Corrosion of Steel Pilings in Soils

ElevationSoil resistivity and pHResistivitypRFIGURE 6. Sandblasted sections of Z-type sheet piling cut fromtwo different locations in the Vicksbunj Floodwall afterexposure for 7 years.Although cinders were present in the soil at both locations, no significant corrosion occurred.B101, Section removed from floodwall at station 16+32.B102, Section removed from floodwall at station 23+83.Condition of piles:Approximately 30 to 40 pel-cent of the steel surfaceswas covered with mill scale. Soil adhered in manyareas of about 1-in. 2 like barnacles, beneath whichappeared uniform metal attack or shallow pitting.Pitting up to 40 mils in depth was widely scatteredand confined to areas of less than 1 in. 2 At elevation83.5 to 81.5 ft, there were a few pits with depthsbetween 40 and 45 mils. Measurements made onthe three most corroded 1 in. 2 areas showed an average reduction in the cross section of the web of 4 to 6percent. A section removed from the most corrodedarea of the pile is shown in figure 6.STATION 23 + 83Piling exposed: A 41-in. width of the flood wall wasexposed between elevation 89 and 80.5 ft.Surface elevation: 93 ftWater table elevation: 80.5 ftSoil characteristics:93 to 84 ft: Bluish-gray clay with nodules of brownclay dispersed throughout.84 to 83.5: Plastic and sticky light brown toreddish brown clay containing some cinders.83.5 to 80.5: Dark gray silty sand mixed withappreciable quantities of cinders, gravel, stones, andbricks. This horizon appears to be a fill material.Free water at bottom of trench.ft93 to 83..- ----___93 to 73 _______93 to 63- _______ .89 to 8488.5_____________83.5. _. ..-__„__-.83_______________81____-__-_______83.5 to80.5_______MinMaxAvgMinMaxAvgMinMaxAvgMinMaxAvgMinMaxAvgOhm-cm2,800 (4-pin)____________9,200 (4-pin)._ __________5,000 (4-pin)____________1,300 (4-pin) ___ _________3,700 (4-pin)___ _________2,800 (4-pin) ___ _________740 (4-pin)____________2,000 (4-pin)_____ _______1,400 (4-pin).____ _______825 (Shepard Canes) _ _ _725 (Shepard Canes) _ _910 (Laboratory) _ _ _ - _1,700 (Shepard Canes) _ _ _3,900 (Laboratory) ______1,050 (Laboratory) ______1,100 (Shepard Canes) _ _ _1,400 (Shepard Canes) _ _ _1,300 (Shepard Canes) _ . .7. 18. 67. 7Condition of piles:Mill scale was intact on about 70 percent of thepiling surfaces. There was no difference in theappearance of the surface at the different horizons.Where the mill scale had been removed, there was afilm of red rust which was brushed off with ease.Under the rust, the steel surfaces were smooth. Nomeasureable pits were found on the exposed pilings.Measurements fail to show a perceptible reduction inwall thickness (fig. 6).c. Sardis Dam Spillway, Sardis, MississippiHistory:An excavation was made to expose a 7-ft widthof pilings from the upstream wingwall on the eastside of the Sardis Dam Spillway. The structureconsisted of arch-type sheet piles with a 15-in.driving width and a wall thickness of % in.Date piles driven: Early 1940Date of inspection: March 1960Age of piling: 20 yearsPiling exposed: A 7-ft width of the wingwall wasexposed from elevation 307 to 302 ft.Surface elevation: 312 ftWater table elevation: 305 ftSoil characteristics:312 to 305 ft: Fill soil consisting of uniform reddishsandy loam.305 to 302: Natural soil, reddish brown tightimpervious plastic clay.