Geotechnical Engineering

DHARM

SETTLEMENT ANALYSIS 399

Time-rate of settlement: Time-rate of settlement is dependent, in addition to other fac-
tors, upon the drainage conditions of the clay layer. If the clay layer is sandwiched between
sand layers, pore water could be drained from the top as well as from the bottom and it is said
to be a case of double drainage. If drainage is possible only from either the top or the bottom, it
is said to be a case of single drainage. In the former case, the settlement proceeds much more
rapidly than in the latter.

The calculations are based upon the equation:

T =

Ct

H

ν
2 ...(Eq. 11.10)
Again, the use of this equation and the notation have been given in chapter seven.
A large wheel load passing on a roadway resting on a clay layer will cause immediate
settlement, which is, theoretically speaking, completely recoverable after the load has passed.
If the load is applied for a long time, consolidation occurs. Judgement may be necessary in
deciding what portion of the superimposed load carried by a structure will be sustained long
enough to cause consolidation.

In the case of foundation of finite dimensions, such as a footing resting on a thick bed of
clay, lateral strains will occur and the consolidation is no longer one dimensional. Lateral
strain effects in the field may induce non-uniform pore pressures and may become one of the
sources of differential settlements of a foundation.

11.3.3Secondary Settlement or Secondary Compression

Settlement due to secondary compression is believed to occur during and mostly after the
completion of primary consolidation or complete dissipation of excess pore pressure. A few
theories have been advanced to explain this phenomenon, known as ‘secondary consolidation’,
and have already been given at the end of chapter seven. In the case of organic soils and
micaceous soils, the secondary compression is comparable to the primary compression; in the
case of all other soils, secondary settlement is considered insignificant. Further discussion of
the concept of secondary settlement, being of an advanced nature, is outside the scope of the
present work.

*11.4 CORRECTIONS TO COMPUTED SETTLEMENT

Certain corrections may be necessary for the computed settlement values—for example, for
the effect of the construction period and for lateral strain. These and the accuracy of the com-
puted settlement are dealt with, in brief, in the following subsections.

11.4.1Construction Period Correction

The load from the structure has been assumed to act on the clay stratum instantaneously; but
the application of the load is rather gradual as the construction proceeds. In fact, there will be
a gradual stress release due to the excavation for the foundation and the net load becomes
positive only after the weight of the structure exceeds that of the excavated material. No
appreciable settlement occurs until this point of time. The ‘‘effective period of loading’’ is reck-
oned as the time lapse from the instant when the load becomes positive until the end of the
construction; the loading diagram during this period may be taken approximately a straight