I am currently designing a 300 sq ft cob cottage. I am wondering about building a rubble trench, my concern lies around how wet the soil is. The whole property is clay soil, with marshy plants. The site I am looking at os sloped facing south east. It is the rainy/wet season and when I dug test holes (4 feetish) they fill with water almost 2 thirds of the way and don't drain at all. Basically I am wondering if a rubble trench as well as french drains along the roof overhang will be enough to make moisture less of am issue. I am hoping to have an earthen floor on a bed of gravel a foot or so thick, wondering of the moisture would come up. Basically I am wondering if its a bad idea to build here and if i am just going to waste my time.
It shouldn't be a terrible problem, as long as you address the water-table issues.
Planning a French drain at the roof overhang might not be enough, and you have to think about where the water is then going to run off too.
Already saturated clay generally won't take extra water, not fast enough for draining purpose anyways. Generally there's a weeper tile drain in a rubble trench, and adding the French drain includes run-off, this has to go somewhere. If there's no county/town/village supplied ditch because your property isn't right along the road, this generally means your going to have to dig a drainage ditch somewhere near the house or along the property, and run the weeper drainage into that from your house site. Then that still has to run somewhere too.
So, take another look at your site where your planning on putting your house, and at your slope.
How steep is your slope and if things are that wet, is there going to be a want of a retaining wall in the future? You might want to rethink the placement of the house.
Digging drainage ditches around the property, or at least around your homesite, should work on diverting the water away from the house itself. But then you have to consider where those ditches are draining -too- as well, and that depends on how close you are to a road system, or if your basically off-grid and can get away with.
But, there's professionals on here who'll get to you eventually, so don't take my word on anything. Ask your neighbours if you can how they've dealt with it, someone's going to have wanted a basement or at least a half-basement crawlspace and run into it in your area.
My parents built their 'new' home by hand, as we tend to do around where they live, on a damned -bog- because it was the cheapest land they could afford that the government would clear. It's basically shaped like a standard acre, a long lot. Even with the village ditch out front, the water table was stupidly high. Dad had to dig a ditch along both sides of the property(within 30 feet of the house), and across the back of the house(not the back line of the property because it went to far back about 50ft back), that then drained into the village supplied ditch in front of the house, it's only now 20 years later, that the ditches are running somewhat dry, and that's more to do with the alder bushes clogging them and the main bog out back of the house running dry.
As for your earth-floor, if you address enough drainage, it should stay dry enough, but some areas have issues where the clay smells, and it works it's way up thru the floor(it's horrible around here. The clay is a blue/grey beachy clay and it -stinks-). So you might want to think about testing it(I think the easiest way is just filling a clean bucket half full, putting a lid on it and letting it sit for a few hours, taking off the lid and smelling the bucket - it might smell like rot, wet dirt or hopefully, nothing at all). Either way, your going to want to insulate it somehow.
Not growing or raising anything at the moment, but I'm here doing research for the future.
I agree with Alice's advice. Saturated clay soils can be tricky to build on, and you need to drain as much as possible as deep as possible to protect your foundation. Some clay may be particularly expansive (shrinks and swells a lot as it gets drier or wetter), and this would be disastrous to build a cob house on. This is a case where I would heed the professional engineer's advice and get a soil test to determine the characteristics of your base.
Beside that, we need some details to advise further. What is the difference in height from the low edge of your proposed house site to the upper edge? What is the difference to the lowest spot nearby, and how far away?
I think you may need to build up the ground more than dig down. Remove the topsoil, but mostly add rocks and gravel, starting with big material at the bottom. A seriously wet location probably needs a couple of feet of stone with plenty of airspaces to prevent moisture from wicking up, and allow vapor to escape sideways.