Laying wastewater in deep topsoil

[swazimatt]

I have a project where my 150mm dia (6 inch) PVC ww gravity pipe will go through a section of very thick topsoil (1m plus below the pipe invert).

My question is should i undercut the full 1m+ depth until i get to bottom of topsoil and fill the trench to pipe bedding or will i be ok if i undercut by 300mm, lay some geofabric and backfill to pipe bedding with good material like we would if we hit a patch of poor soil/wet uncoheasive soil?

 

[HTURKAK]

The question for gravity pipe . You can imagine that gravity pipe flow is more prone to differential settlement. My suggestion would be, extension the granular bedding to the bottom of top soil and compaction of the trench bottom.

 

[swazimatt]

i knew that was the answer, just hoping someone would come up with a cheaper one LOL

It is at minimum grade as well so not much room to accommodate settlement