1. I am beginning to understand that what is called "frost heave", is the process whereby freezing soil at the surface of the ground can only expand UP as the water freezes. The freezing soil "grabs" the post and "pulls" it out of WHATEVER is around it below the soil surface.
2. Repeated freeze/thaw cycles of the surface soil "jacks" the post upward.
3. The final thaw at the end of the season results in a contraction of the soil (as the ice melts) which will allow the post to wobble.
4. There is a belief that if you extend the post below the frost depth, the unfrozen soil is stable and will grab the post bottom sufficiently to prevent the surface soil from pulling it upward. This would seem to imply that the gripping force on the portion of the post below the frost line is greater than the gripping force on the post at the freeze/thaw depth.
5. If all the above is true (and I'm just guessing - correct me if I'm wrong), then it would seem that frost heave might be prevented by simply wrapping the upper portion of the post in something slippery (like polyethylene) which would prevent the surface freezing from gripping the post. Which may be why someone's (I can't recall whose system it was) method of using PVC pipe works. The PVC is slippery enough that the ice can't grab ahold of the pipe. (As well as Gordon Watson's method of using steel galvanized pipe).
Just my thoughts. Any comments/criticism welcome.
royce in SB - where I will NEVER have to deal with frost heave.
Rich wrote:
I do not concur with your analysis. We are discussing a small area post and the placing the base of this post below the "frost line" will have little benefit to stability. As the layout does not cover a large area it would be reasonable to assume that the conditions are homogeneous. The depth of placement will not have any large impact on individual side thrust and thrust from the freeze / thaw cycle will have mostly equal impact on all posts. Posts placed on severly sloped ground will tend to shift due to the movement of the soil with time and depth will not make a lot of difference.
I will not comment on the benefits / disadvantages of placing a post into concrete.
As an aside. I am living out in the country in very hilly ground that has terrible bearing ratings. It liquifies when wet. And the soil is almost impenetrable when dry. I can look down about a quarter mile of fence from where I am sitting. This fence has been in place for probably 30+ years and is composed of original oak posts, replacement metal T posts and treated replacement wood posts. The fence was built by hand in the late summer. I do know who constructed it. The fence is as straight today as it was when first constructed. The original oak posts are about 2 foot into the ground as the original intent was to rebuild this fence the next spring. The barbed wire is holding it up.
I would suggest placing the posts deep enough into the ground on a gravel bed to provide the lateral support required by the layout design and back filling with dirt. Lateral motion is going to be a larger problem than any frost induced vertical motion or frost induced differential motion.
On Wed, 05 Jan 2005 14:10:30 -0500, Peter Foley wrote:
->At 10:48 AM 1/5/05 -0600, Rich wrote:
->>OK, now here is my take on this "frost heave" item. To have any 'heaving" ->>there has to be either differential expansion or contraction. This will ->>not happen across a free floating post that is in the ground almost any ->>distance at all.
->
->I have to disagree, Rich. To be sure that a post (a fence post being the ->one we are all most familiar with, I think) will remain rigid (not wiggle ->when you push on the top) and remain at the elevation it was installed at, ->the post and/or post plus footing has to extend past the maximum depth of ->frost penetration. All the concrete does is provide an un-compressible ->collar around the post to stop it moving side to side and better frictional ->resistance than the post alone. If you look along a long fence run ->(particularly one with a continuous top rail), you invariable notice the ->post that has moved - look at the base and you will see the concrete ->footing popping up out of the ground. The amount a post with an inadequate ->depth footing will move due to frost is a product of how much water the ->soil around it holds - topsoil is the worst for this because of the high ->organic content. Silt is second worst and sand the best. Clay may be good ->or bad, depending on the depth of the natural water table and the degree of ->surface slope - flat areas on clay soil where water ponds and stays after a ->rain are bad for frost movement, sloped area where the water runs off ->easily have few problems with frost heave.
->
->The differential expansion reasoning that you quote refers to the movement ->of two dissimilar materials that will be caused by a change in ->temperature. The most common example people would be familiar with is a ->fracture of a reinforced concrete slab due to differential expansion ->between the steel reinforcing and the concrete. This is really due to ->inadequate cover of concrete over the reinforcing or inadequate slab ->thickness to resist the forces of differential expansion. This term ->doesn't really apply to the issue of frost movement of a post.
->
->The second thing that stops a post from moving from frost action is the ->friction between the footing and the ground around it. This is why a ->concrete footing will perform better in this regard than a post with native ->or granular backfill around it - the frictional resistance to movement ->between the concrete and the ground is higher than between the post and the ->backfill, because the post has a smooth, relatively impervious surface and ->concrete is rough.
->
->For people who have had good results with what, on the surface, appears to ->be an inadequate 'design', it is more a matter of good luck with your soil ->conditions than anything else. Unless the person you are advising has very ->similar conditions to you, it is unreasonable to assume that they will have ->a similar performance, which leads me back to my original suggestion - do ->it 'right' the first time and you won't have to do it over again.
->
->The idea of a completely floating installation (deck blocks on the ground), ->is quite workable and will perform acceptably provided the soil conditions ->(type of soil and depth) and surface slopes are relatively uniform across ->the site. Where they aren't it will be necessary to do minor adjustment to ->the level of the track every year or two or three. If this level of ->maintenance is acceptable to you, and it's the best priced alternative, ->it's the one to choose.
->
->I'm with Dave on this one - I over engineered my own because I'd rather ->play with my trains than work on my railroad. Chacun a son gout! (sorry, ->no accents available in my mail program)
->
->regards,
->
->pf
->
