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basement slab thickness

basement slab thickness

basement slab thickness

I'm noy familiar with basement construction to begin with; however, I'm asked to do a quick costing on basement carpark slab. What is the minimum thickness if the ground water table is above the slab? I'm using grade 30 concrete.  
Any site I can browse for such information?
What sort of hydrostatic load from the bottom of the slab is expected under this circumstance?
Is subsoil pipe necessary under the slab to relieve the upheaving pressure?


RE: basement slab thickness

In the circumnstances you mention I wouldn't propose nor for bottom slabs nor for wall less than 35 cm thickness. Ideally one should externally damp-proof the whole vessel, but this may not be in the reach of every owner, that then so must be made officially aware of the risks, in written.

In any case use the stricter measures for crack control (max mechanically caused crack width 0.1 mm) is recommendable, and also to get the more impermeable concrete available, what may mean using special concretes with mass impermeabilllizers, or important quantities of cementitious materials, significant part of them fly ash to help pore oclusion.

Rubber stops may be required, or alternative sealing of joints with special damp-proofing mortars.

RE: basement slab thickness

The groundwater table above the slab causes several problems. It's hardly feasible to install piping under the slab, as it will be constantly submerged and the water will have to be pumped out faster than it will recharge itself.  The hydrostatic pressure is dependent upon the difference in elevation of the groundwater level to the slab level. Bathtub theory tells us that the pressure is equal to the displaced water.  With required uplift resistance factor of safety between 1.25 and 1.50, you'll need approximately 1 foot of concrete for every 2 feet of water differential. And you'll need to control the water from seeping up through the slab and through the joints.  Better a boat builder than a slab designer.

RE: basement slab thickness

thanks for the response.
More questions, supposed the ground water is 10 ft above the basement slab, does that mean the ground water pressure act on the bottom of the slab is 10ft by water density by factor of safety?
f of s X water density x elevationtion difference = pressure acted on the slab from the ground?
if so, what foundation system is appropriate?
I was thinking of the conventional one way joist-slab system with pile cap at column point; however, I found out that I need a beam of 5ft deep by a foot width main beam since the tributary area is about 640 sq ft between 2 columns(27ft c/c on main beams, floor joist of 27ft to the main beam, the grid is a 27 ft by 27 ft main beams with a 27 ft joists in the middle of the grid).
I thought of adding intermediate pilecaps to reduce the bending & shear of the main beams but that would mean tension to the intermediate the plie caps.   

RE: basement slab thickness

If you can, equilibrate the loads with the walls plus bottom mat. If subpressure (say 3500 kgf/m2) can't be counteracted this way you go for piles in tension. These you need as well if some upwards deflection is excessive for use. I normally wouldn't count friction, but you can make a mat wider than the plan and whatever earths above the structurally projected cantilever you can count...but better submerged weight -since submerged- or about 1 metric tonne/m3. Further than the actual plant I wouldn't count anything equilibrating, but some do, as well as counting friction. I don't.

Respect the safety factor if not enforced by code, can be minimum, since the flotation force from water is very exactly known. Then 1.2 may be enough, but ensure you count all the volume that can be submerged to generate flotation.

The tension piles need however be dimensioned with about the usual safety factors to give the allowable tension atop. I usually wouldn't consider a safety factor under 2 for the piles in tension except if an in situ pull out test has give me direct assessment of what to expect.

RE: basement slab thickness

Your on the right track with the one-way slab and tension pile method.  Realize that resisting 10 ft of water is not a typical problem, so don't assume typical framing members.  If you aren't happy with the sizes of beams, then adjust pile spacings. I would first estimate the pile tension capacity, and try to optimize the pile spacings (this will be the most expensive component of the job). Then frame between the piles and add the slab.
You can also use the dead load of the superstructure to resist the uplift.

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