Anisotropic Hydraulic Conductivity Research Articles

Comparison of preloading by fill surcharge and ground water lowering based on a case study

AbstractPreloading by fill surcharge is a widespread and rather simple ground improvement method for the anticipation of settlements brought on by additional loads from new constructions. The magnitude as well as the area of surcharge is adapted to the final construction, whereas in general about 120–140 % of the final load is applied as surcharge load. Both factors have an influence on the effective depth for which soil improvement and an anticipation of settlements is possible, respectively. In case of large construction areas with significant loads, for which a greater depth effect needs to be considered, a large earth volume for surcharge is necessary. It is a cost‐intensive and environmental‐relevant aspect, when earth material is not available in sufficient amount and near distance of the construction site. In order to avoid this, it was investigated whether lowering the in situ groundwater level and thus increasing the effective stresses over depth could be a possible alternative. Therefore, a case study was carried out to examine these two different types of preloading techniques and their efficacy based on known subsoil conditions. Furthermore, the influence of the possible anisotropy of hydraulic conductivity in the weak soil layer on the preloading process was investigated.

Finite strain elastic visco-plastic consolidation model for layered soils with vertical drain considering self-weight loading and nonlinear creep

Prefabricated vertical drains (PVDs) combined with vacuum and/or surcharge loading have been widely adopted to improve the strength of soft soils. Precise consolidation analysis is the theoretical basis for the design of preloading method with PVD. Current consolidation theories for layered soils with PVD seldom consider the influence of large strain, nonlinear creep, and self-weight loading simultaneously. This paper, thus, presents a finite strain elastic visco-plastic consolidation model, called RCS-EVP, for radial consolidation of layered soils with PVD. RCS-EVP is developed based on the piecewise-linear method. It takes into account nonlinear creep with limit creep strain, variable boundary conditions, anisotropy of soil hydraulic conductivity, and variable compressibility and hydraulic conductivity during the consolidation under self-weight, time-dependent surcharge and/or vacuum loading. The performance of RCS-EVP is evaluated by comparing with the results from finite element simulations and a laboratory physical model test. The variations of settlement and pore pressure of a soft soil ground improved by vacuum preloading with PVD are estimated using RCS-EVP. The results indicate that RCS-EVP provides good estimates of long-term consolidation of layered soils with PVD under both laboratory and in-situ conditions.

Modeling of seawater intrusion into Salalah coastal plain aquifer, sultanate of Oman

Excessive groundwater pumping has become a concern in Salalah coastal aquifer in Oman causing seawater intrusion at different farms and abstraction wells near the shoreline. MODFLOW and MT3DMS were calibrated and validated for Salalah coastal aquifer to assess aquifer behavior and seawater intrusion pattern with new input data and fine grid spacing to improve the accuracy of predictions as compared to the previous modeling studies in this region. The study was conducted over an area of 640 km2 of Salalah Plain representing around 83% of its total area. Injection of tertiary treated wastewater, rainfall and dams were considered as sources of aquifer recharge while groundwater abstraction occurred for agricultural, domestic, and commercial uses. The calibration and validation of the models were done in steady and transient states using observed groundwater levels from year 2000–2019. The aquifer properties such as hydraulic conductivity, specific yield, and vertical anisotropy were calibrated. MODFLOW results were utilized by MT3DMS to simulate solute transport at transient state. The longitudinal dispersivity and porosity of the aquifer were obtained by calibration and validation of the model. Over the three aquifer layers, the hydraulic conductivity ranges from 10 m/day to 100 m/day, vertical anisotropy is 4 and the specific yield ranges from 0.01 to 0.1. The calibrated values of the longitudinal dispersivity and porosity range from 500 m to 50000 m, and 0.4, respectively. The study indicated a clear impact of seawater intrusion at coastal side of the Salalah plain. The predictive modeling indicated an average of 1.5 m drop in hydraulic head across the whole study area by 2040, while seawater intrusion effect was expected to increase near the coast with potential extension to the central area of the Salalah plain. The validated model can be used as a prediction tool by the decision makers to manage the aquifer considering different scenarios, for integrated water resources management in Salalah city.

Combined influence of rotated transverse anisotropy in mechanical parameters and hydraulic conductivity of soils on reliability evaluation of an unsaturated slope under rainfalls

Natural slopes often exhibit tilted stratification with rotated transverse anisotropy in multiple soil properties, including the mechanical (e.g., friction angle φ and cohesion c) and hydraulic properties (e.g., saturated hydraulic conductivity ks). This phenomenon indicates that to achieve an accurate assessment of slope reliability under rainfall infiltration, the anisotropic spatial variation of the shear strength parameters and ks should be incorporated in a combined manner. Thus, this paper discusses the combined influence of rotated transverse anisotropy in the shear strength parameters and ks on the reliability and failure mechanism of an unsaturated slope under rainfalls. It is found that for a slope with tilted stratification, the failure mechanism and reliability estimator are dominantly influenced by the rotated transverse anisotropy in the shear strength parameters, while the influence induced by that in ks is slight. In particular, only incorporating the rotated transverse anisotropy in ks may lead to an incorrect reliability estimator of a slope with tilted stratification. Herein, the reliability index β would be estimated to be dramatically higher, and β of a slope with horizontal bedding would be higher than that of an anti-dip slope, which is inconsistent with the engineering experiences. Nonetheless, the rotated transverse anisotropy in ks should not be ignored in slope reliability assessment, because ignoring the rotated transverse anisotropy in ks would lead to overestimation of the reliability index β, which is adverse to the safety design of a slope.

The impact of hydraulic conductivity anisotropy on the effectiveness of subsurface dam

Hydraulic conductivity is a key parameter controlling groundwater flow and solute transport, and anisotropy in hydraulic conductivity is common in aquifers. Previous studies have investigated the effectiveness of subsurface dams in combating seawater intrusion (SWI) and associated environmental concerns. However, the quantitative impact of hydraulic conductivity anisotropy on the performance of subsurface dam has not been explored. This study goes beyond previous research by examining, for the first time, the effects of subsurface dams on SWI prevention in anisotropic settings. Furthermore, the study optimized subsurface dam designs in anisotropic coastal aquifers through numerical simulations that take into account the environmental impact. Our findings reveal the mechanism of hydraulic conductivity anisotropy's influence on the discharging zone and demonstrate that as the anisotropy ratio (rk) decreases, the coastal discharging zone expands landward, reducing SWI and ultimately lowering the minimum effective dam height (MEDH). This process effectively prevents SWI while maximizing fresh groundwater discharge. Increasing the rk values simultaneously expands both the MEDH and the peak fresh groundwater discharge (PFGD). Additionally, increasing the horizontal hydraulic conductivity of the aquifer significantly increases the range of PFGD, while the distance between the dam and shoreline emerges as the main factor influencing the extent of the MEDH.

Efficient representation of transient tidal overheight in a coastal groundwater flow model using a phase-averaged tidal boundary condition

Ocean tides cause water table overheight near the seaward boundary of coastal aquifers which can have a large impact on groundwater flows and saltwater intrusion. Despite this, regional-scale coastal groundwater flow models often neglect the effects of ocean tides or alternatively assume that the influences of ocean tides and sea level are constant over time. These simplifications are often required because simulation of the phase-resolved tidal signal at a coastal boundary is computationally demanding. Our objective was to derive a phase-averaged tidal boundary condition for application along gently sloping coastlines that enables simulation of real, complex tidal signals combined with sea-level changes due to meteorological effects. The boundary condition extends an existing analytical solution of tidal overheight by including an empirical correction function for conditions where the assumptions of the existing solution are violated. The correction function was developed by conducting a parametric study on an idealized two-dimensional cross-sectional coastal aquifer model and considering the effects of parameters including horizontal hydraulic conductivity, vertical anisotropy, specific yield, aquifer thickness, and beach slope. The performance of the new phase-averaged tidal boundary condition was assessed using a three-dimensional groundwater flow model of the island of Spiekeroog, Northwest Germany, which comprises complex coastal morphology with non-planar beach slopes. Simulations applying the new boundary condition were compared to simulations that adopted a phase-resolved tidal boundary condition and to observed groundwater levels. Accounting for time-varying changes in the tidal signal and sea level was found to be critical to simulate observed data and to adequately reproduce the transient tidal overheight simulated by the phase-resolved model. The performance of the new phase-averaged boundary condition in the regional-scale model varied depending on how the coastal morphology was represented in the boundary condition with local morphological features increasingly important for lower intertidal vertical infiltration capacity (low isotropic hydraulic conductivity or high vertical anisotropy) or specific yield. In conclusion, the new boundary condition presented overcomes current limitations in simulating transient tidal overheight in regional-scale groundwater models.

Two-phase modeling of fluid injection inside subcutaneous layer of skin

The motivation of this present theoretical investigation comes from the delivery of various drugs and vaccines through subcutaneous injection (SCI) to the human body. In the SCI procedure, a medical person takes a big pinch of skin of the injection applicable area of a patient to pull the subcutaneous layer (SCL) from the underlying muscle layer to smooth the execution of the procedure. Generally, the human skin, particularly SCL, is a heterogeneous and anisotropic living material. The major constituents of the SCL are adipose cells or fat cells and interstitial fluid. These adipose cells are oriented in such a way that the hydraulic conductivity of the SCL exhibits anisotropy. Consequently, one can adopt the field equations of mixture theory to describe the continuum nature of SCL. This mathematical modeling involves a perturbation technique where the small aspect ratio of the SCL provides a valid perturb parameter. This study highlights the issue of the mechanical response of the adipose tissue in terms of the anisotropic hydraulic conductivity variation, the viscosity of the injected drug, the mean depth of subcutaneous tissue, etc. In particular, the computed stress fields can measure the intensity of pain experienced by a patient after this procedure. Also, this study discusses the biomechanical impact of the creation of one or more eddy structure(s) due to the high pressure developed, increased tissue anisotropy, fluid viscosity, etc., within the area of applying injection.

An evolutionary-based polynomial regression modeling approach to predicting discharge flow rate under sheet piles

The discharge flow rate beneath sheet plies is an essential parameter in designing these water retaining structures. This paper presents a unified framework for modeling and predicting discharge flow rate using an evolutionary-based polynomial regression technique. EPR (Evolutionary Polynomial Regression) is a data-driven method based on evolutionary computing to search for polynomial structures representing a system. The input parameters in the modeling procedure included the sheet pile height, upstream water head, and the hydraulic conductivity anisotropy ratio. Due to ever-increasing demand for water, a widely held view on predicting and controlling the available water behind reservoirs, dams, barrages, and weirs is of vital importance. To this end, the sheer novelty of the current study has been worn off through the development of a comprehensive model to predict the flow rate considering the most effective variables in the seepage issue. To the best of our knowledge, the research conducted in the literature has yet to cover the whole seepage problem using a comprehensive database extracted by numerical methods; thus, a comprehensive finite-element-based artificial database including 1000 data lines was created using the Scaled Boundary Finite Element Method (SBFEM) by simulating seepage beneath sheet plies covering a considerably wide range of seepage-related real-world values. The database was then employed to develop and validate the EPR flow rate prediction model. Data were divided into training (used for creating the models) and testing (for validating the developed models) data based on a statistical process. The procedure for preparing the data and developing and validating the models is presented in detail in this paper. The main advantage of the proposed models over a conventional and neural network and most GP (Genetic Programming)-based constitutive models is that they provide the optimum structure for the material constitutive model representation as well as its parameters, directly from raw experimental (or field) data. EPR can learn nonlinear and complex material behavior without any prior assumptions on the constitutive relationships. The proposed algorithm captures and transparently presents relationships between contributing parameters in polynomial expressions providing the user with a clear insight into the problem. EPR-based model predictions demonstrated an excellent agreement with the unseen simulated data used for validating the developed model. A parametric study on the presented models was conducted to investigate the effects of the contributing parameters on model predictions and the consistency of the parameter relationships with the database. Results of the parametric study showed that the effects of variations in the contributing parameters on EPR predictions are in line with the expected behavior. The merits and advantages of the proposed technique are discussed in the paper.

Cover crops effects on anisotropy of unsaturated soil hydraulic properties

No tillage (NT) is frequently implemented in simplified agricultural systems, which has negative effects on soil physical quality. Among the suggested practices to preserve the physical fertility of soils under NT are winter cover crops (CC). Soil physical degradation affects the soil pore configuration and therefore it can generate a change in the directionality of the properties that depend on it. The objective of this study was to determine the effect of cover crop incorporation under NT on the soil pore system configuration and on the anisotropy of the unsaturated hydraulic properties in a Typic Argiudoll. The results showed that the inclusion of cover crops modifies the soil pore configuration increasing the structure porosity. The inclusion of CC modifies the anisotropy of unsaturated hydraulic conductivity in the medium saturation region. The pore size distribution behaves as isotropic as expected for a scalar variable.

Revisiting MODFLOW's Capability to Model Flow Through Sedimentary Structures.

Sedimentary structures have unique geometries and anisotropic hydraulic conductivity, both of which control groundwater flow. Traditional finite-difference simulators (e.g., MODFLOW) have not been able to correctly represent irregular, dipping and anisotropic structures due their use of a simplified conductivity tensor, causing many modelers to turn toward finite-element codes with their sophisticated meshing capabilities. However, the release of MODFLOW 6 with its flexible discretization and multipoint flux approximation scheme prompts us to revisit its capability to compute flow through complex sedimentary structures. Through the use of a novel benchmark and case study, we show that when versions previous to MODFLOW 6 are applied to dipping structures, modeled fluxes and hence flow through the system, can be significantly over or underestimated. For example, effective conductivity for a 30° dipping layer with a 100:1 conductivity ratio is reduced to only 2% of its inputted value. We show that MODFLOW 6, with its XT3D capability and flexible discretization options is far superior to its predecessors, allowing flow through complex sedimentary structures to be simulated more accurately. However, on vertically offset grids, which have been available in all versions of MODFLOW and are often used in practice, loss of accuracy is still a concern when the vertical offset is large, that is, the dip of the sedimentary layer is steep, particularly if the layer is much more conductive than the surrounding material. The hypothesis that vertically offset grids lack sufficient hydraulic connectivity between adjacent model layers to accurately simulate the steeply dipping, highly heterogeneous case is a topic for further investigation.

A new semi-analytical method for estimation of anisotropic hydraulic conductivity of three-dimensional fractured rock masses

• A semi-analytical method based on Snow's peameability tensor model is proposed. • The new method can predict anisotropic hydraulic conductivity of complex DFNs. • The new method can deal with engineering level problems with low computational cost. The hydraulic characteristics of rock masses usually show strong anisotropy due to the existence of oriented structure planes. A new semi-analytical method is proposed for estimation of the anisotropic hydraulic conductivity of fractured rock masses based on the discrete fracture network (DFN) description and the Snow’s model ( Snow, 1969 ). In this method, the number of intersections on a fracture is a key parameter to construct correction coefficient, which are used to adjust the contribution of each individual fracture to the macro hydraulic conductivity tensor. In this way, the initial Snow’s model is extended from infinite connectivity conditions to finite connectivity conditions. The correction coefficient is determined by using a mathematical optimization method in the condition of networks following isotropic orientation distribution and polydisperse size distribution. Then, the new method is applied to estimate the directional hydraulic conductivity of engineering rock masses with complex anisotropic oriented fracture networks. The results show that the directional hydraulic conductivities predicted by the proposed semi-analytical method are in good agreement with the referential numerical results. The proposed semi-analytical model can be used to predict the anisotropic hydraulic conductivity of complex fractured rock masses with less processing time and lower memory requirements than numerical models.

Equivalent Permeability Tensor of Heterogeneous Media: Upscaling Methods and Criteria (Review and Analyses)

When conducting numerical upscaling, either for a fractured or a porous medium, it is important to account for anisotropy because in general, the resulting upscaled conductivity is anisotropic. Measurements made at different scales also demonstrate the existence of anisotropy of hydraulic conductivity. At the “microscopic” scale, the anisotropy results from the preferential flatness of grains, presence of shale, or variation of grain size in successive laminations. At a larger scale, the anisotropy results from preferential orientation of highly conductive geological features (channels, fracture families) or alternations of high and low conductive features (stratification, bedding, crossbedding). Previous surveys of homogenization techniques demonstrate that a wide variety of approaches exists to define and calculate the equivalent conductivity tensor. Consequently, the resulting equivalent conductivities obtained by these different methods are not necessarily equal, and they do not have the same mathematical properties (some are symmetric, others are not, for example). We present an overview of different techniques allowing a quantitative evaluation of the anisotropic equivalent conductivity for heterogeneous porous media, via numerical simulations and, in some cases, analytical approaches. New approaches to equivalent permeability are proposed for heterogeneous media, as well as discontinuous (composite) media, and also some extensions to 2D fractured networks. One of the main focuses of the paper is to explore the relations between these various definitions and the resulting properties of the anisotropic equivalent conductivity, such as tensorial or non-tensorial behavior of the anisotropic conductivity; symmetry and positiveness of the conductivity tensor (or not); dual conductivity/resistivity tensors; continuity and robustness of equivalent conductivity with respect to domain geometry and boundary conditions. In this paper, we emphasize some of the implications of the different approaches for the resulting equivalent permeabilities.

Experimental study on anisotropy of hydraulic conductivity for municipal solid waste.

Anisotropy of hydraulic conductivity is an important parameter controlling fluid movement in municipal solid waste (MSW) landfills, while measurements of anisotropy are rare. In this study, a laboratory-scale enhanced reactor was built to create MSW samples with different degrees of degradation. Vertical and horizontal hydraulic conductivities of these samples were measured in a self-designed permeameter to study the effects of compression and degradation on anisotropy of MSW. CT scanning was performed to observe the internal pore-structure of MSW under compression. A prediction model of anisotropy under compression was established. It was found that as degradation time increased from 0month to 18months, the dry mass percent of 0D particles increased from 12.3% to 38.8%, while 2D particles content decreased from 78.7% to 47.2%. As vertical stress increased from 50kPa to 400kPa, dry unit weight (γd) increased from 3.26 kN/m3 to 5.51 kN/m3, anisotropy (A) increased from 1.26 to 5.17. It was because that the size and continuity of pores decreased and the angle of pore arrangement tended to be horizontal as the vertical stress increased. The relation between anisotropy and vertical stress could be well fitted with the prediction model. When degradation time increased from 0month to 18months, A decreased linearly from 5.02 to 2.75 due to the decreasing content of 2D particles. Anisotropy also decreased with the decreasing C/L. Compression has much greater influence on waste anisotropy than that of degradation. Anisotropy of MSW at different depths of landfills could be determined based on the trend lines in this study.

Estimation of anisotropic hydraulic conductivity using geophysical data in a coastal aquifer of Karnataka, India

AbstractEstimation of hydraulic parameters is essential to understand the interaction between groundwater flow and seawater intrusion. Though several studies have addressed hydraulic parameter estimation, based on pumping tests as well as geophysical methods, not many studies have addressed the problem with clayey formations being present. In this study, a methodology is proposed to estimate anisotropic hydraulic conductivity and porosity values for the coastal aquifer with unconsolidated formations. For this purpose, the one‐dimensional resistivity of the aquifer and the groundwater conductivity data are used to estimate porosity at discrete points. The hydraulic conductivity values are estimated by its mutual dependence with porosity and petrophysical parameters. From these estimated values, the bilinear relationship between hydraulic conductivity and aquifer resistivity is established based on the clay content of the sampled formation. The methodology is applied on a coastal aquifer along with the coastal Karnataka, India, which has significant clayey formations embedded in unconsolidated rock. The estimation of hydraulic conductivity values from the established correlations has a correlation coefficient of 0.83 with pumping test data, indicating good reliability of the methodology. The established correlations also enable the estimation of horizontal hydraulic conductivity on two‐dimensional resistivity sections, which was not addressed by earlier studies. The inventive approach of using the established bilinear correlations at one‐dimensional to two‐dimensional resistivity sections is verified by the comparison method. The horizontal hydraulic conductivity agrees with previous findings from inverse modelling. Additionally, this study provides critical insights into the estimation of vertical hydraulic conductivity and an equation is formulated which relates vertical hydraulic conductivity with horizontal. Based on the approach presented, the anisotropic hydraulic conductivity of any type aquifer with embedded clayey formations can be estimated. The anisotropic hydraulic conductivity has the potential to be used as an important input to the groundwater models.

New insights into the drainage of inundated ice-wedge polygons using fundamental hydrologic principles

Abstract. The pathways and timing of drainage from the inundated centers of ice-wedge polygons in a warming climate have important implications for carbon flushing, advective heat transport, and transitions from methane to carbon dioxide dominated emissions. Here, we expand on previous research using a recently developed analytical model of drainage from a low-centered polygon. Specifically, we perform (1) a calibration to field data identifying necessary model refinements and (2) a rigorous model sensitivity analysis that expands on previously published indications of polygon drainage characteristics. This research provides intuition on inundated polygon drainage by presenting the first in-depth analysis of drainage within a polygon based on hydrogeological first principles. We verify a recently developed analytical solution of polygon drainage through a calibration to a season of field measurements. Due to the parsimony of the model, providing the potential that it could fail, we identify the minimum necessary refinements that allow the model to match water levels measured in a low-centered polygon. We find that (1) the measured precipitation must be increased by a factor of around 2.2, and (2) the vertical soil hydraulic conductivity must decrease with increasing thaw depth. Model refinement (1) accounts for runoff from rims into the ice-wedge polygon pond during precipitation events and possible rain gauge undercatch, while refinement (2) accounts for the decreasing permeability of deeper soil layers. The calibration to field measurements supports the validity of the model, indicating that it is able to represent ice-wedge polygon drainage dynamics. We then use the analytical solution in non-dimensional form to provide a baseline for the effects of polygon aspect ratios (radius to thaw depth) and coefficient of hydraulic conductivity anisotropy (horizontal to vertical hydraulic conductivity) on drainage pathways and temporal depletion of ponded water from inundated ice-wedge polygon centers. By varying the polygon aspect ratio, we evaluate the relative effect of polygon size (width), inter-annual increases in active-layer thickness, and seasonal increases in thaw depth on drainage. The results of our sensitivity analysis rigorously confirm a previous analysis indicating that most drainage through the active layer occurs along an annular region of the polygon center near the rims. This has important implications for transport of nutrients (such as dissolved organic carbon) and advection of heat towards ice-wedge tops. We also provide a comprehensive investigation of the effect of polygon aspect ratio and anisotropy on drainage timing and patterns, expanding on previously published research. Our results indicate that polygons with large aspect ratios and high anisotropy will have the most distributed drainage, while polygons with large aspect ratios and low anisotropy will have their drainage most focused near their periphery and will drain most slowly. Polygons with small aspect ratios and high anisotropy will drain most quickly. These results, based on parametric investigation of idealized scenarios, provide a baseline for further research considering the geometric and hydraulic complexities of ice-wedge polygons.

A Sensitivity Analysis of the Anisotropy of Hydraulic Conductivity to the Seepage, Deformation and Stability of Anti-dipping Layered Rock Slopes: A Case Study of the Pulang Area in Southwestern China

In this paper, in order to study the influence of anisotropy ratios and anisotropy directions on the seepage, deformations and stability of the anti-dipping layered rock slopes, Geo-studio software was used in this study for the numerical analysis of carbonaceous slate slopes on the unsaturated seepage, fluid–solid coupling, and stability theory in Pulang area. The results showed that the maximum surface water content of the layered rock slopes gradually decreased with increases of the water conductivity anisotropy ratio and decreases in the anisotropy angle of the anti-dipping layered rock slopes. In addition, the rainfall infiltration depths in the middle sections of the slopes were observed to be the most affected by the anisotropy ratio and dip angles of the rock formations. Meanwhile, the bottom sections of slopes were the least affected by the anisotropy ratio and the dip angles of the rock formations. In regard to the anti-dipping rock slopes, it was found that the anisotropy ratio and rock layer dip angles should be considered in the deformation and stability analyses. When the seepage of an anti-dipping layered slope was considered to be isotropic, the safety factors often were overestimated. As the anisotropy ratio decreases and the anti-dipping angles of the layered planes increases, the safety factors of the slopes will gradually decrease. This study provided a feasible scheme for evaluating the seepage, deformations and stability of the anti-dipping layered rock slopes in southwest China’s Pulang area.

Evaluating the hydromechanical responses of seabed–pipelines with rotated anisotropic heterogeneous seabed properties

Seabed sediments are inherently heterogeneous and present different fabric patterns including rotated anisotropy. This work aims to investigate the effects of rotated anisotropy in seabed properties on the hydromechanical responses of seabed and pipelines. Rotated anisotropic seabed property fields (e.g., hydraulic conductivity and shear modulus fields) are generated using the random field theory, and are then coupled with finite element models to simulate the hydromechanical responses of seabed–pipelines under dynamic nonlinear wave-induced loadings. Monte Carlo simulations are performed to approach a probabilistic analysis of the effects of heterogeneous seabed fields. The pore pressure, liquefaction, seabed displacement, and pipeline stress are analyzed for systematic varying rotation angles. The results show that the upper bounds of the liquefied area would decrease by about 15% and 8% with increasing rotated anisotropy in the hydraulic conductivity and shear modulus fields, respectively. For rotated anisotropic shear modulus fields, the von Mises stress is minimum when the rotation angle is 45∘. The transverse anisotropy in hydraulic conductivity or shear modulus is the most unfavorable situation, the results of which could be relied upon for a conservative design if the rotated anisotropy of a seabed property is unknown in a field project.

Improvement of vacuum pressure on pumping performance of vertical wells in municipal solid waste landfills.

The pumping performance of the traditional vertical well is often poor in municipal solid waste (MSW) landfills due to the blocking effect of landfill gas on leachate migration. To improve the pumping performance, a vacuum vertical well was designed and then installed at the Tianziling landfill. When the leachate was drawn out through submersible pump, the landfill gas was simultaneously extracted through vacuum pump to form vacuum pressure in the well. The vacuum pressure could increase the hydraulic gradient of leachate flow as well as the relative liquid permeability of MSW. Pumping tests were carried out to explore the effectiveness of the vacuum pressure on improving the pumping performance of vertical well. When the vacuum pressure increased from 0 kPa to - 30 kPa, the steady leachate pumping rate increased from 1.58 to 2.34 m3/h, and the steady leachate level drawdown increased from 5.9 to 10.3 m at the distance of 5 m. The vacuum pressure mainly affected the leachate level drawdown within the distance of 15-20 m. When the vacuum pressure in the pumping well was - 30 kPa, it attenuated to - 14.7 kPa and - 6.6 kPa at the distance of 5 m and 10 m, respectively. The influence radius of vacuum pressure was about 15 m. Numerical modeling indicates that the leachate pumping rate and drawdown will decrease with the increase in decreasing rate of hydraulic conductivity with depth, degree of heterogeneity, and anisotropy of hydraulic conductivity of waste. The experimental and numerical results demonstrate the effectiveness of vacuum pressure and provide working parameters for the application of the vacuum wells in MSW landfills.

Analytical analysis of the leachate flow to a horizontal well in dual-porosity media

Horizontal wells are gradually used for dewatering at municipal solid waste (MSW) landfills in recent years. A theoretical model of leachate flow to horizontal well is required to better evaluate its dewatering performance. In this study, a dual-porosity model of the leachate flow to a horizontal well in MSW landfills is established. The proposed model divides waste into fracture and matrix domains. Leachate in these two domains flows horizontally and vertically into a horizontal well together with leachate exchange occurring between them. An analytical solution of leachate level drawdown under horizontal well draining is obtained through Laplace integral transformation and Separation of variables. The analytical solution is verified with the numerical solution obtained by finite element method in COMSOL Multiphysics software. A sensitivity analysis is performed to investigate the effect of model key parameters, including anisotropy of hydraulic conductivity kr/kz, hydraulic conductivity ratio of fracture domain to matrix domain kfr/kmr, and proportion of fracture domain in total domain wf, on leachate level drawdown. A case study of the horizontal well draining test at the Tianziling landfill indicated that the dual-porosity model performed better than the single-porosity model in leachate level drawdown estimation, especially near the horizontal well. The average value of hydraulic conductivity ratio of fracture domain to matrix domain kfr/kmr is fitted to be 419 for typical MSW with high kitchen waste content in China.

Characterization of macropore structure of remolded loess and analysis of hydraulic conductivity anisotropy using X-ray computed tomography technology

The pore structure of loess is crucial for understanding the transport of water in soil and affects soil permeability. In this study, X-ray computed tomography (X-ray CT) technology was used to establish the macropore structure of remolded loess, and analyze the influence of macropore on permeability. A new specific and efficient workflow for constructing macropore structure of loess samples was established. The workflow improves the precision of image segmentation and provides a new method for creating 3D soil macropore structure. The macropore structure showed that the macro-porosity variation of loess sample was clearly anisotropic. Based on the macropore structure model, a modified Kozeny–Carman equation was suggested to investigate the hydraulic conductivity anisotropy of the loess sample. Subsequently, permeability test was used to verify the results calculated by equation. Both the results showed that hydraulic conductivity was positively correlated with the uniformity of the porosity changes in each direction. It provides a new method to estimate the anisotropy of hydraulic conductivity based on macropore structure model.