Filling in a Basement 3

[scrammblur]

Sorry, if this is a dumb question I’m an EE on the wrong board… but will concrete blocks last underwater?

After much review I’ve decided I need to fill in my basement with 3ft of pea gravel and make it a 5ft crawl space. It is just too far below the water table and I cannot pay the electricity for two constant pumps. The floor has about 10,000 cracks and water just comes through making a stream from one end of my house to the sump crock. I’ve talked to four contractors in the area and I keep getting mixed information over what to do with the cement blocks. Half say I should fill the cores and waterproof the insides others just say just leave the blocks as is and they will last. The house was built in 1999 it’s a 30x50 1500sqft ranch, the 10” CMU blocks look to be in great shape from the inside no visible leaks, cracks or bowing in the blocks. So should I do something to treat the blocks or just let them under the water table? Any help is appreciated…

 

[StructDave]

Welcome to engtips! Great question. Better give us 2-3 days for someone to come along with answers for you. Conc blocks are usualy kept dry, though.
Are you in an area where freezing will be a factor? I.e., If you let the water table remain so high, will frost penetrate to the water? This would reduce the life of the blocks.
How did your house get a building permit with gw so high? Or did someone regrade nearby, creating this problem?
How will you contain moisture in the fill, from penetrating your floor framing etc; planning on a conc slab over fill?

 

[scrammblur]

I’m in Michigan so it does get cold, but I don’t think frost will make it to the water table. Based on previous nightmares I don’t want to go into I believe the water will go up to about 20-25 inches. This gives 60” for frost and I believe the Michigan frost lines to be about 42”. I plan on going up to 3ft which will put my basement about the same as my neighbor whose sump pump has only runs when it rains. Also, the water has traditionally gone down in the winter except for this one.

How it was built here I don’t really know. First off it’s a small village in the country and from what I gather they didn’t really jump on the bandwagon of ‘formal’ building codes until the early 2000’s. I ‘m in the lowest spot around, but I believe the problem was exacerbated by the subdivision that was put in behind my house in 2002. It brings the whole subdivisions water to a pond 2-3 lots away and I believe raises the ground water faster than it did before. I’ve talked with a local civil and he said there wasn’t much I or even the county could do to divert the water.

My current plan is to (1) build up the existing 5 center footings 3-4ft somehow and replace the pillars (might also need some guidance on doing this in my swamp), (2) perforate the basement floor to release pressure from underneath, (3) stone sling 150 tons of pea gravel into my basement (4) build a inner drain tile and sump pit at the top level of the rocks (vent the sump with a radon pump for moisture and radon), (5) poor a rats slab of concrete on top. There is also some details about the furnace and hot water heater I need to take care of.

 

[ToadJones]

Well,
for the money you are going to spend, perhaps there is something you can do outside the house to fix this.

Back in my Joe-builder days ....
On several projects with bad ground water we installed what is essentially a dry moat filled with gravel. This was dug completely around the house but maybe 10'-15' away from the house. This was dug below the footing elevation and drain tiles installed just like the basement footing should have. Problem with this is, you have to have somewhere to run the drains. Chances are in your situation, you have nowhere to run them (otherwise, you probably wouldn't have this problem and you'd have gravity footing drains and not a sump).
Over time, the water table should go down.
At any rate, I'd try something outside first.

Sounds like you may have built a house in a pond!

 

[StructDave]

scrammblur (your handle reminds of an AMC vehicle from the late 60's-early 70's) - coupla further thoughts. I trolled around some masonry sites and didn't find anything on the use of conc block that will remain underwater. However I don't like it. Masonry is generally not used this way; we structural types go to great lengths to keep it dry.

Any contaminants in the groundwater (flowing, by the sound of it) could further attack the cementitious content of your blocks. What's worse, it'll be out-of sight, out-of-mind, and you won't know something is failing until there is drastic above-ground evidence. Then, it will be very difficult to repair the wall, and nearly impossible from outside only.

The suggestion of filling the block voids (perhaps in conjunction with parging and waterproofing the masonry surfaces) would alleviate part of the concern. However...

It may get worse, depending how your house is situated: If you have for example a walk-out side to your basement such that lateral soil forces are unbalanced, and your foundations are bouyed by ground water, displacement of some part of the foundation would be a possibilty.

You mention building up the interior column foundations; any situation of a column on top of another column can be tricky. Make sure the top of the extended pier is laterally supported or designed for rigidity from the base.

Therefore, and maybe taking into view TJ's suggestion of doing something outside, I think you might be well advised to hire a local engineer to assess this and confirm direction. Aside from probably improving your chances of a cost effective solution, when you sell you will have documentation to support whatever you do now.

Good luck, and keep us posted!

 

[AELLC]

I suppose the greatest threats to portland cement are acid, sodium and sulfate chemicals, and at least the sulfates can be neutralized by the method here:

http://www.communities.gov.uk/documents/planningandbuilding/pdf/703049.pdf

Try a Google search such as "prevent concrete deterioration" to get ideas instead of an expensive remediation such as filling all the block cores (that sounds overkill to me).

 

[AELLC]

Ooops I meant sodium chloride, not just sodium

 

[scrammblur]

Please believe me that there is no other remedy to my constant pumping water woes than to raise my basement floor. That is why I'm at a structural engineering board and not a civil engineering forum. I've had the country drain commissioner/engineer another local civil engineer and both have told me the water table gets too high and I'm SOL. We'll they did have some ideas but the costs would 2-3 times the cost of my house and take 2-5 years.

StructDave, sitting at Christmas dinner I got an idea about the posts in the basement. I was wondering if i would have better support in the center of my home if I put new footings in/on the pea gravel 2-3ft above the existing footings. Weight dispersion isn't my thing but i think i might be better off using the new pea gravel instead of relying on the old footings that are probably poorly designed for the swamp they are in. Instead of stacking them I can fill the basement leaving what i have now, install some temps and remove the old posts one by one, install a new footing and a shorter post.

AELLC, thanks for the pointers.. I'm assuming the Damp Proof Materials suggest in that paper could be something like dry-lock waterproofing product? I imagine if water is sill slowly getting through the membrane would at least limit the amount of minerals being passed.I ran that Google search and all I can find is floor-mats for my garage do you have any other keywords i can try?

Thanks everyone for your help so far..

 

[AELLC]

Here you go -

http://www.google.com/search?q="che....,cf.osb&fp=dc9072f4d1d34f4c&biw=1224&bih=746

 

[Ron]

scrammblur...be careful with filling that much in your basement.

First, since 1999, the soil below the basement floor has only felt its current loading. If you add 150 tons of weight to the floor, you can expect some settlement of the house. You might not want to do that.

Consider that your current slab could serve as a "work slab" that can be waterproofed and another slab placed on top of it. You'll have to artificially lower your water table to below the existing slab for the time it takes to waterproof and place the slab. In the process, the block can also be filled and waterproofed as well.

This will yield more usable basement area and prevent the settlement issue noted. It will also reduce your construction cost and time considerably.

 

[scrammblur]

Ron, i guess one thing i was thinking about was to jackhammer the old slab away from the footings so when the added weight is on the floor the floor could settle and not take the footings with it. Although there will be some additional weight on the footings it shouldn't be as much. Anybody have an idea on this?

 

[AELLC]

Ron - the added weight is just ballast. It doesn't add much weight to the footings because it is all below existing grade anyway.

It is similar to self-weight of footing concrete is weight of concrete minus weight of surrounding soil (Theoretically after that insignificant amount of settling as the footing concrete hardens).

To me, it is a non-issue. I suppose a Geotech Engr could shed some more light on this - Scrammblur, why don't you edit a portion of your OP and post it in the Geotech forum of Eng-Tips?

 

[Ron]

AELLC....the soil below the added weight will likely settle at least a bit. The stress increase from the loading will extend under the existing footings, thus increasing their potential for settlement by a similar amount. For the soil to act as you are perhaps thinking, there would have to be a clear shear plane at the edge of the loading with no influence transfer beyond the footprint of the loading...not likely.

As for your example, it is the self weight of the concrete - the unit weight of the soil replaced by the concrete...usually about a 40 to 50 percent increase in weight on the soil at the bottom of the footing. Not usually a big deal, as long as the soil reaction is elastic.

In this case, the stress increase on the soil will be about 200psf...not a big deal unless it is clayey or silty...but we know nothing of the soil conditions, so your recommendation to get a geotech opinion is right on.

As for the question regarding deterioration of the concrete masonry. Unless the groundwater is aggressive (either acidic (not likely) or contains sulphates (possible), then the masonry will fair OK if below the water table. Wetting and drying cycling causes problems with masonry because it is so porous.

Sodium chloride won't hurt concrete or concrete masonry...only steel reinforcing if it is in there.

 

[scrammblur]

So should I be looking to ask a Geotech engineer if the soil will support the footings and the added weight of the rocks?

I think my soil is mostly sand which is part of my problem with water, as there is not much in the way to slow down water.

 

[Ron]

scrammblur...almost any soil will support the added weight; however, you have to be concerned about the potential for induced settlement. This is where the geotech who is familiar with local soil conditions can help you.

 

[hwgaston]

scrammblur, I am in an almost identical situation and am curious to hear if you have an update or what you may have tried/found out?

Thanks