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Factors from tSet A1 on structural actions; £Set A2 on geotechnical

Factors from tSet A1 on structural actions; £Set A2 on geotechnical

Few engineers would dispute that active earth pressures on the retained side of a wall are an unfavourable action and hence should be multiplied by yg. Thus in Design Approach 1, Combination 1 and Design Approach 2, active earth pressures are increased from their characteristic values by 35% (since yg = 1.35), but are left unchanged in Design Approach 1, Combination 2 (since Yg = 1.0). In Design Approach 3, earth pressures from the ground's self-weight are treated as a geotechnical action, for which yg = 1.0.

The question arises, however: are passive earth pressures acting on the restraining side of the wall a favourable action or a resistance? The answer to this question has a significant bearing on the verification of limit states GEO and STR. Figure 12.10 shows some of the possible outcomes depending on the answer to this question.

The top half of the diagram shows the bending moment (M) and shear force (V) that occur along the length of a cantilever embedded wall retaining 5m

50 100 550

Bending nwmenl (kNm/m)

Figure 12.10. Bending moments (left) and shear forces (right) for (top) a cantilever and (bottom) a propped embedded wall, both retaining 5m of soil of coarse soil with characteristic angle of shearing resistance = 30° and characteristic weight density Yk = 20 kN/m3. The bottom half of the diagram shows M and V for a propped wall also retaining 5m of coarse soil (with identical properties). The numbers on the curves refer to five different assumptions (or 'cases') summarized in the table below.

Assumption made about passive earth pressure (Op)

Partial factors applied to...

Shearing resistance (tan <pk)

Earth pressure

Active (oak) Passive (opk)

Shearing resistance (tan <pk)

Active (oak) Passive (opk)

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