Simulating constructions directly in contact with the ground

[Sami Roikonen]

Hello!

I was wondering if there's a good, established way of simulating the ground and constructions connected to it, particularly floor slabs.
I'm currently trying to simulate a in-situ concrete slab and see how it behaves with different coating materials.

My structure is as follows: 200 mm 8-16 mm gravel to stop capillary water, 150 mm XPS insulation, 80 mm concrete slab and varying coating materials such as linoleum, ceramic tiles, etc...

At first I solved this by completely cutting the gravel off since I found the humidity bit problematic. Then I added some insulation to compensate for the thermal conductivity of gravel, and set a constant RH 100 boundary condition to the edge of the insulation.

The results were nothing surprising, but I had my doubts about my model. So I thought I should try to simulate it with the gravel layer, but I'm not sure how to.

The thermal side of the gravel shouldn't be a problem. I could use some thermal conductivity between that of stone and air depending on the size of the air pockets between the pieces of gravel. But how should I deal with the water movement? A part of the humidity will be in the air space between the pieces of gravel and some will be in the gravel itself. Humidity will also move differently in the air than in the gravel.

How should I proceed with the moisture in the gravel layer?