CREEP AND CONSOLIDATION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 79

Consolidation model
At the beginning of consolidation, the response to a sudden load step is a

u (excess pore pressure). 27 During the
consolidation process, the response to loading by excess pore pressures is gradually
transferred to the granular skeleton (effective stresses), until the excess pore
pressures have vanished (end of consolidation). Compression or settlement related
to these effective stress increments shows up as a time-dependent process. 28
Thereafter, compression may yet continue at a reduced rate with time, which is
referred to as creep, since the state of effective stress is sustained after
consolidation.
During creep soil compacts as well, and the corresponding decreasing pore
volume may give rise to pore pressure increase, which will play a role when the
drainage capacity is very small (clay). Even so, during consolidation, creep will
take place as an independent phenomenon. In standard soil mechanics, however,
the process of consolidation and creep are considered separate and successive. As a
simplification, consolidation is called primary compression and creep secondary
compression.

sudden increase in pore pressure

' i ' i ' i ' i ' i ' i

dt dt dt dt dt dt

t t t t t t

u u u u u

S S S S S

' ' ' ' ' '

u u u u u u

q+dq q+dq q+dq q+dq q+dq

z z z z z

du du du du du du

u u u u u u

dz dz dz dz dz

du du du du du

h h h h h

q q q q q

consolidation consolidation consolidation consolidation consolidation creep creep creep creep creep

S i S i S i S i S i S i

t t t t t t

t+dt t+dt t+dt t+dt t+dt t t t t t

S S S S S S

dS dS dS dS dS dS

S & S & S & S & S & S &

Figure 6.2 Consolidation process

When performing an oedometer test, the sample may not be fully saturated and
the response to a sudden load step may give an immediate settlement S i which

is due to compressibility of the pore fluid, as it provokes an immediate increase of
effective stress i . At the end of consolidation, when all excess pore pressures have

27 In laterally confined conditions and for incompressible pore water and solid grains, the
initial state u = holds. In two and three-dimensional situations initially u <
usually applies.
28 Terzaghi noticed this effect on the slow settlement of structures after being built. In his
first article in 1924, he named the apparent stress that caused it effective stress.