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the virgin isotaches. It will be necessary to account for this, when more accurate
predictions of post preloading behaviour are available. In the meantime, a simple
expedience is to reduce the amount of unloading in the calculations to some
empirically decided percentage of the actual unloading.
It should be realised that the central issue in preloading design is creating a
period of grace of sufficient length. Some methods of calculation focus on the latter
part of the post preload curve (the "recompression" curve) and are less accurate
with regard to the period of grace. Unfortunately, no accurate methods presently
exist for the latter.
extra surcharge
surcharge
residual
settlement
H
gross
H
net
H
gross
settlement
at the beginning
when taken in service
Figure 13.9 Definitions for preloading design
There is perpetual confusion about the terminology surrounding preloading. The
word surcharge is often used in the same meaning.
53
The prefix
pre
may denote
either a load that is applied earlier to obtain reduced settlements later, or denote
load that is removed. The prefix
sur
may denote a height of fill that is applied to
compensate for settlements due to the fill itself, or the extra load that is later
removed. A Dutch committee has solved this matter by a new set of definitions
shown in Fig 13.9. Surcharge is now the load applied to compensate for
settlements, and extra surcharge is the load, which is eventually removed. The
word preload is avoided. In the paragraphs above therefore, preload would become
"extra surcharge".
D
STIFF INCLUSIONS
The group of ground improvement methods that apply stiff elements into the soft
soil, is quite large and varied. The following methods are discussed: compound
wooden columns, stone columns, deep mixed columns, compaction grouting,
compensation grouting, and geotextiles.
53
The same unclarity occurs in Dutch: “voorbelasting” respectively “overhoogte”.
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