Installation contractors frequently ask if it will be permissible to use “pea gravel” for the select fill envelope for flexible pipe products. One must consider the practical and structural issues relating to the use of pea gravel as flexible pipe backfill before answering this question.
The primary purpose of a pipe’s select backfill envelope is that of providing uniform and adequate support so the installed pipe is capable of carrying the applied loads. Therefore, the select fill envelope should consist of fill that is well-graded, free-draining, angular, durable material that is properly placed and compacted. Some “pea gravels” fit this description – i.e., they are somewhat angular in nature; they are durable – meaning they can withstand the rigors of placement and compaction as well as perform in the environment intended; and they are certainly free-draining. Unfortunately, other “pea gravels” contain rounded particles of a single size, such as a washed river stone. Such fill material is difficult to compact and to place without adequate confinement to keep it from spreading. Also – many “pea gravels” – due to their open-graded nature – are susceptible to migration of fines from adjacent soils.
Therefore, “pea gravel” that is suitable for use as select pipe backfill should be somewhat irregular in particular size and shape. It must be placed in a controlled, balanced manner and in thin lifts - not simply dumped. The lifts of fill need to be compacted or at least vibrated to ensure the gravel spreads into the haunch areas of the pipe and that specified compaction density levels are achieved.
Contrary to popular opinion:
- Pea gravel does not normally achieve suitable in-place density levels by simply dumping this fill material in the trench alongside the pipe – additional compaction efforts are necessary; and
- It is possible to run compaction density tests on “pea gravel” to determine the in-place compaction density of this fill material. The gravel may need to be confined by stable trench walls or adjacent fill to keep it from spreading during placement and compaction. A fabric separator may be needed between the gravel and adjacent, finer grained soil zones to prevent migration of fines into the more open-graded pea gravel.
Provided these conditions are met, pea gravel fill is a viable option for use as select backfill for flexible pipes.