Thoughts on this alternative pier method

[speedfunk]

Another post got me thinking about a good way to use wood post piers , keep them stable in relation to the structure above and keep them dry so they do not rot.  I tried to use paint to illustrate what I'm thinking. ..... a bit crude but I hope you get the idea.   This design is something I'm thinking of for a rental build in the future.  This would be kind of for an environment like NY where 4' frost line etc

1.  Create in this case 3 long trench's approx 5-6 ft deep in native soil.  Have trenches so that move water away from center of where structure will be
2.  Fill up trenches with around 1 of 2 feet of crushed stone 2" or smaller
3.  Insert Log posts.  Maybe connect with floor joists at this point to insure they are in line/where they need to be for above structure.
4.  Backfill around posts with more crushed stone.
5. Shape earth around to pitch outward from structure as much as possible.


Click on pics to make bigger.  I 'd really like to know what you think...possible good points and bad.  Thanks a lot!!

[Squirl]

Why use piers at all?  It just seems like a waste of the wood.  I believe Alan Gage did just the gravel trench foundation with a grade beam on top.  I have read about it more with straw bale houses.  I sometime forget about this method. The wood just seems like a way to allow for shifting and rotting. I would rather distribute the weight over a larger surface area if possible.

Also you and I are in the same backyard.  You have more experience with other inspectors in the county.  I am familiar with the one inspector you are not.  When I mentioned piers, he gave me a sort of disapproval facial gesture, mumbled maybe, and that I would have to show him the numbers and skirt the building anyway.  He is not a fan.  Crushed gravel is at least an approved code footing method under PWF and is common as part of a foundation.  With a full wall above that, I can't imagine you couldn't walk it past an inspector.  As long as it is under 1500 square feet, you could get away without an engineer stamp.  The stamp alone would have me ordering PWF lumber from picketts.

I think a problem with this and Alan's type is a problem he mentioned.  It is difficult to expand.  When you trench down to add on, it can destabilize the structure. 

Also, I checked into the cost for this on my 20x30.  The mini X had a bucket width of 2 ft (I don't know yours).  A load of crushed stone from the sand and gravel place was $300 per 11 yard load, it would have taken at least 3 loads to fill it to grade.  Although it is a lot less work, cost wise it is only a little less than filing it with concrete blocks and the backfill you just dug out.  When you add it the posts, beams, skirting, and bracing, it would probably be more expensive.

[alex trent]

1. Would not the wood below grade still get wet from infiltration of water into the gravel. So, while this is no worse than burying the post in soil it does not get rid of the wet wood and rot problem you have to deal with.  Wood does not have to be fully saturated every day to rot.

2. Burying the posts and tamping in soil around them inhibits uplift and also provided some measure of bending resistance. This does not seem like it would do that as well...anybody have a table for the uplift and bending resonance of soil, gravel, etc?  I have on a wile back but cannot find it.  i believe DonP sent it to me.  Had a good diagram of how posts behave in the solid under lateral resistance.

3. If the answers to 1 and 2 above end up to be not a problem, why not just do two-foot wide holes around the wood put gravel in and save a lot of digging and gravel filling?

[MountainDon]

The problem with the proposal, as I see it, is that the piers are still relying on soil/dirt/gravel infill to resist lateral forces that will be applied to the top end of the piers. With all that digging / trenching going on, that labor may as well go towards a more conventional foundation that because of the way it is constructed has built in lateral resistance with the forces well distributed throughout. 

The use of wood does not bother me as long as it is foundation grade PT. Rather it is the missing parts that provide for a self bracing rigid foundation unit, as one would have in the PWF foundation or a CMU perimeter, or a poured concrete foundation wall, or a concrete slab of some type.

[speedfunk]

Squirl :  Thanks so much for all the great detail      You bring up a lot of great points esp about cost..which is what I really after.  Thanks guys.  I will check out alans thread now. 

MD: TY very much also.