O

Degree of utilization is a,

GEO,2

rEd rRd

The design is unacceptable if degree of utilization is > 100%

Design Approach 3

Actions and effects

Characteristic values = same as for Design Approach 1 Partial factor from Set A2: yg = 1 ® Design normal stress CTnd = yg x CTnk = 81 kPa

Material properties and resistance

Characteristic resistance = same as for Design Approach 1

Partial factors from Set M2: y^ = 1-25 and Yc = 1-25

Design angle of shearing resistance ^d = tan c'k

Design effective cohesion c'd --- 1.6 kPa

Partial factor from Set R3: YRe = 1

Shear resistance TRd

31.8 kPa yRe

Verification of strength against sliding

Degree of utilization is a

GEO,3

rEd Rd

The design is unacceptable if degree of utilization is > 100%

Traditional design

Traditional global factor of safety is F --- 1.47 w

9.8.2 Infinite soil slope overlying impermeable rock

Example 9.2 considers an infinite soil slope overlying impermeable rock, with groundwater present in the slope and seepage parallel to the underlying rock, as shown in Figure 9.10.

Figure 9.10. Infinite soil slope overlying impermeable rock

(permeable

Figure 9.10. Infinite soil slope overlying impermeable rock

Notes on Example 9.2

O The clay's characteristic weight density (which was 18 kN/m3 in Example 9.1) has been increased here to 20 kN/m3 to account for the water table saturating the soil.

© The pore pressure ratio is calculated on the basis of the anticipated maximum water level during the slope's design working life (i.e. the persistent design situation). This is assumed to be 4m above the soil/rock boundary.

© As for Example 9.1, both the unfavourable and favourable effects of the soil's weight density are assumed to come from a 'single source' and hence a single partial factor (yg) is applied to o'nk and TEk.

© This design is governed by Combination 2, which - with a utilization factor greater than 100% - means that the slope does not satisfy Design Approach 1. Measures to rectify this include lowering the water table to 3.8m or flattening the slope to 1 in 4.1.

Example 9.2 Infinite soil slope overlying impermeable rock Verification of strength (GEO)

Design situation

Consider a saturated hill slope comprising H = 5m of sandy CLAY overlying an impermeable rock. The clay's characteristic saturated weight density is kN*

Yk = 20-V and its characteristic drained strength parameters are m c'k = 2kPa and ^ = 25°. The hill slopes at v = 1m vertically to h = 4m horizontally. Groundwater, which is at height hw = 4m above the soil/rock kN

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