(Yk - Yw)x "2" Total resistance q'ult = q'ult = 233 kPa q ult

Design resistance is q'Rd =-= 233 kPa

Verification of undrained bearing resistance

Degree of utilization


qEd qRd

Design is unacceptable if degree of utilization is > 100% Verification of drained bearing resistance

Degree of utilization


q Ed qRd


Design is unacceptable if degree of utilization is > 100%

Notes on Example 10.3

O The term 'medium strength' is defined as an undrained strength between 40 and 75 kPa in EN ISO 14688-2.23

© In an ultimate limit state, the design water level should represent the most onerous that could occur during the design lifetime of the structure. Hence, here it is taken at ground surface.

© The water pressure acting beneath the footing is a favourable action, since it resists the weight of the foundation.

© For the undrained case, dc and sc are based on formulae developed from finite element studies (see main text). Note that EN 1997-1 does not include depth factors in its recommendations in Annex D. Shape factors for strip footing are normally taken as 1.0.

© The calculation indicates that the drained (long-term) situation is slightly more critical than the undrained (short-term). Combination 2 governs in both cases and is verified, since the utilization factors are less than 100%.

© Partial factors for Design Approach 2 are applied principally to actions.

© A resistance factor of 1.4 is applied to the resistance, in combination with the factors on actions.

© The calculation indicates that the undrained (short-term) situation is more critical than the drained (long-term) situation and is marginally unsatisfactory.

© For Design Approach 3, the partial factors result in a simultaneous increase in actions and decrease in soil strength. Design Approach 3 will therefore always be more conservative than Design Approach 1.

® Both for the drained and undrained case, Design Approach 3 suggests the footing is just satisfactory (degree of utilization . 100%).

10.10.4 Settlement of strip footing on clay

Example 10.4 looks at verification of serviceability of the strip footing from Example 10.3 (see Figure 10.11 on page 337). It is assumed that a rigid layer exists below the foundation, limiting settlement to within that layer.

This example attempts to verify serviceability implicitly (through an ultimate limit state calculation), and then repeats the exercise explicitly (using a serviceability limit state calculation).

Notes on Example 10.4

O For serviceability limit states, the design depth of the water table is the most adverse level that could occur 'in normal circumstances'. Hence, we have decided not to raise the water table to ground level. This is a less severe requirement than for ultimate limit states (see Example 10.3).

© the pore pressure beneath the base is lower for the serviceability limit state than for ultimate (see Example 10.3).

© Partial factors for serviceability limit state are normally taken as 1.0.

© The water pressure acting beneath the footing is a favourable action, since it resists the weight of the foundation.

© For footings on clays (excluding soft clays), serviceability limit states may be verified without an explicit settlement calculation provided a minimum resistance factor of 3.0 is applied.

© The calculation based on a resistance factor of 3.0 does not work for either the undrained or drained situations and therefore an explicit settlement calculation is required.

© The calculation model used24 is one of many that are available and follows the guidance given in Annex F to EN 1997-1.

© The calculation model chosen is commonly used in UK practice but is only consistent with the model given in EN 1997-1 Annex F if E = 1/mv.

© Only immediate and consolidation settlement have been considered. The creep component is considered negligible in this example. The analysis has ignored any correction that may be applied to adjust for consolidation settlements based on one dimensional analysis. A depth correction factor is not normally applied to infinitely long footings.

® The limiting value depends on the structure's specific requirements. In this example, serviceability is satisfied by the explicit calculation (degree of utilization = 92%).

Example 10.4 Settlement of strip footing on clay Verification of serviceability

Design situation

Consider the infinitely long strip footing from the previous example. There is a rigid layer underlying the footing at a depth of dR = 4.5m. The clay's undrained Young's modulus is assumed to be Euk = 600cuk = 27MPa and its

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