Freeze-thaw durability and bearing performance of polypropylene fibre-reinforced Himalayan subgrade soil from Sela Pass

This study investigates the compaction characteristics and California Bearing Ratio (CBR) performance of Polypropylene (PP) fibre-reinforced soil obtained from the high-altitude Sela Pass region of the Eastern Himalayas (SW-SM soil), with emphasis on Freeze-thaw (F-T) induced strength degradation. Fibre contents ranging from 0% to 0.5% (by dry weight) were incorporated, and Standard Proctor compaction and CBR tests were conducted before and after four controlled F-T cycles (-15℃ for 12 h and + 20℃ for 12 h). Fibre inclusion resulted in a systematic reduction in maximum dry density (MDD) from 1.83 to 1.70 g/cm³ and an increase in optimum moisture content (OMC) from 13.2% to 15.72%, indicating a reduction in compacted density and an increase in the moisture required for compaction. CBR increased with fibre content up to 0.4%, followed by a slight reduction at 0.5%. Under soaked conditions, CBR improved from 14% to 23.73%, while unsoaked CBR increased from 22% to 33.70%. Following F-T cycling, untreated soil exhibited significant strength loss; however, fibre-reinforced specimens retained substantially higher residual capacity. At 0.4% fibre content, soaked and unsoaked CBR values were 18.77% and 29.38%, respectively, compared to 9.3% and 16.63% for untreated soil. The results show that low-dosage PP fibre reinforcement maintained higher absolute residual CBR values after four freeze–thaw cycles, indicating improved short-term post-thaw bearing performance under the investigated laboratory conditions. Among the fibre contents tested, 0.4% produced the best overall performance and was identified as the optimum dosage for the investigated soil. This study provides new insights into the behaviour of natural high-altitude Himalayan soil under coupled fibre reinforcement and F-T conditions, supporting the application of fibre stabilisation in cold-region infrastructure.