Clayboard?

[Alan Ambrose]

So, this holds the slab while it's setting and effectively makes a void later, protecting from heave?

 

This assumes the trenches are deep enough, secure, never move, and are strong enough to handle differential movement?

 

 

 

[Gus Potter]

Yes but you would normally have clayboard on one or both sides of the founds as well.

 

Each case needs to be deisgned for the site ground conditions.