Pond Boss Forums POND CONSTRUCTION Soil questions Dock/Bridge Pylons vs Soil Depth

I'm about to put in 4 (4"x6" pressure treated wood) pylons, through the ice. upon which I intend to build a 32' long bridge which will span a 30' wide canal. The canal connects two ponds and is about 7' deep in the center. (i.e. the canal side slopes are sort of steep.) The pylons are to be located about 1/3 the distance from the shore, 2 from each side, where I estimate the water depth to be about 3'. The pond bottom soil is yellow clay and then blue clay.

My question is how deep in the clay should the stanchions (pylons) be so that the ice will not lift them up in the winter? I'm in southern Michigan where the winter ice can get to 8"-10" thick. Any other thoughts on this subject are also welcome. Anyone have any experience with such a construction subject to seasonal ice.

If it makes any difference: I'm not sure how much the wood bridge will weigh but my guess is several hundred pounds due to length and 6' width; basically a wood deck sort of construction.

Here is a wiki article on how to use "jetting" to set the piers....at the bottom of the article is mention of setting the piers deeper to prevent prolems with ice uplift:

http://www.wikihow.com/Install-Posts-in-the-Water-for-a-Dock-or-Pier

This wouldn't be the most aesthetically pleasing alternative, but I wonder if you could just slide a 3 foot long piece of 8 inch PVC pipe over the treated 4X6 and fill the top 8 or 10 inches of the space between the PVC pipe and the wooden pier with vegetable oil???? Since the oil floats it will stay confined inside the pipe and allow the pipe to slide up and down along the pier as the ice layer rises and falls.

I guess if you wanted to, you could fashion some sort of a wooden facia on the outside of the PVC pipe with some 1 inch plank lumber to hide the pipe.

Do you think the PVC would hold up to the external ice pressure?

With a 30' long, 6' wide bridge, I think your bigger concern would be "What will keep the 4x6's from sinking deeper into the bottom with the weight of the bridge on it over time?

esshup, You bring up a good point. Contractors have told me that a clay bottom would support such a structure if the stanchions are down deep enough to reach the compressed clay. I have seen some very hard blue clay come out of the pond when it was being dug. I could opt for PT 6"x6" but that would be more difficult to get more depth. or I might be able to drive in another set of four for additional support but im still concerned about the proper depth to resist ice lift. Maybe 8 4"x4", Who knows?

I've never seen any pier lift due to ice unless the pier was low enough in the water that the ice actually lifted the pier itself, not just pushing on the supports. Are there other piers or bridges in the area that have had that problem?

We have a couple of wood docks on Earl lake where I live, that are pretty big, built on wood stanchions but they are in about 12' to 15' of muck and marrow bottom. They don't seem to lift that I know of, but that is a lot different type of bottom. My apprehension is that the clay will be impenetrable at a much shallower depth and would not therefor offer as much resistance to lift. The contractor I have lined up for the job seems more interested in achieving downward support than the ice lifting potential.

I'm Wondering if maybe I should wrap some sheet metal around each post at the waterline so the ice dosen't have anything to grab onto?

I tend to agree with the contractor, and think that ice lifting will be a non-issue.
Yellow Pine weighs somewhere around 37#-41# per cubic foot (roughly one 2x12x8). How many cubic feet of lumber will the bridge be made from?

You will have 10 sq. ft. of interface between the 4x6's and the ice if the ice is 18" thick.

The other possible problem would be shear forces if the ice is subject to high winds during a thaw or in the spring. Is that area fairly well sheltered to high winds?

The total square footage of the Bridge is 225.sqf. In this neck of the woods structural timber is usually what we call "Hem-Fir", which is most of the time Douglas Fir. Notably PT DF DOES weigh more than untreated DF. I was planning to use 5/4 cedar deck planks for the actual decking. I will also construct a 42" high hand rail on both sides of the bridge, also made of a combination of PT and cedar balistars.

Because both ends of the 12'long bridge ramp sections will be supported from the ground at both the shores, the four stanchions will support the full weight of the 9' center section and 1/2 the weight of the two ramp sections. ie effective bridge area with associated weight bearing load is 153.sqf. I have not completed a material list so I don't know what the board feet will be yet.

Shearing 10.sqf of ice sounds like a very difficult thing to do. I imagine the bridge weight would have to be in the thousands of lbs to break that level of adhesion between the ice and the 4 wood stanchions? On the other hand the ice is usually not that thick.

Yes, for the most part it is somewhat sheltered being that the connecting canal is at an angle about 90 deg from the prevailing west winds.

Cecil had a birds eye view of this recently , but I've observed pier pilings/pipes with open water around them in close proximity even with enough ice on the BOW to safely support a person on warmer, sunny days.

You're right, shearing 10 sq. ft. of ice would be very difficult.

Have you found any reference to how much ice will uplift due to freezing if it has water under it?

OK, for what its worth, I just got back from the bridge site with the contractor who MAY make the install of the four pilings. What he says is the traditional solution for sheering the ice at the piling is to wrap the piling with hemp rope. It seems to me I have seen a lot of pilings around piers before. I never looked close at the water line but apparently that's at least partially why the rope is there. This may be worth a try. It's cheep and easy to do.

Another consideration regarding ice lift occurs when the pond might freeze at a lower level, and then continues to fill during a January thaw (lets say) and lifts the entire ice sheet. This would be in addition to the normal expansion of ice as it freezes.

The contractor say 4' to 5' depth in the clay should be sufficient to reach the "hard pan". He uses air hammers to make the install and just keeps going till the piling wont go any further. They will use 16' - 4"x6" to do the job. Waiting for more ice however.

I'll attach some pics when it happens.

I've never heard that about the rope, interesting! The air hammers sounds like the easiest way to drive the pilings.

Wow, maybe I'm just nuts but I just don't see this lifting occurrence as a threat for your project. We have 2 piers (one of them pretty large with covered pavillion) on a pond that freezes. The water level always rises during the winter freeze. That is when we get our topping from the lows of summer/autumn to bring it up to full pool. We are on 6 x 6 treated timbers. I have noted no appreciable elevation change.
I understand the simple physics in play here, but wonder how much they would really affect a 4 x 6 timber. Of course, I doubt that I have seen 8" of ice encapsulate our verts, either. My only concern has been the more likely event of shifting and wind-blown ice floes.
Interesting concern. I would like to hear more evidence of the threat on your project.

This might make the wood a bit more slippery.....
pilewrap

I think the idea is that the hemp will/should move with the ice sheet and the vertical sheering takes place between the wood piling and the hemp. Sort of like placing a soft bushing between the piling and the ice slab. It is therefor important to wind the hemp rope well below and above any possible ice thickness or elevation on the piling.

My would be contractor wanted $1,100. to put in the 4 pilings. I'm still looking.

Some food for thought.

It is well known that water expands as it freezes. Just check your ice cube tray in your freezer sometime; if you still have one. On my farm (clay loam soil) it is very common for small tree seedlings planted in the fall, to be lifted out of the ground by the seasonal frost; sometimes an inch or more. Have you ever heard farmers say they felt like they were growing rocks when talking about some of there fields? It comes from the frost "heaving" up the stones which may be partially surfaced. If the frost gets deep enough to get under them it will heave them up a little each season due to the expantion of wet/frozen soil.

The same thing happens in the pond only if the piling is straight up and down there is not much lift just due to freezing because most of the expansion goes into a clamping action in the horizontal direction against the piling. However the result is sort of like floating your dock, or in my case, the bridge on top of the ice, sense the ice would have a solid hold on the pilings. Thus, any increase in the pond water elevation during the winter season which would raise the ice slab is likely to lift the structure with it. (Depending on the ice thickness)

In case you are thinking that the weight of the bridge could not be lifted or supported by an ice slab, I have driven my 1400 lb tractor on 4" if good ice with no problem. (I don't recommended it however).

How about spanning the 32' with steel trusses. You could face them with wood if that's the look you're going for. The K-buildings I used to build http://www.kbuildings.com/commercial.php use some very strong, lightweight trusses and the trusses have pockets for attaching roof joists. They're fairly low profile too. I would lean toward using 3 as opposed to 2 for redundancy and lower profile.

Thanks for the link Ryan,

I see the trusses but I wonder if they make them to order, what material they use and of course what they cost. I'll try to call them tomorrow and see what they have to say about my application.

An even simpler solution is to use an old 8x40 railroad car.

http://www.americantrails.org/resources/structures/recyclerrbridge.html

The bridge seperates two ponds unless I build it high enough to get a small boat under it.

A local guy here used an old semi flatbed, I believe he spanned right around 40'. I wish I had some pictures, he painted the trailer, stained the new deckboards and did landscaping. He had been thinking about making it into a covered bridge, I don't know that he got that far. Just a thought.

Bryan

Very cool project and some really neat ideas. I would definitely consider building the bridge high enough to get a small boat under. I would be a pain to pull it around every time you wanted to trek from pond to pond. And wouldn't your ice be thicker than 8-10"? If you're in southern Michigan you'd be about the same latitude as we are and we're looking at ice twice that thick right now. Anyway, can't wait to see the pictures of the project. Very cool.

Neat idea about the van trailer. Most van trailers are 48'.