When soil modification is used as a construction expedient to simply dry soils, a mix design is not required and typical dosage rates are used. When a reduction in PI is desired, mix designs typically consist of adding the selected chemical modification agent to the subgrade soils in varying amounts, allowing the soil to mellow if required, and selecting the application rate that provides the required decrease in PI. The design application rate is often increased 1-2% to account for field mixing.
Mix designs are recommended for all soil stabilization applications. Adjustments may be required in the field to the stabilizing agent design content to obtain optimum performance. Mix designs for bituminous and cementitious soil stabilized subbases are similar to base stabilization and FDR mix designs. ARRA publications FDR201 (under development) and FDR202 may be used for both bituminous and cementitious stabilized soils, respectively. Information on mix designs using lime, fly ash and cement are available from the National Lime Association, the American Coal Ash Association and the Portland Cement Association, respectively. Links to these documents can be found under Additional Resources of the Specifications section. Regardless of the stabilizing agent used, mix designs for stabilized bases should include the following general procedures:
Obtain samples of the soil and any additional materials
Combine the soil and any additional materials to desired percentages and determine gradation, plasticity index, Ph, sulfates and sand equivalent of combined materials
Determine need for and select the modifying/stabilizing agent based on structural and material requirements
Determine maximum dry density and optimum moisture content of combined materials
Mix and compact specimens at desired moisture content with varying percentages of modifying/stabilizing agent