Bedding for 15 Inch Concrete Pipe Laying Underground

The standard practice in the USA is to hire a competent engineering firm to specify and oversee the pipeline bedding, and the entire project, for that matter.

Any good general contractor who deals with 15" pipelines will have the staff for this.

Usually up here in New York we specify pipe bedding and pipe zone backfill using a NYSDOT 50%/50% mixture of No. 1 and No. 2 clean and washed Crushed Stone. If heavy groundwater is encountered in the pipe bedding region we will specify a heavier (larger crushed stone), especially if it's a concrete storm drainage pipe.

Here is the LINK to the New York State Department of Transportation "2008 NYSDOT "STANDARD SPECIFICATIONS". [THIS IS PUBLIC DOMAIN INFORMATION.....]

To obtain the crushed stone sizes, go to Section 700, "Table 703-4 SIZES OF STONE, GRAVEL AND SLAG".

https://www.nysdot.gov/main/business-center/engineering/specifications/2008-standard-specs-us

The State of Virginia Dept. of Transportation may have similar crushed stone specifications....you may want to visit their website to obtain them. Also, you can obtain similar crushed stone specifications on the web at the FHWA and AASHTO.

In the event your pipe trench is unstable and has significant groundwater conditions you will most likely have to install a filter fabric around the crushed stone pipe bedding + pipe zone backfill material in order to provide stability and proper pipe laying foundation......I suggest that you look into Mirafi 140N or 140F filter fabric as a minimum, and if conditions warrant it, a much heavier filter/Geotextile fabric with good permeability characteristics.

Good luck!