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

with other structures, vehicular traffic, existing utilities, and locations vulnerable to damage or
vandalism. See DSG section 5.6.3.12.
5.8.3.12 Line Valves
Each transmission main project should examine the proposed route for the best location of
isolation line valves. Consideration should be given to future operational issues such as draining
the pipeline and isolating a mainline break. SPU recommends placing isolation valves at least
every 2,000 ft and at every intertie location. All line valves should be installed within a vault per
the standard line valve vault detail in the DSG (see Figure 5-5 in section 5.6.3.11).
5.8.4 Seismic Design
This section describes seismic requirements for water system design.
5.8.4.1 Critical Needs Assessment
The purpose of a critical needs assessment is to establish design requirements to provide for the
highest level achievable for continuity of service, i.e. for the transmission pipeline to function
within its design limits with as little service interruption as possible. A critical needs assessment
of the proposed new or refurbished transmission main and associated facilities should be
completed as part of the basis of design. From the critical needs assessment, design criteria
should be clearly established for the following:
Design operational life of the transmission main.
Maximum length of time that the transmission main can be out of service for repairs
or maintenance without negatively affecting the water demand requirements.
Identify seasonal considerations that affect the length of time the main can be out of
service.
Redundancy of critical components or functions, such as line isolation valves.
Accessibility for the transport of repair and maintenance equipment.
Access to the interior of pipes, vaults, and other structures for inspections and
repairs.
Impacts from geologic events (landslides, erosion) and seismic events (ground
movement), including:
Foundation and backfill requirements
Drainage and control of or protection against groundwater
Allowable pipe movement
Allowable joint leakage (if any)
Need for and location of seismic valves
5.8.4.2 Seismic Joints and Connections for Pipe Movement
This section describes potential design issues when SPU water pipes must cross settlement
prone areas, faults, liquefaction zones, slides, or where two types of pipe with different restraint
characteristics must be connected. In both cases, design must allow pipe movement.
5-44 SPU Design Standards and Guidelines