Solute Removal Analysis of a Large-Scale Fracture Plane Considering Different Flow Paths and Different Hydraulic Head Differences

An experimental and numerical study was carried out to investigate the solute removal process through a large-scale fracture plane considering different flow paths and hydraulic head differences.The visualization techniques were utilized in the experiment to capture the removal process images,which were then transferred to binary images.The variations in dimensionless concentration,which is defined as saturation of solute phase,were analyzed.With increasing hydraulic head difference,the speed of solute removal increases and the dimensionless concentration decreases.The flow paths result in different solute distribution patterns and different mechanisms for solute removal such as advection and diffusion,thus the curves of dimensionless concentration versus time are different.The dimensionless concentration over time decreases from approximately 1,which is smaller than 1 due to the existence of bubbles,to approximately 0,which is larger than 0 because the folds of the background are dealt as“solute”.A significant longer time is needed to achieve a certain fixed dimensionless concentration for a smaller hydraulic head difference.With the finite element software COMSOL multiphysics,the solute removal process,flow velocity fields,flow streamlines,as well as the hydraulic pressure fields were analyzed,which shows a good consistency with the experimental results.In practical engineering,when the solute pollutes the underground environment,the removal ability can be more significantly enforced by immediately applying a larger hydraulic head difference along a longer distance between the inlet and outlet boundaries.

Advantages of employing multilevel monitoring wells for design of tunnels subjected to multi-aquifer alluvial

For tunnels being excavated through multiple knowledge of the aquifers’hydraulic head becomes essential for determining groundwater inflow into the tunnel and analyzing its stability,specifically using multilevel monitoring systems.In the multi-aquifer alluvial section of the Glas tunnel(Iran),since the hydraulic head calculations were based on the data obtained from single-piezometer boreholes,the excavation risk was assessed to be at high level and the tunnel seemed to be unstable,thus an incorrect conclusion was derived from the misleading data.To take cost mitigation measures into account,it was necessary to calculate the hydraulic head at tunnel level accurately.By installing nested and clustered wells the mean hydraulic head was measured to be 70 m,significantly different from the 90 m previously determined by boreholes.Considering the updated value,the groundwater inflow and bulkhead load,formerly calculated as 0.65 m^(3)/s and 9.5 bars,were determined to be 0.49 m^(3)/s and 7.5 bars,respectively.

Three-dimensional stochastic seepage field for embankment engineering

Owing to the complexity of geo-engineering seepage problems influenced by different random factors, three-dimensional simulation and analysis of the stochastic seepage field plays an important role in engineering applications. A three-dimensional anisotropic heterogeneous steady random seepage model was developed on the basis of the finite element method. A statistical analysis of the distribution characteristics of soil parameters sampled from the main embankment of the Yangtze River in the Southern Jingzhou zone of China was conducted. The Kolomogorov-Smirnov test verified the statistical hypothesis that the permeability coefficient tensor has a Gaussian distribution. With the help of numerical analysis of the stochastic seepage field using the developed model, various statistical and random characteristics of the stochastic seepage field of the main embankment of the Yangtze River in the Southern Jingzhou zone of China were investigated. The model was also examined with statistical testing. Through the introduction of random variation of the upstream and downstream water levels into the model, the effects of the boundary randomness due to variation of the downstream and upstream water levels on the variation of simulated results presented with a vector series of the random seepage field were analyzed. Furthermore, the combined influence of the variation of the soil permeability coefficient and such seepage resistance measures as the cut-off wall and relief ditch on the hydraulic head distribution was analyzed and compared with the results obtained by determinate analysis. Meanwhile, sensitivities of the hydraulic gradient and downstream exit height to the variation of boundary water level were studied. The validity of the simulated results was verified by stochastic testing and measured data. The developed model provides more detail and a full stochastic algorithm to characterize and analyze three-dimensional stochastic seepage field problems.

Calculation of the Water Distribution Networks Based on the Theory of Slow Transient Flow

Urban water supply network is a modern urban survival and development of the infrastructure of a city,and its normal running conditions have important significance. The actual hydraulic process in the variableload water distribution networks can be treated as the slow transient flow which belongs to the unsteady flow. This paper analyzes the multi-loops network slow transient model based on graph theory,and the link flow matrix is treated as the variables of the discrete solution model to simulate the process of the slow transient flow in the network. With the simulation of hydraulic regime in an actual pipe network,the changing laws of the flow in the pipes,nodal hydraulic heads and other hydraulic factors with the passage of time are obtained. Since the transient processes offer much more information than a steady process,the slow transient theory is not only practical on analyzing the hydraulic condition of the network,but also on identifying hydraulic resistance coefficients of pipes and detecting the leakage in networks.

The Groundwater Model for Part of the Water Supply Source Aquifer for the City of Ulaanbaatar Using FEFLOW Simulation

The cold,semi-arid environment shows a high variability in precipitation and river discharge,with a general tendency towards decreasing water availability due to increasing air temperatures and,thus,rising potential evaporation levels.The main watercourse near the city is the Tuul River,fed by precipitation in the nearby Khentii Mountains.However,due to the absence of precipitation during winter and spring,the riverbed usually runs dry during these seasons,and observations show that the dry period has been extending within the last years.For many decades,the water supply of Ulaanbaatar has been exclusively based on the use of groundwater in the Tuul valley.However,in parallel with the city’s development,the extended groundwater aquifer shows a clear decline,and the groundwater levels drop significantly.Therefore,a groundwater management system based on groundwater model and MAR(Managed Aquifer Recharge)is proposed and a strategy to implement these measures in the Tuul valley is presented.The groundwater model research purposes of artificially recharging the Tuul River aquifer are to provide information for future improvement in solving shortage water supply related issues and to find simple,low cost,cheap,and reliable flow control methods to eliminate the Tuul River drying out in low flow season.

Groundwater Flow Modelling of Galma Basin, Nigeria： Using Finite Element Method

The groundwater flow characteristics of the Galma River Basin were simulated numerically by using the finite element method. The two-dimensional partial differential equation governing transient flow in an unconfined aquifer was modified to incorporate the effect of precipitation as a measurable source as it affects groundwater flow, such that for a given amount of precipitation over the basin, the flow of groundwater can be predicted at any point in the basin. With appropriate initial and boundary conditions, the modified equation was solved and the solution programmed for computer run. After calibration and verification, the borehole hydraulic data for the basin was used to predict flow due to groundwater hydraulic heads for 20 years. Findings revealed that there is a direct correlation of 0.79 and a strong linear relationship between simulated and observed hydraulic heads, and that data availability and choice of appropriate initial and boundary conditions are significant for good numerical modelling results. The contour plot of the hydraulic heads showed variation of heads from higher values at the upstream to lower values downstream, and groundwater flow follows the natural topography of the land from the upstream end of the basin towards the main streams and Galma River.

Determination of groundwater flow regimes based on the spatial non-local distribution of hydraulic gradient:Model and validation

The groundwater flow in natural aquifers can change from the Darcy flow to the non-Darcian flow due to a variety of causes,such as the increase of the Reynolds number in the highly permeable media or the decrease of the hydraulic gradient below a threshold in the low-permeability media,while the representative flow regime cannot be reliably determined using the traditional criteria.To address this challenge,this paper proposes a new term called the equivalent hydraulic gradient(EHG)by generalizing the differential form of the Darcy’s law using a spatial integral of the upstream hydraulic head.The nonlocal spatial variation of the hydraulic head difference between upstream and downstream zones is assumed to be the potential cause of the transition of the groundwater flow regimes.This assumption is analogous to the common assumption used for quantifying the anomalous pollutant transport in the geological media.Applications of this idea show that the EHG concept could distinguish three main flow regimes,namely the Super-Darcy flow,the Darcy flow,and the Sub-Darcy flow,although the Super-Darcy flow regime is rarely observed in the laboratory column flow experiments.Results of this study therefore shed lights on the interpretation of the fundamental dynamics of the groundwater moving in various heterogeneous aquifers,and may lead to the rebuilding of the hydrodynamics of the surface water,the groundwater,and the soil.

Influence of permeability anisotropy of seepage flow on the tunnel face stability

This paper focuses on the influence of permeability anisotropy of seepage flow on the face stability for a shied tunnel.An analytical model has been proposed to present the hydraulic head distribution around the tunnel face in the anisotropic ground,considering the difference of permeability coefficient in the horizontal direction and the vertical direction.The rationality of the proposed model is ver-ified by a series of numerical simulations.Then,an analytical model of face stability for a tunnel under the anisotropic seepage has been established based on the limit analysis upper bound method.Comparisons of the analytical solutions and the numerical simulations are conducted,and the limit support pressure of the two methods is consistent.The effect of permeability anisotropy and water pressure on the stability of the tunnel face is analyzed through the three-dimensional analytical solution.Anisotropy of permeability has a significant impact on the stability of the tunnel face,and its impact gradually decreases.It can also be found that the water pressure coefficient of the tunnel face has a significant effect on the limit support pressure and the failure area when the ratio of the horizontal permeability to the vertical permeability is large.

STOCHASTIC ANALYSIS OF GROUNDWATER FLOW SUBJECT TO RANDOM BOUNDARY CONDITIONS

A stochastic model was developed to simulate the flow in heterogeneous media subject to random boundary conditions. Approximate partial differential equations were derived based on the Karhunen-Loeve （KL） expansion and perturbation expansion. The effect of random boundary conditions on the two-dimensional flow was examined. It is shown that the proposed stochastic model is efficient to include the random boundary conditions. The random boundaries lead to the increase of head variance and velocity variance. The influence of the random boundary conditions on head uncertainty is exerted over the whole simulated region, while the randomness of the boundary conditions leads to the increase of the velocity variance in the vicinity of boundaries.

Three-dimensional theoretical analysis of seepage field in front of shield tunnel face

To evaluate hydraulic head distribution in front of a shield tunnel in a saturated soil layer,theoretical analysis and numerical simulations are carried out in this study.Based on the partial differential equilibrium equation of seepage flow,a three-dimensional(3D)theoretical analytical model of the shield tunnel face and the seepage field in front of it is established using the eigenfunction and the Fourier series expansion methods,and the hydraulic head calculation formula is derived.Combined with engineering cases,the theoretical analysis results and the 3D numerical simulation results are compared and analyzed.The effect of the water pressure of the tunnel face on the hydraulic head distribution is also analyzed.The results of the proposed analytical solution are in agreement with those of the numerical simulation solutions;moreover,the proposed analytical solution requires less time to calculate the seepage hydraulic head than the numerical simulation.The ratio of the initial water table to the diameter(D)of tunnel face has a more significant impact on the hydraulic head distribution at a position 0.5D above the tunnel vault.When the water pressure on the tunnel face is not considered,the values of the hydraulic head are significantly underestimated.

Numerical Simulation of Drawdown and Land Deformation in Pumping-Recovery Tests Performed on a Circular Excavation

A mathematical model, which not only fully couples fluid flow and solid skeleton deformation in unsaturated porous elastic media but also considers deformable diaphragm walls, is formulated in axially symmetric cylindrical coordinates for drawdown and land deformation. Based on this model, pumping-recovery tests in various conditions are numerically simulated to reveal the effects of elastic modulus of soil E and initial saturated hydraulic conductivity K_(sat0) on hydraulic head and land deformation. The heterogeneity with respect to E and K_(sat0) is separately taken into account. Large elastic modulus of soil contributes to both dewatering process and deformation control. Either large or small initial saturated hydraulic conductivity may cause relatively high groundwater table, while the larger one leads to smaller displacements.