Mechanical properties of a reinforced clay subgrade: Experimental investigation and mathematical modelling of fibre distribution patterns

• Polypropylene & recycled kenaf reinforced clay in 7 fiber distribution patterns. • Fiber content variations in layers impacted mechanical properties and deformability. • More fiber content in middle layers improved properties over uniform distribution. • A model was proposed to assess layer location effects on reinforced soil properties. • Layer effectiveness on soil properties increased from edges to specimen midpoint. In this research, a low-plasticity clay soil was reinforced by polypropylene and recycled kenaf fibres. Seven symmetrical fibre distribution patterns (FDPs) were proposed to evaluate the effect of FDPs and the type of fibre on the mechanical properties of the reinforced soil. Prepared cylindrical samples comprised 5 layers and contained 1 % by weight (wt) of fibre. However, the fibre content of individual layers varied symmetrically in the range of 0.4 % to 2 %wt, depending on the adopted FDP. A series of unconfined compressive strength (UCS) tests was carried for the assessment of mechanical properties. Furthermore, the effectiveness of combinations of individual layers with various FDPs on mechanical properties and deformability of the whole specimen structure was determined using a mathematical model. The experimental results showed that fibres led to a decrease in the secant modulus of elasticity, as well as a remarkable improvement in the UCS (up to 82 %) and deformability (up to 175 %) of the unreinforced soil. Variations in the fibre content of different layers affected the deformability and mechanical properties of the reinforced samples. Some of the proposed FDPs (F3, F5, and F7) resulted in specimens with greater strengths up to 8.8 % compared to the control samples, which contained uniform fibre content in all their layers (F1). While confirming the experimental results, the outcomes of mathematical modelling indicated that as distance from the edges towards the mid-point of the sample’s height increased, there was a progressive increment in the effectiveness of layers on the deformability and mechanical characteristics of the specimen. Outcomes of this study, facilitates the identification of subgrade’s specific depths/layers which require more attention during ground improvement activities.