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

Water mains crossing beneath Sound Transit Central Link light rail tracks are encased a
minimum distance of 12 feet perpendicular to the centerline of the track. The tracks
have a 5.5 foot minimum separation between the top of the rail and the top of the
casing. See the Sound Transit Design Criteria Manual.
Casings crossing a light rail line must be electrically isolated from the carrier pipe. A
permanent test station should be installed to perform future isolation checks. See DSG
Chapter 6, Cathodic Protection, Test Procedure (TP) 31 – Light Rail and Street Car
Cooperative Interference Testing.
iii Street Car
The presently used street car designs have the least impact on buried pipelines of the
three types of rail. Street cars are smaller and lighter, but still limit pipeline access and
generate stray current.
The design engineer must consider depth of cover, pipeline size, age, thickness,
material, importance, and access.
The design engineer should consider various pipeline protection methods ranging from
do nothing to casings and protective concrete slabs.
D. Parallel Rail Installations
For worker safety, parallel mains should not be closer than 15 feet from the rail center
line. However, rail installation will likely have to be considered case by case.
5.6.3.8 Permanent Restraint Systems
Restraining of forces due to internal pressure at fittings, valves, or dead ends is a major
consideration in pipe installation. Thrust restraint is by welded or mechanically restrained joints
or poured in place concrete thrust blocks depending on pipe size and type.
All bends, fittings, and line valves must be restrained by a joint restraint system compatible with
the pipe type.
A. Thrust Restraint Calculations
For all projects requiring thrust restraints beyond that required by Standard Plans 330a,
330b, 331a, and 331b, the design engineer must calculate the thrust restraint.
Restrained joint pipe is self-restrained. The restrained length for pipe and fittings
depends on the test pressure, backfill, depth, soil characteristics and pipe coating. The
design engineer must calculate the restrained length for both pipe and fittings.
B. Connecting to the Existing System
In the SPU water system, most connections to existing (non-steel pipe) are unrestrained.
A difference in outside diameters of various materials can create a force imbalance at
the connection similar to that of a reducer.
For example, a 100-year-old cast iron, 20-inch-diameter water main could be ½-inch
greater in outside diameter than a new 20-inch-diameter ductile iron main. This force
imbalance must be accounted for at the connection, especially if corrosion preventative
isolation couplings are used to make the connection. At 100 psi, this difference in
5-24 SPU Design Standards and Guidelines