Just an idea.

Sewer lines in sand, possibly in pea gravel: Our contractor for the sewer line replacement project documented in this article used clean sand for bedding the new sewer piping. Sand was specified by our local building department. Some local codes permit and some contractors like to use pea gravel for plastic sewer line protection, asserting that pea gravel settles less than sand in the pipe trench.

http://inspectapedia.com/septic/Sewer_Pipe_Sand_Bedding.php

Pipe Bedding Specifications

The following is a brief introduction on the subject of pipe bedding and should not be viewed as a definitive guide. Pipe bedding guidelines are dealt with in Part H of the Building Regulations in which several site specific situations are addressed, you should consult the above document and your local building control officer if undertaking any new installation works.

http://www.draindomain.com/pipe bedding specification.html

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Note added at 20 mins (2016-02-16 16:06:21 GMT)
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For plastic pipe use - You will already have 10mm pea shingle or 14-5mm graded shingle as your bedding for your plastic pipe covering up to the top of your plastic pipe. You will need to lay a 100mm covering of selected fill or granular material next when back filling your trench and finally filling with a layer of selected back fill consisting of no stones over 40mm, no clay over 100mm clumps and no frozen material. The last layer can be compacted as it is laid in the trench.

http://www.diydoctor.org.uk/projects/underground-drainage.ht...

You can use 'shingle' instead of 'sand'.