Tarrahan Parseh Transportation Research Institute International Journal of Transportation Engineering 2322-259X 13 4 2026 04 01 Investigation of the Factors Affecting the Unconfined Compressive Strength (UCS) of the Subgrade Soil Stabilized with Copolymers 2355 2372 247004 10.22119/ijte.2026.541849.1705 EN Zohreh Ghafori Fard Faculty of Civil Engineering, University of Yazd, Yazd, Iran 0000-0002-9084-982X Maryam Mokhtari Faculty of Civil Engineering, University of Yazd, Yazd, Iran Mohammad Mehdi Khabiri Faculty of Civil Engineering, University of Yazd, Yazd, Iran 0000-0003-3434-7603 Seyyed Mehrdad Jalilian Iran Polymer and Petrochemical Institute, Tehran, Iran Journal Article 2025 08 22 Since transportation plays a crucial role in the economic and social development of a country and affects many aspects of life, it is considered as a vital artery there. Stabilizing the subgrade soil of roads improves their physical and mechanical properties and increases resistance to traffic load, moisture, and temperature changes. The goal is to extend the service life of roads, reduce maintenance costs, prevent premature failures, and enhance transportation. Most soils are stabilized with additives like cement, lime, and bitumen to improve their strength and durability, but these traditional stabilizers cause significant environmental pollution. Therefore, using environmentally friendly materials is important. This study examined the unconfined compressive strength (UCS) of the soil stabilized with styrene-acrylate and styrene-butadiene, as two eco-friendly emulsion copolymers, in dry and wet conditions and then in wet-dry and freeze-thaw cycles. The results were analyzed using the SPSS and Design-Expert software programs. It was indicated that increasing a pure polymer initially increases UCS, followed by a decrease. Also, moisture, wet-dry and freeze-thaw cycles reduce UCS. According to the independent t-test results, only the reduction of strength due to moisture was statistically significant. Using the equations proposed in this study, it is possible to estimate the UCS of the soil stabilized with the mentioned copolymers under different conditions and determine the optimal type and amount of polymers based on project requirements. Temperature and moisture variation cycles Road subgrade soil Sandy Soil Dry unconfined compressive strength Wet unconfined compressive strength http://www.ijte.ir/article_247004_7eaef0b6792f04e9534c0ff2f922ee89.pdf