9. Retaining walls
the total action, they may be favourable – both situations should be checked). In the design calculations, favourable actions are multiplied by different partial factors to unfavourable actions – see Table 1.
Calculation model A In the first calculation model (see Figure 2), the wall including backfill in block ABCD resists sliding and toppling caused by the earth pressures acting on the vertical ‘virtual’ plane, BF. The ground beneath the wall base must also be strong enough to carry the wall’s self-weight and any tractions (vertical components of force) on the virtual plane. An attractive feature of this model is that, provided the wall heel CD is large enough, the earth thrust Pa (see Figure 2b) is inclined at an angle to the horizontal equal to the ground slope at the top of the wall (i.e. c = b ), provided always that c ≤ h’. The test for the model’s applicability is bh ≥ ha tan (45 – h’d /2), which (if met) means that a Rankine active zone forms within the confines of block ABCD
and the earth pressure coefficient used to calculate the thrust is given by Rankine’s formula. If bh is too small, then the wall stem interferes with the Rankine zone and c should be reduced to c ≤ (h’/3 to 2h’/3) – although strictly Rankine’s theory is no longer applicable and calculation model B should be used instead. For sliding and toppling, the most unfavourable location of any imposed surcharge is as shown in Figure 2a), with the edge of the surcharge coincident with point B. In that position it increases unfavourable earth pressures acting on the virtual plane BF but does not increase favourable vertical forces acting on the wall heel, DC. For bearing, the most unfavourable location of the surcharge is when it extends to the back of the wall, point A.
Calculation model B In the second calculation model (see Figure 3), the wall including backfill in block ACD resists sliding and toppling caused by earth
Figure 2 Calculation model A
a h
Figure 3 Calculation model B
h
69 3