Mechanical Behavior and Microscopic Mechanisms of Glass Fiber–Modified Loess in Seasonally Frozen Regions

To reduce the occurrence of specific disasters, such as the freezing of cracks and the uneven settlement of loess subgrade in seasonal freezing areas, glass fiber is selected as a reinforcing material to study its strengthening effect. The glass fiber was mixed into loess at a weight ratio of 0%, 0.2%, 0.4%, 0.6%, and 0.8% to prepare samples. Through freeze–thaw cycles, triaxial shear tests, scanning electron microscopy tests, and nuclear magnetic resonance tests, the mechanisms and effects of reinforcement are expounded from the perspectives of macroscopic and microscopic combinations. The aim of this study is to reveal the influences of glass fiber content, confining pressure and freeze–thaw cycling on the mechanical properties and microscopic mechanisms of glass fiber-reinforced loess. Research has indicated that at the macroscopic level, reinforcement material significantly enhances roadbed strength. With increasing reinforcement amount, the strength increases gradually. After the glass fiber content reaches 0.6%, the reinforcement effect is stabilized. With the increase in the number of freeze–thaw cycles, the change patterns of the failure strength of the plain loess and glass fiber-reinforced loess are the same: first decreasing and then gradually stabilizing. The cohesion of plain loess decreases from 21.46 kPa to 13.45 kPa, and the internal friction angle decreases from 26.53° to 19.06°. The cohesion of fiber loess decreases from 97.11 kPa to 37.30 kPa, and the internal friction angle decreases from 27.65° to 23.50°. After 10 freeze–thaw cycles, the reinforced loess has better strength than plain loess. The microcosmic reinforcement mechanism of reinforced loess is clarified. The reason for the slow development of cracks in loess is that fibers restrain friction and space. Moreover, the formation of a three-dimensional force network in loess fibers can improve the strength. Therefore, adding glass fiber to loess can effectively enhance its failure strength and frost resistance.