Groundwater flow through fractured rocks and seepage control in geotechnical engineering: Theories and practices

Groundwater flow through fractured rocks has been recognized as an important issue in many geotechnical engineering practices.Several key aspects of fundamental mechanisms,numerical modeling and engineering applications of flow in fractured rocks are discussed.First,the microscopic mechanisms of fluid flow in fractured rocks,especially under the complex conditions of non-Darcian flow,multiphase flow,rock dissolution,and particle transport,have been revealed through a com-bined effort of visualized experiments and theoretical analysis.Then,laboratory and field methods of characterizing hydraulic properties(e.g.intrinsic permeability,inertial permeability,and unsaturated flow parameters)of fractured rocks in different flow regimes have been proposed.Subsequently,high-performance numerical simulation approaches for large-scale modeling of groundwater flow in frac-tured rocks and aquifers have been developed.Numerical procedures for optimization design of seepage control systems in various settings have also been proposed.Mechanisms of coupled hydro-mechanical processes and control of flow-induced deformation have been discussed.Finally,three case studies are presented to illustrate the applications of the improved theoretical understanding,characterization methods,modeling approaches,and seepage and deformation control strategies to geotechnical engi-neering projects.

The Design of a Three-Dimensional Physical Modeling System for Real-Time Groundwater Flows

In the past decades,physical modeling has been widely used in hydrogeology for teaching,studying and exhibition purposes.Most of these models are used to illustrate hydrogeological profiles,but few can depict three-dimensional groundwater flows,making it impossible to validate groundwater flows simulated by numerical methods with physical modeling.

The research of three-dimensional numerical simulation of groundwater-flow: taking the Ejina Basin, Northwest China as example

Water is a primary controlling factor for economic development and ecological environmental protection in the inland river basins of arid western China. And it is groundwater, as the most important component of total water resources, that plays a dominant role in the development of western China. In recent years, the use-ratio of surface water has been raised, the groundwater recharge rate from surface water has been reduced, and groundwater has been exploited on a large scale. This has led to the decline of ground-water levels and the degradation of eco-environments in the Heihe watershed. Therefore, the study on the change in groundwater levels in recent years, as well as simulating and predicting groundwater levels in the future, have become very significant for im-proving the ecological environment of the Heihe River Basin, to coordinate the water contradiction among upper, middle and lower reaches of Heihe River Basin and to allocate the water resources. The purpose of this study is to analyze the groundwa-ter-level variations of the Ejina region based on a large scale, to develop and evaluate a conceptual groundwater model in Ejina Basin, to establish the groundwater flow model using the experimental observation data and combining Modular Three-Dimensional Groundwater Flow Model (MODFLOW) and GIS software, to simulate the regional hydrologic regime in re-cent 10 years and compare various water-delivery scenarios from midstream, and to determine which one would be the best plan for maintaining and recovering the groundwater levels and increasing the area of Ejina oasis. Finally this paper discusses the pos-sible vegetation changes of Ejina Basin in the future.

Groundwater Management at West El-Minia Desert Area, Egypt Using Numerical Modeling

Water shortage is the main problem facing any development in Egypt especially in the desert lands. River Nile is considered the main source of water in Egypt but its water covers only the area of flood plain where its tributaries do not reach to the desert. The desert fringes, west of El-Minia governorate, Egypt, are areas of natural expansion for agricultural, industrial, and civil activities. This implies an increasing demand for groundwater. A numerical groundwater model is one of the main tools used for assessment of the resource potential and prediction of future impact under different circumstances and stresses. In this paper, a transient groundwater flow model in the desert district west of El-Minia, Egypt, was developed. The conceptual model was built by analyzing the hydrogeological data and previous work. Steady state model of year 1990 was used to investigate and calibrate the parameters such as hydraulic conductivities, recharge and conductance of the surface water streams. The storage coefficients are calibrated by the transient model based on the available data observed from 1990 to 2013, which provides insights to understand the behavior of groundwater system in Quaternary Aquifer and to predict spatial-temporal distributions of groundwater levels and groundwater flow in responding to extraction of water. The calibrated transient model will be used to predict the impacts of desert development schemes and water resources management schemes on groundwater in the study area.

Impact of longwall mining on groundwater above the longwall panel in shallow coal seams

Since longwall mining causes subsidence through the overlying strata to the ground surface, the surface water and groundwater above the longwall panels may be affected and drained into the lower levels.Therefore, loss or interruption of streams and overburden aquifers is a common concern in coal industry.This paper analyzed the potential effects of longwall mining on subsurface water system in shallow coal seam. In order to monitor different water level fluctuations throughout the mining period, three water wells were drilled down to the proposed deformation zone above the longwall panel. A GGU-SS-FLOW3 D model was used to predict water table contours for the periods of pre- and post-mining conditions. The field data from the three water wells were utilized to calibrate the model. The field test and numerical model can help to better understand the dewatering of shallow aquifers and surface waters related to ground subsidence from longwall mining in shallow coal seam.

Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin

Although arsenic-contaminated groundwater in the Datong Basin has been studied for more than 10 years, little has been known about the complex patterns of solute transport in the aquifer systems. Field monitoring and transient 3D unsaturated groundwater flow modeling studies were car- ried out on the riparian zone of the Sanggan River at the Datong Basin, northern China, to better un- derstand the effects of groundwater flow on As mobilization and transport. The results indicate that ir- rigation is the primary factor in determining the groundwater flow paths. Irrigation can not only in- crease groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations, but also change the HS concentration, redox conditions of the shallow groundwater. Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer. The maximum value of As enriched in the riparian zone is roughly estimated to be 1 706.2 mg.d-1 for a horizontal water exchange of 8.98 m3.d-1 close to the river and an As concentration of 190 μg.L-1.

Parallel Simulation of Groundwater Flow in the North China Plain

Numerical modeling is of crucial importance in understanding the behavior of regional groundwater system. However, the demand on modeling capability is intensive when performing high-resolution simulation over long time span. This paper presents the application of a parallel program to speed up the detailed modeling of the groundwater flow system in the North China Plain. The parallel program is implemented by rebuilding the well-known MODFLOW program on our parallelcomputing framework, which is achieved by designing patch-based parallel data structures and algorithms but maintaining the compute flow and functionalities of MODFLOW. The detailed model with more than one million grids and a decade of time has been solved. The parallel simulation results were examined against the field observed data and these two data are generally in good agreement. For the comparison on solution time, the parallel program running on 32 cores is 6 times faster than the fastest MICCG-based MODFLOW program and 11 times faster than the GMG-based MODFLOW program. Therefore, remarkable computational time can be saved when using the parallel program, which facilitates the rapid modeling and prediction of the groundwater flow system in the North China Plain.

Direct shear tests on cemented paste backfill-rock wall and cemented paste backfill-backfill interfaces

Even though a large number of large-scale arch dams with height larger than 200 m have been built in the world, the transient groundwater flow behaviors and the seepage control effects in the dam foundations under difficult geological conditions are rarely reported. This paper presents a case study on the transient groundwater flow behaviors in the rock foundation of Jinping I double-curvature arch dam, the world＇s highest dam of this type to date that has been completed. Taking into account the geological settings at the site, an inverse modeling technique utilizing the time series measurements of both hydraulic head and discharge was adopted to back-calculate the permeability of the foundation rocks,which effectively improves the uniqueness and reliability of the inverse modeling results. The transient seepage flow in the dam foundation during the reservoir impounding was then modeled with a parabolic variational inequality（PVI） method. The distribution of pore water pressure, the amount of leakage, and the performance of the seepage control system in the dam foundation during the entire impounding process were finally illustrated with the numerical results.