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Impacts of aquifer heterogeneity on subsidence resulting from engineering dewatering in the Lower Yangtze River plains
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Land subsidence is a typical geo-hazard in Cangzhou, North China Plain, having caused severe damages to transportation networks, public utilities, and other civil infrastructures. The mechanism of land subsidence and its affecting factors were revealed by theoretical model analysis, and doing geotechnical tests and monitoring of soil deformation as well as groundwater level. Theoretical analysis shows that groundwater withdrawal from both unconfined and confined aquifers can lead to land subsidence. Land subsidence of the Cangzhou Plain is mainly caused by the exploitation of deep confined groundwater. Geotechnical tests show a general trend that the compressive modulus increases with the increase of soil depth. The strata of the Upper Pleistocene and Holocene are mostly normally consolidated and partially under-consolidated, which are prone to produce large compression after pumping. Land subsidence in the Cangzhou Plain has a strong hysteresis because of seepage consolidation and creep. The deformation characteristics of strata change at different depth. The shallow aquifers are mainly elastic deformation. The area of severe land subsidence in the Cangzhou Plain is closely related to rainfall, mainly because the large amount of water are used for agricultural irrigation. InSAR results show a sudden change in regional subsidence across the Cangdong fault, which is caused by the difference in the thickness of Quaternary sediments and the difference in the deep confined groundwater level.
The groundwater system in the strata within 800 m of Dezhou city, which is located in the North China Plain (NCP), China, includes four aquifer groups (labeled AgI to AgIV from top to bottom). The exploitation of deep groundwater from AgIII and AgIV is generally considered the primary reason for land subsidence in Dezhou. Although the deep groundwater exploitation volume has decreased significantly, the annual land subsidence has been accelerating in the central area of Dezhou since 2012. The factors resulting in the acceleration of land subsidence are discussed. Presently, AgII and AgIII between 60 and 500 m below the ground surface are the two main layers contributing to land subsidence, the settlement of which accounted for 74% of the total subsidence value. The increasing settlement of AgIII may be due to the soil creep caused by the continuous release of groundwater from AgIII. AgII is characterized by high compressibility, the deformation of which is susceptible to a decrease in the groundwater level. Since AgII is not the historical groundwater exploitation layer, the decrease in the groundwater level of AgII may be caused by the downward leaking recharge to AgIII and lateral recharge to the surrounding cities in the NCP. The study of the deformation mechanism of AgII is essential for subsidence control in Dezhou. Geological investigation and groundwater-level monitoring of AgII and joint control of regional groundwater overexploitation in the NCP should be strengthened.
Surface subsidence induced by underground mining is a typical serious geohazard. Numerical methods such as the Discrete Element Method (DEM) and Finite Difference Method (FDM) have been widely used to model and analyze mining-induced surface subsidence. However, the DEM is typically computationally expensive, and is not capable of analyzing large-scale problems, while the mesh distortion may occur in the FDM modeling of largely-deformed surface subsidence. To address the above problem, this paper presents a geometrically and locally adaptive remeshing method for the FDM modeling of largely-deformed surface subsidence induced by underground mining. The essential ideas behind the proposed method are as follows. (1) Geometrical features of elements (i.e., the mesh quality), rather than the calculation errors, are employed as the indicator for determining whether to conduct the remeshing. (2) Distorted meshes with multiple attributes, rather than those with only a single attribute are locally regenerated. In the proposed method, (1) the distorted meshes are first adaptively determined based on the mesh quality, and then removed from the original mesh model; (2) the tetrahedral mesh in the distorted area is first regenerated, and then the physical field variables of old mesh are transferred to the new mesh; (3) numerical calculation process recovers when finishing the regeneration and transformation. To verify the effectiveness of the proposed method, the surface deformation of the Yanqianshan iron mine, Liaoning Province, is numerically investigated by utilizing the proposed method, and compared with the numerical results of the DEM modeling. Moreover, the proposed method is applied to predict the surface subsidence in Anjialing No.1 Underground Mine, Shanxi Province.
Ever decreasing water resources and climate change have driven the increasing use of groundwater causing land subsidence in many countries. Geodetic sensors such as InSAR, GPS and leveling can detect surface deformation but cannot measure subsurface deformation. A single‐well, single‐depth extensometer can be used to measure subsurface deformation, but it cannot delineate the depths of major compaction and provide insight about the deformation mechanism throughout a complex aquifer system, unless man extensometers at different depths are used. We present a multilayer compaction well (MLCW), installed in a borehole, that uses magnetic rings to detect stratum compaction at 25 depths as deep as 300 m below land surface. Our laboratory and field assessments indicate 1 mm precision and accuracy for one single‐depth magnetic reading. We tested the performance of MLCW by measuring aquifer‐system compaction over the proximal, middle, and distal fans of the Choushui River Alluvial Fan (CRAF) that has long experienced severe land subsidence. The MLCW measurements were used to create time‐depth diagrams of compaction, showing different compaction rates at different layers of aquifers and aquitards to identify the depths of major compactions. The elastic (reversible) and inelastic (irreversible) compactions from MLCW were used in stress‐strain analyses to estimate skeletal specific storages and the safe groundwater levels, below which groundwater extractions have caused irreversible compactions. The hydrogeological parameters derived from MLCW measurements can help governmental agencies to determine effective land‐use and water‐use policies, and ascertain the best strategy for utilizing artificial recharge to prevent land subsidence and achieve sustainable groundwater management.
Due to the large-scale mining of mineral resources, the surface strata in many areas have collapsed and even developed into deformation and subsidence. Based on the conventional geotechnical deformation monitoring technology, the surface deformation prediction and disaster caused by the conventional geotechnical deformation monitoring technology can be avoided. In this paper, based on GIS, combined with the analysis of surface settlement, the surface settlement, slope value, curvature deformation value, horizontal displacement, and corresponding horizontal deformation formula are comprehensively analyzed and studied. In order to make the surface deformation visualization and realize the high approximation processing with the subsidence area, this paper innovatively introduces the 3D visualization settlement data analysis and realizes the calculation result analysis and corresponding retrieval multiple visualization in the subsidence area depth prediction process. On this basis, the prediction data of subsidence area can be optimized and analyzed so as to master the efficient and accurate law of surface deformation. In the experimental part, a mine is taken as the experimental object, and the 3D visual monitoring processing is carried out. In this paper, the main method to prevent electromagnetic leakage of the computer system is the source suppression method. The source suppression method solves the problem fundamentally by reducing or eliminating the emission of computer electromagnetic leakage source. In the actual experiment, the effect of this method is very effective. The experimental results show that the prediction results and corresponding early warning accuracy of the system are significantly improved compared with the traditional scheme, which provides practical significance for disaster prevention and corresponding environmental impact assessment.
Groundwater resources have been exploited and utilized on a large scale in the North China Plain (NCP) since the 1970s. As a result of extensive groundwater depletion, the NCP has experienced significant land subsidence, which threatens geological stability and infrastructure health and exacerbates the risks of other geohazards. In this study, we employed multi-track Synthetic Aperture Radar (SAR) datasets acquired by the Sentinel-1A (S1A) satellite to detect spatial and temporal distributions of surface deformation in the NCP from 2016 to 2018 based on multi-temporal interferometric synthetic aperture radar (MT-InSAR). The results show that the overall ground displacement ranged from −165.4 mm/yr (subsidence) to 9.9 mm/yr (uplift) with a standard variance of 28.8 mm/yr. During the InSAR monitoring period, the temporal pattern of land subsidence was dominated by a decreasing tendency and the spatial pattern of land subsidence in the coastal plain exhibited an expansion trend. Validation results show that the S1A datasets agree well with levelling data, indicating the reliability of the InSAR results. With groundwater level data, we found that the distribution of subsidence in the NCP is spatially consistent with that of deep groundwater depression cones. A comparison with land use data shows that the agricultural usage of groundwater is the dominant mechanism responsible for land subsidence in the whole study area. Through an integrated analysis of land subsidence distribution characteristics, geological data, and previous research results, we found that other triggering factors, such as active faults, precipitation recharge, urbanization, and oil/gas extraction, have also impacted land subsidence in the NCP to different degrees.
Three-dimensional (3D) modeling of geological surfaces, such as coal seams and strata horizons, from sparsely sampled data collected in the field, is a crucial task in geological modeling. Interpolation is a common approach for this task to construct continuous geological surface models. However, this problem becomes challenging considering the impact of the faults on geological surfaces. Existing methods tend to solve this problem through three steps, including interpolating stratum and fault surface, applying a fault modeling method to modify the geological surface, and optimizing the modified surface to pass sample points fallen into the fault displacement zone. This paper presents a more concise method to generate a faulted geological surface, in which 1) a constrained Delaunay triangulated irregular network (CD-TIN) is constructed to facilitate the neighborhood search process of the ordinary kriging (OK) interpolation, 2) the CD-TIN is also directly constrained by horizon cut-off lines formed from theoretical fault displacement profiles, and 3) subsequently, neighbors of the location to be estimated are selected effectively in the CD-TIN considering the fault topology. The proposed method significantly improves the time efficiency of the OK interpolation by utilizing the CD-TIN and incorporates fault effects directly into the interpolation process by inserting fault horizontal cut-off lines into CD-TIN. Moreover, by integrating the fault effects directly into the interpolation process, the surface modeling process is accomplished in a single stage instead of two separate stages of interpolation first and then modifying the surface in the fault area. By this strategy, the proposed method significantly improves the time efficiency of the OK interpolation algorithm and achieves more accurate modeling of the faulted geological surface. Experiments were designed to compare the performance of our method with several commonly used approaches, and the results indicate that the proposed TIN-constrained OK method achieves better accuracy and efficiency in modeling faulted geological surfaces than other methods. This method could also be used in geospatial interpolation studies, such as meteorological data interpolation.
Visual Modflow software was adopted to numerically simulate the groundwater flow in unconfined strata for six case studies of previously executed projects in Egypt that were in need of dewatering systems. This software was then integrated with a modular groundwater optimizer (MGO) as an optimization model, where the genetic algorithm (GA) was the optimization solver. The models run an unmanaged nonoptimal simulation based on the results of a pumping test and the executed pumping wells to reach the observed drawdown values in the piezometric wells at the construction sites. The models run once more for pumping well optimization using the MGO with the objective of minimizing the number of wells, the total amount of extracted water and, consequently, the cost of dewatering systems. Comparing the drawdown depth (D.D) from the two consecutive runs for each project, the optimization results for the unconfined strata proved that the required D.D can be achieved with only a percentage of the implemented wells ranging from 58 to 75%. The results identify the importance of applying the MGO as a robust method in the design of future dewatering projects.
The Vietnamese Mekong Delta was formed by rapid transgression during the second half of the Holocene by deposition of mainly unconsolidated, fine-grained (clayey) sediments undergoing high compaction rates. The natural subsidence can seriously impact the already vulnerable delta plain as its low elevation exposes the delta to global sea level rise, flooding, salinization. Human activities such as groundwater pumping, infrastructural loading, sand mining and dam construction have exacerbated the effects of natural consolidation. Here we present a novel modeling study that has allowed to reproduce the formation and evolution of the Mekong delta over the past 4000 years. Using an adaptive finite-element mesh, the model properly simulates accretion and natural consolidation characterizing the delta evolution. Large soil grain motion and the delayed dissipation of pore-water overpressure are accounted for. We find that natural compaction of Holocene deposits following delta evolution exceeds predicted values of absolute sea level rise. The unprecedented high rates (up to ~20 mm/yr) threaten the lower delta plain with permanent inundation and inevitably reduce the designed service life of flood defense structures along the coast. Total subsidence and sediment delivery to the delta plain will determine its future elevation and vulnerability to relative sea level rise.
Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ~18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr⁻¹, with areas subsiding over 2.5 cm yr⁻¹, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.
The classical aquitard-drainage model COMPAC has been modified to simulate the compaction process of a heterogeneous aquitard consisting of multiple sub-units (Multi-COMPAC). By coupling Multi-COMPAC with the parameter estimation code PEST++, the vertical hydraulic conductivity (Kv) and elastic (Sske) and inelastic (Sskp) skeletal specific-storage values of each sub-unit can be estimated using observed long-term multi-extensometer and groundwater level data. The approach was first tested through a synthetic case with known parameters. Results of the synthetic case revealed that it was possible to accurately estimate the three parameters for each sub-unit. Next, the methodology was applied to a field site located in Changzhou city, China. Based on the detailed stratigraphic information and extensometer data, the aquitard of interest was subdivided into three sub-units. Parameters Kv, Sske and Sskp of each sub-unit were estimated simultaneously and then were compared with laboratory results and with bulk values and geologic data from previous studies, demonstrating the reliability of parameter estimates. Estimated Sskp values ranged within the magnitude of 10⁻⁴ m⁻¹, while Kv ranged over 10⁻¹⁰–10⁻⁸ m/s, suggesting moderately high heterogeneity of the aquitard. However, the elastic deformation of the third sub-unit, consisting of soft plastic silty clay, is masked by delayed drainage, and the inverse procedure leads to large uncertainty in the Sske estimate for this sub-unit.
The vertical hydraulic conductivity (Kv ), elastic (Sske ), and inelastic (Sskv ) skeletal specific storage of aquitards are three of the most critical parameters in land subsidence investigations. Two new analytic methods are proposed to estimate the three parameters. The first analytic method is based on a new concept of delay time ratio for estimating Kv and Sske of an aquitard subject to long-term stable, cyclic hydraulic head changes at boundaries. The second analytic method estimates the Sskv of the aquitard subject to linearly declining hydraulic heads at boundaries. Both methods are based on analytical solutions for flow within the aquitard, and they are jointly employed to obtain the three parameter estimates. This joint analytic method is applied to estimate the Kv , Sske , and Sskv of a 34.54-m thick aquitard for which the deformation progress has been recorded by an extensometer located in Shanghai, China. The estimated results are then calibrated by PEST (Doherty 2005), a parameter estimation code coupled with a one-dimensional aquitard-drainage model. The Kv and Sske estimated by the joint analytic method are quite close to those estimated via inverse modeling and performed much better in simulating elastic deformation than the estimates obtained from the stress-strain diagram method of Ye and Xue (2005). The newly proposed joint analytic method is an effective tool that provides reasonable initial values for calibrating land subsidence models.
The purpose of T-PROGS is to enable implementation of a transition probability/Markov approach to geostatistical simulation of categorical variables. In comparison to traditional variogram-based geostatistical methods, the transition probability/Markov approach improves consideration of spatial cross-correlations and facilitates the integration of geologic interpretation of facies architecture into the model development process. The manual was designed primarily
for geostatistical practitioners, not theoreticians. In our experience, geostatistics is not the primary occupation of most users of geostatistical simulation codes. As such, the manual relies
on references for much of the theoretical details. Large portions of T-PROGS originated from modified versions of the GSLIB codes. Clayton
Deutsch, Andre Journel, and the other GSLIB contributors deserve much credit for developing much robust FORTRAN code that remains intact in T-PROGS. Graham Fogg’s steadfast promotion of T-PROGS has been greatly appreciated. This work was supported by U.S. Army Waterways Experimentation Station, Vicksburg, Mississippi; Lawrence Livermore National Laboratory, Livermore, California;
N.I.E.H.S. Superfund Grant (ES-04599); U.S.G.S. Water Resources Research Grant (14-18-001-61909); and U.S. EPA (R819658) Center for Ecological Health Research at UC Davis.
Although the information in this document has been funded in part by the United States Environmental Protection Agency, it may not necessarily reflect the views of the Agency, and no official endorsement should be inferred. This work was performed under the auspices of the U.S. Department of Energy by University of
California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
Sub-surface construction in urban areas generally involves drainage of groundwater, which can induce subsidence in soil deposits. Knowledge of where compressible sediments are located and how thick these are is essential for estimating subsidence risk. A probabilistic method for coupled bedrock-level and soil-layer modeling to detect compressible sediments is presented. The method is applied in an area in central Stockholm, where clay is the compressible sediment layer. First, a bedrock-level model was constructed from three sources of information: (a) geotechnical drillings reaching the bedrock; (b) drillings not reaching the bedrock; and (c) mapped bedrock outcrops. Input data for the probabilistic bedrock-level model was generated by a stepwise Kriging procedure. Second, a three layer soil model was constructed, including the following materials: (a) coarse grained post glacial and filling material below the ground surface; (b) glacial and post-glacial clays; and (c) coarse grained glaciofluvial and glacial till deposits above the bedrock. Layer thicknesses were transformed to proportions of the total soil thickness. Since Kriging requires data to be normally distributed, the proportions were transformed from proportions (P) to standard normal quantiles (z). In each iteration of a Monte-Carlo simulation, a spatial distribution of the bedrock level was simulated together with the transformed values for the soil-layer proportions. From the iterations, the probability density of the clay thickness (compressible sediments) at each grid cell was calculated. The results of the case study map the expected value (mean) and the 95th percentile of the probability of compressible sediments at specific locations. The resulting model is geologically realistic and validated through a cross-validation procedure in order to be in good agreement with a reference dataset. The case study showed that the method can efficiently handle large amounts of data and requires little manual adjustment. Moreover, the mapped results can provide useful decision support when planning risk-reducing measures and when communicating with stakeholders. Although this novel method is developed for risk assessment of groundwater drawdown induced subsidence, it is useful for other applications involving spatial soil strata modeling.
The demands of domestic and industrial water supply in Ho Chi Minh City (HCM City) have surged rapidly since 1990s with the opening up of Vietnam’s economy. In the same time groundwater extraction has continuously increased and is suspected to have caused land subsidence, which in turn would affect the growing infrastructure of the city. By now only a few monitoring stations have been installed, and both pore pressure and settlement observation data are not yet available. As an alternative, we proposed an approach to estimate land subsidence based on the available data of groundwater drawdown in the pumped aquifer and geotechnical properties of the adjacent clay layers. In this study, a large amount of borehole data were collected to construct a computer-based subsoil database. As results a complete characterization of a six-aquifer system up to 360 m deep and that of the shallower subsoil profile up to 140 m deep were done based on data from more than 3,000 boreholes . Examples of land subsidence prediction were done for three cases, i.e.: (i) at a borehole location that has sign of well protrusion; (ii) along the new MRT line no. 1; and (iii) over a profile crossing the city from NW to SE. The first two cases were to show the procedure of land subsidence prediction, while the third case was to compare the FEM analysis results with those estimated by satellite image analysis. The results of this study showed that the subsoil in HCM city with thick soft clay layers is vulnerable to the groundwater extraction. It is expected that the maps of drawdown and the subsoil database developed by this study in combination with a 1D FEM consolidation program will provide a useful tool to estimate land subsidence of HCM City while the pore pressure and settlement data are not yet available.
Keywords: HCM city, land subsidence, groundwater extraction, subsurface database, FEM consolidation analysis.
This study presents a stochastic inverse method for aquifer structure identification using sparse geophysical and hydraulic response data. The method is based on updating structure parameters from a transition probability model to iteratively modify the aquifer structure and parameter zonation. The method is extended to the adaptive parameterization of facies hydraulic parameters by including these parameters as optimization variables. The stochastic nature of the statistical structure parameters leads to nonconvex objective functions. A multi-method genetically adaptive evolutionary approach (AMALGAM-SO) was selected to perform the inversion given its search capabilities. Results are obtained as a probabilistic assessment of facies distribution based on indicator cokriging simulation of the optimized structural parameters. The method is illustrated by estimating the structure and facies hydraulic parameters of a synthetic example with a transient
hydraulic response.
Stock market price is one of the most important indicators of a country's economic growth. That's why determining the exact movements of stock market price is considerably regarded. However, complex and uncertain behaviors of stock market make exact determination impossible and hence strong forecasting models are deeply desirable for investors' financial decision making process. This study aims at evaluating the effectiveness of using technical indicators, such as simple moving average of close price, momentum close price, etc. in Turkish stock market. To capture the relationship between the technical indicators and the stock market for the period under investigation, hybrid Artificial Neural Network (ANN) models, which consist in exploiting capabilities of Harmony Search (HS) and Genetic Algorithm (GA), are used for selecting the most relevant technical indicators. In addition, this study simultaneously searches the most appropriate number of hidden neurons in hidden layer and in this respect; proposed models mitigate well-known problem of overfitting/underfitting of ANN. The comparison for each proposed model is done in four viewpoints: loss functions, return from investment analysis, buy and hold analysis, and graphical analysis. According to the statistical and financial performance of these models, HS based ANN model is found as a dominant model for stock market forecasting.
In distributed and coupled surface water – groundwater modeling, the uncertainty from the geological structure is unaccounted for if only one deterministic geological model is used. In the present study, the geological structural uncertainty is represented by multiple, stochastically generated geological models, which are used to develop hydrological model ensembles for the Norsminde catchment in Denmark. The geological models have been constructed using two types of field data, airborne geophysical data and borehole well log data. The use of air-borne geophysical data in constructing stochastic geological models and followed by the application of such models to assess hydrological simulation uncertainty for both surface water and groundwater have not been previously studied. The results show that the hydrological ensemble based on geophysical data has a lower level of simulation uncertainty, but the ensemble based on borehole data is able to encapsulate more observation points for stream discharge simulation. The groundwater simulations are in general more sensitive to the changes in the geological structure than the stream discharge simulations, and in the deeper groundwater layers, there are larger variations between simulations within an ensemble than in the upper layers. The relationship between hydrological prediction uncertainties measured as the spread within the hydrological ensembles and the spatial aggregation scale of simulation results has been analyzed using a Representative Elementary Scale (RES) concept. The results show a clear increase of prediction uncertainty as the spatial scale decreases. This article is protected by copyright. All rights reserved.
The plain of Beijing city in China suffers severe land subsidence owing to groundwater overdraft. The maximum subsidence rate could reach 6 cm/year through the 2000s. An integrated subsidence-monitoring program was designed, including levelling survey, borehole extensometers and multilayer monitoring of groundwater level, with the aim to understand both hydrological and mechanical processes and to characterize the land subsidence. From multilayer compaction monitoring, the major compression layers were identified. The major strata contributing to compression deformation are the second (64.5-82.3 m) and third (102-117 m) aquitards, which contributed around 39 % of the total subsidence. Meanwhile, irrecoverable deformations were also observed in the second (82.3-102 m) and third (117-148 m) confined aquifers; they exhibit elasto-plastic mechanical behavior, which is attributed to the thin beds of silt or silty clay. Stress-strain analysis and oedometer tests were conducted to study the aquifer-system response to pumping and to estimate the specific storage of the major hydrogeologic units. The results reveal the creep behavior and elasto-plastic, visco-elasto-plastic mechanical behavior of the aquitards at different depths. The compressibility of the aquitards in the inelastic range is about one order of magnitude larger than for the elastic range.
Geological heterogeneity is a very important factor to consider when developing geological models for hydrological purposes. Using statistically based stochastic geological simulations, the spatial heterogeneity in such models can be accounted for. However, various types of uncertainties are associated with both the geostatistical method and the observation data. In the present study, TProGS is used as the geostatistical modeling tool to simulate structural heterogeneity for glacial deposits in a head water catchment in Denmark. The focus is on how the observation data uncertainty can be incorporated in the stochastic simulation process. The study uses two types of observation data: borehole data and airborne geophysical data. It is commonly acknowledged that the density of the borehole data is usually too sparse to characterize the horizontal heterogeneity. The use of geophysical data gives an unprecedented opportunity to obtain high resolution information and thus to identify geostatistical properties more accurately especially in the horizontal direction. However, since such data are not a direct measurement of the lithology, larger uncertainty of point estimates can be expected as compared to the use of borehole data. We have proposed a histogram probability matching method in order to link the information on resistivity to hydrofacies, while considering the data uncertainty at the same time. Transition probabilities and Markov Chain models are established using the transformed geophysical data. It is shown that such transformation is in fact practical; however, the cut-off value for dividing the resistivity data into facies is difficult to determine. The simulated geological realizations indicate significant differences of spatial structure depending on the type of conditioning data selected. It is to our knowledge the first time that grid-to-grid airborne geophysical data including the data uncertainty are used in conditional geostatistical simulations in TProGS. Therefore, it provides valuable insights regarding the advantages and challenges of using such comprehensive data.
During foundation pit excavation, groundwater is often the most important factor that affects pit stability. Dewatering is widely used in pit excavation to avoid uplift of excavation floors due to high water pressure. The characteristics of seepage in small-scale deep foundation pits of high-rise buildings or in the long narrow foundation pits of subway stations have been extensively investigated. However, the characteristics of seepage in large-scale deep excavations have not been studied. This paper investigates the large deep excavation of the buildings in Oriental Fisherman’s Wharf. The total area of the construction site is 33,917 m2. Single-well and group-well field pumping tests were performed and a numerical simulation by 3D finite difference method (FDM) was carried out. The simulation used results from the field pumping tests. The permeability parameters of the confined aquifer were then revised, based upon comparisons of simulation and observation results. Subsequently, dewatering schemes were simulated by FDM forward analysis. The simulation results show that dewatering schemes can minimize seepage and uplift in large excavation pits, though settlement outside the pit may need treatment measures.
Horizontal directional drilling (HDD) is a widely used trenchless method for underground utility connections. The associated ground settlement triggered by HDD depends on the size, types, and surface texture of pipe, diameter of borehole, and soil conditions. The present study investigates the surface settlement due to the construction of a 1067 mm diameter HDD, which will replace an existing sewer siphon under the SR-60 highway in Chino, California using empirical, and numerical methods. Based on the results obtained from the subsurface investigation, an empirical analysis was conducted first. followed by numerical modeling of the HDD using PLAXIS 2D software. A careful comparison between two different methods indicated closer values of surface settlement between the empirical method (7.3 mm) and the numerical modeling (4.6 mm). In addition, the shape of surface settlement and horizontal settlement curves for the empirical and numerical methods was found to be similar. The minor discrepancy between the two methods resulted as the numerical model can host several soil layers whereas the empirical equation can use only one type of soil. Finally, a parametric study was conducted to evaluate the effect of borehole cover depth, size, and soil parameters on surface settlement. It was observed that soil strength parameters yielded a greater effect on surface movement, whereas modulus of elasticity has a relatively smaller influence with zero contribution from Poisson's ratio.
The distribution of hydrogeological materials in a three-dimensional heterogeneous aquifer system has a large effect on groundwater flow and land subsidence simulations. The hydrogeological information for regions between boreholes embeds a large amount of uncertainty into the hydrogeological model, and thus affects numerical assessment. Quantifying the effects of the heterogeneous system and hydrogeological model uncertainty on groundwater flow and land subsidence simulations is thus important. Here, data from 46 geological boreholes in Huwei Town, Taiwan, were adopted to investigate hydrogeological model uncertainty. The one-dimensional continuous-lag Markov chain and the geostatistical method were used to analyze the spatial characteristics of hydrogeological materials and generate realizations of the hydrogeological model based on the assessment results. Estimated hydrological conditions and hydraulic parameters were applied to mitigate uncertainty not caused by the hydrogeological model. The mean of the results of land subsidence in Monte Carlo simulations showed a more stable distribution than that of individual realizations, for which land subsidence might be far from the mean. Therefore, the mean and variance results provide a reliable assessment with uncertainty information for land subsidence simulations. The coefficient of variation (CV) was used to quantify the hydrogeological model uncertainty. The CV value of land subsidence was larger than that of the hydraulic head because the thickness of clay is not uniform whereas the flow pattern is smooth. The variance and CV distributions of the hydraulic head and land subsidence provide uncertainty information that can be used to guide site investigations. A heterogeneous hydrogeological model with uncertainty quantification should be carefully applied to land subsidence simulations to obtain reasonable results.
Wuhan (China) is facing severe consolidation subsidence of soft soil and karst collapse hazards. To quantitatively explore the extent and causes of land subsidence in Wuhan, we performed multitemporal interferometry (MTI) analysis using synthetic aperture radar (SAR) data from the TerraSAR-X satellite from 2013 to 2017 and the Sentinel-1A satellite from 2015 to 2017. MTI results reveal four major subsidence zones in Wuhan, namely, Hankou (exceeding −6 cm/yr), Xudong-Qingshan (−3 cm/yr), Baishazhou-Jiangdi (−3 cm/yr), and Jianshe-Yangluo (−2 cm/yr). Accuracy assessment using 106 levelling benchmarks and cross-validation between the two InSAR-based results indicate an overall root-mean-square error (RMSE) of 2.5 and 3.1 mm/yr, respectively. Geophysical and geological analyses suggest that among the four major subsiding zones, Hankou, Xudong-Qingshan, and Jianshe-Yangluo are located in non-karstic soft soil areas, where shallow groundwater (< 30 m) declines driven by engineering dewatering and industrial water depletion contribute directly to soft soil compaction. Subsidence in the Baishazhou-Jiangdi zone develops in the karst terrain with abundant underground caves and fissures, which are major natural factors for gradual subsidence and karst collapse. Spatial variation analysis of the geological conditions indicates that the stage of karst development plays the most important role in influencing kart subsidence, followed by municipal construction, proximity to major rivers, and overlying soil structure. Moreover, land subsidence in this zone is affected more via coupling effects from multiple factors. Risk zoning analysis integrating subsidence horizontal gradient, InSAR deformation rates, and municipal construction density show that the high-risk areas in Wuhan are mainly distributed in the Tianxingzhou and Baishazhou-Jiangdi zone, and generally spread along the metro lines.
It is widely accepted that the presence of compressible layers beneath pile groups can substantially increase the settlement of a pile group. Furthermore, as the size of the pile group increases this effect is magnified and further complicated by the load distribution across the pile group. A number of simplified methods, such as the equivalent raft or equivalent pier method, are often employed to estimate the settlement of pile groups founded over a compressible layer, however, such methods can lead to both overly conservative and un-conservative results and large variations in results between methods. More recent methods have explored the use of power law functions or energy principles to analyse piles and pile groups in non-homogeneous soil conditions, however, these methods can be difficult to apply in practice. With the availability of modern computers and the advancement of commercially available numerical analyses packages, it is now possible to use the finite element method to analyse pile groups overlying compressible soil layers in order to better understand this problem. This technical note presents some initial results and associated design charts based on the finite element method to provide preliminary estimates of settlement of pile groups overlying compressible layers. Some dimensionless design charts are provided for a range of commonly encountered geotechnical conditions and pile group geometries.
In this paper, we consider the numerical solution of the poroelasticity problem with stochastic properties. We present a Two-stage Markov Chain Monte Carlo method for geomechanical subsidence. In this work, we study two techniques of preconditioning: (MS) multiscale method for model order reduction and (ML) machine learning technique. The purpose of preconditioning is the fast sampling, where a new proposal is first tested by a cheap multiscale solver or using fast prediction of the neural network and the full fine grid computations will be conducted only if the proposal passes the first step. To construct a reduced order model, we use the Generalized Multiscale Finite Element Method and present construction of the multiscale basis functions for pressure and displacements in stochastic fields. In order to construct a machine learning based preconditioning, we generate a dataset using a multiscale solver and use it to train neural networks. The Karhunen-Loéve expansion is used to represent the realization of the stochastic field. Numerical results are presented for two- and three-dimensional model examples.
The dynamic variation of deep groundwater level and the spatiotemporal evolution characteristics of land subsidence in Dezhou city were systematically analyzed based on a vast amount of monitoring data concerning groundwater level and land subsidence in this area. Taking the hydrogeological engineering characteristics in Dezhou city as the research background, a three-dimensional fluid–solid coupling model of land subsidence was established. The spatiotemporal evolution characteristics, transfer law, and essential reason of land subsidence in sand-clay interbed were analyzed and discussed by adopting the Biot’s poroelasticity theory, and the sensitivity analysis of soil parameters was carried out. The research results are as follows: (1) The soil shows different subsidence characteristics when pumping groundwater from different confined aquifers; (2) The land subsidence is smaller when pumping from the deeper and thicker confined aquifer, and the location of the maximum subsidence is not at the land surface; (3) The lower transfer rate of the additional stress in the aquifuge is the essential reason that the process of land subsidence has usually been lagged behind the pumping process; (4) The response degree and sensitivity of land subsidence to the main physical and mechanical parameters are different. The most sensitive parameter in aquifers is the permeability coefficient, followed by the elastic modulus, followed by the Biot–Willis coefficient and Poisson’s ratio. The most sensitive parameter in aquifuges is the elastic modulus, followed by the permeability coefficient, followed by the Biot–Willis coefficient and Poisson’s ratio.
This study is an application of the integrated numerical model FSSI-CAS 2D, to predict the subsidence of a huge rubble mound breakwater in the west part zone at Yantai port. The modified Cambridge clay soil model and poro-elastic model are adopted to describe the quasi-static mechanical behavior of the seabed foundation, which is consists of muddy-clay, clay and silt soil layers. The model parameters of soil used in computation are estimated based on a great number of laboratory tests (over one thousand). A novel point is that the buoyancy applied on the rubble mounds under SWL (Static Water Line), as well as its nonlinear variation dependent on the deformation of seabed foundation are considered in computation. Through analytical and experimental verification, as well as the comparison with observation data monitored in-situ, it is indicated that FSSI-CAS 2D is reliable to numerically predict the subsidence of offshore structures, and has the ability to be applied in practical engineering.
Land subsidence and earth fissures caused by excessive groundwater exploitation severely damage the surrounding property and environment. To examine changes caused by land subsidence and earth fissures, a large-scale physical-geological model was designed to simulate groundwater exploitation and the conditions of bedrock ridges. This model enables real-time monitoring of the groundwater level, soil strain, and changes to subsidence and fissures. It is based on a case study conducted on the ground surface of Guangming Village in the Su-Xi-Chang region, China. The results revealed the following: (1) the land subsidence lagged behind the decline in water level during the pumping process; (2) a strong agreement between the shape and location of the bedrock ridge and settlement was reached, and the region with the most severe subsidence corresponded to the distribution of the fissures; (3) the tensile-stress concentration of the soil mass was essentially the same as that during the stage when the fissures developed, (which is an important means of determining the fissure development); and (4) the rose diagram illustrating the fissures indicated the fissures in the area of the bedrock ridge propagated parallel to the contour lines of the bedrock ridge. These results are consistent with the changes in the earth fissures examined in the case study and, therefore, provide support for the use of the physical-geological model in future research on ground fissures.
Geological structure is an important factor to explore the underground geological conditions for hydrogeological purpose. Borehole density has great influence on the accuracy and application of geological model. In this paper, Transition Probability Geostatistical Software (T-PROGS) has been used to simulate the four facies distribution of West Liao River Plain. And a quantitative uncertainty model of entropy method is introduced. For getting a reliable geological model with as few as the boreholes, two parts have been given. One is the vertical lithologic variability analysis, and the other is the model correct rate and uncertainty analysis. In geological modeling, the borehole data is too sparse to characterize the lateral heterogeneity, so the actual profiles are added. At last, many equal probability realizations of the geological model using 350 boreholes are built. Depending on the model calibration, uncertainty analysis and simulated profile comparison, the geological models are reliable. Thus, for the simple and single stratigraphy study area without complex fault structures and graben structures of several thousands to tens of thousands of square kilometer scale, establishing a reliable geological structure model requires one borehole at least within an average area of 120.81 km². It is of great significance for decision maker to save manpower and material resources. And we present a workflow to build a 3D Markov chain using boreholes and actual profiles and develop a reliable geological model.
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The urban area of Como (Italy) is particularly susceptible to subsidence. The reason is the particular structure of
the subsoil, combined with the anthropic modification of the lakeshore lands linked to the historical evolution of
the area. Moreover, this phenomenon exposes the lakefront areas to an increasing risk of flooding. The primary
purpose of this study was to develop an effective methodology for the assessment of subsidence in urban areas, to
be used as a support in groundwater management. The development of a coupled hydrogeological-geotechnical
numerical model for the period 2004–2011 allowed recognizing the shoreline results as most susceptible areas to
the phenomenon. Model results were critically evaluated through comparison with time series of PSInSAR and
high precision levelling data available in the area. The anthropic perturbations of groundwater flow linked to the
construction of lake flooding defenses in 2008–2009, enhanced the subsidence phenomenon in a localized area,
pointing out the strong inter-dependence of groundwater circulation, lake level oscillations and geotechnical behavior
of soils. Thus, the model revealed the most critical zones and geotechnical units, demonstrating to be a
potential powerful tool to predict subsidence scenarios (e.g. the future completion of the floods defense works in
the shoreline area).
The main factors that cause land subsidence are groundwater withdrawal and the load of high-rise buildings. Previous studies on land subsidence caused by high-rise buildings have focused on small areas. Few scholars have proposed land subsidence models that combine the effects of groundwater withdrawal and high-rise building load at a regional scale. This work was based on Biot’s consolidation theory and the nonlinear rheology theory. The soil parameters were varied in accordance with the Kozeny-Carman equation and Duncan-Zhang nonlinear model, and applied to a site in eastern China. A three-dimensional finite element method (FEM) program, fully coupling varying soil parameter values and fluid-solid characteristics of land subsidence, was coded using FORTRAN. The program was used to simulate and predict regional land subsidence and to study the coupling effects of groundwater withdrawal and high-rise building load. The results showed that the soil parameters varied in reasonable range and the trend of variation was consistent with the characteristics of soil deformation. The sum of the land subsidence under high-rise building load alone and groundwater withdrawal alone differed from land subsidence under the combined effects of groundwater withdrawal and high-rise building load. The coupling effect of land subsidence caused by high-rise building load and groundwater withdrawal was shown to be nonlinear.
Performances of a braced cut-and-cover excavation system for mass rapid transit (MRT) stations of the Downtown Line Stage 2 in Singapore are presented. The excavation was carried out in the Bukit Timah granitic (BTG) residual soils and characterized by significant groundwater drawdown, due to dewatering work in complex site conditions, insufficient effective waterproof measures and more permeable soils. A two-dimensional numerical model was developed for back analysis of retaining wall movement and ground surface settlement. Comparisons of these measured excavation responses with the calculated performances were carried out, upon which the numerical simulation procedures were calibrated. In addition, the influences of groundwater drawdown on the wall deflection and ground surface settlement were numerically investigated and summarized. The performances were also compared with some commonly used empirical charts, and the results indicated that these charts are less applicable for cases with significant groundwater drawdowns. It is expected that these general behaviors will provide useful references and insights for future projects involving excavation in BTG residual soils under significant groundwater drawdowns. © 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences
Partial expansion was observed in stratified subsidence during foundation pit dewatering. However, the phenomenon was suspected to be an error because the compression of layers is known to occur when subsidence occurs. A slice of the subsidence cone induced by drawdown was selected as the prototype. Model tests were performed to investigate the phenomenon. The underlying confined aquifer was generated as a movable rigid plate with a hinge at one end. The overlying layers were simulated with remolded materials collected from a construction site. Model tests performed under the conceptual model indicated that partial expansion occurred in stratified settlements under coordination deformation and consolidation conditions. During foundation pit dewatering, rapid drawdown resulted in rapid subsidence in the dewatered confined aquifer. The rapidly subsiding confined aquifer top was the bottom deformation boundary of the overlying layers. Non-coordination deformation was observed at the top and bottom of the subsiding overlying layers. The subsidence of overlying layers was larger at the bottom than at the top. The layers expanded and became thicker. The phenomenon was verified using numerical simulation method based on finite difference method. Compared with numerical simulation results, the boundary effect of the physical tests was obvious in the observation point close to the movable endpoint. The tensile stress of the overlying soil layers induced by the underlying settlement of dewatered confined aquifer contributed to the expansion phenomenon. The partial expansion of overlying soil layers was defined as inversed rebound. The inversed rebound was induced by inversed coordination deformation. Compression was induced by the consolidation in the overlying soil layers because of drainage. Partial expansion occurred when the expansion exceeded the compression. Considering the inversed rebound, traditional layer-wise summation method for calculating subsidence should be revised and improved.
Monitoring is required when dewatering underground construction sites to anticipate unexpected events and preserve nearby existing structures and/or buildings. The most accurate and widespread monitoring method to measure displacements is leveling, a point-like surveying technique that typically allows for tens of discrete in situ sub-millimeter measures per squared kilometer. Another emerging technique for mapping soil deformation is the interferometric synthetic aperture radar (InSAR) method, which is based on SAR images acquired from orbiting satellites. This remote sensing technique can provide better spatial point density than leveling, more extensive spatial coverage and lower-cost acquisitions. This paper analyses, compares and discusses leveling and advanced multi-temporal InSAR measurements when they are used to measure the soil deformation induced by the dewatering associated with underground constructions in urban areas. This comparison, which has not been considered in previous works, is of paramount importance to ascertain the most suitable technique (or combination of techniques) in these contexts. To do so, an experiment was performed in the future railway station of La Sagrera, Barcelona (Spain), in which leveling and advanced multi-temporal InSAR were used to quantify ground deformation by dewatering. The results showed that soil displacements measured by leveling and InSAR were not always consistent. In the context of soil deformation measurements produced by dewatering in urban areas, InSAR measurements appear to be more accurate for investigating soil deformation, whereas leveling was more appropriate for quantifying the real impact on the nearby buildings.
Geological uncertainty appears in the form of one soil layer embedded in another or the inclusion of pockets of different soil type within a more uniform soil mass. Uncertainty in factor of safety (FS) and probability of failure (Pf) of slope induced by the geological uncertainty is not well studied in the past. This paper presents one approach to evaluate the uncertainty in FS and Pf of slope in the presence of geological uncertainty using borehole data. The geological uncertainty is simulated by an efficient coupled Markov chain (CMC) model. Slope stability analysis is then conducted based on the simulated heterogeneous soils. Effect of borehole layout schemes on uncertainty evaluation of FS and Pf is investigated. The results show that borehole within influence zone of the slope is essential for a precise evaluation of FS statistics and Pf. The mean of FS will converge to the correct answer as the borehole number increases.
Summary Realistic representations of geological complexity are important to address several engineering and environmental challenges. The spatial distribution of properties controlling physical and geochemical processes can be effectively described by the geological structure of the subsurface. In this work, we present an approach to account for geological structure in geostatistical simulations of categorical variables. The approach is based on the extraction of information from a deterministic conceptualization of the subsurface, which is then used in the geostatistical analysis for the development of models of spatial correlation and as soft conditioning data. The approach was tested to simulate the distribution of four lithofacies in highly heterolithic Quaternary deposits. A transition probability-based stochastic model was implemented using hard borehole data and soft data extracted from a 3-D deterministic lithostratigraphic model. Simulated lithofacies distributions were also used as input in a flow model for numerical simulation of hydraulic head and groundwater flux. The outputs from these models were compared to corresponding values from models based exclusively on borehole data. Results show that soft lithostratigraphic information increases the accuracy and reduces the uncertainty of these predictions. The representation of the geological structure also allows a more precise definition of the spatial distribution of prediction uncertainty, here quantified with a metric based on Shannon information entropy. Correlations between prediction uncertainties for lithofacies, hydraulic heads and groundwater fluxes were also investigated. The results from this analysis provide useful insights about the incorporation of soft geological data into stochastic realizations of subsurface heterogeneity, and emphasize the critical importance of this type of information for reducing the uncertainty of simulations considering flux-dependent processes.
A large number of observation data of land subsidence and underground water level and some results of laboratory soil test are used to analyze the deformation characteristics of soil layers in Shanghai. It's the first time to comprehensively analyze the deformation characteristics of all soil layers and the spatial and time distribution of land subsidence caused by excessive pumping of groundwater. It is found that the deformation characteristics of soil layers are rather different from each other, and the deformation characteristic of some soil layer is different at different time and in different place. Land subsidence is influenced by many factors, so that its spatial distribution is complex. The subsidence mainly occurred in shallow soil layers before nineties of the last century, but mainly occurred in deep and shallow soil layers since nineties of the last century. The subsidence model of every soil layer can be properly developed according to these results.
Fine reservoir description plays an important role in oil and gas exploration and development, reservoir geological modeling is the core of fine reservoir description and the new technology developed in recent years of quantitatively characterizing reservoir as well. Petrel can be used to build three-dimensional visualization of quantitative reservoir by simulating and forecasting the reservoir characteristics and properties of parameters .And the reservoir unit is the targets of system modeling used to guide the re-awareness and tapping of the potential reservoir. This article describes three-dimensinal structure modeling of Lishui Depression East China Sea Basin with the help of powerful 3D visualization capabilities and integrated functional modules of the exploration and development, and results of case study show that the three-dimensinal structure model consists with its actual geological information, thus providing strong support for exploration and development of decision-making.
In order to carry out deep excavations under the water table in urban environments, the safety of the work site and of the adjacent buildings is a major cause for concern. One of the most common and effective methods of undertaking these excavations involves combining the cut and cover method with a dewatering system. The success of a construction depends on the stability of the excavation bottom, the effects produced outside the excavation by dewatering (soil movements) and/or the state of the enclosure (defects in the diaphragm walls). This study proposes a realistic multidisciplinary procedure address these issues. The work emphasizes the importance of soil characterisation and underlines the need to perform a Watertightness Assessment Test (WTAT) before the excavation stage. The procedure was applied to the excavation of a deep shaft of the High Speed Train (HST) tunnel in Barcelona. An earlier geological characterisation at large scale ruled out the use of deep pumping wells. However, a subsequent hydrogeological characterisation, which involved borehole logging, grain size analyses, Natural Gamma Ray and pumping tests, revealed the presence of thin transmissive layers inside the low hydraulic conductivity materials. The dewatering system was designed by considering different model scenarios and the safest design was selected for the excavation. Depths of the enclosure and of the pumping wells differed in accordance with the scenarios. The impacts (settlements due to pumping) and the stability in each scenario were computed. The state of the enclosure underwent a WTAT before the start of the excavation, but after constructing the enclosure, to verify its low permeability. The test consisted in pumping inside the enclosure and monitoring the groundwater behaviour outside the enclosure. Numerical interpretation of this test showed a defect in the diaphragm walls below the excavation bottom. Since this defect was not repaired because of its location (below the bottom of the excavation), the dewatering system had to be redesigned to ensure safety. Surface settlements, which were also a source of concern, were small. They were computed using coupled hydro-mechanical models.
This paper presents numerical simulations of a series of hydraulic
interference tests conducted in crystalline bedrock at Olkiluoto
(Finland), a potential site for the disposal of the Finnish high-level
nuclear waste. The tests are in a block of crystalline bedrock of about
0.03 km3 that contains low-transmissivity fractures. Fracture
density, orientation, and fracture transmissivity are estimated from
Posiva Flow Log (PFL) measurements in boreholes drilled in the rock
block. On the basis of those data, a geostatistical approach relying on
a transitional probability and Markov chain models is used to define a
conceptual model based on stochastic fractured rock facies. Four facies
are defined, from sparsely fractured bedrock to highly fractured
bedrock. Using this conceptual model, three-dimensional groundwater flow
is then simulated to reproduce interference pumping tests in either open
or packed-off boreholes. Hydraulic conductivities of the fracture facies
are estimated through automatic calibration using either hydraulic heads
or both hydraulic heads and PFL flow rates as targets for calibration.
The latter option produces a narrower confidence interval for the
calibrated hydraulic conductivities, therefore reducing the associated
uncertainty and demonstrating the usefulness of the measured PFL flow
rates. Furthermore, the stochastic facies conceptual model is a suitable
alternative to discrete fracture network models to simulate fluid flow
in fractured geological media.
