Cathodic Protection Design for Four New Water Transmission Lines and Pump Station Yard Piping, San Antonio (2017-2021)

Four new water transmission lines in San Antonio cross ground that ranges from fat clay to solid limestone. Working as a subconsultant to the prime design firm, our corrosion engineer tested the soil along them in late 2017 and prepared the cathodic protection design in 2018. The as-built and record drawings were issued in 2020 and 2021.

The pipelines

The project carries water from a new terminal facility to three existing pump stations. As designed, the lines were about 3,700 feet of 36-inch welded steel with a dielectric coating; about 10,670 feet of 54-inch steel, nearly all of it tunneled with the pipe set in grout and only 170 feet laid open cut; about 1,800 feet of 48-inch coated steel; and about 3,720 feet of 24-inch pipe, mostly HDPE with 520 feet of ductile iron in a tunnel. New yard piping from 18 to 60 inches at the terminal and at one pump station was part of the scope.

From clay to limestone

A two-person crew ran Wenner four-pin tests (ASTM G57) at 21 locations in November 2017, about every 1,000 feet where the alignment was accessible plus each pump station, with pin spacings from 2.5 to 20 feet. Layer resistivities ran from 376 ohm-cm on the 24-inch route, in fat clay, to 315,975 ohm-cm at the terminal, in limestone. Averages by line ranged from about 1,000 ohm-cm on the 24-inch route to nearly 85,000 ohm-cm on the 36-inch line. Laboratory tests on boring samples found sulfates up to 3,220 ppm along the clay route, enough that we suggested sulfate-resistant Type V cement for concrete there.

The limestone needed its own reasoning. Undisturbed rock is highly resistive, but crushed rock used as trench backfill is lower in resistivity and higher in oxygen, so pipe laid open cut in limestone still sees moderately corrosive conditions. On the 54-inch line, 170 feet of coated steel joined to 10,500 feet of bare steel in grout forms a mixed-metal cell with a driving voltage that can reach about 0.5 V, and the small coating defects become its anodes. Left alone, that short coated section would corrode fast.

Design choices

Resistivity varied too much along the routes for sacrificial anodes, so the transmission lines got impressed current, and the narrow easements left no room for shallow distributed beds. Each groundbed is a deep well, 8 inches across and 200 feet deep, with mixed metal oxide anodes in coke breeze. The 36-inch and 48-inch lines each got a 10 V, 5 A rectifier with five anodes; the 54-inch line got one 50 V, 30 A rectifier with six. Part of the alignment lies over the recharge and transition zones of the Edwards Aquifer, so we noted that the wells would likely need permits from the aquifer authority.

The 24-inch line is mostly plastic, so it got monitoring only. Yard piping, isolated from the transmission lines at the fence lines, got high-potential magnesium anodes: 32 anodes of 32 pounds at the pump station and 43 of 60 pounds at the terminal. The design also calls for three bond wires per joint on non-welded pipe of 24 inches and larger, three bonding clips per joint on mortar-coated bell and spigot pipe, isolating flanges or monolithic joints at connections, wax tape on buried fittings, sleeves where pipe passes through concrete, and test stations about every 2,000 to 2,500 feet, placed where possible at crossings, valves and isolating joints.

We found little risk of interference. Foreign crossings on the tunneled lines sit in undisturbed limestone with at least 2 feet, and mostly 10 feet or more, of separation, and no line runs alongside high-voltage transmission towers for any real distance. Cased-crossing test stations near the two closest crossings allow periodic checks. The as-built and record drawings were issued between November 2020 and March 2021.

Related services

The soil testing on routes like these is described under soil corrosivity analysis. Rectifier and groundbed sizing are covered on our page about cathodic protection.