峻。     

沿海地区城市发展及地面沉降的系统控制

本文运用系统工程原理,系统论述了地下水合理开发、管理和地面沉降系统控制,这对沿海地区地面沉降控制起到重要的作用。     

人工回灌控制基坑工程地面沉降的数值模拟

人工回灌是控制基坑降水工程引起地面沉降的有效手段,本文以上海地铁2、4、9号3条地铁线所围区域盛大基坑降水为例,探讨基坑降水地下水回灌控制地面沉降的作用机理,并基于下负荷面剑桥理论,建立水-土耦合的地面沉降数学模型,预测基于人工回灌的深基坑工程地面沉降,与实测地面沉降数据对比,得到比较符合实际的数值模拟结果,真实地反映了基坑开挖、支撑和降水共同作用下的地面沉降,值得基坑降水回灌工程推广。     

Spatial and temporal variations in subsidence due to the natural consolidation and compaction of sediment in the yellow river delta,china

ABSTRACT

The Yellow River Delta, which is the second-largest delta in China, has experienced varying degrees of land subsidence since the late 1970s. Although recent studies have identified the natural consolidation and compaction of sediment among the most important contributors to geologic processes, their processes have rarely been quantified. We estimated the sediment compaction over different time ranges to determine the temporal evolution of subsidence parameters (i.e., cumulative compaction). Estimates of primary consolidation, secondary consolidation, and the degree of consolidation in 152 boreholes revealed the spatial–temporal characteristics of sediment compaction and consolidation using geotechnical parameters collected from 152 boreholes, soil mechanics equations and the Kriging interplolation method. In addition, these estimates were partially constrained and cross-validated using the interferometric synthetic aperture radar (InSAR) results from early 2007 to late 2010 which were provided in a previous study. By performing a comparison analysis between theoretical evaluations of compaction for borehole data and InSAR observations, we were able to quantify subsidence due to sediment compaction. The comparison results suggest that the theoretical solutions agreed well with the measurements recorded by the well-validated, advanced InSAR method and that the deviations between the InSAR technique and geotechnical evaluations ranged from ?22 to 3?mm. The results reveal that the land subsidence of the chosen borehole sites during the investigative period was dominated by the primary consolidation and compaction of sediment. The underprediction of subsidence may be explained by fluid withdrawal, oil exploitation and engineering construction. To speculate, more geological disasters may occur if the current subsidence condition extends into the future.     

Effect of Groundwater Fluctuations on Pore Pressures and Earth Pressures on Coastal Excavation Retaining Walls

Pore water and earth pressures acting on retaining structures are investigated using an efficient coastal double-layered excavation model to determine offshore excavation responses to groundwater fluctuations outside foundation pits. Total pore water pressure includes excess pore water pressure (due to groundwater fluctuations) and steady pore water pressure (due to steady seepage) determined using one-dimensional consolidation theory of double-layered soil and one-dimensional steady-state flow theory, respectively. Rankine's active and passive earth pressures are obtained from pore water pressure. This method is applicable to arbitrary groundwater fluctuation conditions. How physical parameters affect pore water pressure is numerically investigated using examples, demonstrating the method's practicality for calculating pore water and earth pressures.     

现代黄河三角洲地面沉降对洲体演变的影响

对现代黄河三角洲地区地面沉降的特点和对三角洲发育演变的影响进行了分析研究。分析认为,现代黄河三角洲地区普遍发生的地面沉降具有多因素和时空不连续性的特点,给沉降区带来了一系列的危害,使多种海岸带灾害的破坏和影响加剧;地面沉降不仅可以造成地面标高损失,影响三角洲的垂向发育,还通过改变地面坡降来影响河道的演变和尾闾的摆动,同...     

Numerical simulation of solitary wave scattering by a circular cylinder array

The interaction of a solitary wave with an array of surface-piercing vertical circular cylinders is investigated numerically. The wave motion is modeled by a set of generalized Boussinesq equations. The governing equations are discretized using a finite element method. The numerical model is validated against the experimental data of solitary wave reflection from a vertical wall and solitary wave scattering by a vertical circular cylinder respectively. The predicted wave surface elevation and the wave forces on the cylinder agree well with the experimental data. The numerical model is then employed to study solitary wave scattering by arrays of two circular cylinders and four circular cylinders respectively. The effect of wave direction on the wave forces and the wave runup on the cylinders is quantified.     

Comparison of four numerical models for simulating seepage from salt marsh sediments

A boundary integral equation method (BIEM) model and three differently formulated finite element method (FEM) models were implemented to explore the spatial and temporal patterns in marsh pore water seepage that each generated. The BIEM model is based on the Laplace equation coupled to a dynamic free-surface condition that assumes that, as the water-table changes, the aquifer instantaneously loses or gains an amount of water equal to the change in head times the specific yield. The FEM models all implement a simplified Richards equation that allows gradual desaturation or resaturation and thus flow in both the saturated and unsaturated zones of the aquifer. Two of the FEM models are based on the governing equation for the USGS model SUTRA and thus take into account fluid and aquifer compressibility. One of these was modified to take into account the effect of tidal loading on the total stress, which is assumed to be constant in the derivation of the original version of SUTRA. The third FEM model assumes that neither the fluid or aquifer matrix is compressible so that changes in storage are due solely to changes in saturation. The unmodified SUTRA model generated instantaneous boundary fluxes that were up to two orders of magnitude greater, and spatially more uniform, than those of the other models. The FEM model without compressibility generated spatial and temporal patterns of the boundary fluxes very similar to those produced by the BIEM model. The SUTRA model with the tidal stress modification gave fluxes similar in magnitude to the BIEM and no compressibility models but with distinctly different distributions in space and time. These results indicate that accurate simulation of seepage from marsh soils is highly sensitive to aquifer compressibility and to proper formulation of the effect of tidal loading on the total stress in the aquifer. They also suggest that accurate simulation may require total stress correction not only for tidal loading but for changes in the water table as well. Finally, to aid the development of methods for the measurement of compressibility, we present a schematic, pore-scale model to illustrate the factors that may govern the compressibility of marsh soils.     

基于ARIMA与LSTM的海岸带地面沉降预测方法

快速的地面沉降是一种地质灾害,它关系到社会的可持续发展,甚至威胁人类的生命财产安全。InSAR技术可以获取地表长时间、大范围的形变数据,可用于分析潜在的地面沉降问题,为预防地质灾害提供了一种可靠手段。如何基于InSAR数据对地面沉降进行预测,一直是研究人员关注的重点方向和难题。为此,本文在前人对地面沉降预测研究的基础上,提出了一种将差分移动平均自回归(ARIMA)模型与深度学习中的长短期记忆单元(LSTM)模型相结合的地面沉降预测方法,即利用InSAR得到的形变量数据与ARIMA模型预测结果作差,然后利用LSTM对该差值进行训练与预测。以杭州湾2017—2019年InSAR监测数据为例验证了该方法,结果表明,与传统的单一预测算法相比,本文方法的均方根误差至少减小了2.23 mm,平均绝对误差至少减小了0.98 mm,平均预测精度至少提升了15.19%,验证结果证实了本文方法的可行性,为地面沉降预警工作提供了新的思路和方法。     

Short-crested waves interaction with a concentric porous cylinder system with partially porous outer cylinder

In this paper, based on the linear wave theory, the interaction of short-crested waves with a concentric dual cylindrical system with a partially porous outer cylinder is studied by using the scaled boundary finite element method (SBFEM), which is a novel semi-analytical method with the advantages of combining the finite element method (FEM) with the boundary element method (BEM). The whole solution domain is divided into one unbounded sub-domain and one bounded sub-domain by the exterior cylinder. By weakening the governing differential equation in the circumferential direction, the SBFEM equations for both domains can be solved analytically in the radial direction. Only the boundary on the circumference of the exterior porous cylinder is discretized with curved surface finite elements. Meanwhile, by introducing a variable porous-effect parameter G, non-homogeneous materials caused by the complex configuration of the exterior cylinder are modeled without additional efforts. Comparisons clearly demonstrate the excellent accuracy and computational efficiency associated with the present SBFEM. The effects of the wide range wave parameters and the structure configuration are examined. This parametric study will help determine the various hydrodynamic effects of the concentric porous cylindrical structure.     

杭州湾沿海地区地面沉降时序InSAR监测与分析

Time-series InSAR analysis(e.g., permanent scatterers(PSInSAR)) has been proven as an effective technology in monitoring ground deformation over urban areas. However, it is a big challenge to apply this technology in coastal regions due to the lack of man-made targets. An distributed scatterers interferometric synthetic aperture radar(DSInSAR) is developed to solve the problem of insufficient samples and low reliability in monitoring coastal lowland subsidence, by applying a spatially adaptive filter and an eigendecomposition algorithm to estimating the optimal phase of statistically homogeneous distributed scatterers(DSs). Twenty-four scenes of COSMO-Sky Med images acquired between 2013 and 2015 are used to retrieve the land subsidence over the Shangyu District on south coast of the Hangzhou Bay, Zhejiang Province, China. The spatial pattern of the land subsidence obtained by the PS-InSAR and the DSInSAR coincides with each other, but the density of the DSs is three point five times higher than the permanent scatterers(PSs). Validated by precise levelling data over the same period, the DSInSAR method achieves an accuracy of ±5.0 mm/a which is superior to the PS-InSAR with±5.5 mm/a. The land subsidence in the Shangyu District is mainly distributed in the urban areas, industrial towns and land reclamation zones, with a maximum subsidence rate –30.2 mm/a. The analysis of geological data, field investigation and historical reclamation data indicates that human activities and natural compaction of reclamation material are major causes of the detected land subsidence. The results demonstrate that the DSInSAR method has a great potential in monitoring the coastal lowland subsidence and can be used to further investigate subsidence-related environmental issues in coastal regions.     

Evaluation of Effective Depth of PVD Improvement in Soft Clay Deposit: A Field Case Study

This article presents a case history of determination of effective depth of prefabricated vertical drains (PVDs) under embankment loading on a very soft clay deposit in central China, near Jiujiang, Jiangxi Province. The height of the embankment was 5.3 m and construction time was about one year. The PVDs were installed to a depth of 8.5 m at a spacing of 1.5 m in a triangular pattern. Field observations and the finite element method (FEM) were employed to analyze the performance of the soft deposit during embankment construction. The influential depth of the embankment loading was evaluated based on settlement, excess pore pressure, and stress increase in subsoil, both from the observed data and FEM analysis. The effective PVD depth was determined in the following ways: (1) the depth of 5% subsoil settlement of surface settlement; (2) vertical stress increase in subsoil of 25% in-situ stress; and (3) consolidation time/PVD depth relation by FEM. Based on the analysis, the effective depth of PVDs was determined to be between 10 and 12.8 m for this field case.     

Irregular wave-induced seepage action on cylinders resting on rubble mound foundation

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黄河小浪底傍河地下水源地资源评价的有限体积法

为准确评价小浪底傍河地下水源地的可采资源。根据小浪底傍河地下水源地的水文地质条件,应用有限体积法评价该水源地的可采资源,提出2种开采方案。结果显示：此方法对该区地下水资源所做的评价正确,反演计算的水文地质参数合理。     

点源二维各向异性地电断面的直流电场有限元解法

本文给出点源二维各向导性地电断面的直流电场有限元解法。首先,用付氏变换将点源二维各向异性地电断面的三维边值问题转变成二维边值问题;然后导出与二维边值问题相应的变分方程;最后用有限元法解变分方程并用付氏反变换,就获得三维空间中的电位 本文用此方法给出二个算例,并与解析解进行对比。     

完全非线性波浪与二维固定结构物作用的IGN-BEM耦合分析模型

为高效准确地对完全非线性波浪与二维固定结构物的相互作用进行模拟分析,建立了二维完全非线性时域耦合模型。耦合模型将计算域划分为靠近结构物的内域和远离结构物的外域,每个区域均采用满足完全非线性自由水面边界条件的波浪模型进行求解。在内域使用Laplace方程描述流体运动并采用高阶边界元法（BEM）对其进行求解;而在没有结构物的外域,波浪运动的控制方程为Irrotational Green-Naghdi（IGN）方程并采用有限元法（FEM）对其进行求解。内域和外域通过一段重叠区域进行耦合,从而实现模型间变量的传递。首先利用耦合模型分别对规则波的传播、直墙前立波的生成以及相关物理模型试验进行模拟,数值结果与精确解和试验结果的良好吻合验证了耦合模型耦合方式的合理性以及处理非线性问题的准确性;然后使用耦合模型模拟分析了波浪与固定结构物间的相互作用,并将结果与线性解析解以及完全非线性BEM模型的结果进行了对比分析,进一步证明了耦合模型的正确性与高效性。     

拖曳系统拖曳动态过程仿真分析

对于海洋缆索系统,论文针对传统有限段法的不足,提出改进的缆索有限段法,缆索离散为若干弹性缆段组成的多柔体系统,根据缆索的特点选择适当的参考系和广义速率,引入有限元法中的形函数描述段内各点位移,根据Kane方程推导改进缆索有限段模型的运动方程。基于改进的缆索有限段法,提出了模拟拖曳缆索释放一回收过程的变拓扑结构模型,即用可变长度缆段长度的变化和缆段数量的改变建模缆索的释放和回收过程。文中对一海洋拖曳系统进行了动力学仿真,与海洋试验结果比较验证了模型的正确性。     

Time-Dependent Deformation Behavior of Jiangsu Marine Clay

In this article, the mechanical behavior of a Jiangsu marine clay was investigated by drained triaxial tests, traixial rheological tests, and one-dimensional compression and swelling tests. A visco-plastic model, the Bingham model combining two yield surfaces model, was proposed to describe the time-dependent deformation behaviors of the marine clay. The governing equation of Biot's consolidation theory for the visco-plastic soil is solved using a finite element code which incorporates the visco-plastic model. Using the finite element method, settlements of a typical embankment on the Lianxu expressway in China are calculated. Settlement calculations using the visco-plastic model are in agreement with the measured settlements in the field. The results demonstrate that the visco-plastic model is appropriate for calculating the visco-plastic deformations of Jiangsu marine clay. Theoretical and experimental studies show that the visco-plastic deformation of Jiangsu marine clay is substantial.     

Numerical Simulation of Spatial Lag Between Wave Breaking Point and Location of Maximum Wave-Induced Current

A quasi three-dimensional numerical model of wave-driven coastal currents with the effects of surface rollers is developed for the study of the spatial lag between the location of the maximum wave-induced current and the wave breaking point.The governing equations are derived from Navier-Stokes equations and solved by the hybrid method combining the fractional step finite different method in the horizontal plane with a Galerkin finite element method in the vertical direction.The surface rollers effects are considered through incorporating the creation and evolution of the roller area into the free surface shear stress.An energy equation facilitates the computation process which transfers the wave breaking energy dissipation to the surface roller energy.The wave driver model is a phase-averaged wave model based on the wave action balance equation.Two sets of laboratory experiments producing breaking waves that generated longshore currents on a planar beach are used to evaluate the model's performance.The present wave-driven coastal current model with the roller effect in the surface shear stress term can produce satisfactory results by increasing the wave-induced nearshore current velocity inside the surf zone and shifting the location of the maximum longshore current velocity landward.