Can the pipe handle up and down movement? If not, maybe a 48" diameter concrete caisson could be made deep enough, easy to form and you could get by with minimal steel reinforcing on the skin.
My rule of thumb is that one inch of rigid insulation provides frost protection equal to one foot of soil. The (slightly) tricky part is determining the horizontal extent of the insulation. Keep in mind that frost can penetrate at an angle that is not perpendicular to the ground surface.
