Seepage Modeling with SEEP/W - GeoStudio 2007 version 7.22

Chapter 3: Geometry and Meshing
SEEP/W
particle to move a significant distance relative to the element size, while at the same time not have the
time step size be so large as to allow the particle to jump across several elements. The particle should, so
to speak, make at least one stop in each element. In CTRAN/W this is controlled by the Peclet and
Courant criteria.
In a simulation of consolidation, the time step size for the first time step needs to be sufficiently large so
that the element next to the drainage face consolidates by at least 50 percent. Achieving this is related to
the element size; the larger the element the greater the required initial time step. If the time step size is too
small, the computed pore-water pressures may be unrealistic.
The important point in this section on meshing is to realize that meshing, more particularly element sizes,
comes into play in a transient analysis. Rules and guidelines for selecting appropriate time stepping are
discussed elsewhere with reference to particular types of analysis.
3.7 Finite elements
Discretization or meshing is one of the three fundamental aspects of finite element modeling. The other
two are defining material properties and boundary conditions. Discretization involves defining geometry,
distance, area, and volume. It is the component that deals with the physical dimensions of the domain.
A numerical book-keeping scheme is required to keep track of all the elements and to know how all the
elements are interconnected. This requires an ordered numbering scheme. When finite element methods
were first developed, creating the mesh numbering was very laborious. However, many computer
algorithms are now available to develop the mesh and assign the element numbering. Developing these
algorithms is in some respects more complex than solving the main finite element equations. GeoStudio
has its own system and algorithms for meshing, which are designed specifically for the analysis of
geotechnical and geo-environmental problems.
Some human guidance is required to develop a good finite element mesh in addition to using the powerful
automatic meshing algorithms available. One of the issues, for example, is mesh size. Computers,
particularly desktop or personal computers, have limited processing capability and therefore the size of
the mesh needs to be limited. Variable mesh density is sometimes required to obtain a balance between
computer processing time and solution requirements. Ensuring that all the elements are connected
properly is another issue. Much of this can be done with the meshing algorithm, but it is necessary for the
user to follow some fundamental principles. In finite element terminology this is referred to as ensuring
mesh compatibility. GeoStudio ensures mesh compatibility within a region and for the most part ensures
mesh compatibility across adjacent regions, but it is still possible to create a situation whereby mesh
incompatibility exists. The user needs to provide some guidance in ensuring compatibility between
regions.
The purpose of this chapter is to introduce some of the basic concepts inherent in meshing and outline
some procedures which must be followed when developing a mesh. An understanding of these
fundamentals is vital to proper discretization.
Much of this chapter is devoted to describing the meshing systems and the features and capabilities
available in GeoStudio. In addition, there are also discussions on the selection, behavior and use of
various element types, sizes, shapes and patterns. A summary of practical guidelines for good meshing
practice are also outlined.
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