Installing underground plumbing requires removing existing materials to place the piping system. However, this effort moves the designer/engineer into territory not typically part of their knowledge base: “Backfill and Bedding.” Additionally, the requirements will likely differ depending on the interior, exterior, or public right-of-way location. Code sections, rules, and engineering standards will likely address each area. Additionally, the piping materials manufacturers' Operations and Installations Manual will provide specific recommendations to maintain the manufacturer’s warranty. Hence, the designer/engineer must understand the geotechnical soil conditions and be able to select the appropriate materials for backfill to protect the system while maintaining the buildings or ground surface integrity.

Also, one must understand the site and field conditions under which these systems will be installed. Remember that the installed work must remain stable throughout the construction process. Simply installing the piping system does not relieve the installing contractor from protecting that installed system, along with the designer/engineer understanding the means and methods necessary to ensure the system will function as designed once the project is complete.

I am bringing this topic up for discussion because it was part of my presentation to a design-build contractor with a field problem. The contractor had installed a PVC sanitary system within the building. The installing sub-contractor had used “grits” as their choice of backfill material. As the sanitary line was installed toward the center of the building, its burial depth was less than three feet of cover. As each of us should know, underground plumbing is one of the first systems to be installed. While “grits” is a relatively fine granular material that is not compactable and does not generally settle, it is also unstable. Something will move unless the contractor provides protective measures to the area where the underground plumbing is installed.

Following the manufacturer’s installation instructions is always the best practice when installing underground piping. These instructions will typically override any guidance or requirements provided within the codes. The manufacturer will tell you that proper bedding and backfill include ensuring that the trench is of sufficient width to allow for proper installation. The trench should be slightly over-excavated to provide a layer of compact bedding to support the piping. The pipe should then be placed with compacted fill to a depth of the “spring line.” This will ensure that the piping slope and alignment are maintained. When in doubt, the installing contractor should refer to ASTM D2321 (Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications), which may be included within the contract specifications for the contractor's convenience. It is important to note that by placing this standard within the body of the specifications, it is incumbent upon the engineer/designer to familiarize themselves and understand the standard’s requirements.  Also, it is important to note that additional steps are necessary when utilizing PVC or ABS materials. These materials do not have adequate stiffness to resist earth and live loads that may be imposed on the installed piping. Failure to meet these requirements may result in system collapse and failure. And, if the backfill is not stable, the ability to maintain an appropriate slope will likely be compromised as well. 

Backfill is generally defined as soil or controlled low-strength material used to fill an excavation. The choice of backfill material is driven by the location in which it is placed: open ground area, under structural foundations, under slab on grade, under sidewalks, or paved surfaces.  The bedding course, used as a subgrade to the piping material, is a compactable aggregate layer acceptable to the piping manufacturer. Depending on the location, backfill should be placed in 6” lifts and compacted to protect the installed piping and structural support for what will be placed on top of the trench: slab, grade beam, pavement, sidewalk, etc. The compaction should conform to ASTM D-1557 for cohesive soils and ASTM D-2049 for cohesionless soils.

If construction equipment will disturb this utility trench, such as inside the foundation, protection should be taken to prevent the buried piping from being disturbed. The final installation is only as good as the effort to protect it during construction. This is especially true when “grits” are used as the backfill media. The grits are not compactable and remain unstable until covered by the slab. These grits will shift as heavy equipment travels over the trench. This material shift will telegraph down through the grit backfill, potentially shifting the installed piping.