Foundation below water table

[sfred11]

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?

 

[BigH]

Are you planning too to put the floor slab below groundwater level? Why? - is it a basement? If so and you insist on doing it, you'll need a good below floor drainage system, basement wall drainage system - postive outlet for the collected water or sumps, and a capacity in excess of the anticipated inflows (and with a good factor of safety on the flow volumes). As of this post, we do not know what soils you are going to be in. If clay, then you'll be able to handle the water volumes fairly readily - if sands or more permeable, you may have to design for larger flows - but for a residential foundation, think twice about putting the floor below the groundwater level.

 

[sfred11]

Thanks for the input. The site is entirely ledge. We had to blast to a depth of 25' to accomodate a basketball court in the basement (the client has more money than he knows what to d0 with). Upon encountering groundwater, we (myself, architect, geotech.) recommended against building below groundwater but the client insists. Drainage is being addressed, as well as waterproofing (Volclay panels). I'm more concerned about the design of the slab for possible uplift. I'm looking for any references (ACI, etc.) that would offer guidance or details. I'm assuming a thickened slab, not only for structural purposes but to reduce the liklihood of cracking), a water stop at all joints, grade beams, etc.

 

[civilperson]

I would recommend a continuous trench to daylight from the base of the excavation for positive drainage. If not feasable due to topography, then a trench to a sump pit with a dual pump for normal use and back-up.

 

[oldestguy]

Since you are in bedrock (ledge), why not install some anchorage into the rock to help against the uplift. Remember you are talking about a concrete boat. You certainly can design that lower slab to resist the uplift pressures. I would assume water table all the way to the adjacent ground elevation. Don't depend on the ground water leaking out at the lowest adjacent land elevation, but use the highest adjacent land elevation for future 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?

 

[kslee1000]

Oldesetguy is correct in the drainage system will fail. I have seen underground concrete water tank (emptied for maintenance)been pushed out of ground after a major storm with electricity outage, and the subdrain system was badly clogged.

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.

 

[sfred11]

Thanks for all of the input. I was actually trying to find out if anyone had any good references (CRSI, ACI, PCA publications) that would provide some typical design examples/details.

 

[miecz]

PCA - Rectangular Concrete Tanks is meant for liquid containing structures, and has examples. Your forces will just be opposite. You'll probably be better to follow ACI350 instead of ACI318 for the design. Only test I know of is by R.D. Anchor "Design of Liquid Retaining Concrete Structures. It is based on British Standards and uses metric units. Bouyancy may be an issue, so you'll have to decide on your safety factor against bouyancy. Good luck.

 

[miecz]

Yes, that's text, not test.

 

[Geotechnical76]

I am looking into a two analysis method for bearing capacity(spread footing). I used the two-layer method in Bowles book and it seems that it gives high bearing capacities with low sensitivity to the thickness of the top layer top layer. I understand that the effect of the bearing capacity influence extends 1B to 2B below the footing elevation where B is the width of the footing. Bowles' method can be used for a two layer bearing capacity problem. Has any one had experience with this in the past and if so, what is the best method to use. Also, what if one has more than two layers (do not want to average the layers below the footing).
Thanks.

 

[projectEST]

MINI-Piles

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