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

Chapter 5 Water Infrastructure
2. Applying external bonded coatings or polyethylene film encasements (unbonded
film)
3. Using a cathodic protection system
See DSG Chapter 6, Cathodic Protection, for detailed discussion of that protection method.
5.6.8.1 Soil Resistivity
Resistivity refers to the resistance of the environment to promote electrical current flow. It is
the inverse unit measurement used to determine conductivity/corrosiveness of the internal or
external environment in contact with the pipe surfaces. By selecting appropriate backfill (soil) or
modifying chemical properties of the water being carried within the pipe, resistivity can be
adjusted. When resistivity is adjusted, the corrosion rate on pipe surfaces is reduced. See DSG
Chapter 6, Cathodic Protection, Tables 6-9, 6-10, and 6-11.
For the soil pipe interface, adjusting soil resistivity is usually neither possible nor practical.
Providing select backfill has a short-term effect. Over time, constituents in the soil surrounding
the pipe will degrade the backfill and resistivity will approach that of the surrounding material.
Soil corrosivity is based on resistivity measurements of the soil in the pipeline location. Typically,
several measurements are taken and an average value is determined. Where soils are very near
a classification break point, engineering judgment is required to classify the soil (Table 5-6).
Where resistivity tests in one area vary, greater weight is given to the lower values found.
Table 5-6 Corrosion Protection Requirements for Water Piping Systems*
Soil Resistivity (ohm/cm)
0 – 900
901 – 3,000
3,001 – 10,000
10,001 – 50,000
> than 50,000
Soil Corrosivity Classification
Severely-corrosive
Very-corrosive
Moderately-corrosive
Mildly-corrosive
Non-corrosive
*Large projects may have several soil classifications and will need to accommodate each type appropriately.
Table 5-7 shows SPU requirements for water pipelines where coating is dictated by resistivity.
Table 5-7 SPU Corrosion Protection Requirements for Corrosive Soil
Soil Classification
Severely-corrosive
Very-corrosive
Moderately-corrosive
Mildly-corrosive
Non-corrosive
Notes:
Corrosion Protection Requirements
approved bonded coating
joint bonding - #2 AWG
insulating coupling or flange kit @ connections
full dielectric covering on service
lines to meter setter
electrolysis test station
class b bedding (sand)
magnesium anodes @ 18’ intervals (17 lbs high potential magnesium anodes)
[same as above – except anodes are spaced @ 36’ intervals]
[same as above – except anodes are spaced @ 100’ intervals]
- polyethylene encasement – 8 mil
- class b bedding (sand)
- no protection required unless organic or clay materials are encountered
- additional soil tests may be required for proper recommendation
SPU Design Standards and Guidelines
5-37