HOW WE CAN CONNECT HDPE AND GRP PIPES TOGETHER ?

HOW WE CAN CONNECT HDPE AND GRP PIPES TOGETHER
AS I NEED TO SHOW THERE IN BETWEEN SPEC BRAKE
AND WANT TO ADD JOINING PART IN SPEC.
where GRP (Glass Reinforced Plastic Pipe) is above the ground level and HDPE (High Density Polyethylene) below the ground level and I want to connect two different pipe spec.

THANKS A LOT SIR FOR YOUR QUICK RESPONSE !!!

I've never seen a "flat face" HDPE flange, what I have used are something like a "Lap-joint Flanges ...OR Van Stone Flanges for you older types" ;D=-=- , where the Lap-joint stub is HDPE then they use a cast iron backing ring to position & hold the flanged connection together (see link)
genord.en.ecplaza.net/back-up-fl ... 93598.html
SO connected to a FRP piping system with this HDPE flange, this connection could be a problem. What you could do is this, they make a "transition piece" HDPE to Steel", it's about 30" long (give or take ...see link).
cnchuangrong.en.made-in-china.co ... ition.html
You could attach(welded or threaded) a steel "flat face" flange and as JOP has indicate with a full face gasket and a flat face FRP flange, and it would work fine. OR ...you could dump all the flanges and use a Victaulic couple and avoid the issue altogether! ...Good Luck!
miconeng.com/downloads/ANATOMY_O ... _JOINT.pdf