Field performance of fly ash stabilised subgrades

Class C fly ash has been used to improve the properties of subgrade soils for several decades. This report contains a summary of the results of a study to quantify the level of improvement provided by Class C fly ash and the degree to which those improvements are effectively permanent. A series of dynamic cone penetrometer values were obtained for 12 streets with fly ash treated subgrades, and for five streets with untreated subgrades. Streets ranged in age from zero to nine years. For subgrades with fly ash the penetration resistance was recorded for the fly ash treated layer and the untreated soil beneath. Higher strengths were recorded for all fly ash treated subgrade layers than for the untreated soil beneath. No deterioration with age was observed for the subgrades evaluated. Laboratory and field testing of soils treated with fly ash also showed that fly ash contributed to soil strength and stiffness while plasticity and swell potential were reduced, although swelling was not eliminated.