9.7 Ultimate limit state design

9.7.1 General

(1)P The design of retaining structures shall be checked at the ultimate limit state for the design situations appropriate to that state, as specified in 9.3.3, using the design actions or action effects and design resistances.

(2)P All relevant limit modes shall be considered. These will include, as a minimum, limit modes of the types illustrated in figures 9.1, 9.2, 9.3, 9.4, 9.5, 9.6 for the most commonly used retaining structures.

(3)P Calculations for ultimate limit states shall establish that equilibrium can be achieved using the design actions or effects of actions and the design strengths or resistances, as specified in 2.4. Compatibility of deformations shall be considered in assessing design strengths or resistances.

(4)P Upper or lower design values, whichever are more adverse, shall be used for the strength or resistance of the ground.

(5) Calculation methods may be used, which redistribute earth pressure in accordance with the relative displacements and stiffnesses of ground and structural elements.

(6)P For fine grained soils, both short- and long-term behaviour shall be considered.

(7)P For walls subject to differential water pressures, safety against failure due to hydraulic heave and piping shall be checked.

9.7.2 Overall stability

(1)P The principles in Section 11 shall be used as appropriate to demonstrate that an overall stability failure will not occur and that the corresponding deformations are sufficiently small.

(2) As a minimum, limit modes of the types illustrated in figure 9.1 should be considered, taking progressive failure and liquefaction into account as relevant.

9.7.3 Foundation failure of gravity walls

(1)P The principles of Section 6 shall be used as appropriate to demonstrate that a foundation failure is sufficiently remote and that deformations will be acceptable. Both bearing resistance and sliding shall be considered.

(2) As a minimum, limit modes of the types illustrated in Figure 9.2 should be considered.

9.7.4 Rotational failure of embedded walls

(1)P It shall be demonstrated by equilibrium calculations that embedded walls have sufficient penetration into the ground to prevent rotational failure.

(2) As a minimum, limit modes of the types illustrated in Figure 9.3 should be considered.

(3)P The design magnitude and direction of shear stress between the soil and the wall shall be consistent with the relative vertical displacement, which would occur in the design situation.