Frost susceptibility of the ground

To what extent the ground is susceptible to frost depends both on the properties of the soil material and on local conditions, such as layering and ground water level. Normally a high ground water level, water- containing layers or a mixture of coarse and fine soil layers increases the risk of frost heave.

Frost heave occurs when ice-layers (ice lenses) are created during freezing of the soil below the foundation. This implies a sufficient supply of ground water and sufficiently high capilliarity and permeability of the soil. Thus soils with a high content of silt or clay are those giving the greatest risk of frost heave. These types are considered frost-susceptible.

Fat clay (clay content > 40 %) is less susceptible to frost heave due to its low hydraulic conductivity.

In general, a geotechnical examination of the ground conditions at the building site to the depth of frost penetration is necessary.

A rough assessment of the frost susceptibility of a soil can be obtained on the basis of grain size distribution, as illustrated in Figure D.1, which shows the percentage of grains passing through sieves of different sizes.

Estimation of frost susceptibility of the basis of grain size distribution

Key

1 % through sieve
2 Size of sieve
A Silt
B Sand
C Gravel
D Stones

Figure D.1 — Estimation of frost susceptibility of the basis of grain size distribution

With reference to Figure D.1:

1) if the grain size distribution is such that grains of size less than 0,02 mm comprise less than 3 % of the soil, the soil is normally non-susceptible to frost;
2) if the grain size curve lies completely within region 1, the soil is always frost-susceptible (except for the "fat clay" region 1L where the frost susceptibility is low);
3) if the grain size curve falls completely inside regions 2, 3 or 4, the soil is non-susceptible to frost, provided that in the case of region 2 the capillary rise is also checked and is less than 1 m;
4) if the lower part of the grain size curve permanently passes the boundary of the next region on the finer side, the soil is frost-susceptible;
5) it is necessary to examine borderline cases using more exact methods.

The grain size distribution can be used in this way to classify the soil as either frost-susceptible or non- susceptible to frost. Borderline cases which do not fall precisely into either of these two limiting classifications should either be regarded as frost-susceptible for the purposes of design, or the frost susceptibility should be determined by laboratory tests or by representative frost-heave observations in-situ.

Further information about frost susceptibility and testing methods may be found in [3] to [6] in Bibliography.

ISO 13793-2001 Thermal performance of buildings Thermal design of foundations to avoid frost heave