The Materials Sections in all six ITD Districts have historical records of numerous R-value tests conducted for the strength characterization of subgrade, subbase, and base-course materials used in the design of highway pavements. In addition, some historical data are available regarding expansion pressures of compacted soils, which have been used in conjunction with R-values to help design pavement sections by prescribing the thickness and construction controls needed to reduce pavement distortions due to expansive subgrade soils.
Much of this historical information is contained on soils profile drawings archived in the District offices, and which has never been analyzed comprehensively to determine if usable statistical correlations can be identified between fundamental soil properties and R-value (and expansion pressure). Such correlations obtained from neighboring states are of little use to Idaho, because the exudation pressure used for ITD R-value testing (Idaho T-8) has been 200 psi (1380 kPa), rather than the more commonly prescribed level of 300 psi (2070 kPa) per AASHTO T 190 and ASTM D 2844.
Furthermore, there is some concern that the ITD older paper archives may be physically deteriorating or considered surplus materials and, thus, may be lost to engineers and researchers before the historical test results can be gleaned and analyzed. Thus, a focused study is needed to "mine-out" these historical records and conduct statistical analyses to develop predictive models that can serve as guides in the Districts for tentative/preliminary design of pavement sections, as needed in the geotechnical component of Phase 1 Reports prepared for all ITD highway construction projects.