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

Chapter 4 SOIL SETTLEMENT
Poulos & Davis (1974) gave a suite of elastic solutions for determining the coefficient 'f' for various
load applications and stress distributions in soils and rocks.
The increase of stress in soils due to foundation load can be calculated by assuming an angle of
stress dispersion from the base of a shallow foundation. This angle may be approximated as a ratio
of 2 (vertical) to 1 (horizontal) (Bowles, 1992; French, 1999). The settlement of the foundation can
then be computed by calculating the vertical compressive strains caused by the stress increases in
individual layers and summing the compression of the layers.
A time correction factor has been proposed by Burland & Burbidge (1985) for the estimation of
secondary settlement. Terzaghi et al (1991) also give an equation for estimating secondary
settlement in a similar form. The commencement of secondary settlement is assumed to commence
when the primary settlement completes, which is taken as the end of construction.
4.3 ESTIMATION OF PRIMARY CONSOLIDATION IN COHESIVE SOIL
From the types of settlement described above, generally the most significant settlement is
consolidation settlement. Consolidation settlement is time dependence. For low permeability soil
with reasonably thickness, the primary consolidation may take very long time e.g., exceeding 10
years. Therefore, improvement method by shortening the consolidation process is essential to avoid
distresses or failure due differential settlement after construction.
Table 4.1 Typical Allowable Total Settlements for Foundation Design
Type of Structure
Typical Allowable Total Settlement, δ a
(in)
(mm)
Office Buildings
0.5 – 2.1 (1.0 is the most 12 – 50 (25 is the most
common value)
common value)
Heavy Industrial Buildings 1.0 – 3.0 25 – 75
Bridges 2.0 50
(Source: Donald P.Coduto [19])
In general, lowering of the ground water table will leads to settlement of the ground. In finegrained
soils, prolonged lowering of water table will cause an increase in the effective stresses by
extrusion of water from the voids leading to ground settlement.
Primarily Consolidation, S c (herein refer as ‘consolidation’) is a process when sudden application of a
load to a saturated soil produces an immediate increase in pore water pressure. Over time, the
excess pore water pressure will dissipate, the effective stress in the soil will increase and settlement
will increase. Since shear strength is related to effective stress, it may be necessary to control the
rate of construction to avoid a shear failure. The rate at which the excess water pressure dissipates,
and settlement occurs, depends on the permeability of the soil, the amount of water to be expelled
and the distance the water must travel (drainage path).
The determination of consolidation is commonly based on the one-dimensional laboratory
consolidation test results. Typically, the results are expressed in an e-log p plot which is the socalled
“consolidation curve”, an example of which is as shown as in Figure 4.2. The followings
parameters r may be obtained from the consolidation curve:
a) Initial void ratio, eo
b) Compression index, Cc
c) Recompression index, Cr
d) Preconsolidation pressure, p c
March 2009 4-3