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Development and Validation of the Thaw-weakening Index

In northern climates, frost action is a major cause of pavement deterioration. It is well known that frost action in frost susceptible subgrade soils can cause considerable heaving in pavements. When spring thaw occurs, segregation ice melts generating high pore pressures in frost susceptible subgrades. The bearing capacity loss can be substantial, leading to important structural damage to pavement exposed to severe and uncontrolled loading conditions. Extensive research efforts have been devoted to the characterization of soils and material properties and their variation as a function of moisture and temperature variations. These research initiatives are typically conducted in the field using deflexion measurements or in the laboratory using cyclic triaxial testing. The results of these tests are used for the assessment of seasonal fatigue damage using analytical techniques to compute stresses and strains at critical location in pavement structures combined with empirical damage models. A new approach based on a mechanistic index, the thaw weakening index, is proposed. The index takes into consideration the amount of water accumulated by the freezing process, the rate of thawing in pavement layers and in the subgrade soil and the rate of consolidation of the pavement structure. Based on a number of observations found in the literature and on measurements made on test roads, good correlation was found between the index and the increase in pavement total deflection during spring thaw. A good correlation was also found between the index and the loss of stiffness (resilient modulus) of the subgrade soil. The paper will describe the theoretical development of the new index and will present correlations between the thaw-weakening index and observed weakening on in-service test roads during spring thaw.