Cathodic Protection in the San Antonio Area

From fat clay to limestone

The four transmission lines cross ground that ranges from fat clay to solid limestone. Along them we ran Wenner four-pin tests at 21 locations, and layer resistivities ranged from 376 ohm-cm in fat clay to more than 300,000 ohm-cm in limestone at the terminal. 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 has lower resistivity and more oxygen, so pipe laid open cut in limestone still sees moderately corrosive conditions. In New Braunfels the north end of the route sits on limestone while most of it is fat clay, lean clay and clayey gravel, and the lowest layer measured 321 ohm-cm.

Tunnels, mixed metals and the aquifer

Large water projects mix materials. On one 54-inch line, 170 feet of coated steel laid open cut joins about 10,500 feet of bare steel set in grout. That pairing forms a mixed-metal cell with a driving voltage that can reach about 0.5 V, and the small defects in the coating become its anodes. Resistivity varied too much along the routes for sacrificial anodes, so the 36, 48 and 54-inch lines got impressed current from deep anode wells, and the 24-inch line, mostly plastic, got monitoring only. 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.

Testing what was built

At one of the pump stations the specifications required a corrosion engineer to witness the CP testing, and we took that over from another engineer in 2023. We watched the crew open every test station and install GPS-synchronized interrupters on every anode station. Several deficiencies remained, and by our return in 2024 they had been resolved.

In New Braunfels we also noted that overhead distribution lines run along the whole alignment, and unbalanced loads on them can induce AC on a metallic pipeline.

Grounding near San Antonio

At a corrugated packaging plant near San Antonio we measured the ground path from the building ground rod to nearly every piece of equipment, about 150 readings, and none failed. Four readings above 5 ohms were flagged as worth watching, and we recommended repeating the survey about once a year.