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ABSTRACT: The western and northern parts of Palm Beach County, Florida have shallow layers of organic silts and peat under existing roadways, which often exhibit large amount of cracking, distortion and differential settlement in a relatively short period of time after reconstruction work. A coordinated field and laboratory investigation, comprising of Piezocone penetration tests (CPTu) along with Porewater Dissipation Experiments (PDE), and concurrent laboratory consolidation and secondary compression tests, were conducted to evaluate the characteristics of organic soils and peat existing under SR 15/US 98 roadway. The horizontal Coefficient of Consolidation, Ch was predicted from PDE data using several theoretical interpretation models. It was found that a unique relationship exists between Cc and Ca at any stress level, with Ca/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 these problematic soils.
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1PROJECT BACKGROUND
The western and northern parts of Palm Beach County, Florida have shallow layers of organic silts and peat under existing roads, which often exhibit large amount of cracking, distortion and settlement in a short period of time after reconstruction work. Organic layers under SR 15/US 98 roadway are shown in Figure 1.
The roadway alignment of interest is approximately a 6-mile segment of SR 15/US 98 from the Palm Beach Canal Bridge (MP 19.674) to the Palm Beach/Martin County line (MP 26.519). USDA soil survey data indicate that the surficial soils of the site are mapped as Torry muck and Adamsville sand, organic subsoil variant over hard limestone.
The muck is described as black or dark reddish brown, sapric material with about 2 to 50 percent fiber and 10 to 70 percent mineral content, is sticky to very sticky and is slightly to strongly acidic.
The pavement structure of SR 15/US 98 consists of nearly 305 mm of asphalt, 305 mm of limerock base, and 915 mm of silty sand subgrade, underlain by organic silts and peat with thicknesses ranging from 3 to 5.5 meters. Generally, the primary consolidation process in organic layers is quite rapid, followed by significant secondary compression stages under sustained effective overburden...