Foundation below water table
Foundation below water table
(OP)
I'm designing a residential foundation that is to be about 5' below water table. It's going in a hole that has been blasted out of ledge, but the hole has filled up with groundwater. Does anyone have any good references for the design of the floor slab for uplift?





RE: Foundation below water table
RE: Foundation below water table
RE: Foundation below water table
RE: Foundation below water table
I'd do this in addition to any drainage and pumpage schemes, since they can fail when weather is bad and water is plentiful.
then, comes the big question. What does your building weigh? If it does not have sufficient weight, when empty to be well in excess of that uplift on the bottom, the buiding will float. I've seen this happen with swimming pools when empty.
You may try some "anchorage" by using the backfill to the sides of the basement, but with a big safety factor and submerged unit weights also. That resistance might not be there on some occasions in the future.
That is why the recommendation to install extra uplift resistance with rock bolts into rock below the slab. The "ledge" rock that they are in should be assumed to be submerged also, with the submerged unit weight as doing the resistance to uplift. With many such anchors, your slab will not hve to be so strong in bending resistance.
Your structural question for slab design is easly answered, but more importantly will the building float and what are you doing to resist the floatation?
RE: Foundation below water table
With or without subdrainage, the base mat should be designed for the full uplift. You can treat the mat as upside down floor slab and design it accordingly, except watch out for reduced dead load combination required by code for using strength design method.
RE: Foundation below water table
RE: Foundation below water table
RE: Foundation below water table
RE: Foundation below water table
Thanks.
RE: Foundation below water table
Look into them as means for an anchoring system. As per your drainage, OldestGuy answered better than i could.
But the mini piles are small, can be any length, can be drilled/driven easily and anchored into a foundation. It should be suitable for any uplift.
if you have any site condition pictures, id love to see.
Joe
Project Engineer