Thursday, July 12, 2012

Worried about what the frost will do.



I wrote back to Scott answering his recent inquiry about the preparing the foundation. Wondering if anyone else has some thoughts on this as well. If so please post them in the form of a comment below. Thanks.


Hi John,

I'm a graduate of the Algonquin masonry program and am now an avid follower of your blog posts. Your blog has been one of a handful of sources of traditional masonry inspiration - I'm very grateful that you take the time to share your thoughts and pictures. 
I am currently working on a wall in New Brunswick which is, unfortunately, located on a site with clay soil and poor drainage. I laid the foundation stones excavating 12'', packed 6'' of 5/8'' minus and laid 6'' foundation stones, 2'' extended from the base. We have since had a good downpour and I was discouraged to find water up to the top of the foundation stones 2 days following the rain. That was in mid-June, when the water levels are high. However, I'm worried about what the frost will do in the winter. This photo was taken today. It is of the almost complete first wall. I still have an identical wall to build on the other side. Do you have suggestions for building foundations in low-lying, clay soils... before I get started on the next wall?

Thanks,
Scott



6 comments:

  1. I've got the same dilemma. Heavy clay soil in a low lying area that floods with water. I recall Dan Snow's advice about digging foundation below the frost line- about 4 feet. I'm wondering If I'll just create a large trough that would still fill with water and heave dramatically in winter. Considered a drainage pipe, but where would the water be carried off to- it's already the lowest elevation in the area. Thinking of putting an extreme batter and extra width on the wall. The "wild card" is the moon gate feature that has me concerned. Seems like an extra 12 inches on the footprint of the base would be wise. I've got the Civil Engineers in this area stumped as well. I guarantee my work for life and may just have to go back and rebuild this one "down the road".

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  2. The foundation should be 2' deep, and 6" wider than the wall, and filled with random size stone up to 12" or so and packed in well. Think of this as a wall on top of a french drain. It is better if the foundation has a smaller aggregate anywhere where water may flow in to avoid silt washing in and filling up the foundation. Better yet if the water can be drained out of the foundation at any point. Another potential problem depending on local soil types is swelling and shrinking of the clay soil. This will also help drain the soil for easier gardening around the site.

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  3. The 5/8 minus will hold moisture especially when packed. 3/4 clear would allow moisture to drain under the wall, even adding an O pipe directing the water away wouldn't really hurt. Looks like the wall is a bit too far to make changes to the base now anyways. Monitor it over the winter. Hopefully there are throughs, a good amount of batter, solid hearting, no long running joints and all stones running deep into the wall. The buried foundation course Should be a good base for helping the wall ride the waves of frost and the wall should hold if all basic rules were followed.

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  4. Thanks folks. All good suggestions and thoughts.

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  5. Thanks everyone. I'm going to make the switch to 3/4 clear for the next wall and will add big O along the perimeter of both walls. I'd like to have a better understanding about the effect of deepening a foundation. It would improve drainage to a lower depth but, as long as the bottom of the gravel is above the frost line, it will still heave the wall. Also, would it create a larger surface area to push the foundation laterally?
    In planning this wall I sort of blindly followed the rules I was taught - no consideration of soil type and elevation. It's important to think about!
    Thanks for your feedback,
    Scott

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  6. Hi. Just read a relevant article by Brian Post of Vermont USA (published in Sean Adcock's Stonechat that JSR just posted link to). Great info about using a french drain type arrangement to allow water to exit the foundation basin. I think the Ampitheater at last year's Dry Stone Wall Festival incorporated the same type of system- with the pipe exiting out downgrade at open air to drain the whole area.

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