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8.3.1 General Considerations

Since the stability and safety of a structure—more specifically, retaining wall structures— depend upon the proper performance of the foundation, it is important that an adequate foundation investigation be made. The purpose of the investigation is to provide the designer with information concerning the engineering properties of the subsurface condi­tions. Generally, a retaining wall extends for a considerable length. Accordingly, the amount and type of foundation investigation that should be made and/or which the owner can afford must be considered. The owner must understand that once an exploration crew is dispatched to the site of a proposed wall, the investigation should be sufficiently complete to allow for the selection of an appropriate wall type.

W...

The active pressure coefficient Ka is given by Coulomb theory as

where 0 = angle of slope of back wall to horizontal, degrees ф’ = effective angle of internal friction, degrees 8 = angle of wall friction, degrees P = angle of back slope, degrees

Refer to Figure 8.12 for the force diagram. The resultant horizontal earth force is to be determined for a design case wherein the following assumptions apply:

Design assumptions

ф’ = 34°

8 = 25°

P = 0°

0 = 90°

Y = 125 lb/ft3 (19.6 kN/m3)

H = height of wall = 20 ft (6.1 m)

Soil type = 1 (see Table 8.1)

Computations

sin (0 + ф’) = sin (90° + 34°) = sin 124° = 0.8290 sin2 (0 + ф’) = sin2 (90° + 34°) = sin2 124° = 0.6873 sin (ф’ + 8) = sin (34° + 25°) = sin 59° = 0.8572 sin (ф’ – P) = sin (34° – 0°) = sin 34° = 0...

For yielding walls, lateral earth pressures can be computed assuming active conditions and wedge theory, using a planar surface of sliding defined by the Rankine theory. Table 8.1 provides soil properties for computing active earth pressures for five types of soil. Table 8.2 provides friction factors and adhesion for dissimilar materials. See Figs. 8.10 and 8.11 for the magnitude and location of resultant forces on retaining walls considering various types of soil backfill and backslide geometries. The pressures presented in these figures assume mobilization of the soil shear strength along the entire Rankine active failure plane, extending uninterrupted from the ground surface at the base of the wall or to the location on the wall at which the total earth load is being computed. Figure 8...

Lateral earth pressure loadings are applied in various states—specifically, active, at-rest, and passive states. The state of pressure to be considered varies with the wall type.

Yielding Walls. Yielding walls are free to translate or rotate about their top or base. For such walls, the lateral earth pressure may be computed assuming active conditions and wedge theory. In general, the lateral displacement at the top of a rigid wall of height H necessary to develop the active state varies from 0.001H in dense cohesion­less soils to as much as 0.004H in loose cohesionless soils. For clay soils, a greater displacement on the order of 0.01H to 0.02H, for stiff and soft soils, respectively, is necessary to develop an active state. See Figs. 8.7 and 8.8.

Thus, it is noted that the amount of di...