Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter
4 SOIL SETTLEMENT
p c
Figure 4.2 Typical e – lop p Curve
It should be noted that before this laboratory test results are used, it is very important to
correct
the consolidation curves for the
effects of sampling. The proceduree for correction could be
readily
found in most foundation engineering textbooks e.g. Holtz and Kovacs and is not discussed here.
The response of the soil to settlement also depends on the magnitude of the existing effective
stress relative to the maximumm past effective stress at a given depth. The overconsolidation ratio,
OCR, which is a measure of the
degree of overconsolidation in a soil is defined as
OCR = pc /
po
(4.3)
where
pc = preconsolidation pressure (obtained from an e-log p plot)
po = initial effective vertical stress at the center
of the layer
considered.
The value of OCR provides a basis for determining the effective stress history of
the clay at the time
of the proposed loading as follows:
OCR = 1 : – the clay is considered to be “normally consolidated” under the existing load, i.e., the
clay has fully consolidated under the existing load (p c = p o ).
a)
b)
OCR > 1 : – the clay is consideredd to be “overconsolidated” under the existing load, i.e.,
the clay has consolidated under a load greater than the load
that currently exists (pc > p o ).
OCR < 1 : – the clay is
consideredd to be “underconsolidated” under the existing load, i.e.,
consolidation under the
existing load is still occurring and will continue to occur under that
load until primary consolidation is complete, even if no additional load is
applied (p c < p o ).
4-4
March 2009