6.6 Serviceability limit state design
(1)P Account shall be taken of displacements caused by actions on the foundation, such as those listed in 2.4.2 (4).
(2)P In assessing the magnitude of foundation displacements, account shall be taken of comparable experience, as defined in 18.104.22.168. If necessary, calculations of displacements shall also be carried out.
(3)P For soft clays, settlement calculations shall always be carried out.
(4) For spread foundations on stiff and firm clays in Geotechnical Categories 2 and 3, calculations of vertical displacement (settlement) should usually be undertaken. Methods that may be used to calculate settlements caused by loads on the foundation are given in 6.6.2.
(5)P The serviceability limit state design loads shall be used when calculating foundation displacements for comparison with serviceability criteria.
(6) Calculations of settlements should not be regarded as accurate. They merely provide an approximate indication.
(7)P Foundation displacements shall be considered both in terms of displacement of the entire foundation and differential displacements of parts of the foundation.
(8)P The effect of neighbouring foundations and fills shall be taken into account when calculating the stress increase in the ground and its influence on ground compressibility.
(9)P The possible range of relative rotations of the foundation shall be assessed and compared with the relevant limiting values for movements discussed in 2.4.9.
(1)P Calculations of settlements shall include both immediate and delayed settlement.
(2) The following three components of settlement should be considered for partially or fully saturated soils:
- s0: immediate settlement; for fully-saturated soil due to shear deformation at constant volume, and for partially-saturated soil due to both shear deformation and volume reduction;
- s1: settlement caused by consolidation;
- s2: settlement caused by creep.
(3) The sample methods for evaluating settlements s0 and s1 given in Annex F may be applied.
(4) Special consideration should be given to soils such as organic soils and soft clays, in which settlement may be prolonged almost indefinitely due to creep.
(5) The depth of the compressible soil layer to be considered when calculating settlement should depend on the size and shape of the foundation, the variation in soil stiffness with depth and the spacing of foundation elements.
(6) This depth may normally be taken as the depth at which the effective vertical stress due to the foundation load is 20 % of the effective overburden stress.
(7) For many cases this depth may also be roughly estimated as 1 to 2 times the foundation width, but may be reduced for lightly-loaded, wider foundation rafts.
NOTE This approach is not valid for very soft soils.
(8)P Any possible additional settlement caused by self-weight compaction of the soil shall be assessed.
(9) The following should be considered:
- the possible effects of self-weight, flooding and vibration on fill and collapsible soils;
- the effects of stress changes on crushable sands.
(10)P Either linear or non-linear models of the ground stiffness shall be adopted, as appropriate.
(11)P To ensure the avoidance of a serviceability limit state, assessment of differential settlements and relative rotations shall take account of both the distribution of loads and the possible variability of the ground.
(12) Differential settlement calculations that ignore the stiffness of the structure tend to be over-predictions. An analysis of ground-structure interaction may be used to justify reduced values of differential settlements.
(13) Allowance should be made for differential settlement caused by variability of the ground unless it is prevented by the stiffness of the structure.
(14) For spread foundations on natural ground, it should be taken into account that some differential settlement normally occurs even if the calculation predicts uniform settlement only.
(15) The tilting of an eccentrically loaded foundation should be estimated by assuming a linear bearing pressure distribution and then calculating the settlement at the corner points of the foundation, using the vertical stress distribution in the ground beneath each corner point and the settlement calculation methods described above.
(16) For conventional structures founded on clays, the ratio of the bearing capacity of the ground, at its initial undrained shear strength, to the applied serviceability loading should be calculated (see 2.4.8 (4)). If this ratio is less than 3, calculations of settlements should always be undertaken. If the ratio is less than 2, the calculations should take account of non-linear stiffness effects in the ground.
(1)P The following causes of heave shall be distinguished:
- reduction of effective stress;
- volume expansion of partly saturated soil;
- heave due to constant volume conditions in fully saturated soil, caused by settlement of an adjacent structure.
(2)P Calculations of heave shall include both immediate and delayed heave.
6.6.4 Vibration analysis
(1)P Foundations for structures subjected to vibrations or to vibrating loads shall be designed to ensure that vibrations will not cause excessive settlements.
(2) Precautions should be taken to ensure that resonance will not occur between the frequency of the dynamic load and a critical frequency in the foundation-ground system, and to ensure that liquefaction will not occur in the ground.
(3)P Vibrations caused by earthquakes shall be considered using EN 1998.