223
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Reappraisal of Surcharging to Reduce Secondary Compression
Remise en cause de l'imposition de frais supplémentaires pour réduire la Compression secondaire
Feng T.W.
Department of Civil Engineering, Chung Yuan Christian University, Taiwan, R. O. C.
ABSTRACT: For structures on soft soils with relatively high secondary compression characteristics, post-construction settlement may
be excessive so that application of surcharging may be called for to reduce secondary compression. The principle of surcharging is
simple, but the post-surcharge secondary compression behavior requires further study. This paper looks into effects of surcharging
loading history on post-surcharge secondary compression characteristics of soils. Current knowledge on this subject is primarily
based on unloading from surcharge stress to permanent stress. There could be other types of surcharging stress history. For example,
all preloads, including permanent load and surcharge load, are removed before the permanent load is reapplied. Laboratory data of
secondary compression with different surcharging stress history are presented and discussed. These data provides useful information
for understanding the post-surcharge secondary compression behavior of soils.
RÉSUMÉ : Pour les structures sur des sols mous avec des caractéristiques de compression secondaire relativement élevé, règlement
après la construction peut être excessive pour que l’application d'imposer des frais supplémentaires peut-être être appelée pour réduire
la compression secondaire. Le principe de l’imposition de frais supplémentaires est simple, mais le comportement de compression
secondaire post-surcharge nécessite une étude plus approfondie. Cet article se penche sur les effets de surcharge chargement histoire
sur les caractéristiques de compression secondaire post-surcharge des sols. Les connaissances actuelles sur ce sujet repose
essentiellement sur le déchargement de stress en supplément à un stress permanent. Il pourrait y avoir d'autres genres d'imposer des
frais supplémentaires histoire des contraintes. Par exemple, toutes les précharges, y compris la charge permanente et la charge de la
surtaxe, sont supprimés avant que la charge permanente est réappliquée. Données de laboratoire de compression secondaire avec
différents histoire surcharge de stress sont présentées et discutées. Ces données fournit des informations utiles pour comprendre le
comportement de compression secondaire post-surcharge des sols.
KEYWORDS: Surcharging, precompression, soft soils, secondary compression.
1 INTRODUCTION
Surcharging is a special condition of precompression or
preloading technique (e.g. Jamiolkowski et al. 1983, Johnson
1970) for soft ground improvement. The philosophy of
surcharging is quite simple that a state of overconsolidation in
soil is generated after surcharging executions. It is expected
that post-construction settlements of structures are thus reduced.
Although, there are some unsuccessful surcharging applications
(e.g. Chang 1981, Sower 1964) around the world in which
surcharge loads are removed too ealry during primary
consolidation so that post-construction settlements resulting
from both primary consolidation and secondary compression are
still excessive. It is apparent that the philosophy of surcharging
is not followed in these cases. To bring the soil to a state of
overconsolidation, primary consolidation should have been
eliminated completely at the time of surcharge removal. The
other situation is that the part of settlement resulting from
secondary compression of soil is not usually considered in
practice. It should be emphasized that the magnitude of
secondary compression settlement may still be significant if
conditions allows it to occur. Equation (1) can be used for
calculation of secondary compression settlement:
p
c
c
tt
He
C CC s
log
1
0 0
(1)
Where
is the secondary compression index,
is the
compression index,
e
0
is the initial void ratio,
H
0
is the initial
thickness of soil layer,
t
p
is the time of end of primary
consolidation, and
t
is any time during secondary compression.
A sample calculation can be made with Eq. (1) for a normally
consolidated soil layer of 10
m
thick doubly drained from both
top and bottom, with
C
c
C
05.0
c
CC
,
,
e
0
= 1.2,
coefficient of consolidation
cm
2
/sec. The time of
end of primary consolidation
t
p
is then estimated as 9 years and
the secondary compression settlement at consolidation time of
20 years is calculated using Eq. (1) as 0.08
m
. If vertical drains
were used to speed up primary consolidation, the time of end of
priamry consolidation would be greatly shortened. It is
assumed for the sample calculation that
t
p
is reduced to 6
months. Then the secondary compression settlement at
consolidation time also of 20 years is calculated again using Eq.
(1) as 0.36
m
. The above sample calculation demonstrates that
shortening the time of end of priamry consolidation has an
undesirable effect on the magnitude of secondary compression
settlement. Secondary compression of natrual
soils has
been
observed both in the field and in the laboratory. As can be seen
from Eq. (1), the magnitude of secondary compression
settlement is in direct proportion to secondary compression
index, thickness of soil layer, and consolidation time considered
for calculation. In general, structures on thick soft soils with
high natural water contents are highly susceptible to excessive
secondary compression settlement. It is easier to estimate
secondary compression settlement during design than to face
unacceptable post-construction long term settlement problems.
0.1
c
C
001 .0
v
c
