Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 401

11.4.2 Correction for Lateral Strain
The assumption that the soil does not undergo lateral strain made in the one-dimensional
consolidation theory, is true for the oedometer sample; however, such a condition may not be
true for the clay stratum in the field. This condition may still be achieved if the clay layer is
thin and is sandwiched between unyielding layers of granular material or if the loaded area is
large compared to the thickness of the compressible layer. Otherwise, lateral deformation
takes place and the consolidation will no longer be one-dimensional, which could lead to errors
in the computed settlements using the Terzaghi theory.
Skempton and Bjerrum (1957) have suggested a semi-empirical correction, based upon
Skempton’s pore pressure parameter A. The correction factor may be obtained from Fig. 11.5
for different H/B ratios, H being the thickness of the clay layer and B the width of the foundation.

Over-

consolidated

Normally

consolidated

Highly

sensitive

and soft

Heavily

consoli-

dated

1.2

1.0

0.8

0.6

0.4

Correction factor, C0.2

0

0.2 0.4 0.6 0.8 1.0 1.2

H/B = 0.5

H/B=1

H/B=4

Pore pressure coefficient, A
Fig. 11.5 Correction factor for the effect of lateral strain on consolidation settlement
(After Skempton and Bjerrum, 1957)
The corrected consolidation settlement Scc is obtained from:
Scc = C. Sc ...(Eq. 11.11)
where C is the correction factor and Sc is the computed consolidation settlement.
It may be difficult to evaluate the A-parameter accurately; in such cases it may be advis-
able not to apply the correction, owing to uncertainty.

*11.5 FURTHER FACTORS AFFECTING SETTLEMENT

There are a few other factors which affect the settlement and which need consideration. Two

of the important factors among these are the rigidity of a structure and the horizontal drain-

age; these two are considered in the subsections to follow.

11.5.1Rigidity of a Structure

Considerable judgement must be used in choosing the values of load which are effective in

causing settlement; the average load with respect to time must be used rather than the