Hello Rienzie,
I wish that the experts will see your post because I am no expert.
Since your dealing with GRP, Caesar II should be a good starting point according to a related post I read during those days on this forum.
Check your elastic modulus, this should be given by the GRP vendor, the same with all other physical and mechanical properties.
Allow the pipe to expand/contract laterally, enough to reduce the loads by installing soft fill materials (e.g. vermiculite, calcium silicate chunks) on both sides of the pipe bend. I used the Caesar II underground modeler in this process to at least have an idea of what might happen. Since I dont have the exact stiffnesses of the soft materials that I mentioned, I reduced the stiffnesses of the soil around the pipe instead as a conservative approach.
Also, if possible, install your piping at a tie-in temperature nearest your operating temperature. Check the most probable months by which your piping will be installed.
In the end, consult with the GRP vendor and let the experts check your design. :-)