John Clark of Clark Engineers, Conroe TX, made a presentation on the design of buried structures. Mr. Clark spoke about his work with flexible culverts and fiberglass tanks, such as those used for underground gasoline storage. He gave examples of calculations necessary to check if a pipe is considered flexible or rigid, and for showing how to check for both soil failures and buried flexible pipe failures.

Mr. Clark presented results of full scale testing of 92-inch ID underground fiberglass tanks loaded to failure, showing the usual failure mechanism that occurred. He said ideally they prefer to backfill all around flexible pipes a minimum of one radius with pea gravel. When they use pea gravel, there is no compaction required.