Geoscience Reference
In-Depth Information
compressive stiffness, swelling is limited. The reduction of potential head within
the soil mass by the vacuum has the added advantage of increasing the stability of
the sides of the embankment.
Preloading
Preloading is a commonly used technique to speed up consolidation of
embankments and extensive sand fills on soft soils. The applied overburden is
increased temporarily (II) to a thickness above what is eventually needed (III), and
is removed after settlements have increased sufficiently (Fig 13.8). The idea behind
achieving more settlement in the construction period is that a "period of grace" is
obtained during which very little settlement occurs. This period is the beginning of
the service life of the structure (road, railway, embankment, etc) and, if it can be
made to last long enough, reduces maintenance costs drastically.
II
loading
III
I
t 0
log t
I
II
period of grace
III
A
C
B
settlement
D
Figure 13.8 Stages during preloading
There are various views with regard to the duration of the period of grace, and
whether or not swelling occurs when the preload is removed. View C in Fig 13.8
expects settlements to completely cease until the time it would have taken load III
to achieve the same settlement. This is certainly too optimistic. View A
incorporates swelling due to the unloading, but is otherwise equal to view C. It is
also optimistic. View D is the most likely behaviour: first swelling occurs due to
reversed consolidation, and then creep reasserts itself, more quickly than expected
in view C. The curve B may occur if the preloading is very minor or is removed too
quickly. Then swelling may not occur. Applying vertical drains in the preloading
period helps to more quickly transform the extra overburden into effective stresses.
Then a longer period of grace is obtained.
The isotache model or abc-model (see Chapter 6) is useful to design preloading.
Such models predict a behaviour according to curve A, because the virgin isotaches
(i.e. the relationship between effective stress, incurred strain and creep rate of
strain in virgin conditions) are also applied to the unloading and the recompression
after swelling. During the unloading however, the virgin isotaches are disturbed,
and higher rates of compression occur after the end of swelling than predicted by
 
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