How to seal water out from an underground structure

so if I dig a hole in the ground lets say 9ft deep and I put a pollytunnel on top what would I need to do to seal up the hole from water leaking in from the dirt? I was thinking about taking pond liner and lining the hole and then along the pond liner just build cob walls and a cob floor. Would having the pollytunnel on the top of the hole be enough to keep the soil below dry? Would I need to install some sort of sump pump for when it rains?

Thanks for your help!

Water is a lazy substance... it allways follows the path of least resistance... invite her to follow a diferent path.

Make trenches all around your greenhouse on the underground floor and don't forget the drainer(!) and fill them with gravel. Your plastic roof should end on some sort of drainer too in such way to drive away all water.

That's what i would done in that scenario

John is right. Lazy and persistent. If you dont have enough slope to drain the lazy water to daylight at roughly 1/4" per foot, you will want a sump pump. Or.. you could build a sump pit and be ready with a bucket after some heavy rain.

@N Harris , The answers above were on target about water being a very slow enemy , but digging a hole in the ground to build a home in, will almost ensure you have, a moisture problem. I know this goes against the wofati ideas but ; consider building the structure above the ground (on high ground) starting first, with a water tight insulated floor system, extending the water protection materials all the way up to the roof. Once the building is completed, gently bring in the earth, both to the outside & the inside of your foam insulation, gently compact & berm the building's walls with an adequate protection of soil ( 5+ feet ). Then, if you have chosen your location correctly (on high ground) water should drain away from you and also not be able to rise into the structure, (thus, no sump pump required). We talk about this subject also on our Co. blog page and on our facebook page, all are invited to join in there.

Hope that helps you a little :>)

Best Regards,
Joe Woodall, Ecoitect
Georgia Adobe Ecoitecture
http://georgiaadobe.com

wouldnt the walls cave in from the weight of the ground? what about a high water table? a few years ago there were basements in my area that had never flooded before that flooded because of a wet spring and a pretty high water table

The walls that caved in were probably not built to minimum building codes. Even below ground wood framed walls have been known to stay dry for 100 years in high water tables with the right details. Probably the most overlooked detail is providing a drainage layer (protected from silt) right against the wall. The walls drain to footing drains which drain to daylight or a sump pit. There are many ways to do this, many ways to get lucky and many more to mess up.

Weaker (below code) walls can be compensated for by extending the drainage layer further out from the wall which reduces hydrostatic pressure lateral force.

What I've seen is drain tile all around an earth bermed home just outside the exterior walls at the base, but I'd have a drop to the floor and inner drain tile with a sump pump just in case. Nothing I hate more than a wet basement so I definitely wouldn't want a flooded house.