When modelling semi-permeable or impermeable structures like retaining walls or tunnel linings, the permeability of such a structure is not controlled by the structure itself, but by the interfaces surrounding it. These interfaces, when active in flow, can be defined as fully permeable, impermeable, or semi-permeable. This article shows situations and considerations when modelling semi-permeable structures like retaining walls or tunnels using semi-permeable interfaces.

Practical case: retaining wall

In practical cases, one often has interface elements at both sides of a wall: for example, in the case of an excavation where the wall is, to some extent, permeable. When using the semi-permeable interfaces at both sides of the retaining wall, the hydraulic resistance for both interfaces will be taken into account (i.e. summed).

In such a case it is suggested to:

• assign the correct interface permeability to the ‘outside’ (soil side) of the wall
• whereas the interface at the ‘inside’ (the side that is excavated) is made fully permeable, even along the embedded part of the wall below the excavation.

This will ensure that the retaining structure's hydraulic resistance will not change during (multiple) excavation stages.

This can be done in two ways: a specific material dataset or deactivating the interface for flow.

Option 1: specific material dataset

One way to set this "inside" interface to fully permeable is by defining a specific material dataset and setting the Cross permeability of the interface to Fully permeable:

Figure 1. Setting Cross permeability to Fully permeable

To create this specific dataset:

1. you can make a copy of the "outside" interface material dataset (or the one from the adjacent soil);
2. and then only change the value for the cross permeability in this copy of the material dataset;
3. then, in your Staged Construction phases, you should apply this specific material dataset to the “inside” interfaces.

This implies you need to create a specific material dataset for each soil material in which the plate is embedded.

Option 2: flow conditions option: set inactive

When an interface is set to inactive for flow, it will act as if it is not there, and water can flow across this inactive interface without any resistance. Instead of setting a specific material dataset to the "inside" interface, we can now set the inside "Active in flow" state to inactive (uncheck the checkbox), while we keep the "outside" interface "Active in flow" checked.

Note that this setting is independent of the general active state for interfaces which are used for the deformation analysis:

Figure 2. PositiveInterface_1_1 set to Inactive for flow to behave as fully permeable while the interface state for deformation is set to Active.

We can also easily inspect this configuration visually: when we are in Flow conditions mode, we see the interfaces (Figure 3):

• Drawn in Orange when Active in flow is checked (interface is active for flow)
• Drawn in Grey when Active in flow is unchecked (interface is inactive for flow)

Figure 3. Outside interface (in this case Negative interface) set to active in flow,
and is drawn in orange and inside interface (in this case Positive interface) set to
inactive and is drawn in grey

This setting should then be explicitly checked for your phases.

Tunnel lining

Similarly for a semi-permeable tunnel lining: the correct interface permeability should be given to the interface on the outside of the tunnel lining, whereas the interface at the inside (which will be excavated) should be modelled as fully permeable.

Both approaches mentioned above can be applied here, too.