Figure 9.9. Infinite soil slope overlying permeable rock particular issue as the stresses due to the weight of the ground are both the source of the instability and a key contributor to the resistance.

Notes on Example 9.1

O The contact area is slightly larger than the horizontal area of the slice, owing to the slope angle.

© The calculation of normal stress takes into account both the resolution of the vertical stress normal to the slope and the increased contact area of the slice discussed in ©.

© Here the normal stress is considered to be an unfavourable action, even though its subsequent effect on shear resistance is favourable. Both the unfavourable and favourable effects are considered as coming from a 'single source' and hence a single partial factor (yg) is applied to onk and TEk.

© This design is governed by Combination 2, which - with a utilization factor less than 100% - means that the strength of the slope has been verified successfully for Design Approach 1.

© In Design Approach 2, partial factors are more commonly applied to effects of actions rather than to actions. Therefore, in this calculation, the normal stress is considered a favourable action effect, which subsequently minimizes the shear resistance. The single source principle is not invoked and different partial factors (YG,fav and yg) are applied to onk and TEk respectively.

© Design Approach 2 is the only one which employs a partial factor on resistance > 1.0.

© The design just works for Design Approach 2 (degree of utilization is close enough to 100% not to worry). If the single source principle had been applied (see © above), then the utilization would have dropped to 76%.

© In Design Approach 3, partial factors from Set A2 are applied to all geotechnical actions (there are no structural actions in this design situation).

© The design works for Design Approach 3, giving the same degree of utilization as Design Approach 1.

® The traditional global factor of safety for this situation is «1.5 which would be considered more than adequate for long-term conditions.

Example 9.1 Infinite soil slope overlying permeable rock Verification of strength (GEO)

Design situation

Consider a dry hill slope comprising H = 5m of sandy CLAY overlying rock. The kN

clay's characteristic dry weight density is Yk = 1®-and its characteristic m drained strength parameters are c'k = 2kPa and ^k = 25°. The hill slopes at

v = 1m vertically to every h = 3m horizontally, i.e. p = tan

Design Approach 1

Actions and effects

Contact area per unit width x unit run An =

Characteristic values at soil/rock interface: Vertical stress a^ = Ykx H = 90kPa avkx cos(p)

Normal stress ank =-

0 0

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