What is Frost Heave in Pavements?

Frost heave is the upward movement of a pavement when ice lenses grow in the frozen soil under it.

How it works

An ice lens is a layer of ice that grows inside the soil. Water moves toward the freezing front, often by capillary action, and feeds the lens. The lens grows and pushes the soil and the pavement above it upward. The 9 percent expansion of water as it freezes is too small to explain heave of this size.

The FAA names three conditions for detrimental frost action. All three must exist together.

1. The soil is frost susceptible.
2. Freezing temperatures penetrate into that soil.
3. Free moisture is present in enough quantity to form ice lenses.

If the soil is 70 percent saturated or more, the FAA says frost heave will probably occur. Water can come from the pavement surface or sides, from condensation, or from great depth by capillary action.

Frost-susceptible soil

The FAA sorts soil into four frost groups, FG-1 to FG-4. It uses the soil class and the share of material finer than 0.02 mm by weight. A higher number means a more susceptible soil. Soils with a high liquid limit and a high silt and clay content are more susceptible than gravels or sands. The FAA states that the choice of frost group is subjective.

Effect on airfield pavement

Frost heave is rarely even. The FAA lists non-uniform heave, roughness, drainage restriction and cracking among the effects of frost action. In a flexible pavement, a swelling bulge is usually caused by frost action around dissimilar subgrade materials. In a rigid pavement, frost heave can cause a step at a joint or crack.

A heaved runway can leave the regularity that EASA describes as achievable for a new wearing course. This is no more than 3 mm deviation under a 3 m straight-edge.

The thaw is a second risk. The subgrade loses strength when the frost melts. The FAA and EASA both say that overload operations should be avoided in this period.

Design against frost

The FAA allows three approaches in AC 150/5320-6G section 3.14.

• Complete frost protection: pavement and non-frost-susceptible material together contain the frost. The FAA considers it mainly for runways and taxiways at large hub airports, or where frost penetration is small.
• Limited subgrade frost penetration: non-frost-susceptible material covers 65 percent of the frost depth. The aim is less than 1 inch (25 mm) of heave.
• Reduced subgrade strength: the design assumes about 50 percent of the normal subgrade strength for the thaw period.

In every case the FAA advises a uniform subgrade with gradual changes between different materials. It also advises a geosynthetic separation layer between non-frost-susceptible material and the subgrade when the geotechnical engineer recommends one.