Although this type of test is more difficult, the results provide a more realistic indication of frost heaving characteristics, giving an indirect measure of the size and distribution of soil pores. A theory now held, based on the correlations between pore size and heaving pressures, is that the smaller the pore size the, greater the pressure. The way in which pore size distribution affects the heaving pressure is being investigated. In general it can be said that coarse sands and clean gravels do not heave, while fine sand and silts are very susceptible to heaving. Clays also are very susceptible although they normally heave slowly but often with tremendous pressures.
Silts show a high rate of heave but have much lower heaving pressures.
When silts, sands or gravels are contaminated with clay, however, heaving ability is usually much enhanced and becomes less predictable. At present the most reliable method of spotting a frost-heaving soil is to carry out a laboratory freezing test, although soils that show frost heaving in the laboratory do not always do so in the field. The test is therefore apparently on the safe side, but further research is required before completely reliable predictions can be made. Prevention of Frost Damage Frost heaving is not usually a problem in heated structures since the heat loss from the building keeps the temperature in the soil adjacent to the foundation above the freezing point. Difficulties often arise, however, in unheated detached buildings or in unheated additions to heated buildings.