Utilization of Recycled Carpet Waste in Clay Soil Mixtures: Mechanical Properties and Environmental Benefits

In line with sustainable strategies for soil improvement, this study investigated the mechanical behavior of clay soil reinforced with various percentages of recycled carpet waste (RCW) through compaction, unconfined compressive strength (UCS), and direct shear tests (DST). The results indicated that with increasing RCW content, the maximum dry unit weight (MDUW) decreased, while the optimum moisture content (OMC) increased, primarily due to the lower specific gravity and greater water absorption capacity of the carpet fibers. The UCS and secant modulus at 50% of peak stress (E50) increased up to 1% RCW, reaching 216.7 kPa and 3342 kPa, respectively. However, beyond this content, both strength and stiffness declined, likely due to weakened internal bonding and increased void ratio in the soil-fiber mixture. In terms of shear strength parameters, both cohesion and the internal friction angle improved up to 1% RCW (70.2 kPa and 18.7°, respectively), followed by a decrease at higher dosages. These changes are attributed to the improved interlocking and bonding between soil particles and the distributed fibers at lower contents. Overall, incorporating RCW up to approximately 1% offers a practical and environmentally responsible approach for enhancing the engineering properties of clay soils.