Effects of PET Strip Content, Dimensions, and Cement Stabilization on Clay Soil CBR: Experimental and Numerical Study

Abstract

This study investigates the California Bearing Ratio (CBR) behavior of a low-plasticity clay reinforced with recycled polyethylene terephthalate (PET) bottle strips, focusing on the effects of PET content, strip dimensions, and moisture condition. PET strips measuring 1 × 2 cm and 2 × 2 cm were incorporated at contents of 0.3%, 0.4%, 0.6%, and 1.2% by dry soil mass. CBR tests were conducted under different moisture conditions, including unsoaked specimens prepared at the natural-soil optimum moisture content (OMC), specimens prepared at the individual OMC of each mixture, and specimens subjected to 96 h of soaking. The untreated soil exhibited CBR values of 3.83% and 3.24% under unsoaked and soaked conditions, respectively. At the natural-soil OMC, PET inclusion improved CBR up to 0.6%, with the 2 × 2 cm strips producing the highest PET-only value of 6.63%, whereas the 1.2% mixtures showed lower values than the untreated soil. Under the individual-OMC condition, a measurable CBR of 2.06% was obtained only for the 0.3% PET mixture with 1 × 2 cm strips. Following 96 h of soaking, none of the PET-only specimens exhibited a measurable CBR response. In a subsequent stage, the selected mixture containing 0.6% PET with 2 × 2 cm strips was treated with 4% and 6% cement and tested after soaking, yielding CBR values of 35.94% and 64.21%, respectively. The results demonstrate that PET content, strip dimensions, and moisture condition govern the bearing response, while cement stabilization substantially improves performance under prolonged soaking.