Simplified verification of serviceability

In traditional geotechnical practice, serviceability limit states have been avoided by a variety of means, such as: for foundations, limiting the bearing pressures underneath the foundation to 'allowable' (conservative) values; for piles, by applying large safety factors to base and shaft capacities; and for embedded retaining walls, using 'mobilization factors' to reduce the passive earth pressure assumed to achieve moment equilibrium.

All these methods are fundamentally the same. They attempt to reduce foundation movement by ensuring failure has a sufficiently remote possibility of occurring. Eurocode 7 acknowledges that deformations can be kept within required serviceability limits provided 'a sufficiently low fraction of the ground strength is mobilised'; a value of the deformation is not needed; and comparable experience exists with similar ground, structures, and method of application. Unfortunately, the standard does not specify (other than for spread footings, discussed below) how low this fraction should be. [EN 1997-1 §2.4.8(4)]

Eurocode 7 also gives emphasis to problems associated with heave and vibration, highlighting particular features that must be considered when assessing these issues. [EN 1997-1 §6.6.4 and 6.6.4]

For compression piles in medium to dense soils and for tension piles, serviceability limit states will normally be prevented by verifying that an ultimate limit state will not occur. [en 1997-1 §7.6.4.1 note]

For conventional structures founded on clays, Eurocode 7 requires settlements to be calculated explicitly whenever the ratio of the characteristic bearing resistance Rk to the applied serviceability loads Ek is less than three. In addition, if this ratio is less than two, those calculations should take account of the ground's non-linear stiffness. By implication, if the ratio Rk/Ek is greater than or equal to three, then the serviceability limit state may be deemed to have been verified by this ultimate limit state calculation.

Hence verification of serviceability may also be demonstrated by satisfying the inequality:

Yr ,sls in which Ek = the characteristic effects of actions, Rk = the characteristic resistance to those actions, and yr,sls = a partial resistance factor > 3.

Design actions F„

Design material

Design effect of actions E0

Design resistance R,,

Verification

Figure 8.4. Alternative verification of serviceability

Actions

Simplified verification of serviceability t--

Characteristic actions F,

Representative actions F™

weight density

Geometrical parameters

/ Nominal dimensions

Material properties Characteristic material properties Xt strength

Design actions F„

Design material

Design effect of actions E0

Design resistance R,,

Verification

Figure 8.4. Alternative verification of serviceability

Figure 8.4 shows how the flow chart of Chapter 6 (dealing with verification of strength) can be modified to verify serviceability instead. Partial factors on actions, materials properties, effects of actions, and resistance are replaced by a single partial factor on resistance equal to 3.0. The use of this alternative verification obviates the need to establish limiting values of movement Cd.

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