Evaluation of Mechanically Stabilized Earth Walls with Microwave Synthetic Aperture Radar Imaging
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 143, Issue 5
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 () 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 () 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 and () were individually placed in a sandbox and directly attached to the second wall. The foam blocks were detected and located successfully as illustrated in Fig. 2.
![](/cms/10.1061/(ASCE)GT.1943-5606.0001639/asset/4804b7bc-598c-440b-b8d1-2b1ecccd862d/assets/images/large/figure1.jpg)
![](/cms/10.1061/(ASCE)GT.1943-5606.0001639/asset/ff66e719-5a1b-4ffb-968f-ca074cbfd9b8/assets/images/large/figure2.jpg)
Implications
The proposed nondestructive imaging technique was proven to be a robust and promising method for detecting voids and moisture changes behind MSE walls. Small voids such as can be detected behind a 178-mm-thick reinforced concrete panel. The effects of voids and moisture changes can be distinguished by monitoring the strength of reflected signals. Voids produce weaker than background reflections and appear as dark spots, while moist areas produce stronger than background reflection and appears as bright spots in synthetic aperture radar images. This technique can potentially be applied to retaining walls, pavements, and tunnels.
Acknowledgments
Financial support for this study was provided by the U.S. Department of Transportation under the auspices of Mid-America Transportation Center (Grant No. 25-1121-0003-196).
References
Popovics, J. S., Ham, S., Ghasr, M. T., and Zoughi, R. (2014). “Comparison of synthetic aperture radar and impact-echo imaging for detecting delamination in concrete.” 40th Annual Review of Progress in Quantitative Nondestructive Evaluation, Vol. 1581, AIP, Melville, NY, 866–871.
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©2016 American Society of Civil Engineers.
History
Received: Jun 5, 2016
Accepted: Aug 24, 2016
Published online: Nov 29, 2016
Discussion open until: Apr 29, 2017
Published in print: May 1, 2017
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