Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 419

∴ Si for sand = 2 × 48.7 = 97.4 mm

Clay:

With reference to Fig. 11.3(b),

H 1 = 4.5 m; H 2 = 34.5 m

For H 2 :

L/B = 27/9 = 3;

H

B

2 34 5

9

=. = 3.83 ∴ Is = 0.47

For H 1 :

L/B = 3;

H

B

1 45

9

=

= 0.5, ∴ Is = 0.07

Si = q.

B

E

(1 – ν^2 ) × 4Is × (rigidity factor)

Si, taking gross pressure =

351

16200

× 9 × 0.75 × 4(0.47 – 0.07) × 0.8

= 0.1872 m = 187.2 mm

Heave effect:

Relief pressure due to excavation = 1.5 × 18 = 27 kN/m^2

∴ Heave =

27

62000

× 9 × 0.75 × 4(0.47 – 0.07) × 0.8

= 0.0037 m = 3.7 mm

Net immediate settlement in clay = 187.2 – 3.7 = 183.5 mm

The heave effect is obviously insignificant except for great depth of excavation.

Consolidation Settlement:

The clay layer is divided into five layers of 6 m thickness.

mν ∆σz mν. ∆σz. H

0.000150 263 0.2367

0.000110 197 0.1300

0.000090 146 0.0788

0.000075 112 0.0504

0.000045 87 0.0235

0.5194 = 519.4 mm

Total settlement = (97.4 + 183.5 + 519.4) mm ≈ 800 mm.

Summary of Main Points

1. For a detailed settlement analysis, the soil profile and soil properties at the site of the structure
   and the stresses in the soil before and after loading are necessary.


2. The total settlement may be considered to be composed of initial settlement due to elastic com-
   pression, consolidation settlement due to primary compression and secondary settlement due to
   secondary compression; the latter two phenomena are restricted to cohesive soils.