Building on clay

[Will Shotton <shottonmasonry@zzzzzzzzz>]

Hi Charlie,

As with any masonry wall you'll want to ensure that you have more than adequate drainage. If you're in an area with freeze/thaw cycles this becomes even more critical. I'd usually lay a minimum of twelve inches of compacted granular aggregate (tamped with a jumper or at least a heavy skid compactor every four or six inches) underneath the wall as well as 3/4" gravel over the drain tile and behind the wall, underneath the ground you're retaining.

Some common characteristics of centuries old dry laid walls are large, solid corner stones, through stones and sometimes a wider foundation of flat stones extending anywhere from four to twelve inches on either side of the actual wall.

Dry laid walls are quite forgiving of minor ground movements if they are built properly. Break your vertical joints, put lots of through stones in and if you have the time and stone perhaps batter the backside of the wall or at least construct some buttresses back into the hill.

Some clays are very unstable and can even turn to liquid during minor earth tremors so the best you can do is build the wall high and dry and stick to the tried and true methods of your region. Ice crystals exert something like 22 000 psi when they form so even a concrete base won't help if there's a lot of standing water around your wall.

Good luck,
Will