Field and Laboratory Characterization of Problematic Organic Subgrade Soils in Florida

The existence of soft compressible peat and organic layers under SR 15 / US 98 in Palm Beach County, Florida has been historically recognized as the primary reason for premature cracking, rutting, and differential settlement leading to costly and frequent repair/reconstruction projects. In an effort to develop effective design strategies aiming at long-term preservation of the pavement structure, it is crucial to have a thorough understanding of the strength and compression behavior of the underlying organic silts and peats. Accordingly, a coordinated field and laboratory investigation was undertaken with the following objectives: (i) to conduct Piezocone Penetration Tests (CPTu) along with Porewater Dissipation Experiments (PDE) for in-situ determination of the Coefficient of Consolidation (Cv); (ii) to perform concurrent laboratory consolidation and secondary compression tests to validate the CPTu predicted properties; and (iii) to establish the (Cα / Cc) ratios for Florida organic soils following the well-known Time-Stress-Compressibility concepts. The horizontal Coefficient of Consolidation, Ch (and subsequently the Cv) was predicted from PDE data using several theoretical interpretation models. It was found that a unique relationship exists between Cc and Cα at any stress level, with Cα / Cc ratio ranging between 0.03 and 0.05. Considering the inherent difficulty in sampling and laboratory testing of undisturbed soft organic soils, CPTu holds promise as an effective tool for rapid in-situ characterization of the modulus and compressibility parameters, both of which are useful for mechanistic analysis, forensic interpretations of failures, and the validation of pavement performance models.