Abstract
In places south and southwest of Johannesburg, including the Auckland Park vicinity, the weathered soft and moisture-susceptible residual Parktown Shales and Brixton Quartzite are used as subgrade and base course materials. When traffic loads are applied, the surface and base course material progressively deteriorate, impairing pavement structural integrity. The geotechnical properties of randomly distributed micro (strand) and macro (discrete) polypropylene fibre-reinforced road subgrade soil using calibrated Instron machine and one cubic meter mild steel box (1 m x 1 m x 1m) with a thickness of 25-mm subjected to static and cyclic loading conditions using three 30 mm thick circular plates of 150 mm, 250 mm, and 305 mm diameter were investigated in this study. The bearing pressure-deformation relationship, cycles induced-deformation relationship, stress-strain relationship, the traffic benefit ratio (TBR), bearing capacity ratio (BCR), settlement reduction factor (SRF), subgrade reaction and deformation modulus of unreinforced and fibre-reinforced road subgrade were investigated for different loading pressure plate sizes and at various deformation levels. Lastly, the strain variations calculated from the Instron machine Linear Variable Differential Transducers (LVDTs) under cyclic plate loading conditions were compared to those obtained from the Fibre Bragg Grating (FBG) sensors.
Macro polypropylene fibre significantly improved the recoverable stress, strain, and permanent deformation compared to unreinforced and micro polypropylene fibre reinforced subgrade. FBG sensor instrumentation improved the interpretation of changes in geotechnical properties under cyclic loading conditions. The findings of this study serve as an understanding of potential benefits that could be achieved using polypropylene fibre for soft subgrade reinforcement. The soil-fibre mixture results encourage sustainable engineering applications, such as foundations and roads. The implications of this study in the real-world application are as follows; it is recommended that the top 150 mm road subgrade soil layer is scrapped out and mixed with the polypropylene fibre content that will mobilize the optimum improvement in the geotechnical properties of the road subgrade and then compacted back in place. A coarser subbase layer will then be compacted on top of the fibre-reinforced subgrade layer. An additional asphalt layer would induce more beneficial improvements. Optical fibre sensors may also be embedded at specific locations within the pavement layers to capture and monitor strain and temperature variations over time in the pavement. Hence, the data obtained offers pavement performance evaluation and serviceability in a real-world application.