Specifications for Embankment and Subgrade Compaction
Michael P. McGuire
Filz, George M.
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Six approaches were developed for specifying embankment and subgrade compaction and/or verifying compaction quality on Virginia Department of Transportation (VDOT) construction projects. These approaches, along with VDOT's current practices, were qualitatively evaluated for applicability considering the benefits and risks of each approach. Based on the findings, the use of specifications based on embankment performance measurements is viable for large design-build or design-build-maintain projects. The use of these specifications permits greater innovation on the part of the contractor while shifting more of the overall project risk to the contractor. The contractor's increased stake in the post-construction performance of the embankment aims to promote high quality workmanship with reduced oversight by VDOT. The primary risks faced by VDOT through the use of performance specifications include: (1) disputes between VDOT and the contractor over deficient embankment performance during the warranty period; (2) the difficulty of repairing underlying embankments when finished roadway features, such as pavements, are already in place; and (3) the uncertainty that construction defects will be manifested in a measurable way during the finite term of the performance period. There was considerable support, in both published sources and the responses generated from the survey of experts conducted for this project, for full time visual inspection of embankment construction. To the extent that is feasible considering fiscal and policy constraints, it is recommended that VDOT increase the number of experienced inspectors on embankment construction projects. Three approaches were considered to increase the number of inspectors. The first of these was simply for VDOT to hire more experienced inspectors; however, it is understood that this approach is very unlikely to be feasible. Another approach is for VDOT to outsource inspection work to a private engineering firm. The use of this approach allows for rapid adaptation of the inspection labor supply to changing construction demands. A potential shortcoming of this approach is that VDOT may see an increase in overall project costs compared to hiring its own inspectors. A third approach considered to increase the number of inspectors available on VDOT projects is to have the contractor contract with a private engineering firm to provide outside inspectors. This approach is not recommended based on extensive concerns expressed by the project focus group and the surveyed experts about the potential for a conflict of interest. This study also considered the use of a pay factor for embankment construction to motivate the contractor to deliver high quality compaction. The pay factor developed for this project links the contractor's payment to the results of field density tests. A shortcoming of this approach is that it increases the potential for disputes between the contractor and VDOT because every density test has the potential to influence the contractors pay. Another recommendation of this study is to significantly increase the minimum frequency of field density and compaction moisture content testing for embankment construction. It is important to highlight that an increased test frequency should not be considered as a replacement for observation by an experienced earthwork inspector.