DHARMBEARING CAPACITY 573(ii) Plastic compression of the underlying soil, giving rise to consolidation, settlement of
fine grained soils, both primary and secondary,
(iii) Ground water lowering, especially repeated lowering and raising of ground water
level in loose granular soils and drainage without adequate filter protection,
(iv) Vibration due to pile driving, blasting and oscillating machinery in granular soils,
(v) Seasonal swelling and shrinkage of expansive clays,
(vi) Surface erosion, creep or landslides in earth slopes,
(vii) Miscellaneous sources such as adjacent excavation, mining subsidence and under-
ground erosion.
The settlements from the first two sources alone may be predicted with a fair degree of
confidence.
14.8.2Bearing Capacity Based on Tolerable Settlement
As discussed in Sec. 14.2, the bearing capacity of a foundation is based on two criteria—the
pressure that might cause shear failure of the foundation soil and the maximum allowable
pressure such that the settlements produced are not more than the tolerable values.
The first criterion has already been discussed in detail. For the second criterion, the
tolerable values of the total and differential settlements which a particular structure, on a
particular type of foundation in a given soil, can undergo without sustaining any harmful
effects are to be decided upon. These values have already been specified, basing on experience
and judgement (Chapter 11). Once the limiting values of settlement are fixed, the procedure
involves determining that pressure which causes settlements just equal to the limiting values.
This is the allowable bearing capacity on the basis of the settlement criterion. (It is to be noted
that there is no need to apply a further factor of safety to this pressure, since it would have
been applied even at the stage of fixing up tolerable settlement values).
The smaller pressure of the values obtained from the two criteria is termed the ‘allow-
able bearing pressure’, which is used for design of the foundation.
The bearing capacity based on the settlement criterion may be determined from the
field load tests or plate load tests (dealt with in the next section), standard penetration tests or
from the charts prepared by authorities like Terzaghi and Peck, based on extensive investiga-
tions. More of this will be seen in the section on bearing capacity of sands.14.8.3Construction Practices to Avoid Differential Settlement
A few construction practices are recommended, based on experience, to avoid or minimise
detrimental differential settlements.
(i) Suitable design of the structure and foundation ... desired degree of flexibility of the
various component parts of a large structure may be introduced in the construction.
(ii) Choice of a suitable type of foundation for the structure and the foundation soil
conditions...e.g., large, heavily loaded structures on relatively weak and non-uniform
soils may be founded on ‘mat’ or ‘raft’ foundations. Sometimes, piles and pile
foundations may be used to bypass weak strata.