Geotechnical Engineering

DHARM

414 GEOTECHNICAL ENGINEERING

Example 11.5: A clay stratum of 18 metres thickness was found above a sand stratum when a
boring was made. The clay was consolidated under the present overburden pressure. The hy-
drostatic pressure at top of the clay stratum was found to be 54 kN/m^2. Due to pumping of
water from the sand stratum, the pressure in the pore water below the clay layer was reduced
permanently by 54 kN/m^2. If the void ratios of clay before and after pumping were 0.93 and
0.90, respectively, calculate the ultimate settlement due to pumping.
The settlement is caused due to reduction of water pressure by pumping in this case.
The pressure is thus transferred to the soil grains as effective pressure, as shown in Fig. 11.15(c):

54 54

(54+18×10) (18 × 10) (54)

Pressures in kN/m^2

(a) Neutral pressure

before pumping

(b) Neutral pressure

after pumping

(c) Increase in

effective pressure

Fig. 11.15 Pressure conditions before and after pumping (Ex. 11.5)

∆σ = 54 kN/m^2

e 0 = 0.93 e 1 = 0.90 ∆e = 0.03

aν =

∆

∆

e

σ

=

003

54

.

m^2 /kN

mν =^003

54

1

1093

003

54 1 93

.

(.)

.

.

×

+

=

×

= 2.88 × 10–4 m^2 /kN

Consolidation settlement

Sc = mn. ∆σ. H

=

003

54 1 93

.

×.

× 54 × 1800 cm

≈ 28 cm.

∴ Ultimate settlement = 28 cm.

Example 11.6: A clay layer 24 metres thick has a saturated unit weight of 18 kN/m^3. Ground

water level occurs at a depth of 4 metres. It is proposed to construct a reinforced concrete

foundation, length 48 m and width 12 m, on the top of the layer, transmitting a uniform pres-

sure of 180 kN/m^2. Determine the settlement under its centre. E for the clay is 33 MN/m^2

obtained from triaxial tests. Initial void ratio = 0.69. Change in void ratio = 0.02.

The details of the foundation are shown in Fig. 11.16.