Groundwater Method

The Groundwater Method in Project Settings determines how pore water pressure will be modeled. Six options are available:

Only one Groundwater Method may be selected for a given analysis. However, you may combine the Ru method with either Water Surfaces or Water Pressure Grids, by selecting the Allow Ru with Water Surfaces or Grids checkbox. You may also use the B-bar method to calculate excess pore pressure due to undrained loading, by selecting the Calculate Excess Pore Pressure checkbox.

Water Surfaces

If Water Surfaces is the Groundwater Method, then a Water Table, and / or Piezometric Lines can be used to define the pore pressure conditions for each soil type. See the Add Water Table , Add Piezometric Lines and Water Parameters topics for details.

Ru Coefficients

Ru Coefficients can be used to calculate pore pressure. The Ru coefficient used in Slide is the one widely used, which simply models the pore pressure as a fraction of the vertical earth pressure for each slice. If Ru Coefficients is the Groundwater Method selected in Project Settings, then a Ru Coefficient can be specified for each soil type in the Define Material Properties dialog.

Note: the Ru method can also be used with Water Surfaces or Grids, by selecting the Allow Ru with Water Surfaces or Grids checkbox. See below for details.

Water Pressure Grid

There are three options for defining a Water Pressure Grid in Slide:

A Water Pressure Grid is created (or can be read from a file), using the Water Pressure Grid option in the Boundaries menu. See the Water Pressure Grid topic for details.

When a Water Pressure Grid is specified, an Interpolation Method must also be specified. The Interpolation Method is used to calculate the water pressure at any point in the soil, from the values at the grid points. There are several different Interpolation Methods which are available in Slide. See the Interpolation Method topic for details.

Finite Element Analysis

If the Groundwater Method = Finite Element Analysis, then this will enable the Groundwater Analysis Mode in the Slide Model program. The Groundwater Analysis Mode allows the user to set up groundwater boundary conditions for their model, and then carry out a finite element based groundwater analysis.

The results of the Finite Element Analysis (pore pressure) can then be directly utilized by the slope stability analysis. For more information, see the Groundwater Overview topic.

Tolerance

The Tolerance represents the convergence criterion for the finite element groundwater analysis. The solution is considered acceptable, when the difference in hydraulic head between iterations, at all nodes of the mesh, is less than the Tolerance.

Maximum Number of Iterations

The Maximum Number of Iterations allowed for the finite element groundwater analysis. If the Tolerance is not achieved within the Maximum Number of Iterations, then the analysis will be terminated at the Maximum Number of Iterations.

Convergence History

If this checkbox is selected, the iteration convergence history for the finite element analysis, will be saved to a file, and can be viewed in the Slide Interpret program with the Convergence Plot option. If this checkbox is not selected, the convergence history will not be saved.

Allow Ru with Water Surfaces or Grids

If this checkbox is selected, this allows you to use the Ru method of pore pressure calculation, in conjunction with either Water Surfaces or Water Pressure grids. The desired method can be specified for each material in the Define Material Properties dialog.

For each material, only one method of pore pressure calculation can be used. For example, if you specify a Water Surface for a material (e.g. Water Surface = Water Table), then an Ru value cannot be used for that material. If you are using an Ru value for a material, then you cannot specify a Water Surface for that material (i.e. Water Surface = None). You cannot simultaneously use more than one method of (initial) pore pressure calculation, for a single material.

Excess Pore Pressure

If you wish to model the effect of excess pore pressure resulting from undrained loading, then select the "Calculate Excess Pore Pressure (B-bar method)" checkbox. The B-bar method allows you to account for short term (transient) changes in pore pressure due to rapidly applied loading conditions. These loading conditions may include:

When you select this option, you will be able to define B-bar coefficients for materials in the Define Material Properties dialog. Excess pore pressure is equal to the B-bar coefficient multiplied by the change in vertical stress. The change in vertical stress can be due to any of the above loading conditions. See the Excess Pore Pressure topic for more information.

Rapid Drawdown Analysis

If you select the "Rapid Drawdown Analysis" checkbox, this allows you to simulate the rapid drawdown of ponded water (e.g. earth dams), using the B-bar method. The weight of existing ponded water is used to determine the (negative) change in pore pressure for materials with a B-bar coefficient. The ponded water is then automatically removed from the model when the stability calculations are carried out.

By default, the Rapid Drawdown analysis in Slide represents a complete drawdown scenario, and all ponded water is removed for the stability analysis. A partial drawdown state can be modeled, by defining additional Piezometric lines, if necessary, to define pore pressure, and using distributed loads to simulate the remaining level of ponded water. For a tutorial which demonstrates this option, see the Slide Tutorials page.

Pore Fluid Unit Weight

The default value of the Pore Fluid Unit Weight is determined by the Units of Measurement selected in Project Settings ( 9.81 KN/m3 for metric units, 62.4 lbs/ft3 for Imperial Units). However, the user may change this value, if necessary.

The calculated pore pressure at any given point, is directly proportional to the Pore Fluid Unit Weight, if the Groundwater Method is set to:

The Pore Fluid Unit Weight does NOT enter into the pore pressure calculations, if the Groundwater Method is set to:

The Pore Fluid Unit Weight also determines:

This is always true, regardless of which Groundwater Method is being used.