Finite-Element Groundwater Seepage Analysis

Within the Slide program, Slide now has the capability to carry out a finite element based groundwater seepage analysis for saturated / unsaturated, steady state flow conditions.

The Groundwater Analysis in Slide allows the user to easily define and analyze a groundwater problem, using the same model as for the slope stability problem. The boundaries of the problem only need to be defined once and will be used for both the groundwater analysis and the slope stability analysis.

Contours of pressure head and seepage flow in an earth dam.



Although the Slide groundwater analysis is geared towards the calculation of pore pressures for slope stability problems, it is not restricted to slope geometry configurations. The Groundwater modeling and analysis capabilities in Slide can be used to analyze an arbitrary, 2-dimensional groundwater problem, for saturated / unsaturated, steady state flow conditions.

The groundwater analysis capability in Slide can be considered a completely self contained groundwater analysis program and can be used entirely independently of the slope stability functionality of Slide.

Groundwater Modeling

The groundwater modeling options in Slide are all contained within the Slide MODEL program. In order to enable groundwater modeling, it is first necessary to set the Groundwater Method in Project Settings to Finite Element Analysis. When you do this an Analysis Mode option will be available, which allows you to select either Slope Stability analysis mode or Groundwater analysis mode. When you are in Groundwater analysis mode, the menu and toolbar will present all of the necessary groundwater modeling options.

Groundwater Compute

The groundwater analysis engine in Slide is a separate program from the slope stability analysis engine. You may perform a groundwater analysis in Slide without necessarily performing a slope stability analysis. However, after a groundwater analysis is performed, the results (pore pressures) can be automatically utilized by the slope stability analysis engine in Slide.

Groundwater Interpret

The results of a Slide groundwater analysis are viewed with the Slide INTERPRET program and can be viewed at the same time as the slope stability analysis results. After you perform a groundwater analysis with Slide, it is always a good idea to first use the Slide INTERPRET program to check that the analysis results are reasonable, before you proceed to a slope stability analysis. If not, then you should go back to the Slide MODEL program and check that you have defined your model correctly.

Slope Stability using Pore Pressures from a Groundwater Analysis

Slide contains integrated groundwater analysis capabilities for the purpose of calculating pore pressures for a slope stability analysis. You do not require another program or have to worry about importing results. You also don't have to worry about maintaining and building two separate models which leads to problems with synchronizing both models. Making changes in your slope stability model and forgetting or improperly editing the groundwater model is common in this type of scenario. Since the groundwater modeler, analysis and data interpretation are fully integrated into Slide, only one model needs to be built. And the data from the slope stability and groundwater analysis are always synchronized.

Doing a groundwater analysis with Slide and using the pore pressures in your slope stability analysis couldn't be easier. You just turn it on, mesh the geometry using our one-click automatic meshing, set the material permeability and boundary conditions, and run the analysis. Slide then automatically uses the pore pressures in its slope stability calculations.

Review tutorial number 7 in the tutorial manual for a complete step-by-step explanation.

Unsaturated Shear Strength

After a groundwater analysis has been computed in Slide, the resulting pore pressure distribution may indicate negative pore pressures (commonly referred to as matric suction) in the unsaturated zone above the water table.

It is known that matric suction can contribute to the shear strength of a slope. By specifying an Unsaturated Shear Strength Angle for a material, the shear strength of the material in the unsaturated zone will be INCREASED by the amount:

     ψ tan φb

where ψ = matric suction (POSITIVE value), and φb = unsaturated shear strength angle. (NOTE: by convention, the term matric "suction" implies the POSITIVE, or absolute value of the negative pore pressures calculated in the unsaturated zone.)

By default, the Unsaturated Shear Strength Angle = 0. This means that matric suction in the unsaturated zone WILL NOT have any effect on the shear strength or safety factor.

However, if you enter a non-zero value for Unsaturated Shear Strength, then slip surfaces which pass through a material in the unsaturated zone will have INCREASED shear strength and safety factor.


Hydraulic (Permeability) Properties

Slide utilizes an unsaturated steady-state groundwater finite-element analysis in order to calculate pore pressures and seepage values. This type of analysis requires that you define the permeability behavior in the unsaturated zone above the water table. To facilitate this, Slide offers a number of industry accepted groundwater models and the ability to choose from published data for different soil types. This includes Fredlund & Xing, van Genuchten, Brooks & Corey, and Gardner models, for hydraulic conductivity.

For ultimate flexibility, advanced users can define their own model. This makes it easy for any user to perform a groundwater analysis for the purpose of determining pore pressures for their slope stability analysis.

First choose the groundwater model for each of your materials.



Then define the parameters for the model. If you require help, we offer a list of published parameters for different soil types. The reference and equation are included in the dialog.



You can also plot the model function to see what it looks like.




Mesh Generation

One of the major advantages of using Slide's groundwater finite-element analysis mode is it's unique ability to easily mesh complex slope geometry using both triangular and quadrilateral finite-elements. Not only can it mesh complex geometry but it's as simple as pushing a single button. Simply enter your slope and material boundaries through the CAD interface or by reading in Autocad DXF files and press the Mesh button. Along with automatic meshing, the user is also provided with complete mesh customization capabilities. Custom densification of the mesh allows you to easily place more elements where they are needed. Mapped meshing which allows you to define a regular array of elements in a rectangular or triangular region is also available.

To create a mesh:

1. Define the approximate number of elements you want and the type of element



2. Press the Mesh button



The program will automatically mesh your model with triangles



or quadrilaterals




Mapped Meshing

In conjunction with the fully automatic mesh generation capabilities, Slide offers increased flexibility by allowing the user to define a custom mesh through mapped meshing. A regular array of elements can easily be defined.

A mapped mesh of a dam.




Groundwater Boundary Conditions

The application of boundary conditions to your groundwater analysis model is extremely simple. Just select what type of boundary condition you wish to apply, select the boundary or nodes you wish to apply it to, and that's it. You can apply different conditions to different boundaries all at the same time. No frustrating multiple menu selections and individual node selection.

You can apply total head, zero pressure, nodal flow rates, or infiltration boundary conditions. A special boundary condition that automatically determines whether the surface is a seepage face (zero pressure) or no flow boundary is also available.

One-Click definition of a total head boundary condition (red dots) on the right side of the model.



Boundary conditions can also be set by right-clicking on a segment or vertex and selecting the appropriate type from the pop-up menu.



Slide preserves boundary conditions during modeling so that after applying a different type of mesh (for example) you will always get the same boundary conditions on boundary vertices and segments.




Discharge Sections

In Slide you may also define a discharge section. The quantity of flow across this section is then calculated.

Discharge across a section (green line) in an earth dam.




Integrated Interpreter

The slope stability and groundwater interpreters have been combined into one program. You can now view contours of groundwater data at the same time as viewing slope stability results. This lets you spend less time creating output that is suitable for presentation in a report.

Plot of safety factor in the circular surface grid combined with a plot of pressure head within the slope from a groundwater analysis.




Tolerance History for Groundwater Analysis

You can now view a plot of "tolerance vs. iteration" for finite element groundwater analyses. This plot can be quite helpful if your model experiences convergence problems. You can view the plot and immediately determine if your model was just about to converge (and just needed a few more iterations), or if it was quite unstable (and you need to go back to the modeller and diagnose the problem further).