# Advanced Groundwater

On the **Groundwater** page of the Project Settings dialog, the following advanced groundwater options can be enabled:

- Excess Pore Pressure
- Rapid Drawdown

To enable one of the advanced groundwater options, select the **Advanced** checkbox and choose an option. Only ONE option can be selected, it is not possible to run more than one advanced groundwater option simultaneously. The advanced groundwater options are summarized below.

## Excess Pore Pressure

The **Excess Pore Pressure** option allows you to use the B-bar method to calculate excess pore pressure due to undrained loading. The B-bar method allows you to account for sudden increases in pore pressure due to rapidly applied loading conditions. These loading conditions may include:

- added material weight
- vertical seismic loading
- vertical external loading

When you select the Excess Pore Pressure option, you will be able to define B-bar coefficients for materials in the 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.

NOTE: with **Excess Pore Pressure** analysis you may choose any Groundwater Method to define the initial pore pressure distribution (e.g. Water Surfaces, Ru, Water Grid).

## Rapid Drawdown Analysis

The **Rapid Drawdown** option allows you to analyze the stability of earth dams under rapid drawdown conditions, using one of four methods:

- Duncan, Wright, Wong, 3 stage (1990)
- Army Corps of Engineers, 2 stage (1970)
- Lowe and Karafiath (1960)
- Effective Stress using B-bar

For all methods, an initial water level is specified by defining a Water Table. A second water table (Drawdown Surface) defining the drawdown level can also be specified, or you can assume complete drawdown. The pore pressure is then calculated according to the drawdown method, and the safety factor for rapid drawdown conditions is calculated. For a tutorial which demonstrates these options, see the *Slide3* Tutorials page.

NOTE: with **Rapid Drawdown** analysis you must choose Groundwater Method = Water Surfaces, because the initial and final water elevation must be defined using a Water Table and an (optional) Drawdown Surface. You cannot use the Rapid Drawdown methods with any other Groundwater Method in *Slide3* (e.g. Ru Coefficient, Water Grid).

## Undrained Strength Interpolation Scheme

If you are using the **Rapid Drawdown** option, and you are using either the **Duncan, Wright, Wong, 3 stage (1990)** method or the **Lowe and Karafiath (1960)** method, then you can choose one of the following Strength Interpolation methods:

- Duncan, Wright, Wong (1990)
- VandenBerge, Wright (2016)

Linear interpolation between isotropically consolidated undrained strength envelopes and fully drained (effective stress) strength envelopes is commonly used to calculate undrained strengths for slope stability analyses of rapid drawdown. The Duncan, Wright Wong (1990) linear interpolation method presents numerical problems at low effective normal stresses with soils that possess an effective stress cohesion intercept. The resulting undrained shear strength envelopes are unrealistic at low normal stresses. The VandenBerge, Wright (2016) interpolation method interpolates a single undrained strength envelope for the entire slip surface. The improved interpolation method produces identical results to the 1990 method for cases where c' equals zero.