How would I make a good sealed connection between the pipes if so?
Apart from it being hard work it should be a simple job.
If there aren't any proprietary concrete-plastic connectors....
1 - You could abut the plastic to the concrete as closely as possible and simply backfill around the pipes
2- If you want to do a better job of it...
- Abut the two pipes.
- Wrap some damp proof membrane (DPM), or other strong plastic sheet, around the join and secure with tape. About 6" either side of the join should do - wrap it tight. It is to prevent cement getting into the pipe.
- Wrap some expanded metal lath (EML) around the join - not too tight. This is to hold the cement in place while you do the job and will act as reinforcement once the cement has set.
- Use sharp sand (coarse is better than fine but either will do ). Make up a 2:1 or 3:1 mix of sand to cement and add a waterproof additive to the gauging water - this will make the mix more workable as well as waterproof. You may need as much as 1 to 2 25kg bags of sand per joint.
- Starting at the bottom, have a 1"-2" gap for the cement. Shove some cement in the bottom and push it in, jiggle it about with a stick or similar so it flows to fill the space. Then working upwards, build up the soil to within 1"-2" of the pipe and fill this with cement and jiggle. Keep going all the way round the pipe.
- Cover with plastic to protect from rain and allow the cement to set.
- Next day complete backfilling.
I hope this works for you. Have fun!