Soil Migration and Buried Pipe

Soil migration can create a problem for some buried pipelines. Migration is the movement of small soil particles into the voids of adjacent soils (i.e., crushed rock) that have large openings between particles. The particle movement is precipitated by gravity or the movement of water. If there are enough of the fine-grained particles moving into these void spaces, the behavior of the larger particles can be affected. In the first part of a study conducted by the National Clay Pipe Institute (NCPI) in 2023, the soil migration changed the pipe embedment material from Class I to Class III (ASTM C12), which reduced the support for the pipe. Pipe movement and/or loss of embedment support after installation can be an issue or even lead to a failure for either rigid or flexible pipe products. The study was continued in 2024 to expand on some of the test conditions. The 2023 tests examined various combinations of fine-grained soils and cohesionless free-draining crushed rock. A vitrified clay pipe (VCP) was laid on the surface of the crushed rock/fine-grained soils in a wooden test box, the pipe loaded, and the pipe's settlement measured. The load apparatus was a setup for conducting three-edge bearing tests on VCP. Conclusions from that study showed little effect from clean water or dirty water, an effective difference between uncompacted and compacted bedding, a significant difference using a geofabric separator between soil and rock, and meaningful results for soil migration. The results were useful in evaluating physical testing equipment and procedures for future tests. The 2024 study used a larger pipe, a larger steel soil box, better instrumentation, and a confining pressure on top of the side fill soils to simulate the effect of backfill loading. The study also looked at using a geogrid, rather than a geofabric, as a separator between the soil and the crushed rock. The second phase showed that confining pressure on the bedding beside the VCP in the soil box reduced settlement, and the first phase results should be tempered by that. The second phase tests also showed that adding about 20% soil to the crushed rock increased the pipe settlement about 20%, adding about 40% soil to the crushed rock increased the pipe settlement about 200%, uncompacted bedding settled about 20% more than compacted bedding, and geotextile separators helped on compacted soil but not on uncompacted soil.