Basement In Flood Zone
Basement In Flood Zone
(OP)
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.
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.






RE: Basement In Flood Zone
On design, you should consider both cases - seasonal probable highest ground water level and the maximum flood level. You could, if allowed, adjust the safety factors to reflect the possibilities of occurance of the latter case, so under such condition, the owner shall anticipate some damages, but not catastrophic structural failures.
RE: Basement In Flood Zone
Consider an interior applied membrane. A tiny hole through the wall will cause a bubble to form behind the membrane, which will pressurize up to the static ground water static pressure at this depth. I would be concerned with the membrane then leaking or tearing off the wall. Similarly if the waterproofing is a cement base, water seaping through the wall will tend to push the material from the pores that it is supposed to seal.
Conversely, ground waterpresssure will force an exterior membrane onto a wall surface or force cementitious water proofing into cracks/crevices in the wall surface corking the seepage paths.
Do not forget to waterproof below your floor slab. Use a waterstop in construction joints.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
Yes, it would, and it has the logic behind: let the house float to the ground for ease of relocation, rather than sinking, buried under the mud.
Just kidding. Excellent point.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
Yes, so many disaters occurred during construction. Can only advise the client to avoid flood/raining season, and have emergency plan/equipment onsite.
Depends on who issued the assessment report, you may want to challenge it. However, if there is smoke...Be prudent. If the owner can afford, let him pay the insurance, losing a house to a nature event is not a small loss, you could be liable too.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
RE: Basement In Flood Zone
However, if the house does not float, but floods, intall a large sump pump to dewater the basement. However, the four pile will still have to be installed as the dewatering may cause the house to float.
In lieu of the pile, furring out the concrete walls with lead weights in the stud spaces will help decrease the net uplift.
Hope this helps.
Mike McCann
MMC Engineering
RE: Basement In Flood Zone
We have to dig out some soft soil anyway so I'm proposing backfilling with concrete that can be used to counter the bouyacy effects. I've attached a detail showing how the mass concrete backfill could be tied to the RC Basement. Any thoughts/comments?
RE: Basement In Flood Zone
RE: Basement In Flood Zone
1. I don't see the waterproofing in your sketch.
2. The top slab bars/inside wall bars can't go around the inside corner like you show.
3. I would pour the base slab monolithically rather than in two pours.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
1.the waterproofing is applied (painted/sprayed) to the inside face.
2. yes ok
3. the thinking behind keeping the base slabs seperate was that a thicker monolitchic base would require much greater minmium steel (as it's related to the thickness of slab) to resist thermal shrinkge and cracking.Preventing cracks appearing is obviously quite important here because of the waterproofing system. Although I'm still not convinced about the waterproofing but the client is prepared to take the risk.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
RE: Basement In Flood Zone
A couple of comments on your sketch. You will not need insulation unless your above the frost level. You should have a vapor barrier below the slab. A thick mill plastic installed with taped laps and no holes. Water will permeate the concrete through vapor pressure.
A thought to waterproofing the walls is to have a double wall with waterproofing (membrane sheet) between walls. Only the interior wall would be reinforced for full hydrostatic head. I would also recommend a sump be boxed out in the slab for a pump, just in case. Look at Boston's Big Dig to see a waterproofing disaster by trying to do it from the inside. Don't forget waterstops between pours.
Good luck --what is the soil type--I read your underpinning post. Still think helical piles might do it, depending on available access.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
Also the sketch shows your insulation might end up saturated with water - will it not deteriorate and allow the complete house to settle ??
CM
RE: Basement In Flood Zone
I am not sure why engineers tend to believe such ridiculous claims.
RE: Basement In Flood Zone
RE: Basement In Flood Zone
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.
RE: Basement In Flood Zone
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.
RE: Basement In Flood Zone
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.
RE: Basement In Flood Zone