Technical Booklet D Structure·Page 88·4E.1
Subsoil conditions for strip foundations
The subsoil must be free from made ground, peat, weak layers at depth, and seasonal soil movement that could affect foundation stability.
There should not be –
(a) any made ground, peat or wide variation in the type of subsoil within the loaded area;
(b) a weaker type of soil at such a depth below the soil on which the foundation rests as could impair the stability of the structure; or
(c) soil movement within the loaded area resulting from seasonal weather changes.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet D - Structure.pdf