Frost depth and frost heave

The phenomenon

Water in the ground expands when it freezes. If that happens beneath a foundation, it lifts it — and worse, not evenly.

On thawing the ground loosens and the foundation drops back. That cyclic movement causes the cracking and the movement.

Sensitivity depends on the ground: fine-grained, water-retaining soils — clay, silt — are frost-susceptible; coarse gravel much less so.

The classical solution

Take the foundation level below the frost depth, to a depth the frost does not reach. The depth required is a climatic question, given by local practice and the regulations.

The alternatives

Shallow foundations protected by insulation

Protect the ground from cooling with insulation and the frost line does not reach the foundation level. That allows a shallower foundation with less excavation.

Here the insulation is part of the structure: it cannot be omitted, and it has to be installed accurately.

Drainage

No water, no frost heave. Good surface and groundwater drainage reduces the risk considerably.

Replacing frost-susceptible ground

Swapping the susceptible layer for coarser-grained material.

What it means for ground screws

The capacity of a ground screw has to develop below the frost depth. If the load transfer takes place in the frost zone, frost heave can lift the element — above all where there is tension from the structure anyway.

What counts particularly for a light structure

The low self-weight gives less counterweight against frost heave. A heavy building is partly held down by its own weight; a light one less so.

So frost protection here is not merely a matter of convenience.

What the design has to record

• the depth of the foundation level and the reason for it,
• the arrangement of the insulation, where the protection rests on it,
• the drainage solution,
• the ground level around the structure, because a later change of level also changes the frost depth.

The point

Frost does not attack the capacity of the foundation; it lifts it. The defence is either depth or insulation — but one of them has to be there.