Basement In Flood Zone

[oxbridge]

I'm working on the structural design of a basement. The flood risk assessment stated the threshold to the property at ground floor should be 500mm above existing ground level to cater for flooding (although this has never occured on the site). The design of the basement is such that the top of basement will protrude 600mm above ground level to cater for flooding and will be wateproofed to prevent ingress of ground/flood water. We have been advised by a wateproofing specialist that this can be achieved with an internally applied liquid waterproofing system that can handle a pressure head of 10 metres.

Has anybody experienced a similar situation, and are there pitfalls with this waterproofing system apart from the obvious workmanship issues and are there better alternatives, apart from any involving pumping which the client does not want.

Also, the ground water level was found to be at about 2.5 metres depth and no long term monitoring of water levels was carried out. In accordance with BS8102 I was going to take the water table at 1.5 metres (1metre above ground water level) for structural design purposes. As the seasonal variation could mean the water table actually becomes higher would it be advisable to take the water level at ground level instead. And finally!!! in terms of flood water, if the water table is taken at 1.5 metre depth, should the 500mm of above ground flood water be considered as seperate hydrostatic loading or would it be advisable to assume full hydrostatic loading from flood water level. The ground conditons are clay. Thank you for reading all this.

 

[aayjaber]

An Internal water proofing system for a basement in a flood zone!! This is the biggest hoax you can find for this April.

I am not sure why engineers tend to believe such ridiculous claims.

 

[oxbridge]

thanks for all your comments. it does seem riduculous the more i think about it. but if that's what the manufacturers say and the clients are keen to use it, i guess i can only give them my concerns in writing. i'm not sure if i owe the cleints anymore duty of care.

 

[kslee1000]

ox:

I think most of us here would back you up. You cared enough to go through this excercise here, and voicing your concerns in writing.

 

[SteelPE]

Maybe this was already covered.

Why don’t you extend the slab beyond the walls to pick up the weight of the soil to resist the hydrostatic uplift forces? This way you don’t have to provide the mass concrete.

 

[ron9876]

I would use a bentonite waterproofing on the outside with mono pour for the base slab.

In US we would have to use 0.6D in combo with uplift and any time that I have had to deal with this condition we had to use tension piles. If small enough extending slab may be enough.

If problem with flotation during construction you can flood the basement to offset the head.

 

[MiketheEngineer]

If you have basement drains - watch out!!