Geotechnical Engineering

DHARM

412 GEOTECHNICAL ENGINEERING

25 m

X

50 m

200 m

50 m

25 m

X

50 m

150 m

50 m

1

50 m 2 3

(a) Plan of heap (b) Division into rectangles
Fig. 11.13 Spoil Heap (Ex. 11.2)
Example 11.3: A soft, normally consolidated clay layer is 18 m thick. The natural water con-
tent is 45%. The saturated unit weight is 18 kN/m^3 ; the grain specific gravity is 2.70 and the
liquid limit is 63%. The vertical stress increment at the centre of the layer due to the founda-
tion load is 9 kN/m^2. The ground water level is at the surface of the clay layer. Determine the
settlement of the foundation.
Initial vertical effective stress at centre of layer

= (18 – 9.81) ×

18

2

= 73.71 kN/m^2

Final effective vertical stress = 73.71 + 9.0 = 82.71 kN/m^2

Initial void ratio, e 0 = w. G = 0.45 × 2.70 = 1.215

Cc = 0.009 (63 – 10) = 0.477

Si =

18 0 477

1 1 215

82 71

(^10) 73 71
×

• .
  (.)
  .log.
  .
  = 0.194 m
  = 194 mm.
  This procedure may be used for rough estimate of the settlement of a small structure.
  For large structure, consolidation characteristics must be got from laboratory tests.
  Example 11.4: A footing foundation for a water tower carries a load of 9000 kN and is 3.6
  metres square. It rests on dense sand of 9 m thickness overlying a clay layer of 3 metres depth.
  The clay layer overlies hard rock. Liquid limit of clay is 54%, water content 40.5%, and grain
  specific gravity is 2.70. The saturated unit weight of dense sand is 18.9 kN/m^3. Estimate the
  ultimate settlement due to consolidation of the clay layer, assuming the site to be flooded.