Investigation Of Variations In Corrosion Potential In Mechanically Stabilized Earth Backfill Due To Migration Of Fines

Investigation of Variations in Corrosion Potential in Mechanically Stabilized Earth Backfill Due to Migration of Fines

Departments of Transportation have constructed thousands of mechanically stabilized earth (MSE) walls to support bridge abutments in highway projects and for other applications. These MSE walls often include metal strips or grids as reinforcement, typically galvanized steel strips within granular backfill meeting Federal Highway Administration (FHWA) and American Association of State Highway and Transportation Officials (AASHTO) standards. Utilization of steel strips or grids creates a stronger composite material; however, minerals within the backfill or salts applied at the surface can create a corrosive environment. Excessive corrosion can lead to distresses or premature failure of MSE structures. Corrosion may increase when cycles of water from precipitation promote migration of fines through the granular backfill. Migrating fines have the potential to accumulate at the base of the reinforced fill and clog drainage and retain water, which could accelerate the corrosion process. This study evaluated the potential for accelerated corrosion due to the accumulation of fines. Aggregate approved for use in MSE structures was placed in a test column with internal dimensions of 30 × 30 × 183 cm, which then had water flowed through it. The grain size distribution was measured at different elevations within the column and the resistivity of the aggregate, which is correlated with corrosion rate, was also evaluated at a series of elevations within the column after water has been passed through it. Results from the testing were compared with resistivity results from a test box consistent with current Kansas Department of Transportation (KDOT) use. All aggregates tested had a drained resistivity that was well above the 5,000-ohm-centimeter (ohm-cm) limit. The results of this study show that migration of fines can occur in KDOT aggregates, and that this migration can cause measurable changes in the grain size distribution, water content, and resistivity of the soil column. In addition, as the number of saturation and drained cycles increases for each material, the resistivity also increases. The current KDOT specification limiting the amount of material passing the No. 200 sieve is beneficial in that it limits the fines available for migration. Additional constraints within the specification could further limit the potential for suffusion.

Measuring Corrosion Conditions in Mechanically Stabilized Earth Walls

KDOT extensively utilizes mechanically stabilized earth (MSE) walls, typically with coarse aggregate backfill. Previous projects K-TRAN: KSU-15-6 and K-TRAN: KU-15-5 tested the aggregate material from MSE walls under construction. KU identified that several of the clean aggregate samples contained some fines, likely from crushing of the aggregate, and two of the walls contained over 10% fines. Aggregate backfill is specified because it allows for free drainage, limiting the pore water pressure buildup behind the wall. There is the potential for these fines to migrate towards the base of the wall over time. This migration of fines would not only increase the potential for water retention, but it would also increase the localized corrosion potential of the backfill. This joint project measured the backfill corrosion conditions of 12 MSE walls selected by KDOT. Walls were surveyed using a variety of non-destructive resistivity arrays and by cutting through the wall face, sampling the backfill, and inspecting reinforcing strap conditions. Multiple walls with potentially corrosive backfills (low resistivity) were identified, and several walls had reinforcement with visible corrosion in progress. The resistivity survey method and the physical sampling method were generally consistent in identifying backfills with low resistivity. The results support the potential of using a modified four-electrode electrical resistivity measurement to identify corrosive environments in MSE walls. If used by KDOT, this will improve KDOT’s geotechnical asset management of MSE walls. Results from the physical testing of backfills showed that the finest fraction of the backfill (D10) was strongly correlated with low resistivity values, particularly with the sand backfills. High chloride concentrations were also measured in the three samples with the lowest resistivity. These results support the current recommendation that the percentage of fines (material smaller than the #200 sieve) be limited to a maximum of 5%, and encourages the consideration of an additional specification that the percentage passing the #100 sieve be limited to 10%.

Investigation of Corrosion at 14 Mechanically Stabilized Embankment Sites