2449
Consolidation theory for combined vacuum pressure and surcharge loading
Théorie de la consolidation sous l’action combinée du vide et d’un pré-chargement
Chai J.-C.
Saga University, Japan
Carter J. P.
The University of Newcastle, Australia
ABSTRACT: Atheory describing the consolidation of a uniform clayey deposit with and without prefabricated vertical drain (PVD)
improvement under the combination of a vacuum pressure and a surcharge load has been developed and expressed as closed-form
equations. For the case of a soil layer without PVD improvement, both one-way and two-way drainage boundary conditions are
considered. Laboratory consolidation tests using combinations of vacuum pressure and surcharge load were conductedunder
oedometer conditions with vertical or radial drainage. The measured excess pore water pressures are compared with values predicted
by the theory presented in the paper. It has been demonstrated that the theory is valid and can be used for designing preloading
projects that involve the combination of a vacuum pressure and a surcharge load.
RÉSUMÉ : Une théorie décrivant la consolidation d’un dépôt argileux uniforme avec et sans amélioration par drains verticaux
préfabriqués (DVP) sous l’action combinée du vide et d’un pré-chargement a été développée avec un système fermé d’équations. Pour
le cas d’une couche de sol sans amélioration par DVP, des conditions aux limites drainantes par un côté et par deux côtés sont
considérées. Des essais de consolidation au laboratoire sous des conditions oedométriques ont été réalisés sous vide et pré-chargement
avec des drains verticaux ou radiaux. La surpression interstitielle mesurée est comparée avec les valeurs prévues par la théorie
présentée dans le présent article. Il a été démontré que la théorie est valable et peut être utilisée pour définir des projets de pré-
chargement qui impliquent l’utilisation combinée du vide et d’un pré-chargement.
KEYWORDS:consolidation, vacuum pressure, embankment, laboratory test, soft clay
1 INTRODUCTION
Preloading a soft clayey deposit with the combination of a
vacuum pressure and a surcharge load (embankment fill) has
several advantages, such as increasing the preloading pressure
and reducing lateral displacements of the deposit, etc. (e.g.,
Chai et al. 2006). Its use in engineering applications has
increased in recent years (e.g., Kelly and Wong 2009; Hirata et
al. 2010; Indraratna et al. 2011).
Vacuum consolidation has different characteristics compared
with consolidation induced by direct application of a surcharge
load (Chai et al. 2009). For a soil deposit without any
improvement in consolidation performance that might result
from the installation of prefabricated vertical drains (PVDs),
when a vacuum pressure is applied water is drained out of the
soil layer only at the boundary where that vacuum pressure is
applied. This applies for both cases of one-way and two-way
drainage conditions. However, for a deposit with one-way
drainage constrained to deform under one-dimensional (1D)
conditions, the final state is a uniform vacuum pressure
distribution throughout the deposit and consequently zero flow
rate. But for a deposit with two-way drainage, at the bottom
boundary the excess pore water pressure is fixed at zero and
effectively no vacuum pressure can be applied at this location,
and so the final state involves the steady flow of pore water
toward the boundary at which the vacuum pressure is applied.
Considering these complicating factors, Chai and Carter (2011)
recently derived a consolidation theory for soils subjected to
vacuum pressure. However, their theory cannot be applied
directly for cases that involve a combination of vacuum
pressure and surcharge loading, and therefore there is a need to
develop a reliable theory for such cases.
This paper presents a newly developed consolidation theory
applicable to soils subjected to a combination of vacuum
pressure and surcharge loading. This theory is applicable to the
case of a uniform soil deposit with or without PVD
improvement. Predictions obtained using this theory are
compared with the results of laboratory tests conducted under
oedometer conditions, for cases that involve both vertical and
radial drainage conditions, with the latter designed to simulate
the consolidation of a deposit improved by PVDs. It has been
shown that the theory is valid and can be used for
designingpreloading projects that involve a combination of
vacuum pressure and surcharge loading.
2 CONSOLIDATION THEORY
2.1
Uniform layer without PVDs
Under the same assumptions as those made in Terzaghi’s 1D
consolidation theory (Terzaghi 1943), the governing equation
and the boundary conditions for the generation and dissipation
of excess pore water pressure in a saturated soil layer under a
combination of vacuum pressure and surcharge load are as
follows:
t
u
z
u c
v
2
2
(1)
vac
p t u
),0(
(2)
0 ,
z
tHu
for
(one-way drainage)
(3)
0
t
0 ,
tHu
for
(two-way drainage)
( 3 a )
0
t
