Civil Engineering Reference
In-Depth Information
Chapter 11
Foundation Settlement and
Soil Compression
11.1 Settlement of a foundation
Probably the most difficult of the problems that a soils engineer is asked to solve is the accurate predic-
tion of the settlement of a loaded foundation.
The problem is in two distinct parts: (i) the value of the total settlement that will occur, and (ii) the rate
at which this value will be achieved.
When a soil is subjected to an increase in compressive stress due to a foundation load the resulting soil
compression consists of elastic compression, primary compression and secondary compression.
Elastic compression
This compression is usually taken as occurring immediately after the application of the foundation load.
Its vertical component causes a vertical movement of the foundation (immediate settlement) that in the
case of a partially saturated soil is mainly due to the expulsion of gases and to the elastic bending reori-
entation of the soil particles. With saturated soils immediate settlement effects are assumed to be the
result of vertical soil compression before there is any change in volume.
Primary compression
The sudden application of a foundation load, besides causing elastic compression, creates a state of excess
hydrostatic pressure in saturated soil. These excess pore water pressure values can only be dissipated by
the gradual expulsion of water through the voids of the soil, which results in a volume change that is time
dependent. A soil experiencing such a volume change is said to be consolidating, and the vertical com-
ponent of the change is called the consolidation settlement.
Secondary compression
Volume changes that are more or less independent of the excess pore water pressure values cause sec-
ondary compression. The nature of these changes is not fully understood but they are apparently due to
a form of plastic flow resulting in a displacement of the soil particles. Secondary compression effects can
continue over long periods of time and, in the consolidation test (see Section
11.3.2)
, become apparent
towards the end of the primary compression stage: due to the thinness of the sample, the excess pore
water pressures are soon dissipated and it may appear that the main part of secondary compression occurs
after primary compression is completed. This effect is absent in the case of an
in situ
clay layer because
the large dimensions involved mean that a considerable time is required before the excess pore pressures
