Marquette University, Department of Civil, Construction, and Environmental Engineering
Wisconsin Department of Transportation
Contract or Grant Number
WisDOT SPR # 0092-45-95
This executive summary presents a summary of the findings of all study phases conducted to develop recommendations for the development of specifications for subgrade acceptance based on measured deflections. The rolling wheel deflectomter (RWD), portable truck-mounted deflection measurement systems, and dynamic cone penetrometer (DCP) were utilized on numerous subgrade construction projects between the 1998 and 2001 construction seasons. Comparative nuclear density and soil stiffness gauge readings were also obtained at selected locations on many of the included construction projects.
The research findings indicate that deflection test results may be appropriate for identifying areas of poor in-place stability within constructed subgrades. However, deflection testing alone may not provide all of the data necessary to properly differentiate acceptable and non-acceptable subgrade stabilities. It is important to note that deflection test results are related to the moisturedensity conditions at the time of testing. Soils that show acceptable results (i.e., low deflections) may subsequently weaken due to changes in moisture content, freezing/thawing, etc. In instances where subgrade acceptance is well in advance of base course application, subgrade moisture changes may result in decreased soil support. For those conditions where soil compaction has been conducted at a moisture state near optimum, surface deflections should be correlated to the achieved level of compaction.
Based on the deflection data gathered during this research study from test areas which were considered as passing based on visual observations, a deflection acceptance threshold of 1.50 inches was selected as reasonable to limit associated acceptance errors. For use within project implementations, this threshold value was recommended for use to identify potentially “failed” test locations. It was recommended that the project engineer retain the right to require corrective actions to improve subgrade conditions based on the magnitude and extent of failed readings.