Geotechnical Engineering

DHARM

416 GEOTECHNICAL ENGINEERING

Example 11.7: The loading period for a building extended from Feb., 1957 to Feb., 1959. In

Feb., 1962, the average measured settlement was found to be 117 mm. The ultimate settle-

ment was expected to be 360 mm. Estimate the settlement in Feb., 1967, assuming double

drainage to occur. What would be this result if the measured settlement in Feb., 1962 was 153

mm instead of 117 mm?

The reckoning of time is conventionally done from mid-way through the construction or

loading period. In this case,

S 4 = 117 mm when t = 4 years. Sc = 360 mm.

The settlement is required at time t = 9 years.

Let us assume, in the first instance, that at t = 9 years,

U, the degree of consolidation is less than 50%.

In such a case, U = 1.13 Tν, where Tν is the Time-factor.

S

S

U

U

4

9

4

9

= U

U

T

T

4

9

4

9

= ν

ν

∴ S

S

t

t

C

H

4

9

4

9 2

= ,since ν is a constant.

∴

117 4

S 9 9

= = 2/3

∴ S 9 = (3/2) × 117 = 175.5 mm

Thus, T

S

ν (^9) Sc
9 175 5
360 0
==. <50%
.

. Hence the relationships used are valid.

If S 4 = 153 mm at t = 4 years,

U 4 = 153/360 = 42.5%

For double drainage,

this corresponds Tν = 0.142, since Tν = (π/4)U^2

Then

C

H

v×^4

2 = 0.142, or

C

H

v

2

0 142

4

=

.

= 0.0355

For t = 9 years, Tν 9 = 0.0355 × 9 = 0.3195 ≈ 0.32

Correspondingly U 9 = 0.632

Hence the settlement in Feb., 1967 = 0.632 × 360 = 227.5 mm.

Example 11.8: A building was to be constructed on a clay stratum. Preliminary analysis indi-
cated a settlement of 60 mm in 6 years and an ultimate settlement of 250 mm. The average
increase of pressure in the clay stratum was 24 kN/m^2.

The following variations occurred from the assumptions used in the preliminary analysis:

(a) The loading period was 3 years, which was not considered in the preliminary analy-

sis.

(b) Borings indicated 20% more thickness for the clay stratum than originally assumed.

(c) During construction, the water table got lowered permanently by 1 metre.