Slope angie, p (degrees) Figure 9.12. Taylor's chart for determining the stability of undrained slopes

Notes on Example 9.3

O It has been assumed that Yk x H represents the unfavourable action and therefore yg has been applied to this part of the stability number. For DA1-1, the stability number has been calculated using yg = 1.35 and ym = 1.0; and for DA1-2, yg = 1.0 and ym = 1.4. The net effect is to give a lower stability number for DA1-2 than for DA1-1.

© For the undrained case, the resultant utilization factor is low, indicating that in the short term the slope more than adequately satisfies DA1.

© For the drained case, the stability number for DA1-1 is higher than for DA1-2 as ym x yg is smaller for DA1-2 than for DA1-1.

© As would be expected, the drained situation is more critical and DA1-2 governs the design.

Example 9.3 Road cutting (using design charts) Verification of strength (GEO)

Design situation

Consider a road cutting for a new bypass. Ground conditions in the cutting consist of a uniform CLAY with the following characteristic parameters: kN

weight density Yk = 20-, undrained strength cuk = 75kPa, effective m cohesion c'k = 5kPa, and effective angle of shearing resistance 9>k = 20°.

The slope is dry. The slope stands at an angle Pk = 19° and, at its maximum depth, the cutting is H = 12m deep. The slope passes through farmland and hence surcharge loading at its crest may be ignored.

Design Approach 1

Actions and effects

Partial factors from Sets

VA2 y

Material properties and resistance

Partial factors from Sets

V1.25 y

Design undrained strength cud =-=

v53.6 y

Design angle of shearing resistance ^d = tan kPa

Design effective cohesion c'd =-

0 0

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