# 11.5 Ultimate limit state design

### 11.5.1 Stability analysis for slopes

(1)P The overall stability of slopes including existing, affected or planned structures shall be verified in ultimate limit states (GEO and STR) with design values of actions, resistances and strengths, where the partial factors defined in A.3.1 (1)P, A.3.2 (1)P and A.3.3.6 (1)P shall be used.

NOTE The values of the partial factors may be set by the National annex. The recommended values for persistent and transient situations are given in Tables A.3, A.4 and A.14.

(2)P In analysing the overall stability of the ground, of soil or rock, all relevant modes of failure shall be taken into account.

(3) When choosing a calculation method, the following should be considered:

- soil layering;
- occurrence and inclination of discontinuities;
- seepage and pore-water pressure distribution;
- short- and long-term stability;
- creep deformations due to shear;
- type of failure (circular or non-circular surface; toppling; flow);
- use of numerical methods.

(4) The mass of soil or rock bounded by the failure surface should normally be treated as a rigid body or as several rigid bodies moving simultaneously. Failure surfaces or interfaces between rigid bodies may have a variety of shapes including planar, circular and more complicated shapes. Alternatively, stability may be checked by limit analysis or using the finite element method.

(5) Where ground or embankment material is relatively homogeneous and isotropic, circular failure surfaces should normally be assumed.

(6) For slopes in layered soils with considerable variations of shear strength, special attention should be paid to the layers with lower shear strength. This may require analysis of non-circular failure surfaces.

(7) In jointed materials, including hard rock and layered or fissured soils, the shape of the failure surface can partly or fully be governed by discontinuities. In this case analysis of three-dimensional wedges should normally be made.

(8) Existing failed slopes, which can potentially be reactivated should be analysed, considering circular, as well as non-circular failure surfaces. Partial factors normally used for overall stability analyses then need not be appropriate.

(9) If the failure surface cannot be assumed to be two-dimensional, the use of three-dimensional failure surfaces should be considered.

(10) A slope analysis should verify the overall moment and vertical stability of the sliding mass. If horizontal equilibrium is not checked, inter-slice forces should be assumed to be horizontal.

(11)P In cases where a combined failure of structural members and the ground could occur, ground-structure interaction shall be considered by allowing for the difference in their relative stiffnesses. Such cases include failure surfaces intersecting structural members such as piles and flexible walls.

NOTE In analysing natural slopes, it is generally an advantage to make a first calculation using characteristic values, to get an idea of the global factor of safety, before starting a design. Experiences with comparable cases including investigation procedures should be applied.

(12) Since a distinction between favourable and unfavourable gravity loads is not possible in assessing the most adverse slip surface, any uncertainty about weight density of the ground should be considered by applying upper and lower characteristic values of it.

(13)P The design shall show that the deformation of the ground under design actions due to creep or regional settlements will not cause unacceptable damage to structures or infrastructure sited on, in or near the particular ground.