Comparison of the Stabilization Behavior of Fly Ash and Bottom Ash Treated Expansive Soil

Abstract

Expansive soil swell on absorbing water and shrink when that water gets evaporated. Because of this alternate swelling and shrinkage of expansive soil, the civil engineering structures built on them get severely damaged. Ground improvement using mechanical and chemical methods can be a mitigation measure. In this research, chemical stabilization was used as a ground improvement technique. The variation of the compaction characteristics, Atterberg limits, Unconfined Compressive Strength (UCS) and swell pressure were tested using separately ASTM Class F fly ash (low calcium) and bottom ash as chemical stabilizers at 8 %, 16 % and 24 % of the total weight of the expansive soil. A Scanning Electron Microscopy (SEM) test was also conducted to study the microstructural changes in the expansive soil treated with fly ash and bottom ash. The results indicate that the Maximum Dry Density (MDD) of the stabilized soil increases up to 16 % with fly ash and bottom ash additions and that it begins to decrease thereafter with further additions. The results of the Atterberg limits test reveal that the liquid limit and the plasticity index decrease with both fly ash and bottom ash additions while the plastic limit increases with those additions. The effect of fly ash and bottom ash on the variation of the UCS was observed for three different curing periods (7, 14 and 28 days) as well as for three different percentages of ash content (8 %, 16 % and 24 %). The findings reveal that the UCS increases up to 16 % of ash addition, and that it thereafter starts to decrease with any further addition of fly ash or bottom ash. Furthermore, an increase in the curing period will help to increase the UCS for a given percentage of additions. The microstructure of the stabilized soil becomes more uniform as the the optimum ash content is reached, and beyond this optimum value, the microstructure becomes nonuniform with an abundance of unreacted ash particles. A reduction of the swell pressure by 70% for fly ash and 48% for bottom ash is observed with the addition of admixtures. The main conclusion that can be drawn from this study is that the MDD, UCS and the plastic limit can be increased with the addition of fly ash and bottom ash while swelling, liquid limit and plastic index can be reduced through these additions. Fly ash is also found to be more effective than bottom ash in stabilizing expansive soil.