Spreading Base
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Spreading Base
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First, dig a hole wide enough to fit the spread base — and deep enough to meet or beat the frost depth in your area. Then tamp the bottom flat. Next, place the tube on the base to establish the contact position, and cut the excess rings from the base. Then, attach the tube form to the spread base with several screws, and place the assembly in the hole. Make sure the tube is plumb, and backfill occasionally, using a tamper as you go. Once filled, cut the tube to the desired height, and then it’s time for concrete.
Keep batches manageable by loading two 80-lb. bags at a time into the mixer, turn it on, and add water, aiming the hose toward the back to prevent dry clumps from sticking to the mixer. Mix until the consistency is wet enough to hang together without dry spots, but folds over on itself as the mixer spins. Use a wheelbarrow and shovel to fill the footing. The number of batches needed will vary depending on the size of the form — this footing will take eight bags. As the tube fills, plunge a length of wood into the form — a technique called “rodding” — to help settle the concrete into any voids. To finish, fill the form a little higher than its edges, and flatten the top with a mason’s float.
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The post hole digger who drilled the holes for my deck, used a 12 in auger but used 8 in sono tubes. He cut the tubes about 8 inches shorter than required. Loaded some concrete in the tubes and then lifted them up so that the concrete could flow out at the bottom to form a wide support base that resisted frost heave.--a little cheaper than those commercial plastic bases.
The only issue i see is that the tube between the newly compacted soil will eventually become loose, its better to fill the hole with concrete then use a small section of tube above the soil/grade level. Keep your hole as tight to the diameter of the tube you will be using, less backfill the better.
I would never suggest this method, the side shear from the contact with the soil give most of the bearing capacity and that is jeopardized with tamping backfill.
Anyone who can do all that in "a minute" deserves a medal, or two!
Since most of a pier system's value comes from the "skin friction" of the pier in contact with undisturbed soil, I would want an engineer's approval before proceeding with this solution. I can see it working on a shorter pier, but how "short" would depend on the project's specific loads and the soil/site conditions.
Besides, all that extra diameter excavation and then replacement plus compaction comes at quite a cost. Depending on the size of the piers, it could easily become the difference between a small/moderate sized drill rig and a large rig (even if one is planning on using a post-hole digger), which is very costly AND can become impossible if the site has limited/challenging access issues.
1. This method and the variation that I suggested do pass code and is, in fact, mandatory.
2. The vertical support provided by the wide footing is sufficient, in itself, to withstand the vertical load. (Think about excavating the footings around a house.--much wider than the actual footings and lots of back fill.)
3. The wide footing prevents vertical lift from various forces, even when down below the frost line. As I said, the wide base is mandatory by code, here where I live.
4. The backfilling was done several days later-by me at no cost, and in fact, with little effort.
5. Tubes are left in the ground and remain in contact with the surrounding soil with no deleterious effects. Again, code acceptable.
6. Costly? You're kidding. I live in a very expensive part of this continent. The 5 holes were done using an auger on a "bobcat". I provided the tubes. He provided the labour and cement. Very dense clay soil that I could never do myself even with a power auger. Total, with tax was $300 CAD. Two guys, in and out in about 2+ hours.
7. Too much opinion and not enough fact on this site. This is a structural engineering issue. This is a widely accepted and mandatory method. Where I live, if you don't do it this way, the inspector will make you rip it out. Period, End. I merely suggested a less expensive way. I've also seen plastic bags taped to the the bottom of the sono tubes that allow spread.--also good.
As per "suburban guy" the method is called Haunching .... many engineers will also suggest a piece of #5 re bar embedded in the Sonotube. The weight of a garden shed hardly requires much of a footing spread unless your working with VERY poor soils.
A 12 inch dia footing is allowed to carry no more than 29 sf of non-cantilevered deck by code 2012+. 12" dia circle is 78% OF 1 SF, .78 * 1500 /40 = 29.48 sf. Prove the soil is better than 1500 psf and you fair better.
The shape of the base is insignificant if it sits above the frost line, as all the soil is lifted .
Side shear is insignificant as this is a base bearing footing and not a caisson. So leave the tube in place.
A concrete column footing looks 'widened' at the base because that is it's calculated bearing surface. The smaller column above is only smaller because it can be to save on concrete and soil disposal.
A 12 inch diameter footing is allowed to carry up to 29 sf of non-cantilevered deck by code: 2012+. (12" dia circle is 78% OF 1 SF, .78 * 1500 /40 = 29.48 sf. 200 sf of post supported deck would require 7 of these). Use a bigger diameter of prove the soil supports more than 1500 psf and you fair better.
The shape of the base is insignificant if it sits above the frost line, as all the soil above and around it is lifted by the heaving. In fact, a wider base above the frost line provides more surface area for the frost to heave against and lift the footing, e.g. hydraulics. And that is why caissons have pointed bases not flat ones.
Side shear is insignificant as this is a base bearing footing and not a caisson. So, leave the tube in place and tamp the backfill to 95% compaction (code). Just don't flatten the base by tamping fill. All base bearing footings should sit on undisturbed ground (code). New codes consider existing backfilled foundations as disturbed up to 5 feet from them, so expect to dig those footings at least to the depth of the nearby existing footing no matter where the frost line is.
Vertical steel is only necessary if there is significant concrete column above the surface (susceptible to lateral forces or impact), or a significant difference in diameter of the column and the base footing (key, balancing the spread of the load and reduction of uplift of extended footing). In ground concrete has little tension and steel is always designed/used to withstand tension forces.
In Alaska, it's common practice to wrap the tube on the outside with a piece of good plastic. This helps the dirt from grabbing the tube if there is any dirt movement due to freezing/thawing. It just slides up and down on the plastic. Works for me.
Ktkcad - thanks for the in-depth (get it?) information above. Good stuff.
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