Wet clay soil unsuitable for deck foundation, or not?

[kmoiarty35]

As I've mentioned previously in other threads, I'm in the process of building a deck. At the moment, I've just dug holes 5 feet into the ground in preparation for pouring concrete pier-columns (the foundations which my deck will rest on). However, as soon as these holes were dug I noticed they would partially fill up with water, seeping in from the surrounding (clay) soil). I've drained them once with a sump pump, and I thought the holes would stay dry until I could pour the concrete into them, provided the weather remains dry til then. However, despite continued dry weather conditions, this morning I noticed that the holes have filled with water again. I surmise this to be related to my neighbor leaving his lawn sprinkler on the night previous, and that water seeping into the ground from that is what is entering my holes. All this has got me thinking... Will my concrete pier-columns be stable enough to hold my deck properly since the surface soil they will be buried in is obviously prone to holding water? Right now the weather is dry because it is summer. But most of the year it rains a lot here (Vancouver). If water from a neighbor's sprinkler can saturate my future foundation soil so readily, can I expect it not to turn into an unstable, sinking, muck when the heavy rains return.

Responses please and thanks, Ken

[PipeDown]

Concrete does just fine in wet conditions, It is not the presence of water or not but if that water contributes to a weakening of the soil containing it that is important. This I certainly cannot answer from here. Given that a deck load is not very high, directed straight down and that clay us usually very dense and stays put unless on a slope, MY gut feeling is that

5'deep by what 1.5' wide would probably be enough anchor. The depth of the hole is mainly necessitated by the need to get well below the frost line to prevent movement, otherwise, a shallow, broad support would work just as well if you live in a no freeze zone.

If the deck is to be covered or used as an indoor space than you will want to be more particular about the underpinnings and maybe worth consulting a structural engineering firm for an evaluation. In fact a concrete contractor may be able to advise you enough without expensive tests.

[EXT]

I have found that if I fit a plastic (garbage) bag in the hole and then place the Sonotubes into the hole inside the bag, I can pour the concrete without stirring up bottom mud and contaminating the concrete with the mud and excessively diluting the concrete with excess water.

[Tony Hwang]

Hi, From Calgary. Concrete does not weakens in water. Think hydro dam built with concrete. The question is whether your soil is stable to keep the concrete post in place. Here usually we dig down to frost line minimum for this kind of work. That is ~ 6 feet deep. Tony

[twstanley]

I think the main problem is the holes are to deep. You just need the holes deep enough to get below the frost line. If I were you I would find out what is recommended in your area and refill and pack the holes to that depth.

If 5' is recommended then just go ahead and do like the other poster said, drop your sonotubes in trash bags into the hole, fill with concrete, and put the post anchors on top of the pillars while the concrete is still wet. I wouldn't sink the posts in the ground in the concrete as you have a lot of moisture in your area.

[Bill Gill]

There shouldn't be a problem, as long as the clay stays wet. The problem would be if the clay dried out, because then it would shrink and shift the footings.

Bill Gill

[kmoiarty35]

I'm going to be mixing my own concrete. (I already bought a dozen bags of "Ready-Mix".) Guess I can buy a bag of cement and add extra just the same as the concrete plant would do?

Is this additive also available for the 'mix-it-yourselfer'? If so, what product do I look for?

Okay, I can live with that very small risk (for this part of the country anyway, that is ).

BTW, I forgot to add another, related, query to do with my future intention to build a large shed with a below-grade basement cellar. Given my "wet clay soil", in addition to the inconvenient fact that local bylaw states any foundation drainage flow into public sewer must be by gravity only (i.e. no sump pumps, etc), is there anything special I might need to be aware of here (considering that I intend to use my cellar for storage and therefore needs to be dry)? For example, is foundation drainage always a necessity (because this concrete foundation will be too low to drain into my sewer connection under bylaw constraints)? Should the concrete of the basement cellar walls and floor-slab (my foundation, in other words) require any extra measures be taken to make it sufficiently water-resistant and, if so, how is this usually accomplished? (1) E.g. buying "waterproof" concrete made from "epoxy cement"? Pre-installing a waterproof (e.g. "swimming pool") membrane? Etc...

Thanks, Ken

[Tim Fischer]

I have direct, recent experience with this, as we're also building a deck right now. Our 4' deep holes (7 of 'em) did the same thing. When I first called the building department, they told me "the holes have to be dry and no much for the footing inspection". When I pressed them for how exactly I was to accomplish that when I was obviously below the water table, they forwarded me to the inspector who would actually be coming over, and we came up with a plan:

a) first I sucked the water out of each hole (with a shop vac) and removed the loose muck caused by digging the hole. b) Then the inspector came out to verify the footings, and verify that the soil was indeed clay. c) She then had me place about 3" of river rock in each hole, and tamp it down good into the mucky clay. This didn't raise the level of the footings much (we had plenty of depth anyway as I only need 42" for frost footings and I had 48") but provided a very solid base for the concrete pour. d) The next day when pouring the footings, they had of course filled with water again. I sucked them out, one at a time, before doing each pour.

Can't say how well it will hold up as this was only last week -- but they seem solid so far...

-Tim

[Tim Fischer]

This isn't allowed by many codes-- footings need to be on undisturbed soil.

-Tim

[Tim Fischer]

You're in for a shock. Our 7 4' deep holes took **42** 80 pound bags of quickcrete. You don't say how many holes you have, but at 5' deep you'll certainly need more than a dozen bags. That might get you a hole and a half...

You'll also likely want to rent a mixer. Our HD rents one for about $40 a

24 hour period. You dump in the mix, and some water, then wheel it right up to the hole and dump it in (after mixing, of course). SO much easier than mixing by hand, when you're mixing up 3500 pounds of the stuff... Even so, you'll want help. 80 pound bags get heavier with every one you lift...

-Tim

[wkearney99]

This is one of the many reasons to do the math ahead of time and get stuff delivered. I've seen people nearly destroy their vehicle cramming a ton of bricks into the trunk when they could've just gotten an entire palette delivered for around $50. Likewise with concrete and gravel, check with a local supplier. You can get gravel for about a THIRD of what bagged stuff from the box stores would gouge you.

Renting a mixer for concrete is about the only way to go unless you've got enough to justify ordering up some delivered by mixer truck. Then it's one wheelbarrow at a time to offload it from the hopper to the holes. And you're generally stuck with ALL that came in the load, not just the amount you need. So unless you've got a lotta holes, patio and/or stairway landings then mixing your own is the way to go. Just buy it from a bulk source and have it delivered instead of overpaying the home despot and wrecking the car.

[Tim Fischer]

[SNIP]

Quickcrete's pretty cheap. I had all my deck supplies delivered for $35 (delivery charge) so didn't pay more for delivery. I can't imagine I could have had "bulk pre mix" (does that even exist? I had no interest in trying to mix my own concrete from its component parts) delivered for cheaper than I paid for my Quickcrete, and then I would have had a big pileof dry powder in the garage or wherever, making a huge mess...

I had 40 bags delivered, ended up needing a few more so I sent the wife down to the local BORG to pick up six more while the rest of us worked on the pour, of which we only used two. I'm from the "rather have too much on hand than too little since you can always return it" camp but I guess my footing holes were wider at the bottom than my calculations indicated...

-Tim

[kmoiarty35]

What was the diameter of your holes? Mine are 10" for the column, and about 24" for the pier at the bottom. I have 9 holes. Ken

[Tim Fischer]

Ours were 12" round, with 3 of them having a 17" base (the others roughly

12" -- the city's plan reviewer said those didn't need flareing).

We ended up needing about 6 bags/hole. I'm guessing you'll need more than that, with the deeper depth and wider bottoms, and even with the narrower tops...

-Tim

[LARRY ARNOLD ROGERS]

replying to kmoiarty35, LARRY ARNOLD ROGERS wrote: Hi Ken, I had my deck built for at least10 yrs. now and I did the same as you, I pre-dug my post holes 5 ft deep, I have a tremendous amount of clay soil in my lawn. I used an auger to drill my holes for my supports, and the soil stuck to the auger like glue,but no water. We poured 6 inch columns into the ground for each (3) all together. let the concrete dry for a few weeks before starting on the deck. My deck post themselves are nailed down to the the top of the concrete. I have noticed over the years iregardless of the soil. my deck post have moved considerably and I have to dig back the behind the post ( Damn Clay soil ) !! I have to use a level on the post and hammer them back as straight as I can back into place. So as I see it..........I will never build a deck again on ANY CLAY SOIL OF ANY TYPE. Thanks, Larry Wilder Vt