Terminology

• Bedding: material under the pipe, between the trench bottom and the pipe
• Haunching: material placed up the sides of the pipe to its springline
• Initial backfill: material placed from the springline to above the pipe crown
• Embedment / pipe zone: bedding, haunching, and initial backfill together
• Final backfill: material above the pipe zone to grade
• Soil classes: Class I to Class V embedment materials keyed to ASTM D2487 soil groups
• Overexcavation: digging below the pipe bottom, restored with bedding material
• Tracer wire: conductor laid with buried plastic pipe for locating
• Flowable fill (CLSM): controlled low-strength material that needs no compaction

Standards

• ASTM D2321: underground installation of thermoplastic pipe for sewers and other gravity-flow applications; classes of embedment material map to ASTM D2487 soil groups
• ASTM D2487: classification of soils for engineering purposes
• Bedding thickness: minimum 4 in. in the D2321 table excerpt, 6 in. in rock cuts
• Lift thickness: compact in layers about 6 in. thick maximum
• Particle size: embedment material passing a 1-1/2 in. sieve
• Excluded material: Class V material is not permitted as bedding or backfill for flexible pipe in one state rule
• Haunching depth: one state rule requires haunching and initial backfill at least 12 in. above the crown
• Trench width: one contractor detail citing D2321 uses 1.25 times the pipe diameter plus 12 in.
• Plumbing code: support, overexcavation, rock, soft-soil, tracer wire, and backfill provisions are in the site elements code reference

Materials

• Crushed stone: angular, free-draining bedding and haunching; the base under pipelines
• Sand: trench bedding and fill around pipe
• Gravel: bedding where granular material is allowed
• Native soil: only where it meets the specified class and is free of rock, debris, and frozen lumps
• Flowable fill: where compaction is difficult

Detailing

• Support piping along its full length and never on blocks resting on grade
• Dig bell holes at each joint so the barrel, not the bell, bears on the bedding
• Overexcavate where the trench bottom is rock or soft soil and restore with bedding
• Place and compact bedding, haunching, and initial backfill in lifts on both sides of the pipe
• Keep construction debris, cobbles, and frozen material out of the pipe zone
• Follow the pipe manufacturer's installation instructions where they are more restrictive
• Lay tracer wire with buried plastic sewer pipe
• Protect pipe during backfilling and from heavy equipment until cover is in place
• Test the pipe after backfill where specified

Uses

Specific uses

• Storm sewers, subsoil drains, and drain pipe
• Sanitary building sewers
• Water service lines
• Utility conduit trenches
• Culverts and underground detention piping

Pros and cons

Pros

• Uniform support prevents point loads and pipe breaks
• Granular bedding can drain water along the trench
• Well-compacted haunching limits pipe deflection

Cons

• Open-graded stone can carry water along the trench and needs fabric or a graded filter
• Poor compaction under the haunches causes settlement
• Extra excavation and imported material add cost

Manufacturing and cost

Construction

• Trench excavated to depth and width; bedding placed and shaped; pipe set and jointed; haunching and initial backfill placed and compacted in lifts; final backfill and restoration follow

Main manufacturers and suppliers

• ADS
• Contech Engineered Solutions

Sources

• UpCodes: 2024 IPC Chapter 3 general regulations, Section 306, https://up.codes/viewer/general-services-administration/ipc-2024/chapter/3/general-regulations
• ADS: classes of embedment and backfill materials detail, https://www.adspipe.com/resources/documents/DDA5ADBC-302F-49F0-82005668F61BC4EB
• Contech: standard backfill, ASTM D2321 detail, https://www.conteches.com/media/mf3htpv3/a2000-std-backfill-astm.pdf