Expansive soil guy is back again. Here in Colorado we have, in some places, terrible problems with Bentonite – expansive soils. Plays havoc with everything.
Putting a 8′ wide by 24″ long redwood deck off the side of the house. Question is how do I anchor the 6 x 6 (yes overbuilt but stuff is free) cedar pilings into the ground to support the deck joists in this type of soil.
Options considered after sheeting the soil with plastic membrane to keep the water off the soil:
A) Level the dirt…. place concrete footings on the ground & bolt the cedar to the preformed footings….then pray.
B) Sink the cedar in concrete caissons (how deep) to support the deck. Not fun but will do it it it saves the deck from heaving over time.
Any ideas ? Horizontal structures?… horizontal 6 x 6 to spread the load out over the ground?
Replies
Here in Northern Virginia we also have expansive clay- deck footings in this kind of soil have to go 4 feet deep with 16" square footings, even though the frost line is nominally 2 feet. And 6x6 posts aren't overkill- that's what the code requires here.
Does your local building inspection agency have a web site with information on what's needed? You can visit the one here with clay information at http://www.fairfaxcounty.gov/gov/DPWES/publications/marineclayproblems.htm or http://www.fairfaxcounty.gov/gov/dpwes/publications/unmap.htm for iniformation about engineered and "standard" solutions for problem soils. BOCA and CABO are the codes here, and maybe also where you are, so the information might help you even though you are in a different state.
Poke around the web site for a lot of good information on projects.
You need a shovel.
or
A backhoe.
Your footings need to be below frost level. You can buy precast pyramidal shaped concrete piers for the purpose - 38 bucks each in my area.
Or use a simpler method which requires PT posts. After digging deep enough to get below frost (I'm betting three feet or so in your area), drop in a bag of Sakrete still in the bag so it sets flat in the bottom of the hole. Ground moisture will turn it hard soon enough. Set the post directly on top of it. Wrap the post with plastic or rip some stips of styrafoam to place on the sides of the post underground. This becomes a slipform. It will stop the frost from freezing the soil to the post and lifting it up, necessary since the post is not anchored down under.
I can think of four porches in my area that were built without going deep enough. every winter the posts lift a little higher, soil washes in under the wood so it can't settle back down in spring thaw, and the porch keeps getting a greater lean back towards the house - bad for the drainage plane.