Evaluation of guidelines for subgrade treatments

Title from cover.; "February 2005."; "Final Report No. FHWA/OH-2005/003."; Final report.; Prepared by University of Toledo, Dept. of Civil Engineering in cooperation with the U.S. Dept. of Transportation, Federal Highway Administration and sponsored by the Ohio Dept. of Transportation under; Harvested from the web on 2/22/06ODOT has recently developed guidelines for subgrade treatments to be used during plan development and construction. This study evaluates these guidelines by comparing them with existing guidelines developed elsewhere and validating them through comparison of the actual treatment methods and quantities used with those suggested by the guidelines. Soil boring data and construction record from 7 reconstruction and 2 new construction projects were obtained. Actual treatment methods and quantities were obtained from the project field offices. Dynaflect deflection data, when available, were analyzed to determine their usefulness in subsurface assessments. GB1 criteria for excess moisture content predicted the undercut quantity reasonable well, but the criteria for acceptable moisture content tend to under predict the undercut quantity in many cases, likely due to increased soil moisture content after removal of existing pavement. For reconstruction projects, the average undercut depths (i.e., the overall undercut quantities) versus the corresponding SPT NL values seem to fall reasonably well within the upper bound provided in the Section 204 guidelines. However, the actual undercut depths vary significantly even for soils with similar or same NL values. Actual undercut depth and quantity are somewhat correlated with the average SPT NL value, Dynaflect W5 deflection, and soil moisture content. The regression equation developed has a coefficient of determination (or R-square value) as high as 0.71.ODOT has recently developed guidelines for subgrade treatments to be used during plan development and construction. This study evaluates these guidelines by comparing them with existing guidelines developed elsewhere and validating them through comparison of the actual treatment methods and quantities used with those suggested by the guidelines. Soil boring data and construction record from 7 reconstruction and 2 new construction projects were obtained. Actual treatment methods and quantities were obtained from the project field offices. Dynaflect deflection data, when available, were analyzed to determine their usefulness in subsurface assessments. GB1 criteria for excess moisture content predicted the undercut quantity reasonable well, but the criteria for acceptable moisture content tend to under predict the undercut quantity in many cases, likely due to increased soil moisture content after removal of existing pavement. For reconstruction projects, the average undercut depths (i.e., the overall undercut quantities) versus the corresponding SPT NL values seem to fall reasonably well within the upper bound provided in the Section 204 guidelines. However, the actual undercut depths vary significantly even for soils with similar or same NL values. Actual undercut depth and quantity are somewhat correlated with the average SPT NL value, Dynaflect W5 deflection, and soil moisture content. The regression equation developed has a coefficient of determination (or R-square value) as high as 0.71