Stabilization of expansive Belle Fourche shale clay with different chemical additives

Improving the engineering properties of expansive soils is very important in northern plains, Texas and mid-west regions of the United States. In this study, expansive Belle Fourche clay (B) from South Dakota, was mixed with the class C fly ash (FC), class F fly ash (FF) and lime. Swelling pressure (SWP) and unconfined compressive strength (UCS) tests were conducted on samples that were cured at different periods (0, 7 and 28 days). Furthermore, freeze and thaw (F-T) effects on the swelling and strength properties of the day and selected mixtures were investigated. Results showed that the SWP of the Belle Fourche clay (B) decreased significantly with addition of lime 4% by dry weight of soil from 235 kPa to almost 0 kPa. Mixing fly ashes also reduced the SWP to 47 kPa and 100 kPa with class C and class F fly ashes, respectively. Increase in F-T cycles reduced the SWP, whereas the SWP increased with 2 and 4 F-T cycles for the mixtures with the fly ashes. However, after 4 cycles, the pressure of the same mixtures decreased. On the contrary, to the clay-fly ash mixtures, F-T did not affect the SWP of the clay-lime mixtures. In terms of strength, chemical treatment increased UCS. The overall effectiveness of the treatment under the curing and F-T was in the order of FC, lime, and FF. The UCSs of the clays treated with these additives were 3.58, 1.82, and 1.63 times higher than the non-treated clay. F-T reduced the UCS of the clay and the clay treated with FF. Although the UCS of the FC and lime mixtures increased within 2 cycles of F-T, they did not show the same strength improving performance with more F-T cycles. It was observed that mixtures with higher liquid limit and plasticity index (PI) tended to have higher SWP and lower UCS. This study claimed that chemically stabilized soils with high CaO content, CaO/SiO2, CaO/Al2O3, and CaO/(SiO2 + Al2O3) ratios had higher potential to decrease SWP of expansive soils and increase UCS of weak soils.