# Groundwater

This page contains the answer to commonly asked groundwater-related tech support questions.

**How should I model discharge sections in RS2?**

The discharge calculated for discharge sections is calculated for all elements that are intercepted by the discharge section. The discharge section must go across an element for the discharge value to be calculated correctly. Discharge sections that are put on the boundaries are inaccurate.

Why are pore pressures above the groundwater table negative in a groundwater analysis, and how is this affecting the analysis?

The Interpreter plots total stress minus pore pressure for effective stress, regardless of whether the point is above or below the phreatic surface.

If you use the Negative Pore Pressure Cutoff in the Project Settings, this cutoff will be applied to all materials. When calculating the effective stress the cutoff will limit the maximum suction for all materials to that value.

__Example__

If the Maximum negative pore pressure is set to 20 kPa in the Project Settings, then the maximum suction for all materials is 20 kPa.

*Global Negative Pore Pressure Cutoff in Project Settings*

For Mohr-Coulomb materials you are able to define Phi_b and Air Entry Values. When the suction is less than the Air Entry Value, the calculation of effective stress proceeds as usual â€“ the suction will affect the effective stress as if itâ€™s a saturated media. When the suction is more than the Air Entry Value, the corrections are applied such that the suction beyond the Air Entry Values is cutoff from the effective stress calculation for that particular material. However, for values of suction between the Air Entry Value and the global Maximum Negative Pore Pressure, the effect of Phi_b will be added to the strength.

__Example__

Phi_b = 10

Air Entry = 5 kPa

Maximum Negative Pore Pressure = 20 kPa

Suction < 5 kPa: suction affects the effective stress

5 kPa < Suction < 20kPa: effective stress is modified by Phi_b

Suction > 20kPa: suction effect is capped at the effect of Phi_b for 20 kPa

*Material-specific suction effect*

When applying pressure on a composite liner, how is the pressure applied?

In **RS2**, users have the option to apply additional pressure inside a joint, and there is also the option to apply the pressure to the liner side only.

*Apply Pressure to Liner Side Only feature: Define Joint Properties dialog in RS2*

Consider the case of a model having a primary liner, a secondary liner, and a joint.

If the joint is between the rock and the primary liner, and the Apply Pressure to Liner Side Only option is checked on in the Joint Properties dialog, the pressure is applied to both the primary and secondary liner and not the rock.

If Apply Pressure to Liner Side Only is not checked, then the pressure is applied to both the liners and the rock. Since the liners use beam elements, and the beam elements (primary and secondary beam elements) share nodes on one side of the joint element, the pressure gets applied to both. The beam elements are really two dimensional with no thickness except in the formulation. There are numerous resources available with regards to Timoshenko beam elements.

*Joint between rock and primary liner: Pressure will be applied to both liners*

If the joint is between the primary liner and the secondary liner, and the Apply Pressure to Liner Side Only option is checked, then the pressure is only applied to the secondary liner.

*Joint between primary and secondary liners: Pressure will be applied to Liner 2 only*

**How can I model an impermeable diaphragm wall?**

There are two ways to do this in **RS2**:

- with a low permeability material
- with a structural interface with an impermeable joint.

For the first method, you must use a low permeability material next to the sheet pile wall. In the Anchored Sheet Pile Wall tutorial the sheet pile permeability is set to 1e-20, which is a very low value. Remember that we cannot choose 0, since this will lead to numerical instability.

*Cofferdam model with impermeable diaphragm wall*

For the second method, you need to ensure that the Permeable option is not selected in the joint used in the structural interface. This will ensure a "no-flow" groundwater boundary condition across the joint. Tutorial 39 illustrates this method.

*Define Joint Properties dialog*

When using a piezo line or pore pressure grid, is the unit weight of water accounted for when the load on an element is calculated?

Piezo lines (and pore pressure grids) only affect the pore pressure. They do not increase the weight of the elements that lie below them due to saturation and they do not add weight if they lie above the surface of the model (ponded water).

The weight of the water must be added separately. This is done by defining a saturated unit weight and by using ponded water loads when the phreatic surface is above ground level. See the Add Ponded Water Load help topic for additional information.

Are stress and groundwater analyses separate in RS2 or is the effect of water on field stress included?

**RS2** has two analysis options for Solid-Fluid Interaction:

- Uncoupled
- Coupled (Biot Theory)

The following is taken from the Online Help:

__Uncoupled Analysis__

The default analysis is Uncoupled (i.e. changes in pore pressure do not affect deformation and changes in loading or deformation do not affect pore pressure). This is the case for a total stress analysis (i.e. deformation is a result of changes in total stress).

If the Use Effective Stress Analysis check box is selected (on the Stress Analysis page of the Project Settings dialog), then deformation is a result of changes in effective stress. This is a partially coupled analysis where changes in pore pressure, and thus effective stress, affect deformation but changes in deformation or loading do not affect pore pressure.

__Coupled Analysis (Biot)__

For a fully Coupled analysis based on Biot theory, you must first enable Transient Groundwater in Project Settings. Then you can choose the Coupled Solid-Fluid analysis option. This is used for time-dependent consolidation analysis problems, in which changes in pore pressure can affect deformation and changes in loading or deformation can affect pore pressure.