Soils susceptible to cracking are considered of geotechnical interest due to the implications they have on the stability of engineering works. An emerging and sustainable alternative to improve the mechanical properties of these soils is the addition of fibers from reused synthetic materials such as polyethylene terephthalate (PET). This research adapts experimental techniques to induce tensile stresses in samples of fine soils of lacustrine origin reinforced with variable percentages of PET fibers, monitoring the strain response through imaging techniques. As a result, the optimal experimental conditions are identified, and parameters useful for the study of fracture mechanics and the flexural strength of reinforced soil are determined using methods based on linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM).