Alternatives were explored to strengthen soils in the city of Chiclayo, Peru, especially those with low plasticity and strength. The goal was to study the mechanical properties using sugarcane bagasse ash (SCBA) and high-density polyethylene (HDPE) as sustainable and cost-effective reinforcements. The ash was calcined at four different temperatures, and each sample underwent an energy X-ray fluorescence test. The sample with the highest sum of oxides was mixed with clayey soil in increasing proportions, ranging from 5 to 20% by weight. Subsequently, the SCBA proportion with the best mechanical behavior was selected, combined with increasing proportions of HDPE fibers ranging from 0.25 to 1.0% by weight. The dimensions were kept constant with a length of 25 mm and a width of 15 mm. Chemical and physical stabilization techniques were applied to the study soil. The soil mixed with ash had a direct influence on compaction test parameters and soaked California bearing ratio (CBR) test, showing a significant increase of 32.08% up to 10SCBA. However, beyond this proportion, the strength decreased below the control sample. The results indicated that clay improves its behavior and strength with a ratio of 10SCBA + 0.75HDPE, resulting in a significant increase in soaked CBR of 154%. The use of the optimum dosage of ash and fiber influences in having a subgrade layer of low and high traffic volume; in addition, it minimizes contamination problems by reducing landfills and urban deposits, contributing to environmental sustainability.