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

Chapter 4 SOIL SETTLEMENT
4 SOIL SETTLEMENT
4.1 GENERAL CONCEPT
In geotechnical engineering, in particular foundation works for structures, engineers are interested
in how much and how fast soil settlement will occur. Excessive settlement including (differential
settlement) may cause structural damage as well as impair the functionality or serviceability of the
structures.
Soils whether cohesionless or cohesive, will experience settlements immediately after application of
loads. Whether or not the settlements will continue with time after the application of the loads will
be a function of how quickly the water can drain from the voids as explained in Item 3.2 Long-term
consolidation-type settlements are generally not experienced in cohesionless soils where pore water
can drain quickly or in dry or slightly moist cohesive soils where significant amounts of pore water
are not present. Therefore, embankment settlements caused by consolidation of cohesionless or
dry cohesive soil deposits are frequently ignored as they are much smaller compared to immediate
settlements in such soils.
The total soil settlement. S t can be divided into 3 main components, namely immediate settlement,
primary consolidation settlement, , and secondary compression settlement
S t = S i + S c + S s (4.1)
S i = immediate settlement
S c = primary consolidation settlement (time-dependent)
S s = secondary compression settlement
S i
S c
S s
Figure 4.1 Components Of Total Settlement Versus Log Time
4.1.1 Immediate (Distortion) Settlement
Immediate, or distortion, settlement (S i ) occurs during application of load as excess pore pressure
develops in the underlying soil. If the soil has a low permeability and it is relatively thick, the excess
pore pressures are initially undrained. The foundation soil deforms due to the applied shear stresses
with essentially no volume change, such that vertical compression is accompanied by lateral
expansion.
It should be recognized that most field evidence indicates that S i is usually not important design
consideration especially in cohesive soils. It can usually be reduced by precompression or, to some
extent, by a controlled loading program which allows consolidation to increase the soil stiffness and
reduce the shear stress level in the foundation.
March 2009 4-1