Designing basement floor of a multi-storey building

[GalileoG]

At the site in question, the water table is very shallow roughly 3 ft below grade. Our mechanical/civil fellows will be designing a weeping tile system, however I am wondering if it is standard practice for structural to design the entire basement floor (structural slab, not SOG) for the full hydrostatic pressure (for the event that the weeping tile system fails). Let us assume the basement slab is 5m below grade and that the hydrostatic pressure increases linearly from grade, I am finding that the design pressure for the basement slab would be 50 kPa. This seems like a very high value to design the structural slab for. Your guidance would be appreciated. Thank you.

 

[dik]

YOu can cast 'pop outs' in the floor to alleviate the hydrostatic pressure in the event the underfloor drainage system fails... but things above the concrete may get wet...

Dik

 

[hokie66]

In all similar buildings I have been involved in, the assumption has been that the drainage system will eventually fail. That leaves only two choices: design the building for uplift, or allow the water to flow in and equalize the hydrostatic pressure. In most cases, flooding a basement is frowned upon.

 

[dik]

Depends on what the basement is used for... with may highrises... it is for parking... and generally not an issue... also popouts for SOG type of construction can be square/rectangular... if a concern about them falling into a void, then round is the way to go... all with bevelled edges...

Dik

 

[hokie66]

My experience with high rise buildings is different from dik's. Whether residential or commercial, tenants/users of the building do not want their cars or storage areas flooded, and don't want reduced usage of the basement carparking in the case of required repairs.

 

[grantstructure]

I have had to hold the slab down exactly once--it's not cheap. Provide an active drain to a sump under the slab, and have the sump be separate from the perimeter drain such that the owner can monitor water coming up under the slab separately from water behind the walls.

If the floor is not designed to be held down, you have to provide pop-outs; leaving them out just means the slab will fail before the water rises, but the water will rise either way. You can still be sloped to drain (sump, probably) so that it shouldn't actually rise very high before the owner is aware of the issue.

 

[hokie66]

I wonder if dik and grant are talking about the situation as described by Galileo. He stated that the water table is near the surface, and the uplift would be in the order of 50 kPa, which is 5 metres of water. No "weeping tile system" can cope with that. To depress the water table would require a permanent dewatering system, which would mean pumping with an uninterruptible power supply, and perhaps most problematic, a place to daylight the water. I have dealt with similar sites, and the solution has always been to design the building to resist the hydrostatic uplift, with dewatering only used on a temporary basis during construction.

 

[dik]

I was just offering it as a possible solution, an alternative to designing the slab for uplift. In some instances, it is a viable solution... if there is little value to the contents, or if they can be readily moved, it can serve in the event the main de-watering system fails.

Dik