High Water Table

[dave1949]

It would be a very lucky day around here in our glacial til that an auger could miss all the rocks when boring dozens of holes that have to fall in a particular location and reach a certain depth. Sometimes excavation is the only choice if it is going to be done correctly. A bigfoot form is very useful and cost effective in that situation.

Wind loads would be the force that pushes and pulls laterally and vertically on a sonotube pole footing. Are sonotubes being yanked out of the ground when pole buildings fail, or, are pole buildings wobbling around on their sonotube pilings on a windy day? I don't know the answer to that, but if that is a very rare thing, then the side strength of the hole must be strong enough without effective compaction--with or without a wide foot at the bottom.

In many areas it is impossible to not have water or very wet soil around a pole or sonotube in the ground for at least a portion of the year. I don't know how effective pole sleeves are in preventing the entry of water or moisture, but once it gets inside the sleeve, I assume it isn't leaving any time soon. In some soils, a pole surrounded with gravel is just an invitation for a place that water will fill the voids in the gravel. Concrete is a material that never rots, wet or dry.

I guess it depends on what a person considers or needs the useful lifetime of the building to be, and how much effort or money should be spent to get to that goal. There is no way a wood pole stuck in the ground is a 'forever' building. It could be that 30-40 years is useful enough as a trade-off versus the initial cost. Utility buildings serve a need, and needs are going to change over a period of decades. It costs quite a bit to have a lot of money wrapped up in a 'forever' structure too.

 

[zing]

I am with Dave on this one. Frost action happens in the top couple of feet as the ground freezes and thaws repeatedly. The water will get in the ground anyway sonotube or not, because any tiny bit of moisture will expand the ground around it when it freezes and then when it thaws more water will flow back into the space to take up the expanded space. As this happens over, the path of least resistance is of the water to expand upward, even if that means pushing rocks and stuff upward with it. When you dump cement into the ground without the sonotube there are numerous spaces in the dirt that the cement flows into and create ledges that the frost will push upward on when the ground freezes. Holes with a wider mouth than base also create a place to frost to push upward. The worst is is you dump extra concrete over the top and create a mushroom cap. All of these will lift your post in a few years of freeze/thaw cycles. Having the sonotube keeps the cement in a tight column so the frost has less to grip and push upward.

The thing to remember is that the ground around your post will be pushing updward when it freezes, so the less it has to grip onto the less it will carry your post up with it.

 

[tcreeley]

I put a line of 6x6 pressure treated rough posts for a fence along the barn side. The horses moving all winter along the barn drive the frost deeper. I have the barn on 8" cement filled pvc sauna tubes. The idea is that the frozen ground/ice/water cannot freeze to the pvc tube and heave it up. It is working. The 6x6 wood pt posts down to the same depth 50-55 inches are heaving from 3 to 4 inches a year. While the ground is soft in the spring, I use the underneath of my FEL bucket to push them back down. I think the frost is catching hold of the rough wood on the sides and heaving. Every spring the water table is within a foot of the surface. Dig a hole and withing a couple of hours it is full of water. -It makes full basements below depth impossible unless they are built at grade and backfilled. Even then they have sump pumps.
For horse fencing there is nothing that doesn't heave. White fiberglass posts are good because they can be pushed back down by hand. I use steel posts on corners and gates and the FEL to push those back down. When I first came up I used cedar posts and an auger. The frost over time heaved them all and I went to fiberglass posts.

The ground around here easily heaves 6 " up and down by the end of the winter. You can see the left overs in the spring. -We had a 5" hole open up in the lawn and the melt water was pouring in steadily for a couple of days. Someplace the ground had heaved and hadn't dropped back down again and water was flowing in finding new channels to flow through. There is no hole now that the ground settled again.

 

[Cord]

With the footings that we deal with, there is very little gain with the friction. Nearly all drilled footings are only considered to be end bearing. I have done a few friction piles. Those drilled piers were into bed rock and the friction was utilized for withdrawal due to expansive soil. Even then there needed to be over 20' of penetration into the rock to generate enough friction.

The purpose of the big foot footing is to reduce the amount of concrete. To install the form still requires a straight hole.

So long as the footings are below frost (code), don't worry about the water freezing.

 

[Hiltz]

With the footings that we deal with, there is very little gain with the friction. Nearly all drilled footings are only considered to be end bearing. I have done a few friction piles. Those drilled piers were into bed rock and the friction was utilized for withdrawal due to expansive soil. Even then there needed to be over 20' of penetration into the rock to generate enough friction.

The purpose of the big foot footing is to reduce the amount of concrete. To install the form still requires a straight hole.

So long as the footings are below frost (code), don't worry about the water freezing.

Thank you for the insightful and educated explaination.

 

[boggen]

if your hitting the water table. do some good grading of the entire site, so when it does rain / snow melts. water runs away from the shed. good grading now before anything goes into the ground. can save a lot of extra work later on trying to get water to run away from the building. . only dig as deep as you need to go, try to avoid over digging and then adding dirt back and trying to compact the dirt. if you plan for a concrete floor either once built or later down the road. good drainage away from shed now. can keep the concrete slabs from cracking and shifting during frost heaves later down the road.

if good drainage and slopping of your area means putting in some sort of "french drain" setup to drain water away. then getter done and over with. if it means bring in some soil to properly slope the entire site tfor rain water / snow melting runs away from shed then do that for proper sloping of land and compaction,

simply bring in soil to get yourself out of the water table *shakes head no*

compaction of brought in soil / any loose soil is a must. bad compaction of soil = major frost heaving of soil later down the road that may never be resolved. without re digging the area up and properly compacting the soil.

 

[Cord]

Thank you for the insightful and educated explaination.

I just looked up the drilled piers for the job I referenced above. For that project the design assumptions were 30,000 psf end bearing capacity and 3,000 psf for skin friction. This was into competent bedrock (the first few feet being weak). The piers were 12" end bearing with a 4 bar cage. The lateral design values were 75 tons per cubic for clay or engineered fill and 700 tcf into bedrock.

Mind you, this was only a 1 story building built upon really crappy soil.