4 Design principles

Soil is fully frozen when all the water in it is frozen. This is assumed to have occurred when the temperature of the soil reaches –1 °C (see annex D). The data given in clauses 8 to 10 apply when the foundations are to be designed so that no fully frozen soil occurs below the foundation during the design winter. Alternative data based on a criterion of 0°C are given in annex C.

This design condition may be achieved in one of four ways:

  • 1) arranging for the foundation depth to be greater than the depth at which fully frozen soil occurs;
  • 2) removing frost-susceptible soil from below where the foundations will be built, to the same depth as mentioned in 1), and replacing this with well-drained material that is non-susceptible to frost;
  • 3) insulating the foundations to reduce heat loss from the soil below the foundations so as to keep this soil unfrozen;
  • 4) using heat loss from the building, or special heating measures, to keep the soil below the foundations unfrozen.

For the purposes of this standard, 1) and 2) are equivalent and are covered in clause 7. Furthermore, the solution adopted can be a combination of 2), 3) and 4). Thus, the thickness of any layer below the foundations that is non-susceptible to frost may be included in the foundation depth Hf when using this standard to decide whether frost protection measures are necessary and, if so, what insulation is needed.

NOTE 1 If option 4) is chosen, a combination with 3) is usually necessary to restrict heat loss.

The insulation required by options 3) and 4) can be determined by:

  • a) using the tables and graphical presentations in this standard (see clause 8, 9 or 10, depending on the type of building), or
  • b) undertaking numerical calculations conforming with the principles given in annex B.

It is also permissible to use a combination of a) and b), for example determination of insulation required at corners by a) and (two-dimensional) numerical calculations to determine the insulation required elsewhere.

Heat emission from floor heating systems, heating cables in the ground, or similar, is not allowed for in the design procedures of clauses 8 to 10. Numerical calculations shall be undertaken when such heat emission is to be considered.

NOTE 2 If the design procedures of clauses 8 to 10 are applied to such situations, there will be an additional margin of safety as regards frost heave, but perhaps additional heat loss.

The foundations shall be designed to avoid adfreezing of the soil, thus preventing frost heave by transfer of shear forces, for example by having a layer of material that is non-susceptible to frost adjacent to the walls of the foundation or basement.

If the building envelope is not completed and/or the building is not heated before the frost season, additional insulation measures shall be undertaken to protect the foundations.

NOTE 3 One way of achieving such additional protection is to design the foundations as for unheated buildings using a design freezing index for non-permanent structures (see 6.1).

The parameters relevant to frost protection are:

  • climate, especially freezing index and annual average temperature;
  • frost susceptibility of the soil;
  • thermal properties of the ground, both frozen and unfrozen;
  • insulation of the floor;
  • internal temperature in the building;
  • the geometry, and especially the overall dimensions, of the building, and the type of foundation used.

NOTE 4 Snow cover has the effect of reducing the frost penetration depth, but since snow cover cannot be assured for design purposes, no allowance for it is made when assessing the design criterion.

Some examples are illustrated in Figure 1.

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