8.3.1 Frost Heave — Introduction
Frost heave occurs in roads having fine graded, so called frost susceptible material, at a depth to which the freezing front reaches during the winter. The frost heave typically causes an uneven road surface and results in reduced travelling speed and comfort. The main problem though usually arises upon thawing when ice lenses involved with the frost heave melt and result in high water content in the pavement. The increased water content often means reduced bearing capacity and spring-thaw load restrictions are imposed to avoid severe pavement deterioration.
Granular pavement layers normally show a substantial decrease in stiffness with increasing values of moisture. Once thawing commences in the spring season, the granular layers often reach a state of near-saturation that substantially reduces the load carrying capacity. During the winter, short thawing periods have similar effect, especially on granular base courses. Therefore, seasonal changes cause a significant variation in the ability of a pavement to support traffic loads. During the thawing
period, water is melted from the ice lenses and since the layers where the ice lenses are formed have high fines content, the stiffness can drop dramatically. Since the road thaws primarily from the surface downwards, the free water can not drain through the still frozen underlying layers. Another effect of frost heave is that the road layer where the ice lens was formed loses its compaction (density) which is gradually regained under traffic load.
Determination of frost-sensitivity of soils is generally carried out using frost heave tests, such tests are mainly used to classify soils, according to their frost — sensitivity. However, prediction of the mechanical behaviour of such soils in pavements (heave during the frost period or loss of bearing capacity during thawing) is much more complex, because it depends on the climatic and moisture conditions, and on the characteristics of the whole pavement structure.
The principal characteristic of stiffness variation due to environmental effects is given in Fig. 8.7. During winter short thawing periods can cause temporary decreases in the aggregate base and sub-base stiffness. If the thawing penetrates down to the subgrade it also loses its stiffness. As the freezing starts again both layers regain stiffness. During spring thaw, the stiffness of the granular base and sub-base again lowers but the bearing capacity regains soon after the spring thaw period is over. However, if the subgrade has high fines content, it can take longer for the bearing capacity to recover.
