Coupled chemical–mechanical stabilization of expansive subgrade soil using cement kiln dust and polypropylene fibers for flexible pavement performance

Expansive soils pose significant challenges to construction of pavements because of their swelling behaviour, low strength and high moisture sensitivity. This paper examines the use of cement kiln dust (CKD) and polypropylene fiber (PPF) as chemical and mechanical stabilizers respectively to enhance expansive subgrade soil and assesses the implication of this on the thickness of flexible pavement based on the IRC guidelines. Laboratory tests were conducted in a sequence in order to evaluate swelling, plasticity, strength and bearing properties, and the pavement design was made according to mechanistic-empirical IRC processes. The findings revealed that CKD has a great influence on the behavior of soil in terms of cation exchange and pozzolanic reactions, in which the behavior of clay becomes less active and subsequently increases its stiffness and the load-bearing capacity. The polypropylene fibers added further mechanical restraint by fixing microcracks and reducing the volumetric deformation and thus increasing ductility and post-peak performance. The integrated stabilization system altered the soil to low-plasticity and dimensionally stable subgrade that can support pavements in compliance with IRC standards. The optimal combination of 20% CKD and 1.5% PPF significantly improved the performance of expansive soil, reducing DFS from 75.38 to 9.48% and PI from 32.2 to 4.5%, while improving UCS from 280 to 1340 kPa after 90 days curing and soaked CBR from 3.47 to 15.3%. The resulting changes led to the reduction of pavement thickness from 595 to 530 mm, which represents a savings of about 10.92% in material. The inclusion of the stabilized subgrade properties into IRC pavement design reported a significant decrease in total pavement thickness across all traffic classes, up to a maximum of approximately 10.9% in 20 msa traffic. The research also provides a direct correlation between sustainable soil stabilization and structural pavement response, and indicates the possibility of CKD-PPF systems to be cost effective and environmentally friendly systems of pavements construction. The microstructural analysis established that cementation (C-S-H/C-A-H formation) of the clay matrix due to CKD and polypropylene fibers contributed the crack-bridging restraint, which, together with the cementation of the clay matrix, contributed to strength enhancement, volumetric stability, and decreased pavement thickness.