Me and the other partners here at Greenshire plan to build a greenhouse, sticking to Mike's design as closely as possible. I have his book and have read through it but didn't find answers to my questions.
Our problem is that we have a high water table. The soil is about 3-4 feet of sandy loam, and then it hits a chalky, gravel/clay mix that doesn't drain well. Mike Oehler's design calls for the design to be 4 feet below ground at the front, and 2 feet below ground level at the growing area. I should mention that the site for the greenhouse is on almost completely flat ground. Late last summer we dug a test pit 4 feet down and next spring thaw water filled about 3 feet deep at worst, levelled off to about 2 feet deep during mid-spring and then disappeared when the heat of summer came on. This excessive accumulation of moisture could have purely been from the pit being exposed to the elements, and could have been mostly melted snow which wouldn't enter the equation if there's a greenhouse overtop. But it also could have been made up of significant amounts of ground water. I'm not sure.
If my worst fears came true and I did get standing water in the area inside the greenhouse, wouldn't it all just collect in the deepest area at the front walk in space, leaving the growing area saturated but still suitable for plant growth? I don't mind having to wear waders . Would this amount of water compromise the structural integrity, even if we followed Mikes instructions to a tee?
In an effort to avoid standing water...Would I lose too much R value if I raised everything up by 2 feet, so that the front walk-in section is 2 feet below ground level, and the growing bed is at ground level? I would shore soil up around the structure over-top of the existing ground level but I'm worried that wouldn't be enough to match the heat value of the 'by the book' depth.
Just a little experience with earthbermed structures. I had a passive solargreenhouse at our last place, but it was not earthbermed. I have read Mike's book and like most books, I tend to take what works or what I think is best from several referances to adapt to each use. We have a fairly flat property with problems of water not soaking into the ground quickly this time of year. With that in mind I am a bit leary of going too deep in the ground. We do have a full basement under our house and it stays pretty dry if the down spouts are drained away from the house. I have yet to dig deep enough to find the water table, but the deepest I have dug is only 30" for fence posts. I would guess that where I want to put our greenhouse that the water table is fairly deep if I guage that by the water level in our well. Some of the things that I want to add to the green house is to heat the bed with a RMH and on the other side of the north wall I will put a root cellar. The north wall will be earthen, either rammed tires or gabions filled with earth. on the other side of this will be a ferro cement vault that is insulated with straw bales. I want to duct the air from the cold sink into part of the root cellar, it will have two parts at different temps. Because of how flat the ground is at the greenhouse site I am leary of drainage problems in the cold sink and the access to it and do not have anyplace to drain it to that is close. I need to just spend a day to dig a hole at that location to see what drainage is like, I might be stressing over nothing. You might want to do some searching on green houses, different needs, and situations can make different options better. Some places I like to look at for ideas are: builditsolar.com flyingconcrete.com the earthship books and straw bale building sites. I saw a decent straw bale greenhouse somewhere recently could have been in backwoods home magazine that just came.
Wading in a pit filled with meltwater, even with rubber boots, is going to freeze your feet. It's enough to keep you from using the greenhouse until the water level drops. If you are tough enough to take it, standing water in a trench will erode the sides, filling in the trench. You'd have to dig it out perhaps every year.
You could go with an earthberm: raise the land in the greenhouse area, build on the manmade hill.
You could build on flat ground, but have a raised floor. Allow the cold air to gather under the floor. 10' to 12' lumber for the walls or just a stem wall a few feet high. With this plan you have easy access to the entire structure during construction. Pile up soil around the wall when you are done for some added insulation.
Seed the Mind, Harvest Ideas.
I dug two more test holes in the spot where the greenhouse is set to be; one that is 1 foot deep, and another thats about 2 feet deep. The 1 foot hole stays drained, and the 2 foot hole is about half filled with water. So I think we're settled on the cold sinks bottom being 1 foot below the current ground level, with a false floor thats at ground level, and having a hugelkultur bed that'll be built to end up with a height about 4 feet higher than the current ground level. Then we'll just shore up soil as thick as possible around the structure. We have extra soil from constructing a round pen that should suffice.
The greenhouse has been built and is planted, though only the front of the greenhouse is buried in soil. The back and sides are insulated with hay bales and yard waste bags full of hay, all wrapped up with reused white greenhouse plastic. I plan on posting more of a full update with video and/or pictures in the near future.