Parking Garage Slab - Hydrostatic Pressures

[cvanoverbeke]

I understand the physics of concrete slab uplift from installing it below the water table. It is the rho g h concept. In the case we are looking at the geotechnical report indicates that the parking garage should be installed at 321.7 metres above sea level. The water table is 0.5 metres below this level there 321.2 m ASL. The top of the parking garage floor is at 317.5 m ASL. Preliminary calculations give a load of over 1000 lb/sq.ft. This is quite high.

Can anyone provide example calculations for the uplift pressue on this slab?

Can anyone provide examples of alternative construction technologies besides making the slab 3 to 4 feet thick to resist the uplift pressue. We have considered permannet dewatering but the risk of not getting regulatory approval is too high to consider this option.

Regards,
Cvanoverbeke

 

[hokie66]

Well, you know how to calculate the hydrostatic uplift, although I don't know why you mix your units. I would either stick with metric (m and kPa) or English customary (ft and lb/ft2). For each foot above the slab soffit, the uplift is 62.4 lb/ft2, and for each metre, it is 10 kPa.

The uplift forces can be resisted in 3 ways:

1) Mass of basement slab acts as ballast, taking advantage of concrete density being about 2.4 x water density.

2) Mass of entire building acting as ballast, by designing the basement slab to span between the columns (think of it as an upside down floor)

3) Similar to (2), except with part of the uplift force resisted by piles or rock anchors in tension.

Note that for methods 2 and 3, you must monitor the ground water to assure that the uplift force is limited during construction.

 

[oldestguy]

Sounds like this had better be looked at as a concrete but heavy enuff to sink.

Why not look at drainage under the slab to remove that uplift and all the grief of resisting seepage thru cracks or joints?

Sure there may be sump pumping costs, but is that water table always there?

This form of flotation resistance also can be provided by anchorage, as with rock anchors. If H-piles are used, be sure to consider only the submerged unit weights of the soil below. A rough way to figure uplift resistance from an H-pile would be to use the soil within an inverted cone surrounding the pile, with side angle from vertical no more than 20 degrees. Your geotech should be in on these calcs.

 

[civilperson]

Weep holes through the walls with filter geofabric and gravel/sand backfill and floor drains will negate the uplift problem. Use plenty, (300% of need), and instruct maintenance on frequency of cleaning,

 

[cvg]

in order for the weephole plan to work, you will need a continuous bed of drainage material under the slab and behind the walls up to the anticipated groundwater level so that water can migrate to the drains. Suggest 6 - 12" of sand or gravel. Pumps will then be required to remove the water accumulating in the basement and as previously indicated, a dewatering permit and monthly logs of the amount of water pumped may be required.

 

[hokie66]

I suggest all drainage solutions have the problems that drains can block, pumps can fail, etc. The most direct solution, making the building heavy enough or tied down well enough to resist the uplift, is the best way.

 

[Mickney]

Drainage will work, but how will you daylight the water? It may have to include a sump area with pumps instead of directing the water to a storm drain system. Depending on the amount of water, this could be a pretty large system that would also require a back-up plan incase of a power outage. Is the groundwater contaminated?

Also, I am envisioning a temporary well point system for dewatering the site for construction. This dewatering may impact the adjacent construction if the water table is lowered substantially beneath any adjacent structures.

 

[dicksewerrat]

Raise the building. Or live with water in the garage

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com