Investigation of Number 10 Screenings for Subgrade Soil Improvement

About the report:

Poor subgrade often requires in-place stabilization or removal by excavation and replacement with suitable material.  Chemical stabilization with cement or lime is a common remediation practice.  This study investigated the use of No. 10screenings, a quarry byproduct finer than the 4.75-mm (No. 4) sieve, as a means to improve poor subgrade conditions. 

Three sources of screenings with two marginal soil types at various mixture rates were studied in the laboratory to determine an appropriate field application.  The engineering improvements were determined using the California bearing ratio, density, plasticity reduction, and resilient modulus.  At a mix rate of 50% by weight, increased density, reduced optimum moisture content, and reduced plasticity were observed.  Some improvements for the clayey soil as measured by the soaked California bearing ratio were observed, but none for silty soil.  Improvement in the resilient modulus, which is the subgrade support value for mechanistic pavement design, was not obvious and depended on the field moisture condition.  No improvement in resilient modulus was observed at the field equilibrium moisture condition predicted by the Mechanistic-Empirical Pavement Design Guide using the Thornthwaite moisture index model, as amended subgrade seems to achieve equilibrium at or near 90% degrees of saturation.  

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