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Old 05-12-2014, 06:30 PM   #1
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How to keep river out of basement?


I have a Victorian house that has four to six inches of water in the basement whenever the nearby river overflows its banks, usually twice a year. I find it is not rain water, since the basement is dry when the river is down. So the sump pump, if I get it working, will be running continuously when needed at all. That's probably why the previous owner's one burnt out. Will cementitious waterproofing (like hydraulic cement) stand up against this kind of pressure? Will it be necessary to parge the brick walls with it, or just the cement floor?

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Old 05-12-2014, 06:41 PM   #2
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How to keep river out of basement?


Not much you can do with being in a flood plain.

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Old 05-12-2014, 06:52 PM   #3
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How to keep river out of basement?


Gregs bottom line right....

You might be able to mitigate the flooding volume some with hydralic cement and even some waterstop paint.... and maybe mitigate/lessen the work your sump pump has to work....

if it's just a matter of lessening the water infiltration....
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Old 05-12-2014, 07:36 PM   #4
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How to keep river out of basement?


Water weighs about 62 pounds per cubic foot. The pressure of 1 ft of water is therefore 62 pounds per square foot. The pressure of 6 inches of water is about 30 pounds per square foot. If you were successful in waterproofing the foundation so no water got in, the upwards pressure on the slab would be 30 psf from only 6 inches of water. Most slabs would crack under that load.

This means that if you do plan to waterproof the foundation, you need to reinforce the slab also, to prevent cracking during flood conditions. This is far from simple, and usually requires replacing the existing slab with a heavily reinforced slab, carefully tied into the walls. Usually not worth the cost.
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Old 05-12-2014, 09:26 PM   #5
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How to keep river out of basement?


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Water weighs about 62 pounds per cubic foot. The pressure of 1 ft of water is therefore 62 pounds per square foot. The pressure of 6 inches of water is about 30 pounds per square foot. If you were successful in waterproofing the foundation so no water got in, the upwards pressure on the slab would be 30 psf from only 6 inches of water. Most slabs would crack under that load.

This means that if you do plan to waterproof the foundation, you need to reinforce the slab also, to prevent cracking during flood conditions. This is far from simple, and usually requires replacing the existing slab with a heavily reinforced slab, carefully tied into the walls. Usually not worth the cost.
Dan.... I highley respect your analysis..... but 30 /PSF will crack a slab???

I'm having trouble comprehending that.... I do understand that that is not a compressive force in play.... just seems allfully little for a normal 4-5" SOG with bar.

Just a comment/discussion

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Old 05-12-2014, 10:35 PM   #6
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How to keep river out of basement?


mtn remodel. Majority of basements out there are just poured concrete, no rebar or mesh in them. Very easy to crack them under 30psf underneath or on top.
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Old 05-12-2014, 10:47 PM   #7
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How to keep river out of basement?


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mtn remodel. Majority of basements out there are just poured concrete, no rebar or mesh in them. Very easy to crack them under 30psf underneath or on top.
Gregor.....Maybe in your area slabs are not reinforced.... they sure are around here ... and generally we are on decomposed granite with no earthquake or bentonite heaving issues.
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Old 05-12-2014, 11:56 PM   #8
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Gregor.....Maybe in your area slabs are not reinforced.... they sure are around here ... and generally we are on decomposed granite with no earthquake or bentonite heaving issues.
Back in the day, there was no such thing as reinforcing basement slabs. Even today they do not reinforce them in my area. Just pour them and be done. After the forms are removed, French drains and any DWV that needs to be installed is done.

Typical slab in my area is no thicker then 4", for a basement or even finished crawl space area.
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Old 05-13-2014, 02:18 AM   #9
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How to keep river out of basement?


having lived in & restored a 1865 house in upstate ny next to the susquehanna, IF i were you, i wouldn't do anything including installing pumps,,, that house has stood for yrs as blt,,, take out the water when it floods only adds to the exterior pressure risking severe damage.

our old house still stands,,, 4-6" you can live with failing that, you can find zoeller m-53 pumps on eBay for $ 110 new + $ 20 shipping,,, having a spare on the shelf would be a good idea imho

any parging is a waste - again, only mho

irs
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Old 05-13-2014, 06:38 AM   #10
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How to keep river out of basement?


The slab will not crack if the basement floods is because the downward pressure of the water compresses the slab against the soil, and concrete is very strong in compression. Unfortunately, if the water table is 6 inches higher than the basement, the pressure is upwards on the slab. The edges of the slab are locked in place by the walls, so the slab acts like a large diaphragm, under upward pressure. Now the slab is in bending, and the top of the slab is in tension, and concrete is very weak in tension, hence it is likely to crack.

Even if a slab is reinforced, most slabs are so thin (less than six inches) that the rebar is at approximately the midpoint of the slab, which is the neutral axis, and the rebar offers no resistance to tension on the upper part of the slab. In some cases, the rebar was not supported on chairs, so it ends up at the bottom of the slab, where it is totally useless in resisting bending from upward forces.

Conclusion: Typical basement slabs were never designed to resist upward forces, and rebar or not, are almost certain to crack under even minimal upward loads like 6 inches of water pressure.

Side note: Once the slab cracks, water comes up through the crack, and the slab is no longer under upward loading, so the cracking stops. Unfortunately, the basement is flooded, and there is probably a big hump somewhere in the basement that has to be fixed.
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Old 05-13-2014, 07:15 AM   #11
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How to keep river out of basement?


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Side note: Once the slab cracks, water comes up through the crack, and the slab is no longer under upward loading, so the cracking stops. Unfortunately, the basement is flooded, and there is probably a big hump somewhere in the basement that has to be fixed.
In my opinion it's fortunate the slab cracks from upward loading and floods. If it doesn't allow water in there is potential the house will float and we really don't want that for several reasons. In ground pools in high water table areas, where water must remain in the pool, are good examples.
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Old 05-13-2014, 07:31 AM   #12
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How to keep river out of basement?


Just some info on an experience in my life. When I was a kid, in the mid '60s my folks bought a rancher in a new development....like only the 3rd or 4th house built new development. From the very beginning the basement would flood. Turns out (my dad was told by a coworker after the fact) the area where the house was built, would flood every time there was any significant rainfall due to an underground stream. After dealing with it for a few years, with just running sump pumps, my dad had enough. He had 6" of gravel put on top of the existing floor, then had a 6" reinforced slab poured on top. Can't remember if there was plastic sheeting on top of the gravel or not. Anyway....water would run BETWEEN the floors, and out of the basement by way of 4....yes 4 sump pumps. The basement has been dry ever since. He runs dehumidifiers, and although he gave up a foot of headroom, the basement is just like any other USEABLE finished basement. Pool table, storage, everything.....you just can't be over 6'3", and you have to remember to duck when going through the doorways!
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Old 05-13-2014, 07:58 AM   #13
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How to keep river out of basement?


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Originally Posted by Daniel Holzman View Post
The slab will not crack if the basement floods is because the downward pressure of the water compresses the slab against the soil, and concrete is very strong in compression. Unfortunately, if the water table is 6 inches higher than the basement, the pressure is upwards on the slab. The edges of the slab are locked in place by the walls, so the slab acts like a large diaphragm, under upward pressure. Now the slab is in bending, and the top of the slab is in tension, and concrete is very weak in tension, hence it is likely to crack.

Even if a slab is reinforced, most slabs are so thin (less than six inches) that the rebar is at approximately the midpoint of the slab, which is the neutral axis, and the rebar offers no resistance to tension on the upper part of the slab. In some cases, the rebar was not supported on chairs, so it ends up at the bottom of the slab, where it is totally useless in resisting bending from upward forces.

Conclusion: Typical basement slabs were never designed to resist upward forces, and rebar or not, are almost certain to crack under even minimal upward loads like 6 inches of water pressure.

Side note: Once the slab cracks, water comes up through the crack, and the slab is no longer under upward loading, so the cracking stops. Unfortunately, the basement is flooded, and there is probably a big hump somewhere in the basement that has to be fixed.
Dan.... Thank you for the more detailed and full explanation. As usual, makes excellent sense and sure helps me better understand the forces.

I partially understood the concepts, was just surprized that 30/psf was sufficient to crack that slab... guess I intuitively thought it would be in thr range of 100-200/psf... and I now better understand the role of reinforcement also.

Bottom line.... Thanks!

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Old 05-13-2014, 08:08 AM   #14
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having lived in & restored a 1865 house in upstate ny next to the susquehanna, IF i were you, i wouldn't do anything including installing pumps,,, that house has stood for yrs as blt,,, take out the water when it floods only adds to the exterior pressure risking severe damage.

our old house still stands,,, 4-6" you can live with failing that, you can find zoeller m-53 pumps on eBay for $ 110 new + $ 20 shipping,,, having a spare on the shelf would be a good idea imho

any parging is a waste - again, only mho

irs
Ed.... Sorry for my poor/insufficient thought that you could mitigate some flooding with hydraulic cement.

Luckily, Dan and It's Really were around to further advise as to potential cracking.

Guess I was just reflecting my very limited success with hydraulic cement with ground leakage at the stemwall/footer, and did not consider that your flooding was completely under your slab and "floating it".

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Old 05-13-2014, 10:53 AM   #15
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How to keep river out of basement?


Since the house was built in 1896, it really doesn't have a slab. Where the previous idiot broke out the basement floor to make a sump the cement (no aggregate) is less than one inch thick. When the water does drain out you can see the cracks in the floor are still wet.
As the current idiot, I didn't think of the fact that keeping the water out will put pressure on the brick basement walls. I suppose I'll have to let in pour in and pump it out like all my neighbors do. Or just give my kid a toy boat and let him play in the basement. The furnace is already up on bricks -- that should have told me something.
Thanks for the help.

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