Frost Boiling Mechanism
Based on the comparisons of the pavement performance during the monitored four years, the water sources that were available to form the frost boils came from the thawing of in situ ice lenses that developed in the pavement structure due to frost heave in the previous winter. The moisture content of the pavement structure at the beginning of the freezing process was one of the major factors that determined the intensity of the frost heave and the subsequent thawing in the next year. The larger the fully saturated zones were in the pavement structure, the more suction or negative pore water pressure (due to water expansion during the freezing process) it would generate. Since the freezing process penetrated the pavement structure from the top to the bottom, the only water source must be from the shallow water table beneath the pavement structure. Higher suction values would further increase the moisture content in the pavement structure and cause a zone of over saturation. The melting water from the over-saturated zone would provide sufficient water during the following spring to create soft spots at the surface, because the water was forced to the road surface when the soil beneath was still frozen.

Another factor influencing the severity of the frost boils was when the thawing front penetrated down to the bottom of the pavement structure. The thawing front penetrated to the bottom of the pavement structure in late July or early August, which was almost three months after the thawing season began. Because the frozen soil in the west side of the roadway held a large amount of frozen water, it took a larger amount of solar energy to melt the frozen soil. The only drainage path for the melting snow and runoff water was to flow through the pavement structure. This would further reduce the soil stiffness and intensify the frost boil issue. Furthermore, the center of the pavement structure formed a hard, frozen core during the melting season. The frozen core altered the water flow direction and trapped a large amount of water in the west side of the pavement structure, which intensified the frost heave action during winter time.

It was also worth noting that a large amount of rainfall would cause another issue called pressurized water overflow, which might also have generated soft spots on the road surface in summer time. Rainfall duration served as a more deteriorating factor to the pavement performance than rainfall intensity. The soft areas would heal up if there were periods of no rain. The moisture contents in the pavement structure beneath 0.47 meter (1.5 feet) experienced short periods of time of overly saturated. By carefully examining the rainfall events summary in Table 2, it was seen that there were several days of rainfall before the sudden increases in moisture content. Since the road prism was built on a side hill, the water naturally flows from west to east. Also, the 11° downhill slope made the hydraulic gradient the highest at the test section. These factors were evidence that the sudden increases in moisture content were due to pressurized water overflow to the road surface. Although the two issues presented the same superficial phenomena, the mechanisms causing the phenomena were different. Wicking Fabric Performance during Rainfall Events