A long-time customer called to ask if I’d take a look at a dry stone wall I’d built for him many years ago. He was concerned that a woodchuck had taken up residence nearby and that its excavations had undermined the wall. Upon inspection, I could see that the critter’s digging hadn’t compromised the structure, but a section of the wall did have a decided lean to it. The straight line of the 4’ high, free-standing wall was bowed and was listing badly. It looked as though it had gone slack in the July heat. The wall was still holding it’s own, but might give up the fight and let go at any time. Since there was going to be a wedding party on the property in the not-too-distant future we agreed it would be best to pull down the section in question and rebuild it. So, that’s what I did today.
The mystery of the slumping wall preoccupied my mind as I stripped out the stone and set to work. I immediately dismissed the woodchuck theory for lack of hard evidence. Woodchucks do get rightly blamed for damage around a place, although it’s most often in the garden, involving the munching of green, leafy things. In our discussion, the customer reminded me that the grade in that area had been artificially elevated when the house was built. Could that section of wall have been built atop un-compacted earth? That could account for the slow motion “slumpage” over the years. A real possibility, I supposed, but other evidence steered me toward water. Two roofs shed rain into the area in question. But, there was no runnel in the earth, or signs of fill being washed through the stone work.
As I worked, I kept looking up at the eaves, and the pitch of the metal roofs. The stone wall was only 8’ from the house. I tried to imagine what it would look like there on a snowy January night, and what it would sound like when the snow pack slid from the roof, cascading to the ground. The weight of a winter’s worth of snow pressing against one side of the wall could potentially bend it without breaking it. The increase, year after year, in the wall’s bow would be incremental. I don’t know if my diagnosis of Chronic Avalanche Fatigue is correct, or not, but it sure was refreshing to be contemplating lofty piles of fresh white snow on such a sultry summer day.
Like most architects, I'm always slightly paranoid about foundations and drainage. What kind of subsurface base would you normally create for a wall of this sort, Mr. Snow, in order to prevent frost heaving from causing irregular vertical shifts of the component stones?
ReplyDeleteI was just looking at one of the few diagrams I have in my architecture library that actually illustrates a drystone wall, and it shows a a 12" deep base of gravel in a trench below the lowest stones. Presumably this gravel base would have to be coarse enough that water could not move into the gaps between stones by capillary action, and therefore one wouldn't see the formation of ice lenses that cause a dramatic heaving effect (which can break a conventional masonry wall, or more commonly crack a concrete sidewalk in my neighborhood somewhat south of you).
This might be a slightly simplistic theory, but it seems to me that if some factor caused the drainage to fail below just a small portion of the wall (say, an animal -- a villainous woodchuck -- dug into the gravel and mixed enough of it with fine soil or loam to allow a small ice lens to form during a hard winter freeze), a few stones of the lowest course immediately nearby might heave up, just slightly and imperceptibly at first. But over time and successive wet winters that insignificant heave might be enough to change the equilibrium of the wall elements bearing on those lower stones.
Would that be enough to create the bow you are describing?
Hi Lewis,
ReplyDeleteYes, a combination of factors may have contributed to the failure. And you've pretty well summed up the likely results of cyclical frost heaving. I would add that it's the let-down as much as the heave-up
that creates the settlement. As frozen soil particles thaw, they are suspended in liquid (water). The weight of the wall squishes them out, right along with the melt water, allowing the wall to settle into
the resulting voids. Best to use a foundation material that doesn't absorb water. I most often use 1 1/2" crushed stone. The spaces between the crushed stone may fill with water and freeze but the stone stays put during the freeze/thaw cycles. For depth of footing I use - "1/2 height of wall." So, for a 4' high wall, a 2' crushed stone footing.
Dean, in Ontario, sent this message regarding the post- "I've been considering your issue with the slumped wall. We often have walls here that are abused all winter with snow blowers and plows. I'll be interested to see what the effects are on them."
Best,
Dan