Evaluation of granular backfill materials for large-diameter, high-density polyethylene pipe
Traditionally, the Texas Department of Transportation (TxDOT) has used concrete or corrugated metal pipes in highway subsurface drainage applications. In recent times large-diameter HDPE thermoplastic pipes have emerged in the marketplace as a viable option. However, these pipes require a more care and control during installation. Because of this reason, and due to the lack of statewide experience with this type of pipe. TxDOT has taken a conservative approach and specified flowable backfill as the only acceptable type of backfill material for thermoplastic pipe. Flowable backfill, which is a mixture of sand, cement, fly ash and water, is considerably more expensive than conventional pipe backfill materials such as sand and gravel. As a result when flowable backfill is used, HDPE pipe is no longer an economically attractive option. TxDOT therefore initiated a research project to investigate the possibilities of using alternative backfill materials, chiefly granular materials. The research described in this thesis is part of the work accomplished in the above project. The primary objective of this research was to develop specifications to allow the use of thermoplastic pipe in TxDOT construction projects. This standard specification must address issues such as the minimum cover required, maximum fill heights to be allowed and allowable backfill materials and field compaction procedures. Such alternative installation methods : (a) should ensure integrity of the pipe system during both their installation and long-term service, (b) utilize backfill material that is readily available statewide, (c) must be cost effective, and (d) applicable to a broad range of soil conditions. One of the main objectives of the research is to develop specifications for backfill type and compaction in such a manner that density checks, backfill compaction control, or pipe deflection measurements to verify proper installation would not be needed.