WUFI

Hi everyone,

What would your approach be to model a basement wall with groundwater? Would it be to define a soil layer with a moisture source? Or define 100% RH on the outer surface of the assembly?

Additionally, is there a way to define temperature and/or RH of the air in an air change source?

Any help appreciated!

- Lucy

Hi Lucy,

the best approach would be to model a soil layer which has the initial water content set to free saturation and which includes a small moisture source which replaces any water taken up by the wall. In the simulation, the wall will absorb liquid water by capillary forces, it is assumed that the ground water itself is pressureless.

100 % RH at the surface would only correspond to saturated air, not liquid water which could be absorbed by capillary forces (unless you deliberately set up conditions where condensation occurs on the surface, but that would be a very roundabout method).

An air change source uses the temperature and RH specified by the boundary conditions with which the air change source has been connected. Beyond the choice of appropriate boundary conditions there is no way to modify the temperature and RH.

Regards,
Thomas

Thanks Thomas!

As this moisture transfers through a material adjacent to the soil layer and into the assembly, am I able to model a drainage sheet to remove the moisture, perhaps by creating a moisture sink that corresponds to the flow capacity of the drainage sheet?

I am aware I may be able to just reduce the amount of groundwater in the soil but I'm curious if I can model the drainage plane or not.

-Lucy

If the drainage layer is in standing ground water, it will be filled with water and as far as capillary water absorption is concerned, the situation is the same as with the saturated soil. (An alternative approach to model the saturated soil or the water-filled cavity would be to omit the soil or the cavity and to supply water available for capillary uptake by applying "rain" in a quantitiy which is larger than what can possibly be taken up by capillary absorption, see below).

If the drainage layer always remains dry (i.e. without any liquid water content), the soil need not be modelled and it is sufficient to model the wall surface as the "outer" surface, with climatic conditions applied which describe the air in the drainage layer.

But it seems you wish to describe a "partially successful" drainage layer which succeeds in suppressing direct capillary contact between the wall and the saturated soil, but which still admits a certain leakage rate of liquid water for absorption by the wall. Thinking in terms of flow rates in the ground water will not work for this purpose, because the rate of liquid absorption by the wall is only determined by the amount of available liquid and by the capillary forces in the pores of the soil and the wall. It is independent of any flow rates within the ground water.

If you want to model a certain "leakage rate" through the drainage layer, you can omit both the soil and the drainage cavity and add a fictitious "water-releasing material" to the surface which contains a moisture source which releases water at the desired rate (the other material properties for this fictitious auxiliary material may be modeled after soil properties).

An alternative might be to omit this fictitious material as well, and to expose the wall surface to "rain" with the desired rate. This fictitious "rain" results in water which is available for capillary absorption, but which is only supplied at a certain rate: If the wall cannot absorb water at this rate, the absorption rate of the wall is the limiting factor for the water absorption. If the wall could absorb water at a higher rate than what is available, the available water rate is the limiting factor. This may be what you inted to model. See the topic "Reference | Climate Data | Rain" in WUFI's help file for a description of the rain absorption.

Regards,
Thomas

Hi Thomas,

Thank you for your reply.

This is the build up I have currently defined in WUFI:
Soil (saturated with small moisture source to replace moisture taken by the wall), a layer of site concrete, a drainage layer made up of an air layer and an impermeable HDPE sheet, a structural concrete wall, a ventilated air cavity, and an interior cavity wall of various materials.

I had defined a moisture sink at the drainage layer (in the air layer) to remove the moisture that transfers through the site concrete from the soil layer. Would I be able to do this, or would it be too difficult to predict how much water is being drained away?

- Lucy

lucyadams wrote: ↑Sun Nov 17, 2019 5:08 pm -1100 This is the build up I have currently defined in WUFI:
Soil (saturated with small moisture source to replace moisture taken by the wall), a layer of site concrete, a drainage layer made up of an air layer and an impermeable HDPE sheet, a structural concrete wall, a ventilated air cavity, and an interior cavity wall of various materials.

I had defined a moisture sink at the drainage layer (in the air layer) to remove the moisture that transfers through the site concrete from the soil layer. Would I be able to do this, or would it be too difficult to predict how much water is being drained away?

Why do you need to remove moisture from the air layer? The air is not capillary-active and therefore no liquid transport is possible across the air layer, both in reality and in the WUFI model. No liquid water can intrude into the air layer by liquid transport, so there is no need to actively remove any liquid water. The capillary-breaking effect of the drainage layer (which breaks the capillary contact between the soil and the structural wall) is automatically modelled by the fact that the air layer has no liquid transport properties. The additional real-world ability of a drainage layer to drain away water from other sources (such as rain) is irrelevant in the one-dimensional WUFI world where no other sources of liquid water exist which could add liquid water to the drainage layer.

Are you concerned about vapor transport across the air layer? About possible condensation on the surfaces of the drainage layer?

Regards,
Thomas