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

Chapter 5 Water Infrastructure
5.8.2.2 Geotechnical Report
With a new or major refurbishment to transmission main, a geotechnical engineering study
must be done for the proposed route (alignment) and documented in a geotechnical report.
Consult the SPU Materials Lab for recommendations on what the report should cover. The
Materials Lab geotechnical staff must review findings on all projects, even when an outside
consultant completes the geotechnical evaluation.
5.8.3 Materials
Transmission mains must be designed to withstand internal working pressure, external loads,
and transient pressures. Design should minimize the use of pump stations.
5.8.3.1 Minimum Pipe Size
Transmission mains must be sized to carry the designed peak flow required including fire flow
without exceeding the design velocities or head losses.
5.8.3.2 Material Types
A. Pipe
Water transmission mains are typically constructed of steel or class 52 ductile iron. Both
ductile iron pipes and steel pipes are to be cement mortar lined in most instances. For
more detail on materials for water transmission mains, see Standard Specification 9-30.
Note: ductile iron pipe is only manufactured up to 48-inch diameter. SPU does not use
pre-stressed concrete cylinder. Large diameter steel pipe (>48-inches), fittings, and
structures housing valves and appurtenances should be designed or overseen by a
licensed engineer.
See DSG section 5.6.7.
B. Casing
See DSG section 5.6.3.7.
5.8.3.3 Pipe Cover
Transmission mains are subject to special pipe strength design considerations and analysis by
the design engineer. The minimum depth of cover for water transmission mains is given in the
contract for each project. Use AWWA M11 or other applicable design manual to meet the
requirements of the project. SPU typically buries transmission mains with 4 feet of cover to
allow smaller utilities to cross over the pipeline and to reduce live-load on the pipe.
5.8.3.4 Bedding and Backfill
See DSG section 5.6.3.4.
A. Standard Trench Section
For detail on a standard trench section, see Standard Plan 350.
B. Controlled Density Fill
See DSG section 5.6.3.4B.
SPU Design Standards and Guidelines
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