Geotechnical Engineering

DHARM

418 GEOTECHNICAL ENGINEERING

18 m

9m

54 m

27 m

30 m

Hard rock

Clay

G.W.T. 3m

6m

1.5 m

Sandgsat= 18 kN/m ; N = 18^3

m = 0.00015 m /kNv^2

m = 0.00009v

m = 0.000075v

m = 0.000045v

m = 0.00011v Clay

E = 16.2 MN/m

E = 63 MN/m

2

2

swelling

(a) Plan of foundation (b) Soil profile at the site

Fig. 11.17 Raft foundation (Ex. 11.9)

The foundation is split into four rectangles as shown and Fadum’s’ chart is used:

Depth (m) B/z L/z Iσ 4Iσ ∆σz (kN/m^2 )

3 3 9 0.247 0.988 320

9 1 3 0.203 0.812 263

15 0.60 1.80 0.152 0.608 197

21 0.43 1.30 0.113 0.452 146

27 0.33 1.00 0.086 0.344 112

33 0.27 0.82 0.067 0.268 87

Immediate settlement (Sand):

N = 18 σ 0 = 4.5 × 18 – 1.5 × 9.81 = 66.3 kN/m^2

Cs = 1.5

Cr

σ 0

But Cr = 400 N = 400 × 18 = 7200 kN/m^2

Cs =

1 5 7200

66 3

.

.

×

= 162.9

S 1 =

H

Cs e

.log

σσ

σ

0

0

F +

HG

I

KJ

∆

=

(.)

.

log.

.

615

162 9

66 3 320

66 3

−+F

HG

I
e KJ m = 0.0487 m = 48.7 mm
Since most part of the sand below the foundation is submerged customarily it is as-
sumed that the settlement will be doubled.