Oh ye gods of concrete, hear my prayer!
The excavation for the basement for my addition is finished, and the good news is I’ve got a very stiff clay till to build on. So stiff that the existing house has sat on it with only minor settlement issues for the past 70 years, despite having no footings! The existing basement walls are 9-10″ cast concrete, and wood forms were used only on the inside and on the outside above grade- below grade, the form was the wall of the excavation. The new hole is deeper than the existing basement, properly sloped from the base of the existing wall, and ready for underpinning.
The bad news is that I’ve got some water to contend with. It’s just perched water, draining in from the surrounding soil as the last bits of frost come out of it- we’re nowhere near the permanent water table. I’ve got a sump pit dug and some trenches to drain the water to the pit, and the inflow rate is absolutely no problem for my pump to contend with- but I know I’ll be pumping this hole forever. The base of this hole isn’t currently and will never be completely dry. The base of the hole is reasonably level, but not perfectly so, and there are low spots (some of which are located where footings will go) where water is ponded and seems to stay that way despite my best efforts.
I know I need to set my footings on “undisturbed soil”, and any of this soil which has been “disturbed” turns into nasty clay mud when it gets wet. So my plan is set the outboard form boards, scrape off as much mud as I can until I get down to the hard underlying native soil, set the inboard form boards, and pour the footings.
My question is this: the low spots will still have some water in them, and I doubt I’ll be able to get 100% of the mud out in every location. I know that I’ll displace some of the water when I dump the concrete, so things don’t need to be scrupulously dry before I pour. But how fussy do I have to be about keeping mud out of my forms, given that there’s no way I can get and keep every part of the bottom of this hole completely dry?
Your experience and advice will as always be of great value to me, and I thank you in advance.
Replies
any of this soil which has been "disturbed" turns into nasty clay mud when it gets wet.
You hit the nail on the head sharply with that comment.
To stack your house on top of theis layer of muddy ooze is asking for problems as the weight increases and the wet plastic clay squeezes out from under the footings.
And just because you get the footings poured your problem does not stop there because any added water that is not drained from the overdig and the prospective floor area will cause more clay to turn from rock hard to plastic. The excavated clay soil is now in the "active zone" and water activates it to a plastic medium unsuitable for building loads.
Pay close attention to draining all seep and rain water from the excavation as quickly as possible. Coat the poured walls and install the perimeter drain if not using the forma-drain system.
When you pour the floor remove all the mud, then add the 4" of gravel and pour asap. Do not add the gravel to the mud thinking it will create a proper base.
Your backfill must wait until the floor is framed and decked as the weight of wet clay will wedge an unsupported poured wal out of position.
Be prudent in this matter or you will see the bad building results for the rest of your life and of the building's.
...............Iron Helix
we pour in water everyday, we pour underwater too. they is a method to this. subgrade must be tight. Just because water is in the form does not mean the water has mix with the subgrade. a stiff clay is perfect for this. Place a stronger mix instead of 3000 place a 4000. have your slump dry, about a 3. just enough so it will roll. start at one end and roll the concrete to the other forcing the water out the form. do not vibrate the conccrete and water, just vibrate the concrete. At the end the water will try to get on top of the concrete. Thats fine, just leave it. DO NOT VIBRATE THE WATER WITH THE CONCRETE. it works the same way with underwater concrete but you need a pump truck, stick the hose in the mud and start pumping.
"subgrade must be tight"
I agree that the water will be displaced, but the plastic layer of clay may not, therefore there might not be a tight substrate for the footings.
The clay that has been described is similar to what is common in this local and it requires that the footing trench be free of plasticized clay and that water is kept drained to prevent the clay from becoming plasticized.
If the clay is dry, we chop the footings with an axe to finish tweaking the trench forming, and then "crumb-out" the residues. If there has been standing water left after rainy days it will produce 2-3 inches of ooze, we "gum-it out" and then pour.
Sure like to time it to omit the "gum-it out" part of the cycle cause the goo sticks to the shovel like gum and you spend more energy shaking goo off the shovel than digging! We won't even talk about the PITA wheelbarrow full of goo!
Other local areas have a gravel/sandy soil that has a greater load capacity than the clay plus it does well when there is standing water in the excavation and allows for the concrete to push out the water. That's an easy day compared to the clay.
.........Iron Helix
Iron Helix et al- thanks for your help!
Here's the current state of affairs:
I'm pumping the hole reliably now. Pump's working great (had do dig the pit with a crowbar- this stuff is STIFF until it's disturbed!). The trenches I cut are doing a good job of draining the areas where infiltrating water was ponded on top of hard clay. Of course, the areas that were covered with crumbly stuff left over after excavation are now a layer of plasticized clay ooze, and that junk will not drain period!
The soil fillets supporting the existing foundation wall are high and dry and are not plasticizing, so no danger of my existing house falling into the hole (yet!).
I'm tarping the hole completely whenever there's a threat of rain, with the edges of the tarp running to eavestrough (gutters to you Americans) draining out to the street. So far, very little rain and lots of sun expected in the forecast, but it's spring and that will change soon!
I've got boards in the hole to walk on and a pallet at the base of my ladder, and not walking anywhere else. There's no walking or other disturbance going on any of the surface which will later support footings- no walking, no ooze-raking etc.
I'm thinking of setting footing forms and pouring all except the sections currently covered by the fillets of soil adjacent to the existing foundation wall (to be underpinned ASAP). The original thought was to wait until the last set of underpinning was done before pouring the addition footings, so they can be poured as a unit, but now I'm thinking that it would be better to get these footings down NOW rather than waiting, scraping away yet more ooze and doing them after the u/pinning is completed. I'll put some rebar in the footings to make sure the joint between the section I'm about to pour and the section I'll pour later is stable
Should I scrape off and bin the clay ooze in my floor area, then dump in my under-slab gravel layer like Piffin recommends? Will this keep the remaining clay from plasticizing and provide me with stable walking surface, or will the gravel start to go to China in the way you were describing? If I do dump in the gravel now, I'll have to store the spoils from my underpinning excavations on top of plywood on top of the gravel and tarp it every night until I can muck it out.
Guess I'd better talk with the inspector pronto. He's going to want to inspect the footing forms prior to the pour, and I want those two things to happen in quick succession so I don't have to muck out TWICE!
Any more thoughts? Thanks again for your help-
Time and water are your worst enemies.
Get in....pour footings on good substrate.....set walls and pour.....put in perimeter drain..... crumb floor area, fill w/gravel & pour.....get out of the hole!
If things are "good" and the job isn't too large....5-7 dry working days.
We get 40+ inches of rain a year, and in the spring it is almost impossible to have a "good" week for a basement.
Don't clean the "goo" after every rain....clean only for the pour. Clay usually has a terrible percolation rate so the water only effects the first inch or so.
Chit chat with your inspector about what he has seen and knows about your soils. Ask what the local SOP is for basement installations. And of course it is nice to know what he expects to see to pass on the work.
.................Iron Helix
Good advice. Speed is of the essense. Too bad the underpinning won't let me go that fast, and the inspector may slow me down more- 48hrs min warning time to get an inspection. Code-required curing time on the underpinning footings (before digging the next set of trenches) will keep me at this for at least a week longer than your 5-7 days, even if I go at the rest of it like a madman.
So I'll set my exterior wall footing forms and pour them as quickly as Mr. Inspector will allow, then I'll get the perimeter Big O pipe in place and covered with washed stone level with the top of the footing- I can add more once the walls are poured. That ought to keep the infiltrating water at bay at least. Tarp over the hole should take care of direct rain onto my floor area from that point onward, reducing the mucking work. I'll forget about the floor area until the underpinning's done and signed off by Mr. Inspector. Think I may dump some gravel in there per Piffin's advice.
Ditto what you said . But if he has a small puddle (less than a foot dia.) I think that will not be a problem. He said that he would scrape the wet claymud from the footing area. Just my opinion worth what he paid and then some ,maybe.
Here's what i do -
I scrape the excavation relatively smooth to hard clay with a flat shovel. niot easy and you can't keep mucking around disturbing more with your feet.
Then I add a few inches of stone usually 3/4" and walk it in. This makes it nice walking surface for working to set form boards and it is firmed in good by time you get the froms set. That pushes the water aside and/or gives you a practically paved surface to walk/work on
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That is exactly what our excavator guy does. In a site with all clay the entire hole will get 4" or more of stone. It is not cheap but is essential in any area that has standing puddles. This time of year that is any hole.
The entire hole bottom gets 4" of stone? Does that include the area that later bears the footings?
I know that crushed stone dumps to ~ 90% of final density, but would I really want to found my addition on a layer of crushed stone? Is "walking it in" enough, or would I want to rent a compactor?
Again, the underpinning complicates things a bit- still a fair bit of spoils to take out of this hole, and I don't want to contaminate the top of my nice stone layer with this sh*t.
By the time the forms are set, the stone has been walked on plenty and there is a fair amount of muck that has squeezed into the stone. Frankly I don't know of any way around this, except to wait until the conditions are dry. That is not usually an option; it could be six months.
The alternative around here is to do nothing or to muck out the bottom of the footings before the pour. I don't like either of these options.
We dug a perimeter trench which seems to be catching most of the infiltrating water from the sides of the excavation and taking it nicely to the pump. We're blessed with a week of dry weather and had a beautiful, warm, sunny weekend. Hope my luck holds! This place could get very ugly very quickly with an extended heavy rain...
The first three underpinning trenches have been dug with much effort, and I'm running around setting the outer footing forms for the addition so I can pour them at the same time. This is taking some time because I want these footings LEVEL and have to put in the effort to make that happen.
We found the source of water running to the centre of the hole: there's clay weeping tile UNDER the old wall, on the INSIDE- remember there are no footings under here- they dug a trench, put in some coal or slag, laid the weepers, dumped some more coal/slag on that, then set the forms and poured in the earth trench. Thank God, it doesn't appear to be coming up from underneath... The old installation proves that there's no real value to weeping tile in a tight clay soil like this unless you slope it properly and run it out to daylight- they did neither of these things and really should have had a pump. These old weepers are actually doing the reverse of what they're designed to do- right now they're actively draining water from the backyard into my hole! Fortunately a few more trenches have dealt with most of that, and the sun is baking down the sludge in the floor slab area which accumulated before we found the weeping tile.
Once Mr. Inspector gets there and gives it his blessing (48 hrs leadtime), and the first three underpinning footings and the exterior footings for the addition are poured, I'm dumping four inches of crushed stone into the area where the slab will go, out of necessity. It'll pretty much be like "building a road" down there, and won't be as good of a "capillary break" as it might otherwise be, but I don't have much choice. I'll walk that in during the rest of the underpinning digging and the ICF stacking, add more if I need it, then put my 2" of XPS foam, poly VB and slab on that.
Hey- there's a question: I'll need "chairs" to support my welded wire mesh and the PEX tubing which will be wire-tied to it. Doesn't that mean I'll be poking tons of holes in my VB? Doesn't that more or less defeat the purpose? Is there a kind of chair which doesn't act as a stake? Guess the foam itself will act as somewhat of a VB, as long as I don't puncture it through.
Is there a kind of chair which doesn't act as a stake?
The ones I use have plastic feet and wouldn't easily puncture anything. Check with your concrete material supplier.PAHS Designer/Builder- Bury it!
Thanks VaTom. What you're describing is exactly what I want. Got to go shopping in the right places...
Too bad you're not a little closer. I bought a pallet, stacked 4' high, of 4' long chairs with nice plastic feet. More than a lifetime supply for me, for $2. Auctions are fun.
Before chairs, it was common to use brick or rock to raise the rebar. PAHS Designer/Builder- Bury it!
Unfortunately we're not building any plants going to Virginia at the moment, nor do we have anything coming in from that direction, or we could have the truck swing by your place on his way back!
That's half the problem- the right stuff is usually in the wrong place! Bet there's something I've got in my salvage storage that you need worse than those chairs, but the shipping doesn't pay!
You got it. I run an informal material salvage here. Lots of donations, quite a bit of free material going to new homes.
Although I like the plastic feet better for keeping the steel away from moisture, here's a Simpson chair I hadn't seen until I Googled:
PAHS Designer/Builder- Bury it!
Edited 4/13/2005 8:59 am ET by VaTom
You do NOT want a compactor. with only a few inches of stone, it will bury itself as the vibrations bring in more water and stir it into the muck.If you are going the compactor route, you need to excavate another 18" and fill back up in eight inch lefts, compacting the fill. Note too, that I carefullysaid, "This is what I do"I know it works, but it is possible that your inspector could have "issues" with just walking it over.Another solution is to pour a "mus slab" which is a weak concrete slurry to give you a walking surface. 3" just screeded in to settle the mud. You can still drive a steel stake in it to set footing forms. I've never done this but learned about it here, when somebody asked what a mud slab is.
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