The Brew Cabin

i am busy converting an upstairs kitchen into an ensuite. i have just dug up the old gully trap and was wondering the best way to connect a new plastic soil stack to the exsisting drains. the exsisting drains of clay (salt grlazed i think). the is a gully trap into a 90 degree bend going down and joining another bend into the drains. i have removed the gully trap and 1st bend as they are damaged, as is the flange on the next bend. can i join into this bend, and what do i back fill with as what i have dug up is just soil with big stones in.
thabks

Use an adaptor coupling to connect new to old. Your local BM will advise on which coupling is most suitable.

The soil stack is connected to a 'rest bend', aka a 'long radius bend' which trnslates the pipe from a vertical linme to a more horizontal profile. This rest bend rteally ought to be bedded on a spadeful or three of concrete, and haunched with same. Connecting the rest bend to the remaining salt-glazed system is a matter of finding the requisite combinmation of bends and straights.

Normally, after covering over the new pipework with protective material such as pea-gravel or a selected, sandy fill, you'd backfill with whatever was dug out. The key point is to ensure that no big stones or bricks are able to come into direct contact with the pipework. If you are really concerned about the quality of the excavated material, you can replace it all with a selected fill or whatever your local BM has that's cheap and inert.