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.

 

[kslee1000]

Unless the waterproofing specialist can provide track records of success under similar circustances, I am keeping my doubtful eyes wide open to his proposal - a risky one to me, I would prefer a positive, more traditional, water proofing system from outside.

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.

 

[slien]

I agree with kslee 1000's skepticism regards the waterproofing. I would also recommend an external waterproofing system.
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.

 

[civilperson]

If the water proofing works, the building will float out of the ground!

 

[kslee1000]

civilperson:

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.

 

[oxbridge]

The site will be dewatered during construction and there is sufficient weight to resist uplift once the basement lid is on. Fingers crossed it won't flood during construction though!!! actually the site has never been flooded apparently so I think the hydraulic report is a bit OTT and will lead to the usual over design at the clients cost.

 

[kslee1000]

ox:

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.

 

[hokie66]

Don't believe anyone who says he can waterproof from the wrong side. When the water comes through, he will just point to the cracks and joints in your slab and walls and say that it is not his fault.

 

[oxbridge]

Thanks for all the advice so far. I'm very impressed with this forum and the quick responses.

 

[msquared48]

There's the answer - assuming the waterproofing works and it floats, install four 8" diameter steel pipe pile, one at each corner with U shaped hooks attached to the house to keep it in place during the flood. When the flood subsides, it will settle back into place.

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

 

[oxbridge]

I've done some numbers and worked out that the basement needs alot of concrete to prevent it from floating-even when all the superstructure is on.

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?

 

[kslee1000]

Extend the slab to make a ledge around the basement would help further.

 

[hokie66]

Comments:
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.

 

[RWF7437]

Why build a basement in a flood plain below the estimated level of flooding placing a possibly habitable space in harms way ?

 

[oxbridge]

hokie66,

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.

 

[hokie66]

Make sure the risk is solely the client's, not yours, as internal waterproofing can only lead to disappointment and acrimony. If your client is the builder rather than the owner, you are not off the hook. I make a good living writing reports about leaky basements when people have made stupid decisions in the design. Lawyers like it too.

 

[oxbridge]

good advice-thanks

 

[BSE05]

oxbridge,

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.

 

[oxbridge]

Thanks steelylee. Actually the other post is a different basement if you hadn't worked that out. They seem to be the rage at the moment.

 

[corrosionman]

Last weekend I visited a house built into a hillside with the garage rear wall below soil level. There is an internal sprayed lining but water pours in when it rains.

Also the sketch shows your insulation might end up saturated with water - will it not deteriorate and allow the complete house to settle ??
CM