Hydrostatic Pressure From Ground/Flood Water

[cvanoverbeke]

We are designing a one level undergound parking garage under a 6 storey condo. The building footprint is smaller than the parking garage therefore the building loads are transferred by point loads to pad footings.

A major issue for us is the hydrostatic pressues from a high ground water table. The stabilized ground water level gives of approximatley 12 feet of lateral pressure from ground water. Since the parking garage walls are only supporting about 3 feet of soil the vertical loads are not as large as the lateral loads.

A major other issue is the lateral pressures from flood waters since the garage will be under the flood plain.

We are looking at using a caisson wall shoring system in order to hold back the ground water.

QUESTIONS:

1) Since we have to design for a 1 in 250 year flood event, would the opposing water forces ground counteracting against flood water reduce the thickness of the foundation wall i.e. net water pressue effect.

2) Based on a 12 foot head for ground water, if we DO not use caisson shoring to hold back the ground water pressue, how thick do you think a reinforced cast in place concrete foundation wall would be?

Any thoughts on this design case would be appreciated.

Regards,
Cvanoverbeke

 

[JedClampett]

I don't quite understnad your statement "Since the parking garage walls are only supporting about 3 feet of soil the vertical loads are not as large as the lateral loads." Does this mean the garage is buried and the walls carry a soil load?
But in general you have lots of issues with ground or flood water. If you talking about drilling caissons and bracing the walls against them, I've never done it, but I suppose it possible. In my industry, water and wastewater treatment, we deal with internal and external hydrostatic forces all the time. First of all, you need to make sure that the structure is watertight. If it's not, the water will leak in and all your forces cancel. Of course, all the cars in the garage are underwater. All construction joints must have PVC or rubber waterstops. A sump pump system is not a bad idea for incidental leakage.
Then you need to make sure the structure is stable. There must be enough anchorage (piles, soil nails, rock anchors, etc.) or mass to keep the structure from floating away. You can use the building weight, but don't be too liberal in assuming live loads, furniture or vehicles.
Then you can design the walls. If there's just hydrostatic (not submerged soil)lateral forces, 12 inch thick walls are probably OK. But a bigger problem is the local design of the slabs. They're seeing more total load upward than the walls.
This is going to be one expensive garage. You might want to start preparing your client for the bad news.

 

[cvg]

I'm not sure why you would design the basement to keep the 250 year flood out. let it in and reduce your structural section thickness. sure, you will damage a few cars during the big flood, but they will be flooded even if they were parked outside.