Improvement of bearing capacity of shallow foundation on sand reinforced with polyethylene terephthalate (PET) flakes

This study investigates the impact of adding recycled PET flakes on shear strength properties and the ultimate bearing capacity of sandy soil. The sandy soil was reinforced with different percentages of PET flakes (0, 0.5, 0.75, 1.0, and 1.5 by dry weight). Vertical load tests were conducted in a laboratory on square model footings in a soil tank. Tests were performed on both unreinforced sand and reinforced sand. The sand samples in the tank were prepared at three different relative densities (Dr): 40%, 65%, and 85% for each percentage of PET flakes. The effect of reinforcement depth beneath the model footing was also investigated to determine the optimum reinforcement depth that can improve the bearing capacity of model footing. To support the experimental results, numerical analysis was also carried out using Plaxis 3D software based on the finite element method. In addition to experimental tests and numerical analysis, three well-known bearing capacity equations from the literature were also used. The bearing capacity equations used to calculate the ultimate bearing capacity of model footings were Terzaghi (1943), Hansen (1970), and Vesic (1973). According to experimental results, it was found that the optimum percentage of PET flakes that gives the best improvement in ultimate bearing capacity load of model footing was found to be 0.75%. It was found that reinforcing sand with 0.75% PET flakes resulted in a considerable increase in the ultimate bearing capacity, at a relative density of 40%, there was an impressive 84% increase in ultimate bearing capacity. At a relative density of 65%, a substantial 64% enhancement was observed, and at a relative density of 85%, the ultimate bearing capacity improvement was 56%. Furthermore, this reinforcement led to a notable reduction in settlement of model footing, with decreases of 14.1% at a relative density of 40%, 11.3% at a relative density of 65%, and 9.6% at a relative density of 85%. It was also revealed that the reinforcement sand beyond a depth of 2B beneath the model footing has a negligible effect on the results. A comparison was made between the results of experimental, theoretical, and numerical analysis. It was found that experimental results closely matched the results of Vesic's equation at relative densities of 40% and 65%, and predominantly aligned with Hansen's equation at 85% relative density, and all results notably deviated from Terzaghi's equation. Numerical analysis results showed slightly higher ultimate bearing capacity loads and settlement than experimental results but overall there was a good agreement between them. Interestingly, the difference between experimental and numerical results was more pronounced at lower relative density (Dr = 40% and 65%) compared to higher relative density (Dr = 85%).