DHARM404 GEOTECHNICAL ENGINEERING
The effect of this horizontal drainage is to accelerate the time-rate of consolidation when
compared with the situation of one-dimensional flow or drainage. It has no effect on the total
settlement.
The magnitude of this effect is dependent upon the size of the structure relative to the
depth and thickness of the clay stratum, the effect being greatest when the plan size of the
structure is small. This effect also depends greatly on the relative magnitude of the soil prop-
erties in the horizontal and vertical directions.
The effect of horizontal drainage, as presented by Gould (1946), based on his investiga-
tion with reference to a simple building, 21 m square, is shown in Fig. 11.7.
It can be observed that the effect of horizontal drainage is to make the settlement pro-
ceed faster than in the case of one-dimensional drainage; however, this effect may not be
important in many cases.
11.6 OTHER FACTORS PERTINENT TO SETTLEMENT
Three other matters pertinent to settlement—viz., accuracy of computed settlements, permis-
sible settlements and remedial measures are dealt with in the following sub-sections.
11.6.1Accuracy of Computed Settlement
The accuracy of the computed settlements is naturally dependent upon the degree or extent to
which the assumptions involved in the theories made use of in the analysis are valid in any
given case.
Assumptions made for the interpretation of the geological profile, especially values used
for thickness of the strata, may lead to errors if they are incorrect. Similarly the use of soil
properties obtained from partially disturbed samples, especially the consolidation characteris-
tics may lead to errors in the estimate of total settlement as well as speed of settlement. But
these are inaccuracies in data and not in the analysis, as such. The primary assumption re-
garding the one-dimensional nature of the compression may be valid only in the case of deeply
buried clay strata; in other cases, the effects of lateral strain may be considerable.
The theories of stress distribution in soil, used in the settlement analysis, involve as-
sumptions which may not be true in practice. For example, the assumption that soil is per-
fectly elastic, homogeneous and isotropic is nowhere near the facts. However, it is considered
that the accuracy is not affected significantly by this erroneous assumption.
In conclusion it may be stated that settlement analyses usually give results which are at
best crude estimates; however, even a crude estimate may be considered very much better
than a pure guess or conjecture, which may be the only alternative.
11.6.2 Permissible Settlements
It is not easy to decide what value of settlement will have a detrimental effect on a structure.
This is because uniform settlement will have little adverse effect on the structural stability,
but even small differential settlement may cause trouble. There are two main ill-effects of
differential settlements: (i) the architectural effect (cracking of plaster, for example) and (ii)
the structural effect (redistribution of moments and shears, for example, which may ultimately
lead to failure).