Replies

1. 
   DanH | Aug 17, 2004 10:49pm | #1

   Retaining walls in this situation don't fail (mostly) due to frost heaving of the footing, they fail because the earth behind the wall pushes it out when frozen. A deeper foundation will help to control this, though I'm not sure there's any way to prevent it entirely.

2. 
   WayneL5 | Aug 18, 2004 03:33am | #2

   Insulation won't help.  All insulation does is to slow the transfer of heat, so, for example, in your home you can stay warm with a much lower heating bill.  But, in the ground, with no source of heat to contain, the whole thing will freeze solid down to the frost depth during your winter.  You may have underestimated your frost depth.  It's approximately 40 inches along the St. Lawrence River.  Probably more elsewhere.

   I don't know that you necessarily have to construct below the frost depth for a retaining wall.

   Drainage beneath the wall, and behind the wall, are critical to reducing frost heaving.  But, I don't know enough to recommend a specific design.

1. 
   BillHartmann | Aug 18, 2004 04:07am | #3

   Insulation will work.

   The earth is the source of the heat.

   Just as you don't get much frost depth under snow (insulation) covered ground.

   It is called frost proof shallow foundations. Most commonly used for heated buildings. But there are designs used for unheated buildings.

1. 
   WayneL5 | Aug 19, 2004 12:49am | #6

   Yes, but you'd need to cover quite a large area to have any useful effect.  Just covering a few square feet of the retaining wall footing won't do much.

2. 
   kclarson | Aug 19, 2004 04:41am | #7

   Bill, as another reader mentioned, insulation only slows the rate of heat transfer, it does not prevent heat transfer. Place a peice of rigid insulation in the freezer and wait a few days. Take the insulation out and what do you have...a peice of frozen insulation. If the ground is frozen, you can bet your bippie that the concrete stem wall will be frozen as well. This isn't the issue though. As mentioned in another post, frost heave due to poor drainage and expansive soils is an issue as are active soil pressures. Retaining walls can fail in the sahara desert if not designed properly.

1. 
   BillHartmann | Aug 19, 2004 04:45am | #8

   But it does work.

   Putting it in the freezer is unlike putting over earth that supplies heat.

   As some one else pointed out, it probably is not practical because of the width that is needed. But the concept works.

3. 
   IronHelix | Aug 18, 2004 01:56pm | #4

   Try this resource...............

   There are some excellent illustrations of retaining wall details in the JLC Field Guide, page 91-95.  Complete with explantions and charts!

   ....................Iron Helix

4. 
   SethArgon | Aug 18, 2004 09:26pm | #5

   I would go below the frostline put in drain tile to daylight and backfill with gravel againts the wall. Keeping the moisture out of the fill side will help reduce the soil pushing against the wall when it is frozen. Dont forget the filter fabric.

5. 
   hacknhope | Aug 19, 2004 09:15am | #9

   In this part of Canada, a fair percentage of homeowners have a retaining wall of 8 inches to 4 feet at some point in the 9 foot distance between the sidewalk and front porch.  Very, very few of these are mortared.  Fewer still have anything by way of footing (nicely compacted gravel base and, maybe, one row of the same blocks sitting below grade).  Many of them only fail 30 years later when the roots of the maple tree, planted 4 feet away, forces the wall apart.     

   Unless the harm caused by a wall failure would be catastrophic (such as for a structure) just use a prefab concrete locking block product, as per directions (no real footings).   In fact, the more rugged of these products are rated for terrain way more extreme than your 2 foot change in elevation.

   The frost depth is more like 4 feet.  You are unlikely to want or need to dig deep/wide enough to get below it, or to achieve any meaningful amount of ground-based heat in winter.  You can hand-excavate and rebuild a stacked wall 4 times for that money, but it's unlikely you'd have to.  

   e.g.,  http://www.unilock.com   http://www.pavestone.com   See installation videos and pamphlets associated with retaining-wall products.  They show how to achieve water drainage around the wall and ways to incorporate extra strength. 

   Unilock must be printing money in this neck of the woods, but they've got great products that work.  A few are even good looking although the toughest look a bit industrial.

   Good luck.