2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

Chapter 5 Water Infrastructure
5.6.3.4 Bedding and Backfill
The design engineer must require sand bedding for water mains unless another agency dictates
otherwise. Sand bedding creates a less corrosive environment around a pipe than does native
soil. Sand bedding also eliminates point loads on the pipe caused by stray rocks. Sand bedding is
typically Class B, Sand Mineral Aggregate Type 6 or 7 unless otherwise specified. Note: Type 9 is
for transmission mains.
Backfill is either suitable native material, Mineral Aggregate Type 17, or other material as
approved by the design engineer. For suitable native backfill material, see Standard Specification
7-10.3(10) for requirements. For requirements for Mineral Aggregate Type 17, see Standard
Specification 9-03.16.
For more information on bedding and backfill, see Standard Specifications 7-10.3(9), 9 03.12(3)
and 9-03.16.
A. Standard Trench Section
For requirements for a standard trench section, see Standard Plan 350.
B. Controlled Density Fill
Sometimes an outside agency, time constraints, or compaction will require that a water
main be bedded and backfilled in controlled density fill (CDF). When this requirement
outweighs the benefit of using sand bedding, a metallic water main must be coated both
where embedded in CDF and for some distance beyond the CDF encasement. Typically,
SPU uses either two layers of 5-foot-wide visqueen or two polyethylene bags over the
main to keep it separated from the CDF (Figure 5-4).
When CDF is used near the metallic pipe, a corrosion specialist should be consulted
because CDF can create a corrosion cell.
The CDF used to encase the water main must be the weaker (diggable) CDF mix. All CDF
must be ⅓ sack mix, less than 200 psi, and preferably less than100 psi. SPU has
approved various types and uses of CDF. CDF can be used as a trench plug, trench
backfill, or for grouting an annular space. Each use has a different mix ratio. The design
engineer must reference the City Standard Plans and Specifications for each CDF use.
See Standard Specification 9-01.5.
When CDF is used to fill pipe and the annular space between two pipes, it must have
100 psi strength at 28 days. See Standard Specification 9-05.15.
SPU Design Standards and Guidelines
5-19