Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 391

which gives an idea of the depths of various characteristic zones of soil at the site of the struc-
ture, as also the relevant properties of soil such as initial void ratio, grain specific gravity,
water content, and the consolidation and compressibility characteristics. The second part con-
sists of the analysis of the transmission of stresses to the subsurface strata, using a theory
such as Boussinesq’s for stress distribution in soil. The final part consists of the final settle-
ment predictions based on concepts from the theory of consolidation and data from the first
and second parts.

11.2.1 Soil Profile and Soil Properties

The thicknesses of the various strata of soil in the area in which the structure is to be built
have to be ascertained carefully and presented in the form of a soil profile. Sufficient number
of borings should be made for this purpose and boring logs prepared from the data. The loca-
tion of the water table and water-bearing strata are also to be determined. In case the boring
data show some irregularities in the various soil strata, an average idealised profile is chosen
such that it is free of horizontal variation. Adequate boring data and good judgement in the
interpretation are the prime requisites for good analysis. Usually five bore holes—one at each
corner and the fifth at the centre of the site are adequate. The depth should not be less than
five times the shorter dimension of the foundation. A relatively simple but typical profile is
shown in Fig. 11.1.

sand

Ground

surface

O

15 m

–6m

Medium sand :

G = 2.67

e = 0.90

w = 5% (above water table)

Soft clay :

G = 2.76

LL = 63% ;

P = 36%

w = 42%

I

Soft

clay

40 m

Fine sand

Fine sand G = 2.66 : e = 0.63 ; w = 24%

Medium

Fig. 11.1 Typical soil profile
The special feature of this profile is that a clay stratum with no horizontal variation is
sandwiched by sand strata.

The soil properties of the compressible strata, especially clay strata, are also to be ascer-
tained by taking samples at different depths. If the thickness of a compressible stratum is
considerable, the properties of the soil taken from different depths are obtained and if there is
significant variation, their average values are considered for the analysis.

The existing void ratio is computed from the grain specific gravity and water content, on
the assumption of complete saturation, which is invariably valid for relatively deep strata