Research on the Frost-Heave Feature of Roadbed Soil Reinforced by Polyurethane Using Distributed Fiber-Optic Sensing

Polyurethane (PU) has proven to be an effective material for reinforcing frozen-soil roadbeds; however, the excessive use of PU increases cost and contamination and limits its large-scale application in practical projects. To fill this gap, laboratory tests were conducted to determine the optimal content that achieved the best reinforcement effect at the lowest cost. A continuous frost-heave strain profile and its variation features were obtained through laboratory tests using advanced Rayleigh optical frequency-domain reflectometry technology (OFDR). A calibration method for OFDR at negative temperatures was introduced. The influences of the PU content, water content, and ambient temperature on frost heave were determined based on distributed measurements. The results indicate that a linear function is suitable for describing the relationship between the strain shift and temperature variation above 0 °C, whereas a cubic function is suggested below 0 °C, with a fitted R² of 1. When the moisture content is 4.7% and the ambient temperature is -20 °C, compared with the original reinforced soil, the frost-heave displacement decreased by 33.27%, 47.43%, 71.65%, and 72.77%, respectively, after reinforcement with PU contents of 4%, 8%, 12%, and 16%. When the moisture content increased from 4.7% to 10% and the ambient temperature was -20 °C, compared to the original reinforced soil, the frost-heave displacement of the reinforced soil with PU contents of 4%, 8%, 12%, and 16% increased by 49.34%, 14.93%, 7.48%, and 0.16%, respectively. When the PU content was less than 4%, the reinforcement effect was insignificant. The freezing point and frost heave rate decreased after the addition of PU owing to the changes in the pore structure and matric suction.