This article answers common user questions about how to specify the initial conditions in a groundwater analysis.
The saturation (or volumetric water content) is not included in the formulation of groundwater flow. For the case of unsaturated steady state flow, the “permeability coefficient” is dependent on the matric suction in the unsaturated zones. In transient cases the variation of volumetric water content with matric suction is taken into account just to evaluate the “storage coefficient” used in the formulation.
It’s quite common that people will use a finite-element product to compute excess pore pressure and then import this into Slide as a pore pressure grid. As long as the pore pressure distribution is correct, and make sure you check it using the Supplemental contours option in the Data menu of the Interpreter, the slip surfaces will be valid. This also assumes your effective strength criterion is correct.
The output from a seepage analysis is total heads (or pore pressures) throughout the model. So when you have pore pressures at particular points, as you do with piezometer readings, you want to correlate the results of your model with the measured piezometer readings. So you end up modifying your groundwater model in terms of permeability’s, permeability model (i.e. simple, van Genuchten, Brooks and Cory), and far field boundary conditions, until you get a fit between your model and the pore pressures measured at certain locations.
What some customers do incorrectly, is apply pressure boundary conditions at the points where the piezometers are located. Although this forces the pressure at these points to be the measured values, this also creates a source or a sink at these locations resulting in fluid being injected or removed at these locations.