Improving Quality Assurance of MSE Wall and Bridge Approach Earthwork Compaction

Abstract

The objective of the study was to investigate the efficacy of new devices for quality assurance (QA) of Class 1 backfill in mechanically stabilized earth (MSE) wall and bridge approach earthwork compaction. Extensive testing at two construction sites, an MSE wall project near Golden, CO, and multiple MSE wall/bridge approach projects near Wheat Ridge, CO, revealed that the light weight deflectometer (LWD), dynamic cone penetrometer (DCP), and Clegg Hammer are all capable of assessing the compacted state of Class 1 backfill. Field test data also revealed that the current Colorado Department of Transportation (CDOT) practice of single position nuclear gauge (NG) testing is inadequate. An evaluation of field data demonstrated that target values (TVs) exist for the LWD modulus (ELWD), Clegg Impact Value (CIV), and DCP penetration index that could serve as surrogates for the current 95% compaction requirement. The DCP exhibited two key limitations: moisture sensitivity and penetration resistance from placed geogrid (MSE walls) and geofabric (bridge approach). Because the LWD and Clegg Hammer do not possess the same limitations, these two devices were deemed most suitable for QA of MSE wall and bridge approach earthwork compaction. The authors recommend CDOT implement a pilot study using the LWD and Clegg Hammer in conjunction with NG testing with the following objectives: (1) identify ELWD and CIV TVs for various soils, site and moisture conditions, seasons, etc.; (2) evaluate if/how TVs change with these conditions; (3) populate a database of TVs; and (4) allow CDOT inspectors, consultants, and contractors to evaluate the devices.