I am replacing my well house this weekend. It's about 5-6' from the cabin. The cabin is built up on a gentle slope about 2' higher than the surrounding ground. So, when we get a heavy rain, my well house "holds" about 12" of water. It's never gotten high enough to enter the well pipe, but close. I think the problem is simply the construction of the well house, which is dug out of the slope and has no floor. It is simply a dirt floor. None of the ground around the well house (the lawn) holds water after a heavy rain...it just runs off into the rest of the yard.
So, I'm thinking I can simply fill in the "hole" that's currently in the well house, keeping it sloped towards the rest of the lawn. Then, I'm going to build a new floor with 6"x6" green-treated posts. Considering I need to heat this in the winter, I'll then insulate the floor (with styrofoam insulation) over the 6x6 posts, along with the walls, etc.
My question is regarding the drainage. I'm leaning towards just filling it in with a compactable Class 5 type gravel from the local gravel pit. They describe it as what's used for roads. That way, any rain that does run under the floor will just run off into the lawn. My thinking is that if I fill in that hole, I won't be providing a collection area any more. I'm pretty sure it doesn't collect water for any other reason than the fact that there's currently a hole to collect water. I'd rather not make it more elaborate than needed. I considered putting in larger, drainable rock with PVC pipe to drain...but that seems like much more work than required.
A photo would help but it seems like you have the right idea. Drain the submerged floor the best that you can and fill it in with soil and gravel on top. Putting styrofoam on the floor will probably only float when it's wet.
Porky Cutter, MGWC
(Master Ground Water Consultant)