post & frame shed-- related to roadbed stabilization! 2

[schreibs]

OK guys, Here goes again.

I last asked for your help stabilizing the roadbed for the 1/4 mile lane to my planned home across virgin soil. Now, I'm building a simple post and frame shed in the same area as this roadbed, laterally in the same position at the base of a small forested hill. The location is in the same field I referred to in the previous thread.

Here is the problem:
-- building 200 sq ft shed(keeping it small for now) using only 8, 4x6 posts, simple lean to shed but must place the post bases below frost level-- 42" here. The grassy field here drops about 6" per 10 ft. Placement is about 20 yards from a wooded hill, with an expected increase in the rise up.
-- augering the post holes yesterday I did not get far, just the first hole. At about 3 ft I hit mud, looking in, I could see water flowing in, a few minutes later the sides of the hole were caving in! After the dust(ummm, water) settled the water level was only 18" from the top of the hole.
-- I tried again closer to the hill about 30 ft from the first (i.e. uphill slightly, maybe a foot ) and hit water again. So, now, two artesian wells! This one settled out about 3 ft from the top. So, the water table recedes a bit as the land goes uphill. No surprise.

I really want to build this shed to house some equipment over the winter before building the home next spring. Moving up the hill into the forest before the hill rises drastically is a possibility but it some real downsides I do not need to detail just now. However, doing so would likely get to a point above the water table based on auger drills.

I don't want this to be expensive for a simple and small shed. I don't want to get in a big cement truck and wreck my newly fixed lane when I KNOW now that the lane also has a high water table beneath it and was likely the main cause for all the work I put into it. Any ideas how I can place 8 dang posts for this shed without having to place concrete piers or some other costly foundation support?? OR, another way to address this is not to place the posts into the ground but ONTO something(?). . . I have some thoughts on this but will hang fire until I hear your own!

 

[Ron]

Auger down through hard section into "mud". Put in 8" PVC pipe till you can't push any more. Fill pipe with concrete. You can drop in "Sakrete" or similar from the bag...let the groundwater provide the moisture to hydrate. Add single rebar down middle before concrete sets. Attach wood posts to concrete after curing. Use knee braces in the structure since this method provides no lateral resistance at the bottom of the posts.

 

[SlideRuleEra]

Ron - That is a great answer! Dry-Sankrete-In-Groundwater is a good trick to have up your sleeve. Also comes in handy as backfill for setting things like fence posts, mail box posts, poles, etc. in wet, poor soil.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[schreibs]

Hey Ron. That may be just the sort of solution I am looking for. However, before I jump on this, let me clarify my understanding and you can keep my running straight here. . .

NOTES: I'm concerned that reaching the point of being "muddy" will occur significantly before Minnesota min frost depth of 42" thus making it impossible to get below frost depth for footing.
Just the same here is how I would consider doing this.

an expanded process description:
Plan-- 42" is frost depth here and I want 4" additional height for drainage slope from building to surrounding land. If my original post height for a post and frame building was 150" I will cut my post by 46"(104" now) and make up the difference with a 46" tall PVC formed concrete pier. All piers must hit the same top elevation. I realize I cannot use pre-cut 46" tubes because some will go deeper and some shallower after pile driving. I will plan to use the auger bit / skid steer to press it deeper till fair resistance is noted. Kind of like a pile driving . . . After driving and leveling, each post will have to be custom cut at top to hit planned elevation of laser line.

In order:
-- cut, say, a 60" long x 8" dia PVC tube and 80" piece of rebar.
-- set up laser to mark line for final elevation of top of all posts.
-- auger till muddy(24", 38", 43". . . ??? who knows?) , place 8" PVC pipe.
-- determine what the difference in height is between the 60" tube and the 46" needed for laser level height for top of 46" pier; cut PVC to place top at that level. (If can only hit 32", say, too bad it will be above frost footing.)
-- using only bags of dry Sacrete fill the 8" tube; plumb and level it, brace it in two directions at 90°. Add water as necessary to the column of powdered Sacrete (as the water in the bottom of the hole is dried up) and rod the mix till it reaches correct level line; drop in #4 rebar about 20" too tall and plumb it at center of pier.
-- wait a few days till the pier cures completely before further work.
-- cut off the rebar so about 4 to 6" protrudes and drop wood post with a centered, 5/8" hole in it over the rebar to center it.
-- use grade board to connect posts at bottom outside faces of posts; also mount knee braces to inside of grade boards at 45° up to posts for lateral bracing.
-- for the 2nd thru 8th posts use same final height of PVC tube regardless of penetration depth into soil(mud?), regardless of ability to reach below 42" frost depth.

Is this correct or CLOSE? Please "fix" and paste in "improved" version with a repost. thanks!

 

[Ron]

Thanks, SRE.....like you, I deal with groundwater a lot!

 

[oldestguy]

Shreibs:

That 42" "frost depth" is just a cook-book thing to prevent sitting conventional footings on soil that may lift the footing. . Once freezing front gets to that water table, the freezing progress down is much slower, due to heat of fusion given off as that plentiful water content freezes. I'd not worry at all about some magic "frost depth", since the heaving, if any is above that, more likely gripping your posts sides. You have minimal "footing" at the base of the posts, so possible frost lifting from there is very unlikely if you get a little ways into the water table. So going deep to reach some magic depth is impractical as I see it. Placing proof loads on the posts as you apparently will do should be sufficient.

On depth of freezing, it all depends on many things and can vary in your area from near the surface (as in a swamp) to 8 feet or more in clean low moisture sand, depending on other things equal, such as temperature of air, insulation on surface (snow depth and grass). In your Minnesota lakes have you ever heard of 42" thick ice under a foot of snow? Very unlikely.​

 

[schreibs]

Sounds like I have a an adventure in front of me here. I plan to go for it. My biggest concern is accurately placing the pile centerlines and making each plumb. Considering using a pile & beam construction now instead of just extending the pile up into a post due to probability of poor accuracy. I can place the beam over inaccurately set piles and still extend my posts up from the beam with accuracy by only having to level and square the beam.

In addition, with the one long side of the shed(22 ft) running parallel to the hill and placed on a higher perched water table, during the rainy season it is more likely to drop into the mushy ground than the other long side resting on an area where the water table is a bit farther from the surface. I can place shims between the beam and the piers as time goes by if the low side settles. . . Extending each post up from a pier could be more problematic with settling as each pier could settle differently whereas I have better load sharing with the beam plan.

AS to the frost depth, thanks, that makes a lot of sense, not to mention the overall temp of earth being about50°F preventing the water table from freezing and the fact that there is water movement through the water table continually refreshing it with constant temp water, significantly above freezing point. My biggest hope here is that the friction of the pile is enough but feel with such a small structure it should be no problem.

thoughts on these?

 

[oldestguy]

Not sure of your details, but for a shed, maybe no point in gilding the lily. If it were to become more of a thing for good appearance in years to come, some thought of wind situations. Tying tying building down then should be considered now. The buried part of posts will have resistance to uplift. That's more likely a loading that will take it down than snow. If you will have a floor slab that can be the anchor also.

 

[allgoodnamestaken]

Just an fyi - 42" is the frost depth for the region, I'm assuming, but frost under roads can reach significantly deeper. The lack of greenery, snow cover, and movement above can do that.

Based on what you've described all the conditions are met for potential problems for frost action - presence of free water from the high water table, mushy non granular soils that sound frost susceptible, and cold enough environment to cause freezing. It can cause substantial heaving and upward movement on lightly loaded piles. I don't have enough experience with this to comment on it - maybe another forum member can tell me if I'm totally wrong.

 

[schreibs]

thanks guys. Still looking for input on the pier and beam construction idea vs extending posts all the way up from the pier. . .

 

[oldestguy]

I could argue with "all good ..." with my field experience, but won't. As to type of construction, considering the potential for wind, I'd take a look at how pole barn buildings are built, generally with the pole extending from in the earth and up to roof zone providing good resistance to lateral loads as well as uplift. As to designing the pole depth, look at the post I added to a thread that dealt with this, under the general subject of geotech formulas.

 

[oldestguy]

there is a link that came up under a search for "pole barn details". I think it may answer some questions for you.

http://www.coloradochaptericc.org/pole-barn2k09.pdf

 

[allgoodnamestaken]

I could argue with "all good ..." with my field experience, but won't.

I'm not worried about arguments. I read this forum to read about situations I haven't gotten into personally, and the way other engineers think about fixing or addressing issues.

It sounds to me like he's in a place where frost action could be an issue, but I could be totally wrong. From an experience standpoint I don't have enough to be confident, and I said so at the end of the post I wrote. If you have experiences that tell you it wouldn't be an issue, I'd like to hear about it and what criteria I should be looking for when identifying something like that.

 

[oldestguy]

"All-good..." Interesting subject, but my "argument" about "frost depth" is that 42" is not necessarily meaning anything at this site, due to shallow water table. For example, in LaCrosse, WI, on the Mississippi, with deep sands there was a heating gas explosion in winter before Christmas. As a part of my investigation as to why, I measured below 32 degree F ground temps down to 8 feet below sidewalk then in non-frost susceptible soils and there was no heave. Just one example that there may well be reason to not take the 42" "frost depth" as gospel all places. A Google search of frost heave and soil freezing brings up lots of info. One of the more complete references shows the complexity of the subject.

http://web.mit.edu/parmstr/Public/NRCan/CanBldgDigests/cbd026_e.html

At the job site there may be some frost heave, but it more likely will grab those posts and do what ever lifting happens. That's known as frost jacking. But for a shed, I wouldn't care if it was mine. Going down say 6 feet, it well may still happen. I'd just go to suitable bearing in warm weather and forget about winter.

 

[schreibs]

OK, I'll try again. . . how about pier and beam guys? See attached pic

The idea being to NOT extend the posts up from each pier but let the piers support the beam and build the posts up from the beam.

One more thing. Forget about heaving, I believe I got that covered. Besides the pointers already made about mass of earth at 50°f, heat of fusion, water moving through the water table there also is the great heat conduction of moisture keeping the pier area warm, and, better yet my plan is to use LOW frost footing technology also. Basically, that is placing a layer of EPS foam insulation about 12" under the soil top surrounding the pier thus preventing frost penetration. Used a LOT in Scandinavia.

LOOKS like this for a home. I will just make it circular!
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1446756972/tips/Frost_ftg_wro6dn.pdf[/url]

 

[oldestguy]

Schereibs: Post and attachment makes sense if you are building an enclosed structure that will be heated and there are conventional footings, etc. For the insulation around the buried posts, yes, a good idea. A note about my experiences. Dealing mainly with highways in Wisconsin, it appears that, in general, if you can keep the water table more than 5 feet below grade (of road), there will be no noticeable frost heaving (assumes frost susceptible soils). Thus, unless you insulate under a unheated building, the interior will experience frost heave due to that shallow water table, assuming the soil is frost susceptible. Roughly if the minus P-200 sieve size is more than 5 percent, frost susceptible soil is present. The more silt size, the worse it gets. In clean sand, no noticeable heave with shallow water table. When I talk "heave: I mean many inches, visible to the eye.

 

[Ron]

In your post of 11/5 at 2:44pm, your procedure is fine. I would add that you should coat the top of the concrete and the rebar dowel into the wood post with asphalt mastic (roofing cement). Add a couple of large washers under the post to give you a little capillary break and allow drainage. Install the washers before the asphalt mastic.

As for your PVC casing....drop a water hose down the pipe with a spray nozzle on it. If you keep the hose at the bottom of the hole and on "full blast", you'll be surprised how deep you can push that pipe with the help of your skid steer hydraulics. Cheap form of assisted jetting.

You will need to consider uplift of each post, but you can just strap them down to the concrete piers.

 

[schreibs]

Ron, thanks for guiding me here, especially for reviewing my "process plan"!

I understand Simpson makes some great post mounts that have a slot in them. Requires threaded rod, J-rod etc. instead of rebar. This makes sense to allow me to fix inaccuracy of pier placement. What do you think?

This is out 1/4 mile from any utilities so jetting would be tough without an electric pump, of course I could rent a gas pump. I will try the first hole and consider the options you offer.

 

[Ron]

screibs...yes, Simpson post anchors are good. You can put a threaded rod down the center of the pier instead of rebar. Increase the threaded rod size to 5/8" instead of 1/2" that you have for the rebar.

 

[oldestguy]

Hope progress is good before weather sets in. One thought that may or may not apply, Do a proof load on your foundation posts before trimming. If possible take your skid-steer loader and apply the full load of a full bucket on each one. Not just for a second, but longer if possible.

If one has a post already set in the ground and wants to do this, the use of a chain wrapped around the part sticking out of ground makes a good grip. Attach chain to loaded bucket. Sort of the reverse of if you wish to pull fence posts. Works even on slippery stuff like PVC. Even only two wraps with both ends connected can work, but more is better.