Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 395

The use of charts: The actual number of blows, N, from the standard penetration test

has to be corrected, under certain circumstances to obtain N′, the corrected value. Thornburn

(1963) has given a set of curves to obtain N′ from N. He also extended the graphical relation-

ship given by Terzaghi and Peck (1948) between the settlement of a 305 mm square plate

under a given pressure and the N′-value of the soil immediately beneath it, as shown in Fig. 11.2.

This can be used for determining the settlement, Sf, of a square foundation on a deep

layer of cohesionless soil by using Terzaghi and Peck’s formula:

Sf = Sp 2

03

2

B

B+

F

HG

I

. KJ

...(Eq. 11.3)

where Sp = Settlement of a 305 mm-square plate, obtained from the chart (Fig. 11.2) and,

B = Width of foundation (metres)
The chart is applicable for deep layers only, that is, for layers of thickness not less than
4 B below the foundation.

For rectangular foundations, a shape factor should presumably be used. It is as follows:

Table 11.1 Shape factors for rectangular foundations in cohesionless soils

(After Terzaghi and Skempton)

L/B Shape factor Shape factor

(flexible) (rigid)

1 1.00 1.00

2 1.35 1.22

3 1.57 1.31

4 1.71 1.41

5 1.78 1.49

Note: Settlement of a rectangular foundation of width B = Settlement of square foundation of
size B × shape factor.
Immediate Settlement in Cohesive Soils
If a saturated clay is loaded rapidly, excess hydrostatic pore pressures are induced; the soil
gets deformed with virtually no volume change and due to low permeability of the clay little
water is squeezed out of the voids. The vertical deformation due to the change in shape is the
immediate settlement.
The immediate settlement of a flexible foundation, according to Terzaghi (1943), is given
by:

Si = q. B^1

F −^2

HG

I

KJ

ν

E

I

s

. t ...(Eq. 11.4)

where Si = immediate settlement at a corner of a rectangular flexible foundation of size L × B,
B = Width of the foundation,
q = Uniform pressure on the foundation,
Es = Modulus of elasticity of the soil beneath the foundation,
ν = Poisson’s ratio of the soil, and