Glass fiber reinforced thermosetting polymer plastic (GRP) is widely used in the construction industries due to the advantages of their superior mechanical and physical characteristics. Most of pipes are installed for long-term purposes and they should be safely installed in consideration of installation conditions because there are unexpected various terrestrial loading conditions. In this paper, we present the result of investigation for 253 days pertaining to the structural behavior of flexible pipes buried underground. From the buried test results, we predicted long-term, up to 60 years, ring deflection of GRP pipes buried underground based on the method suggested by the existing literature. It was found that the GRP flexible pipe is appropriate because 5% ring deflection limitation of 60 years could be satisfied.