How to stop a sinking deck post?

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Old 08-28-10, 02:20 PM
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How to stop a sinking deck post?

Hi all,

I have a problem with a deck corner post that is sinking. I tried to fix it once, but it continues to sink, I am hoping someone has a permanent solution. The specifics are:

I built a 9' high 10'x27' deck 10 years ago. My house is a daylight basement house and the ground slopes away from the house. The deck has held up well, except one corner started to sink 4 or 5 years ago. Last year I dug down below the footing, jacked the deck and footing up 2 1/2 inches and placed bricks under the footing. That fix lasted for about 9 months (I checked for level every month). In the last three months the post has started to sink again. The post has now sunk almost to the level it was at before last year's fix.

The deck is built using 4x6 posts on 1'x1' round footings. The footings are 1' deep into the ground (i.e. the top of the footing is at ground level. The frost level is at ground level here in Portland, OR). The posts are attached to the top of the footings using a j bolt in the footing and a metal post bracket. The posts show no sign of decay. The deck header is built up from two 2x10s with the post sandwiched in between (I cut a 1" ledge on each side of the 4x post at the top of each post, then bolted the 2x10s to the top of the post with the 2x10s resting on the newly cut ledge, with the remainder of the post in between the 2x10s). The joists are 2x10s and are spaced 12 inches apart. The decking is 1x6 Ipe. The posts are spaced 9' apart. I dug down to undisturbed dirt (house was 24 years old at the time I built the deck) to pour the footings. The ground is clay, which may be causing my problem.

The only solution I think might work is to again dig down to the bottom of the footing, jack up the deck, remove the bricks I placed there last year. Then pour cement under and around the existing footing. By doing this the weight of the deck corner will be distributed over a larger area and hopefully will not sink again. I have a few questions.

1. Does this fix sound like it will work?

2. How large should I make the hole around the existing footing before I pour the cement?

3. How deep below the existing footing should I dig before I pour the cement.

3. Should I use hydraulic cement under the existing footing?

4. Any one have a better solution.

Sorry for the long post, but I wanted to be sure to include enough detail so hopefully someone will have a good solution.

Thank you for your time.

Jim
 
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Old 08-28-10, 04:51 PM
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Sounds like your footing is built on fill. The only thing you can do is put in temporary support, remove the post and old footing, then dig down to virgin soil and place a new footing at this level.

To determine when you are at virgin soil you can get a piece of re-bar from the local lumber yard or big box store and bend it into an "L" shape. When you can no longer push this into the soil, more than an inch or so, using your own body weight you should be at virgin soil. At least that's how we determine it around here. There may be a different procedure depending upon soil type in your area, and you might have "bad soil" in which case you would need an engineer to suggest a remedy.

I am guessing that you did not get a permit for the deck, correct? A footing inspection should have caught this problem.

Good luck,

Bill
 
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Old 08-28-10, 05:13 PM
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I did get a permit and the deck was inspected (it took six months to get the permit because a ditch, or intermittent stream as the county calls it, was within 200 feet of where I originally wanted to build the deck, but that is another long story ).

Because the ground does not freeze here, code allows deck footings to rest on top of the ground. But I went the extra step to dig a hole down to undisturbed dirt before pouring the footing using a 1' diameter sonotube.

If it is fill dirt, it has had 30 plus years to settle since the house was built in 1976. The other footings are on the same slope and have not sunk at all.

Any other suggestions?

Thanks for the reply.
 
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Old 08-28-10, 05:29 PM
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You had fill dirt there and it can vary widely in terms of stability and shrinkage/expansion becaue is not a controlled and compacted. The nearby water could also have an effect depending in the water amounts over the last few years.

Some soils never compact by themselves over time.

Because you probably did more than normal, the inspector did not question the soil at every hole and force density/compaction testing.

In any event that particular post location is on poor soil. and the brick just pushed down into a soft soil because there were carrying more load to the soil.

Your plan sounds good. Go as wide as possible to put in a spread footing while the load is removed from the deck. Make the footing as thick as possible, but the minimum thickness is at least the width of the distance from the face of the 12" tube and preferably more because it probably not placed, vibrated and finished like most concrete footing poured when there is better access.

The added area will reduce the load on the soil and sure beats replacing a post and getting an excavator in if it is done by the book.

Dick
 
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Old 08-28-10, 07:22 PM
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Thanks for the reply.

I think you are right on the inspector, it took all of two minutes to do the inspection. All he said was it does not take him long when the job is done right.

I think I can dig around the existing footing enough to create a 2' diameter footing and may be able to dig under it another 6-12 inches. Hopefully that will be enough to distribute the weight better and stop the sinking.

Thanks for the help.

Jim
 
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