Draining clay from underneath a floating slab.

I'm building a cordwood sauna (ala Rob Roy). In removing the topsoil, the ground i've exposed to build on is poor-draining clay. The surrounding grade in my building area is quite flat only slightly sloping in the northward direction. I'm thinking that I will fill clay into the area that I have exposed, creating a claypad on which to build a well-percolating sandpad. The claypad will be level with the topsoil around the excavated site. This will give me some elevation to install a drainage system that includes perforated drainpipe laid into the claypad from which the sauna's slab may also drain. Im hoping this will prevent any heaving in the future. The only concern with this is: will the sand pad between clay and the floating slab erode away over time? Do I need to dig into the clay to make a "bowl" to hold the sand pad in place?

I know you permies are smart!

,Kriev

I don't know but I know someone who knows. Hubby used to build bridges and parking decks for a living. I'll e-mail your question to him and get back to you.

Here is hubby's answer: 'You are better off using a 57 stone and crusher run mixture for drainage. It will pack as good as sand and will allow better drainage without wash out issues.'

He said he would be happy to answer anymore questions if you have them.

now... Having my sauna in the middle of a slight slope, I do expect some water to sneak under my foundation. I want to make sure this water will drain out. So I will slope my clay subpad so that water runs down the slope underneath my stonepad and slab into a perforated drain pipe. Thus draining away any foreseeable trouble.

I'll get hubby to look at your photos in the morning. He doesn't 'do' posting and forums but he is interested in following your project.

I'm going to essentsially "screed" the clay as smooth as possible before building up the pad for the foundation so any water freely flows into the trough i've dug.

I concur with Jeanine. #57 is a much better choice as a drainage layer. Sand can actually wick up moisture and will hold onto it readily when surrounded by wet clay. I would also suggest surrounding your drain pipe with 57 and wrapping the whole thing in silt fabric to protect it from the sand and clay which can clog drain pipes over time.

Kriev, sorry I took so long to get back with you. I just showed my husband your pictures. He said it looks good, liked the drain pipe, and he thinks it is a good plan so far.

I forgot to add a permies alternative! Perhaps the sandbed could work better with your drainage pipe and a 57 substitute like crushed brick, stone or concrete. Anything that packs tight, preventing settling and still creates trapped air pockets for the water to flow through drained to daylight. Still recommend a silt fabric to prevent it from clogging over time.