Evaluation of Mechanically Stabilized Earth Walls with Microwave Synthetic Aperture Radar Imaging

Mechanically stabilized earth (MSE) walls have been increasingly used in geotechnical engineering due to their cost-effectiveness, construction efficiency, and tolerance to differential settlement. MSE walls have previously been observed to fail in the presence of voids and moisture behind them. To date, effective detection of voids and moisture changes with nondestructive evaluation is yet to be developed. In this study, a wideband (1–4 GHz) microwave synthetic aperture radar imaging technique (Popovics et al. 2014) is implemented to generate two-dimensional (2D) slice images of an MSE wall system at various depths into the wall. Two typical $\mathrm{1,520}\times \mathrm{1,460}\times 178\mathrm{mm}$ ($\mathrm{length}\times \mathrm{width}\times \mathrm{thickness}$) reinforced concrete wall panels with backfill sands were investigated. The maximum size of coarse aggregates in concrete was 25 mm. The front face of each wall panel included a decorative layer with an average thickness of 38 mm and a surface roughness of 20–50 mm. The effects of the surface roughness of wall panels, moisture, and voids in backfill sands were investigated. The moisture content was controlled by varying the water-to-sand ratio of the backfill sands from 0 to 0.18 with a step of 0.06. As shown in Fig. 1, the change in moisture in a $350\times 250\times 76\mathrm{mm}$ ($\mathrm{length}\times \mathrm{width}\times \mathrm{height}$) sandbox attached behind the first wall was successfully detected. The color scale represents the intensity of reflected microwave signals. To simulate voids in backfill sands, foam blocks measuring $127\times 127\times 51\mathrm{mm}$ and $254\times 254\times 51\mathrm{mm}$ ($\mathrm{length}\times \mathrm{width}\times \mathrm{thickness}$) were individually placed in a $660\times 610\times 51\mathrm{mm}$ sandbox and directly attached to the second wall. The foam blocks were detected and located successfully as illustrated in Fig. 2.