Lesson 1/Learning Event 2
Subgrades can be stabilized by the addition of granular materials or chemicals as admixtures. Consider
stabilization with admixtures only when economical. When granular materials are used, give the
stabilized layer a CBR rating as a subbase material, provided the admixed material meets the
requirements of subbases, as given in selection of design CBR for select material and subbases.
Otherwise, treat the stabilized layer as a layer of select material. Layers stabilized with portland cement
or bituminous materials in proper quantities to produce good quality soil cement or bituminous
stabilization shall be rated as a subbase with a CBR of 50. The surface of sand subgrades is often
stabilized with bituminous materials or coarse cohesionless granular materials to form a "working flour."
The treated layer is generally 4 inches or less in thickness. It should be assigned the same CBR as the
underlying layer. Do not use clay or other plastic soils or highly organic soils (sometimes locally termed
"muck") to stabilize sand subgrades.
TABLE 1. IN-PLACE STABILIZATION OF SUBGRADE SOILS
Stabilization of Soils with Portland Cement
Portland cement stabilization is applicable to granular soils and to low plasticity clays that can be
thoroughly pulverized. In general, soils to be stabilized with cement should have a plasticity index of
less than 20 and a minimum of 45 percent passing the #4 sieve, to permit proper mixing, as indicated