It is a design objective that snowmelt and rainfall should have fast access to the side of the road and the drainage system. This avoids surface ice formation and skidding, but also protects the structure so that as little water as possible filters through to the pavement. Edges, kerbs, channels and runoff barriers must be kept clear by routine maintenance. Sufficient snow storage (and snowmelt) capacity on the road side must be included in the design. Dry structures and the road bed should be effected less by frost.
During spring-thaw there is considerable increase in the moisture of many unbound materials. The magnitude of the spring-thaw weakening (on bearing capacity and slope stability) depends, very much, on the functioning of the drainage system.
Due to snow cover on the road side, frost depth is typically deepest in the middle of the road and this will cause uneven frost heave across the section of road. The heave depends on how long the freezing front remains in the frost susceptible soil, the soil characteristics and also how much water there is (see Chapter 4, Section 4.6). If the frost heave differential is big enough, it can cause surface roughness, cracks, breakage of the surfacing and lifting stones (causing first roughness and later on surface cracking).
The permeability of most soils decreases when frozen. In springtime the frost can form an almost impermeable water barrier in the soil below the road since thawing will be fastest from the top and in the middle of the road. In that case the melted water will flow in a longitudinal direction until it can find an exit route, at which point it will flow to the road side or into the subsoil causing localised seepage and/or erosion problems.
Low permeability side-slopes decrease the infiltration of rainfall, but they can also act as flow barriers for the seepage of water out of the pavement structure and thus increase frost damage as well as decrease the strength of the structure. In that case openings with gravel filling at certain intervals and on low-lying locations can help the situation as in Fig. 13.9. Another option is a drain. A drain is the only option if the road structure is deeper than the open ditch at the side of the road.
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Fig. 13.9 Outlet with gravel filling on a low permeability slope |
If there is no outlet ditch or drain available, and the amount of water is small, the water may be soaked into the soil. A transition wedge of coarse material and non water susceptible structure can be used to avoid frost damages at a low lying location where water is periodically accumulated.
In cold climates, where frost heave in winter is a problem, an open-graded layer can be constructed at the bottom of the pavement to prevent water being pulled from the subgrade due to a freezing front in the pavement. This will lower frost heave in the road construction by decreasing the water content compared to pavements without a capillary break. However, frost-susceptible layers will always heave to some degree if there are freezing conditions. For example, Hermansson (2004) reports a field study where an extremely well drained soil heaved 80 mm during a period of two months. The open-graded layer will not affect frost heave in a susceptible subgrade if the freezing front gets that low in winter, but it will significantly reduce the effect on the highway surface by allowing the road to bridge differential subgrade heaves.
More information on drainage of low volume roads in cold climates is given in a report by Berntsen & Saarenketo (2005).
