This document is drawing 702 Retention-Detention System (Cross-Section) Detail showing the cross-section of a retention-detention system constructed with HDPE pipe.

The drawing shows the layout of the system for the HDPE pipe, the top of the drawing is the grass area with the height needed or the flex pavement height or the rigid pavement height. The drawing indicates the bedding requirements of Class I or Class II material, with a 4” minimum for 12” - 24” pipe and a 6” minimum for 30” - 60” pipe. The material needs to be placed and compacted in accordance with ASTM D2321 in the pipe zone.

This document includes eight drawing notes for reference. Note 1 states: All references to Class I or Class II material are per ASTM 2321 “Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity Flow Applications.”

Reference Note 2 states: All retention and detention systems shall be installed in accordance with ASTM D2321.

Reference Note 3 states: Measures should be taken to prevent migration of native fines into the backfill material.

Reference Note 4 indicates: Use a filter fabric such as a geotextile fabric as specified by an engineer to prevent the migration of fines from the native soil into the select backfill material.

Reference note 5 covers the foundation. If the trench is unstable, the contractor should excavate to a predetermined depth and replace it with suitable material. A design engineer can call for the trench bottom to be stabilized using a geotextile material.

This drawing includes a table for reference to support the drawing. The table provides values for the nominal diameter, the nominal O.D., typical spacing “S”, nominal spacing “C”, the typical side wall “X”, non-traffic “H” height, and traffic “H” height.

Other notes cover information about suitable bedding materials, initial backfill requirements and the necessary minimum cover. Refer to the drawing for more specific details.