Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 403

The flexible building shown in Fig. 11.6(a) will exert on the soil just below it a pressure
distribution which is nearly uniform, shown by curve (1). This will cause a bell-shaped pres-
sure distribution at the top of the buried compressible stratum, represented by curve (2). The
settlement pattern of the surface of the stratum will then be as shown by curve (3). If the soil
above is of better quality as a foundation material than the compressible stratum, the latter
will be the source of practically all the settlement; the settlement pattern at foundation level is
as shown by curve (4), similar to curve (3).

For the rigid building shown in Fig. 11.6(b), the settlement pattern is known and the
curves must be considered in reversed order as compared to (a). The building must settle
uniformly as shown by curve (2); the pressure must then be about uniform, as in (3). By com-
paring with (a), since the bell-shaped pressure distribution results from uniform pressure
distribution just below the foundation, it may be deduced that in (b) the surface pressure
distribution, required to cause pressure distribution at the compressible stratum shown by (3),
should appear somewhat as shown by curve (4).

Thus, under a flexible structure with uniform loading, settlement at the centre is more
than that at the edges, while, for a rigid structure the pressure near the edges of the loaded
area is greater than that near the centre. The differential settlement in case (a) may result in
cracking of the walls; in case (b) the upper storeys are not subject to distortion or cracking. But
the existence of greater pressures on the outer portions of slabs in case (b) should be recog-
nised in the design.

11.5.2Horizontal Drainage

The hydrostatic excess pressures at the same depth may be different at different points under-

neath a loaded area, especially if the structure is supported on piles or columns carrying dif-

ferent loads. This creates horizontal flow or drainage due to the gradients in the horizontal

directions.

10

20

30

40

50

60

70

80

90

0 0.05 0.10 0.15 0.20 0.25 0.30

Time factor, T

Degree of consolidation, U%

I

II

III

I

II

III

: Theoretical curve for one-dimensional case

: Horizontal flow with k = k

: Horizontal flow with k = 4k

hv

hv

Fig. 11.7 Effect of horizontal flow on consolidation

(After Gould, 1946; as presented by Taylor, 1948)