6 Spread foundations
(1)P The provisions of this Section apply to spread foundations including pads, strips and rafts.
(2) Some of the provisions may be applied to deep foundations such as caissons.
6.2 Limit states
(1)P The following limit states shall be considered and an appropriate list shall be compiled:
- loss of overall stability;
- bearing resistance failure, punching failure, squeezing;
- failure by sliding;
- combined failure in the ground and in the structure;
- structural failure due to foundation movement;
- excessive settlements;
- excessive heave due to swelling, frost and other causes;
- unacceptable vibrations.
6.3 Actions and design situations
(1)P Design situations shall be selected in accordance with 2.2.
(2) The actions listed in 2.4.2 (4) should be considered when selecting the limit states for calculation.
(3) If structural stiffness is significant, an analysis of the interaction between the structure and the ground should be performed in order to determine the distribution of actions.
6.4 Design and construction considerations
(1)P When choosing the depth of a spread foundation the following shall be considered:
- reaching an adequate bearing stratum;
- the depth above which shrinkage and swelling of clay soils, due to seasonal weather changes, or to trees and shrubs, may cause appreciable movements;
- the depth above which frost damage may occur;
- the level of the water table in the ground and the problems, which may occur if excavation for the foundation is required below this level;
- possible ground movements and reductions in the strength of the bearing stratum by seepage or climatic effects or by construction procedures;
- the effects of excavations on nearby foundations and structures;
- anticipated excavations for services close to the foundation;
- high or low temperatures transmitted from the building;
- the possibility of scour;
- the effects of variation of water content due to long periods of drought, and subsequent periods of rain, on the properties of volume-unstable soils in arid climatic areas;
- the presence of soluble materials, e.g. limestone, claystone, gypsum, salt rocks;
(2) Frost damage will not occur if:
- the soil is not frost-susceptible;
- the foundation level is beneath frost-free depth;
- frost is eliminated by insulation.
(3) EN-ISO 13793:2001 may be applied for frost protecting measures for building foundations.
(4)P In addition to fulfilling the performance requirements, the design foundation width shall take account of practical considerations such as economic excavation, setting out tolerances, working space requirements and the dimensions of the wall or column supported by the foundation.
(5)P One of the following design methods shall be used for spread foundations:
- a direct method, in which separate analyses are carried out for each limit state. When checking against an ultimate limit state, the calculation shall model as closely as possible the failure mechanism, which is envisaged. When checking against a serviceability limit state, a settlement calculation shall be used;
- an indirect method using comparable experience and the results of field or laboratory measurements or observations, and chosen in relation to serviceability limit state loads so as to satisfy the requirements of all relevant limit states;
- a prescriptive method in which a presumed bearing resistance is used (see 2.5).
(6) Calculation models for ultimate and serviceability limit state design of spread foundations on soil given in 6.5 and 6.6 respectively should be applied. Presumed bearing pressures for the design of spread foundations on rock should be applied according to 6.7.