Effective July 18, 2018, the placement of graded stone beneath prefabricated, gravel-less chambers is no longer required or recommended for disposal field systems, including sand mounds and sand-lined trenches.  This change applies to chambers with an H-10 AASHTO load rating (16,000 lbs/axle).  Graded stone is still required beneath chambers having an H-20 AASHTO load rating (32,000 lbs/axle) to provide structural support. This change is based on manufacturer recommendations and is consistent with other jurisdictional requirements for chamber systems.

Note: AASHTO = American Association of State Highway Transportation Officials

OWMS Program Website: http://www.gov.mb.ca/sd/envprograms/wastewater/industry-group/index.html