Bearing capacity and settlement of shallow foundations - The Mechanics of Soils and Foundations - page 348

Figure 22.4 Loading and settlement of foundations.

the loading and terminate as ρ d as shown in Fig. 22.4(b). Figure 22.4(c) shows the

same loading increased quickly so the loading is undrained and there is an immediate,
undrained settlement

ρ i as shown in Fig. 22.4(d). The undrained loading raises the pore
pressure in the soil below the foundation and dissipation of the excess pore pressures
causes consolidation settlements to occur. The settlement at some time t after the
start of consolidation is ρ t and the final consolidation settlement which occurs after a

relatively long time is ρ ∞ . (Notice that the loadings and settlements shown in Fig. 22.4

are similar to those shown in Figs. 6.9 and 6.10 which describe the fundamental
differences between drained and undrained loading and consolidation.)
Generally, engineers designing foundations will need to calculate all, or some, of the
following:

1. The bearing capacity q c (to ensure that the foundation has an adequate margin of
safety against collapse).
2. The allowable bearing pressure q a and either the drained settlements

ρ d or the

i .
3. For consolidation after loading, the final consolidation settlement

(undrained) immediate settlement ρ

ρ ∞ , and the

variation of settlement ρ t , with time.

22.3 Soil strength parameters and factors for design of
shallow foundations
The main consideration for design of safe slopes described in Chapter 21 is to pre-
vent the slope reaching an ultimate limit state in which it fails. The appropriate soil