THE EFFECT OF DENSIFICATION ON THE ENGINEERING CHARACTERISTICS OF ORGANIC SOILS

Mixtures of clay, silt, and sand with known organic contents were molded at their liquid limits in special cylinders and statically loaded for 48 hours under various simulated overburden pressures. Both unconfined and quick triaxial compression tests indicated usable shear strengths of 0.3 to 0.7 ksf. Static surcharging also increased the dry densities, reduced the moisture contents, and resulted in greatly reduced permeabilities of the clays and silts. Shear strengths generally increased directly with the surcharge and organic content. Rebound after removal of the surcharges was negligible. From standard consolidation tests, only the square root-of-time plots provided classically-shaped curves that could be used to determine the end of primary consolidation. Primary consolidation of organic soils occurs in two overlapping stages. Organic soils gain in stiffness under cyclic loading, generally after six cycles. Extensive appendices provide data for the preliminary estimating of settlement rates. The text gives numerical examples of embankment settlements and shear distribution based on the finite-element analyses. /FHWA/