F C

The components of settlement that should be considered are:

• immediate settlement (s0) due to shear at constant volume in saturated soils (or with volume reduction in partially-saturated soils)

• settlement caused by consolidation (s1)

• settlement caused by creep (s2) [FN 1997-1 §6.6.2(2)] Hence, the previous inequality can be re-written for footings as follows:

SFd = s0 + s1 + s2 < SCd where sEd is the total settlement (an action effect) and sCd is the limiting value of that settlement.

In verifications of serviceability limit states (SLSs), the combination factors ^ applied to accompanying variable actions are those specified for the characteristic, frequent, or quasi-permanent combinations (see Chapter 2), i.e. y =

In ultimate limit state (ULS) verifications, combinations of actions for permanent and transient design situations employ combination factors ^ = Since is numerically greater than for most actions, representative actions are usually larger for ultimate than for serviceability limit states.

Partial factors for serviceability limit states are normally taken as 1.0.

Eurocode 7 states that calculations of settlement must always be carried out for footings on soft clays - and should be carried out for footings on firm to stiff clays1 when the risk is anything other than negligible (i.e. the footing is not in Geotechnical Category 1). [EN 1997-1 §6.6.1(3)P and (4)]

Calculations must consider settlement of the entire foundation and differential movement between parts of the foundation must include both immediate and delayed components. [en 1997-1 §6.6.1(7)Pand 6.6.2(1)P]

Annex F of EN 1997-1 presents two methods to evaluate settlement.

In the stress-strain method, the total settlement of a foundation may be evaluated by, first, computing the stress distribution in the ground due to the foundation loading (using elasticity theory for homogeneous, isotropic soil); second, computing the strain in the ground from those stresses using an appropriate stress-strain model (and appropriate stiffness); and, finally, integrating the vertical strains to find the settlements. [EN 1997-1 §F.1(1)]

In the adjusted elasticity method, the foundation's total settlement 's' may be evaluated using elasticity theory using the following equation:

Em where p is the bearing pressure (linearly distributed) on the base of the foundation; b is the foundation's width; f is a settlement coefficient; and Em is the design value of the modulus of elasticity. [en 1997-1 §F.2(1)]

Other methods for calculating settlement (from in situ tests) are given in the Annexes to EN 1997-2 (see the list in Chapter 4). Eurocode 7 emphasizes the fact that settlement calculations 'should not be regarded as accurate. They merely provide an approximate indication'. [EN 1997-1 §6.6.1(6)]

fStrictly, 'soft' clay refers to 'low strength' clay, 'firm' to 'medium strength' and 'stiff' to 'high strength' - see Chapter 4.

Where the depth of compressible layers is large, it is normal to limit the analysis to depths where the increase in effective vertical stress is greater than 20% of the in situ effective stress. [en 1997-1 §§6.6.2(6)]

As discussed in Chapter 8, for conventional structures founded on clays, Eurocode 7 requires settlements to be calculated explicitly when the ratio of the characteristic bearing resistance Rk to the applied serviceability loads Ek is less than three. If this ratio is less than two, those calculations should take account of the ground's non-linear stiffness. [EN 1997-1 §6.6.2(16)]

The serviceability limit state may be deemed to have been verified if:

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