Foundation holes below watertable
PenobscotMan
| Posted in Construction Techniques on
New problem — just dug two holes for a small deck. Plan to install 8 in sonotubes over a 14 in flat footer. At about 3 feet down the holes began to fill with water, coming in at a pretty good rate (I can bail 2-3 gallons after 5 minutes). The frost line is said to be at about 4 ft here (coastal Maine). We have hd some heavy rain recently, if that is relevant. We also close (50 ft) to a small wetland and ephemeral stream.
I’m stumped. Don’t tell me to move the house! (The house extension foundation, also on sonotubes a few feet away from the water-filled holes seems OK — no frost heave in 5 years or so.)
Replies
I don't know how you do it in maine, maybe Piffen will be along to help.
"We have hd some heavy rain recently, if that is relevant. We also close (50 ft) to a small wetland and ephemeral stream."
Just my two cents worth. How high is the general ground height of the construction in relation to the wetland/stream? Two, three, five feet? Under normal conditions, and I use that term reluctantly, ground water near a wetland will not be much higher than the level of the wetland. Given your recent rains I would guess that the the subsurface water has risen faster than it can leach out. This your perplexing discovery. (How much higher than normal is the water in the wetlands/stream area?)
What kind of soil conditions do you have? A general assumption on my part is that you have some organics and stone and/or fine sands or ???. Since you are considering a sonatube with a footer may I assume further that the bearing capacity (if you hadn't hit water) would be fine.
In digging the holes you unintentionally gave the water an outlet, rather than leach down naturally. If you can I would let the hole drain out and then continue. Should you still have water down to the depth you need I think you found a temporary subsurface "stream" (huge proposed development near me was totally shut down because of just this problem, 3feet down they got nothing but water and the nearest creek was a 1/2 mile away!) There are solutions to high water tables but first we find out about the current conditions.
Lordy, I talk too much, ciao, ted
Excellent questions. We don't have a lot of soil -- just granite ledge, with some soil in the low parts. So, I'm thinking that the depth of the water table depends on the depth of the ledge below the surface. Granite will impede downward percolation of the water, perching the water table. So, where you dig can have big effect. Anyway, this is my thinking. Regarding the little wetland, I believe the level of the "water table" in the holes is above the level of the standing water in the marsh.Not sure why I have two threads going on this, Must be my desperation.
I kind of figured that being in Maine you would have a bit of granite around. That's why I asked about the soil type and bearing. Around here, Northern Ohio, we get so many different soil types that any excavation/hole is an adventure.
If that water don't go down in a short time and time is important then I would consider the plastic cone/tubes the other posters mentioned.
Sounds like a plan. Best of luck. Ciao, ted
Worst case you have to "dewater" the area by pumping continuously for several days.
more like acouple months that way things have been here lately.He has another thread on this I replied to.
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Yeah, it depends on the nature of the soil, and how much effort goes into the dewatering. It's a technique used fairly commonly around here, though, and you rarely see the pumps in place for more than a couple of weeks.
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I saw a pretty cool lookin' bell shaped plastic form, finishing with a tube on top, on one of the PBS home improvement shows, some time back. It's made specifically for deck piers.
I wouldn't be concerned about the water if I had a form like that resting on a dependable surface. I'd just pour the concrete in and let it displace the water.
Having water surround the form will actually help the concrete to cure better.
>>>I saw a pretty cool lookin' bell shaped plastic form, finishing with a tube on top, on one of the PBS home improvement shows, some time back. It's made specifically for deck piers.BigFoot, though there might be others also. I think they advertise in either FHB or Old House Journal. It's a Canadian company, I think. The top is tiered to allow for different sizes of tubes. You attach the tube to the appropriate step, and it allows the footing and column to be made in one pour.
BigFoot,
Clever marketing, clever concept too. I'm glad to have a name for it 'cause I'll sure try some on my next deck job.
I've seen those forms, but not sure how they solve my problem. Piffin (on my other thread) thinks I should go ahead and pour, but support the sonotube with wood rather backfilling a soggy cardboard tube. Hey -- maybe that wouldn't be a problem with the plastic gizmo?
I've seen those forms, but not sure how they solve my problem.
Well it's a single piece form, footing and pier together. Place it in the hole and pour the concrete.
I'd probably put some large stones on the form's bell to hold the base of the form down so that the water inside would be forced to escape out the top of the tube when it's displaced by the concrete.
Is there some other problem that I'm missing?