Volume 36, Issue 5 (September 2008)
Forensic Investigation of Mechanically Stabilized Earth (MSE) Approaches
A forensic study was performed to determine: (1) the extent of voids in the retaining wall backfill in the first 1 m behind the wall panels; (2) the extent of moisture in the suspected wet areas where the District is planning to add turnarounds under the bridges; (3) severity of bridge approach settlement and its ride quality; and (4) whether or not there is global stability concern for FM (Farm to Market) 310, FM 1304, and Chatt Rd. mechanically stabilized earth (MSE) walls. Ground penetration radar (GPR), dynamic cone penetration (DCP), drilling, elevation surveys, and instrumentation was performed in this study. Although voids and soft layers were found under the concrete slab, they are all near the MSE wall. Periodic survey results from April 2006 to March 2007 indicate that there is no significant settlement (<10 mm). The piezometer measurements in the MSE walls indicate that the water table was low. The safety factor was found to be 1.4, which is above the minimum requirement of 1.3. Therefore, it was concluded that there is no global stability issue. Longitudinal profiles collected in June 2006 and July 2007 indicated that the maximum settlement in one year was found to be 6 mm for the Chatt Rd. overpass. Although there are some settlements on other overpasses, they are all less than 5 mm. There are no criteria on annual allowable settlement for bridge approaches. Thus, it was decided to continue to monitor the approach slabs without any immediate rehabilitation.