By Andrew M. Jenkins, E.I.
| Tuesday, March 22, 2016 | 2291 Views |
Dear Tech Abby: Do I have to use a crushed stone backfill for my PVC sewer pipe installation? --That 70’s Engineer
Dear That 70’s Engineer: Good question… the answer is “maybe”…
Well, let me expand on that. First, when talking about PVC sewer pipe, remember that PVC products are commonly used in both gravity flow sanitary and storm sewer applications (typically 4” – 36” diameters). Which application applies to your project? Both applications are covered within the industry’s standard installation specification, ASTM D2321 – Standard Practices for Underground Installation of Thermoplastic Pipe and Sewers and Other Gravity- Flow Applications. In general, the sanitary market is easy; nearly 99% of the governing agencies that use PVC pipe require a manufactured aggregate or crushed stone backfill. ASTM D2321 would classify stone as Class I backfill material. There are two main types of gravity PVC pipes used in the sanitary industry, solid wall SDR and profile wall (e.g. Contech’s A-2000 - www.conteches.com/a-2000) PVC pipe… and both follow ASTM D2321. So, for gravity flow sanitary applications, you are typically required to use the crushed stone backfill.
The storm sewer market is where it gets fun. SDR is rarely used in this application, but A-2000 profile wall pipe is used quite often in many regions of the United States and is typically specified along with Reinforced Concrete Pipe (RCP). Most agencies require RCP installations to use an engineered backfill to the spring line, and then native, or “in-situ”, materials can be used to finish the trench. Due to A-2000’s popularity in the sanitary world, many engineers mistakenly think that you have to backfill with Class I materials. This is not true. In fact, A-2000 can be backfilled using Class I, Class II, Class III and even Class IV in certain situations (manufacturer consultation required). Now, with all that said, what does RCP’s specification using in-situ soils really mean? Do they allow anything? Class IV (clays, silts) or Class V (fat clays, organics)? Bottom line, most engineers do not want their sewer pipe backfilled with fines that could be highly plastic (e.g. swelling, fat). Class IV and V are not great roadway materials, period. Therefore, most in-situ soils that are making the grade probably will fall in the Class II or III category and therefore can be used, with the appropriate compaction effort, with A-2000 as well.
Please note that ASTM D2321 states the minimum cover to be 24”. Contech believes this to be conservative and recommends the minimum cover to be 12” (same as RCP and CMP). Contech further lists that the maximum cover is 30 feet (*). The * denotes that covers greater than 30 feet are capable, but engineering consultation is required. Once you get past 30 feet, many times the sub-grade materials are fully saturated due to ground water and the geology itself may be quite different than the surficial materials. Remember, the material in the trench wall will be important due to the forces incurred at such depths.