July 2005
	In this issue:  Transient Seepage Analysis

Demystifying a Transient Analysis 
Some people panic when they hear the words “transient seepage analysis”. However, if you feel your steady-state seepage results do not reflect what you believe to be happening in the ground, it is likely because you are trying to represent a transient condition with a steady-state solution.

A steady-state solution is one that doesn’t care about time because the pore-water pressures in the ground are unchanging and are at equilibrium with the defined boundary conditions. By contrast, a transient analysis means that the pore-water pressures in the ground are changing with time and you now have the opportunity to look at the conditions that exist at discrete snapshots in time. The transient analysis takes into account how long it takes the soil to respond to the boundary conditions and how much water was stored or lost between time increments.

The variety of situations you can numerically model increases significantly if you’re willing to conduct a transient analysis. A transient analysis does not have to be an especially difficult process, especially if you keep a few key points in mind when developing the analysis. 

1. Initial Conditions – A transient analysis requires a point from which to start moving ahead in time. These are called initial conditions and there are several options. You can either conduct a simple steady-state analysis to determine the initial pore-water pressures in the ground before a transient event is applied; or you can draw in an initial water table. If you use a steady-state analysis, you will need to flag this file as the starting point for your transient analysis. If you draw an initial water table through the profile, SEEP/W will assume that the pressures are hydrostatically positive below the water table and hydrostatically negative above the water table, up to a maximum value that you define when drawing the initial water table. 
   
2. Volumetric Water Content Function (VWC) – A steady-state analysis requires a conductivity function, but a transient analysis requires both a conductivity function and a volumetric water content function. The VWC function describes how water is stored or released in the soil due to changes in pore-water pressure and the conductivity function describes how easily the water will flow through the soil at a given pore-water pressure. There are estimation techniques built into the software that will predict a VWC function based on a simple grain-size curve as well as a database of measured functions for a wide range of materials. The function database is included in the SEEP/W examples folder that is installed along with the software.
   
3. Time Steps – An incremental time sequence is required for all transient analyses and the accuracy of the results depends to some extent on the size of the time step. Since each time step is equivalent to a mini steady-state analysis, large time steps can lead to non-convergence or inaccurate results. If the rate of change in the soil is large, smaller time steps are warranted and when the rate of change is low, larger time steps will provide a reasonable solution. Because it can be difficult to predict precisely when smaller time steps will be required, a feature called adaptive time stepping has been built into SEEP/W, so the software will automatically adjust the time steps as required.

Having the ability and confidence to set up and solve a transient analysis will greatly increase the number of engineering problems that you can model. There are several features that have been implemented in the software to make it easier to develop a transient analysis and we have dedicated several sections of our book, Seepage Modeling with SEEP/W: An Engineering Methodology to the subject. If you have not yet upgraded to GeoStudio 2004, for which the book was written, you can download an evaluation copy of the software from our website and access the PDF file of the SEEP/W book.

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