Groundwater Method

The Groundwater page in the Project Settings dialog is used to configure the main groundwater parameters for your Slide2 model:

• Groundwater Method
• Pore Fluid Unit Weight
• Advanced Groundwater settings

The Groundwater Method and Pore Fluid Unit Weight are discussed below.

The Advanced groundwater options (transient, excess pore pressure, rapid drawdown) are described in the Advanced Groundwater topic.

Groundwater Method

The Groundwater Method determines how pore water pressure will be modelled. Six options are available:

• Water Surfaces
• Ru Coefficients
• Water Pressure Grid (Total Head)
• Water Pressure Grid (Pressure Head)
• Water Pressure Grid (Pore Pressure)
• Steady State FEA (Finite Element Analysis)

Only one Groundwater Method may be selected for a given analysis.

Water Surfaces

If the Groundwater Method = Water Surfaces, 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 Coefficient

If the Groundwater Method = Ru Coefficient then a Ru Coefficient can be specified for each soil type in the Define Material Properties dialog, in order to define pore pressure. The Ru coefficient simply models the pore pressure as a fraction of the vertical earth pressure for each slice. When the Groundwater Method = Ru Coefficient, then pore pressure can only be defined using Ru coefficients.

Water Pressure Grid

There are three options for defining a Water Pressure Grid in Slide2:

• Grid (Total Head)
• Grid (Pressure Head)
• Grid (Pore Pressure)

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

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 Slide2. See the Interpolation Method topic for details.

Steady State FEA (Finite Element Analysis)

If the Groundwater Method = Steady State FEA, then this will enable the Steady State Groundwater Analysis Mode in the Slide2 Model program. The Steady State Groundwater Analysis Mode allows you to set up groundwater boundary conditions and carry out a steady-state finite element groundwater seepage 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.

Steady State FEA Options

If the Groundwater Method = Steady State FEA, you can customize the finite element analysis parameters by selecting the FEA Options button in Project Settings. You will see another dialog which allows you to define the Tolerance and Maximum Iteration settings as described below.

Tolerance

The Tolerance represents the convergence criterion for the finite element groundwater analysis. The solution is considered acceptable when the difference in discharge velocity between iterations, at all nodes of the mesh, is less than the Tolerance. As a rule of thumb, the value of the Tolerance should be approximately equal to (or less than) the permeability (Ks) of the lowest permeability material in the model.

Scale Tolerance According to Time Units

On the General page of the Project Settings dialog you can choose Time Units and Permeability Units for a finite element groundwater analysis. If the Time Units are different from the Permeability time units (e.g. Time Units = Days, Permeability Units = meters/second), then the value of the Tolerance (entered in the Project Settings dialog) should be scaled accordingly so that the groundwater compute engine will utilize a suitable value of Tolerance. This scaling will be automatically applied if the Scale Tolerance According to Time Units checkbox is selected. By default, this checkbox is turned on.

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.

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:

• Water Surfaces
• Grid (Total Head)
• Grid (Pressure Head)
• Steady State FEA

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

• Grid (Pore Pressure) (pore pressure determined directly from the grid data)
• Ru Coefficient (pore pressure determined from vertical earth pressure)

The Pore Fluid Unit Weight also determines:

• The unit weight of ponded water defined by a Water Table. See the Add Water Table topic.
• The unit weight of water used to calculate the hydrostatic force in a water-filled or partially water-filled Tension Crack. See the Define Tension Crack topic.

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