Abstract Numerical modeling of groundwater and geothermal problems has expanded in the past few years due to the increase in computational power and software. The size and complexity of solutions attempted has grown in step with computational abilities. Problems with larger numbers of total nodes, with complex geology involving faulting, as well as coupling of multiple physical processes (geothermal, CO2 sequestration) are now being attempted. Los Alamos National Laboratory (LANL) has invested over 50 man-years of effort into the FEHM control volume finite element solver over the past number of decades. The code has been used on US EPA Superfund sites, low and high level nuclear waste sites, and a variety of fundamental hydrogeological applications. The code allows for complex coupling of processes including non-isothermal models and can solve more complex problems than existing commercial codes. LANL and SoilVision Systems Ltd., have combined efforts to offer groundwater and geothermal numerical modeling solutions of larger and more complex systems. In order to gain confidence in the combined front end, solver, and back end visualization system, a number of benchmarks have been created in order to document performance. This paper presents the results of benchmarks created to test the performance of the new groundwater and geothermal modeling system. Performance of the system is discussed as well as challenges and hurdles encountered in the collaboration. The ability of the system to scale up to model field-scale systems will be discussed.KeywordsFEHMHydrogeologicalGroundwaterGeothermalNumerical modelingCoupled solutions

AbstractThe prediction of actual evaporation (AE) is required when calculating the net moisture flux from the ground surface to the atmosphere. The proposed Wilson-Penman equation appeared to yield reasonable predictions of AE from saturated clayey soils but overpredicted AE from coarse-grained soils in arid regions. In recent years, two distinct approaches have emerged for the prediction of AE from unsaturated soil surfaces. Both approaches are based on the realization that evaporation tends to “shut off” as the natural water content approaches residual water content conditions. The first approach to calculate AE from an unsaturated soil involves the adjustment of the total suction at the ground surface. The second approach to calculate AE involves the determination of the “evaporation-rate reduction point” from the drying soil-water characteristic curve. A vapor pressure reduction factor is then applied to calculate AE. Both approaches are attempts to take the effects of “surface resistance” into consid...

It is commonly understood that a 2-D slope stability analysis will provide a lower factor of safety than a 3-D slope stability analysis. The difference in the calculated factors of safety between a 2-D and a 3-D analysis is generally less than 15% for simple slope geometries. Most past comparative studies between 2-D and 3-D stability analyses have ignored the effect of negative pore-water pressures (i.e., matric suctions) in the soil zone above the groundwater table. In this paper, a comparison is made between 2-D and 3-D slope stability analyses on soil slopes where a portion of the soil profile has matric suctions. The factors of safety on simple geometry slopes and complex geometry slopes (i.e., slopes which have two intersecting slope surfaces), are investigated for a range of shear strength parameters and groundwater conditions. For simple slopes with a low slope angle, the difference in factor of safety between a 2-D and a 3-D slope stability analysis, (i.e., ΔFs/Fs2-D), generally ranges from 9% to 16% when ϕb is equal to 15°. The value of ΔFs/Fs2-D for a steep, simple slope is generally larger than for a low angle, simple slope. When ϕb is 15°, the values of ΔFs/Fs2-D for the simple, steep slope generally range from 12 to 18%. The difference between a 2-D and a 3-D stability analysis was most pronounced for concave geometries where a portion of the soil profile contained unsaturated soils. The values of ΔFs/Fs2-D for corner angle concave slopes with angles ranging between 180 to 270° can be as large as 20 to 59% when ϕb is equal to 15°. Two case histories, (i.e., the highwall stability failure at the Poplar River coal mine and the Kettleman Hills landfill slope failure), were used to illustrate the effect of the unsaturated zone on changes in the factors of safety.

The linear form of the extended Mohr–Coulomb shear strength equation uses a [Formula: see text] parameter to quantify the rate of increase in shear strength relative to matric suction. When the [Formula: see text] value is unknown, a [Formula: see text] equal to 15° is sometimes used in the slope stability study to assess the influence of matric suction on the stability of a slope. In many cases, however, a [Formula: see text] value of zero is used, signifying that the effect of matric suction is ignored. Experiment results have shown that the relationship between the shear strength of an unsaturated soil and matric suction is nonlinear. Several semi-empirical estimation equations have been proposed relating the unsaturated shear strength to the soil-water characteristic curve. In this paper, the results of a study using two-dimensional slope stability analysis along with an estimated nonlinear shear strength equations is presented. The effects of using an estimated nonlinear shear strength equation for the unsaturated soils are illustrated using three example problems. Several recommendations are made for engineering practice based on the results of the example problems. If the air-entry value (AEV) of a soil is smaller than 1 kPa, the effect of matric suction on the calculated factor of safety is trivial and the [Formula: see text] value can be assumed to be zero. If the AEV of a soil is between 1 and 20 kPa, the nonlinear equations of unsaturated shear strength should be adopted. For soils with an AEV value between 20 and 200 kPa, an assumed [Formula: see text] value of 15° provides a reasonable estimation of the effects of unsaturated shear strength in most cases. For soils with an AEV greater than 200 kPa, [Formula: see text] can generally be assumed to be equal to the effective angle of internal friction, [Formula: see text], in applications where geotechnical structures have matric suctions around 100 kPa.

This paper aims at obtaining an advanced formulation of the time -domain Boundary Element Method (BEM) for two-dimensional dynamic analysis of unsaturated soil. Unlike the usual time-domain BEM the present formulation applies a Convolution Quadrature which requires only the Laplace-domain instead of the time-domain fundamental solutions. The coupled equations governing the dynamic behavior of unsaturated soils ignoring contributions of the inertia effects of the fluids (water and air) are derived based on the poromechanics theory within the framework of the suction-based mathematical model. In this formulation, the solid skeleton displacements, water pressure and air pressure are presumed to be independent variables. As there is no analytical solution for the 2D wave propagation in unsaturated soils in the literature, to verify the accuracy of this implementation, the displacement response obtained by the boundary element formulation is partially verified by comparison with the elastodynamics problem.

The cold climate environment can cause damage to engineering structures such as roadway/railways and pipelines. There has been an increased use of finite element numerical models to study and understand the processes involved. The primary difficulty in the application of such numerical models is the lack of proper coupling between the related processes of water, thermal, and airflow. This paper presents classic benchmarks solved with a finite element numerical modeling code which demonstrates the correct coupling of the thermal, hydraulic, and air (THA) processes. The use of this technology potentially opens the use of numerical models to a wider range of applications in artic regions.

The strength of levees can be affected during fluctuations in the water table. It is also possible for the climate to have an influence on the position of the water table in an earth levee. Traditional methods have resulted in approximate methods for dealing with the transient fluctuations of the water table in a levee. These approximations are generally accepted in engineering practice but the question can be rightfully raised as to how these approximations compare to a rigorous transient combined seepage and slope stability analysis. Software technology has significantly changed in recent years and is now at the point where it is much easier to perform transient seepage analyses. There are new questions that can be asked. Does an effective stress analysis diverge significantly from the 3-stage Duncan (1990) analysis? If so, under what conditions? This paper compares the Duncan (1990) three-stage methodology for analyzing rapid drawdown scenarios to a combined transient seepage and slope stability analysis. Traditional limit equilibrium methods will be utilized in the slope stability analysis and the accommodation of saturated and unsaturated pore-water pressures will be considered. Analyses of a number of typical cross-sections will be considered in order to determine the potential influence of geometry. The intent of the paper is to illustrate scenarios under which the Duncan (loc. cit.) methodology produces similar results to the results of a more rigorous analysis.