CREEP AND CONSOLIDATION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 76

' A . 25 The As shown in Fig 6.1, soil is subjected to a loading step ' = ' C

branch AB (elastic) is reloading with a recompressibility index a and ' B is the so-

called preconsolidation stress (ultimate historic loading, preloading pressure). The
branch BC (elasto-plastic) is virginal compression or inelastic compression with
compressibility index b (similar to

, see Chapter 5). When thereafter strain is
sustained, relaxation will occur, branch CF (visco-elastic). When the stress is
sustained, strain may further develop by creep, branch CD (visco-plastic). If a new
load step is applied, the state will move to the line BC first, along branch DE
(creep) or FE' (relaxation). The increase from ' C to ' E due to creep is called
ageing effect, after Bjerrum, i.e. by time during relaxation or creep soil strength has
increased. In this regard, the stress point ' E is referred to as preconsolidation
pressure, since it is not related to historic over-loading. The effect of ageing is
similar as preloading.

B CREEP
Creep occurs when strain continues under constant effective stress. Its rate
decreases with time and when projected against ln(t) , a straight line is found; the
corresponding creep index is c (Fig 6.1b). Similarly, when strain is kept constant,
stress will attenuate with time (relaxation) and when projected against ln(t) , also a
straight line may be found with an index proportional to c/b (Fig 6.1a, lower part).

The abc-model or isotache model
In the uniaxial compression model it is suggested that creep can be visualised in
the stress-strain plot of Fig 6.1a by straight lines each with a specific constant creep
rate (dotted lines). They are called isotache lines. The model is also referred to as
the isotache model. Similarly, in the strain-time plot of Fig 6.1b, it is found that for
most soft soils the creep develops with the same compression index c , independent
of pressure.
A compression creep index C is usually mentioned in literature. The relation
with the index c is based on the way of measurement, identical to (5.16)

C = 2.3 c (1 +e 0 ) = 2.3 c v 0

(6.1)

TABLE 6.1 COMPRESSION CREEP INDEX VALUES
soil type soil type

C value C value

gravel
sand
loam

0.0
0.0
0.002

preconsolidated clay
normally consolidated clay
organic soils, peat

< 0.005
0.005 - 0.05
0.05 - 0.5

25 In the oedometer test the loading is applied in successive steps, one a day. Consolidation
causes the sudden loading step to be transferred by a gradual pore pressure reduction to a
gradual effective stress increase. The period of (primary) consolidation takes about 1 hr in
the oedometer test.