This article addresses many of the questions users have with regards to the theoretical basis of Slide.

How does Slide incorporate compressibility in transient seepage analyses?
The slope of the water content curve that Slide requires in the transient analysis is mw (coefficient of volume change). An associated verification example is presented in the General Modeling Tips article.

What does Slide do with pressure head?
Slide converts pressure head to pore pressure by multiplying by the unit weight of water. Then uses pore pressure to determine effective normal stress on the base of a slice. In the attached hand calculation, u is the pore pressure. It’s standard Terzaghi effective stress theory. Pore pressure is not used in the determination of interslice forces in Slide. Slide uses total forces for everything, only in the determination of shear strength on the base does pore pressure get used.

How does pore pressure influence the results in a slope stability analysis?
What pore pressure does is reduce the effective stress. The effective stress being the amount force being transmitted through the soil skeleton per unit meter of area. For a constant total stress, as the pore pressure increases, the effective stress decreases. Since strength in a frictional material is a function of normal effective stress, an increase in pore pressure generally reduces strength.

What is the difference between the water table and piezometric lines?
Piezolines do not create ponded water. When you add a water table above the ground surface, ponded water is automatically added (as shown in the figure below), including the weight of the water.
Ponded water automatically created by Water Table
Details of this feature can be found in the Add Water Table section of the Online Help.

Is the factor of safety for slope stability affected by negative pore pressures?
Factor of safety is only affected by negative pore pressures from a groundwater analysis if the Phib value is nonzero. For more details see the Unsaturated Shear Strength help topic.