Technical Booklet N Drainage·Page 23·3.8(a)-(b)
Protection for rigid pipes with shallow cover
Rigid pipes with insufficient depth of cover must be surrounded by concrete encasement with movement joints to protect them from ground loads.
Where a rigid pipe of – (a) less than 150 mm diameter has less than 300 mm depth of cover; or (b) 150 mm or more diameter has less than 600 mm depth of cover, it should be surrounded with concrete either 100 mm or the diameter of the pipe, whichever is the greater, in thickness and have movement joints, at not more than 5 m centres (see Diagram 3.4).
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet N - Drainage.pdf