Modification of Highway Soil Subgrades

Major study objectives were to develop highway pavement subgrade stabilization guidelines, to examine long-term benefits of chemical stabilizers, such as cement, hydrated lime, and two byproducts from industrial processes, and to establish a subgrade stabilization program in Kentucky. In developing a program, a number of design and construction issues had to be resolved. Factors affecting subgrade behavior are examined. Changes in moisture content and California bearing ratio (CBR) strengths of untreated and chemically treated subgrades at three experimental highway routes were monitored over a 7-year period. CBR strengths of the untreated subgrades decreased dramatically while moisture contents increased. CBR strengths of subgrade sections treated with hydrated lime, cement, and multicone kiln dust generally exceeded 12 and increased over the study period. At four other highway routes ranging in ages from 10 to 30 years, CBR strengths of soil-cement subgrades exceeded 90. Knowing when subgrade stabilization is needed is critical to the development of an economical design and to insure the efficient construction of pavements. Bearing capacity analyses using a newly developed, stability model based on limit equilibrium and assuming a tire contact stress of 552 kPa show that stabilization should be considered when the CBR strength is less than 6.5. For other tire contact stresses, relationships corresponding to factors of safety of 1 and 1.5 are presented. Stability analyses of the first lifts of the paving materials show that CBR strengths of the untreated subgrade should be about 9 or greater. Guidelines for using geogrids as subgrade reinforcement are presented. Factors of safety of geogrid reinforced granular bases are approximately 10 to 25% larger than granular bases without reinforcement. As shown by strength tests and stability analysis, when the percent finer than the 0.002 mm-particle size of a soil increases to a value greater than about 15%, the factor of safety decreases significantly. Guidelines are also presented for the selection of the design strengths of untreated and treated subgrades with hydrated lime and cement. Based on a number of stabilization projects, recommended design undrained shear strengths of hydrated lime- and cement-treated subgrades are about 300 and 690 kPa, respectively. A laboratory testing procedure for determining the optimum percentage of chemical admixture is described. Correlations of Dynamic Cone Penetrometer and the Clegg Impact Hammer values and in situ CBR strengths and unconfined compressive strengths are presented.