2529
Evaluation of Vertical Drain-enhanced Radial Consolidation with Modified Analytical
Solution
Évaluation de la consolidation radiale améliorée par des drains verticaux par une solution
analytique modifiée
Lee C., Choi Y., Lee W.
School of Civil, Environmental, and Architectural Engineering, Korea University
Hong S.J.
Technology Research & Development Institute, Daelim Industrial Co., Ltd.
aBstract: the installation of vertical drains accelerates the consolidation process by reducing the drainage path and
predominating horizontal flow within the soft deposits. however, the radial consolidation of the vertical drain installed into the soft
ground is governed by the permeability of a smear zone. modification of hansbo's analysis is proposed to analyze the degree of
consolidation on a horizontal plane by considering the properties of the soil within the smear zone in this study. a parametric study is
carried out to investigate the effects of the soil properties on the proposed analysis. the proposed equation is observed to be relatively
insensitive to the uncertainty of the horizontal permeability ratio between the undisturbed and smear zones. the validity of the
proposed analysis is examined by comparison with the settlement data from a field measurement. it is revealed that the proposed
analysis provides a reliable prediction on the consolidation rate of soft ground installed pVd.
rÉsUmÉ : l'installation de drains verticaux accélère le processus de consolidation en réduisant les chemins de drainage et
d'écoulements horizontaux prédominant dans les dépôts mous. toutefois, la perméabilité de la zone
d’influence
détermine le degré de
la consolidation radiale induite par les drains verticaux installés dans les sols mous. Une modification de l'analyse de hansbo est
proposée dans cette étude pour analyser le degré de consolidation dans un plan horizontal en considérant les propriétés du sol dans la
zone
d’influence
. Une étude paramétrique est notamment menée pour étudier les effets des propriétés du sol sur l'analyse proposée.
l'
observation montre que l’
équation proposée est relativement insensible à l'incertitude du rapport entre perméabilités horizontales
entre les zones non perturbées et les zones
d’influence
. la validité de l'analyse proposée est examinée par comparaison avec les
données de tassement d'une mesure sur site. il
s’
est révélé que l'analyse proposée fournit une prévision fiable sur le taux de
consolidation des drains verticaux installés dans les sols mous.
KeYWords: permeability, radial consolidation, smear zone, Vertical drain
1 introdUction
the radial consolidation flow into the vertical drain induces
a reduction in the flow channel and an increase in flow rate
approaching the drain. it causes that the hydraulic head is
dramatically decreased as the distance to the drain decreases.
therefore, the permeability of drain and soil near the drain
control the rate of consolidation by the vertical drain. since the
permeability of drain is generally designed to be larger enough
than that of soil, it is known that the well resistance is negligible
if the discharge capacity exceeds the required discharge
capacity (holtz et al. 1987, lo 1991). on the other hand, the
installation of vertical drains induces a soil disturbance in the
vicinity of the mandrel. the disturbed zone, called smear zone,
is an area where has reduced permeability and increased
compressibility comparing with an undisturbed soil. reduced
permeability in the disturbed zone governs the rate of
consolidation, because the hydraulic head loss in soil near the
drain further increases when the permeability decreases.
many researchers insisted that the soil adjacent to the drain
is remolded, and several researches were investigated the smear
effect by obtaining the permeability of the distrubed zone from
the permeability of remolded clay (tavenas et al. 1983, Bergado
et al. 1991, hird and moseley 2000, sathananthan and
indraratna 2006). in this study, modification of hansbo
’
s
solution is proposed to evaluate the degree of radial
consolidation, considering the consolidation characteristics of
remolded clay. characterisitcs of the modified solution are
discussed, in comparison with hansbo
’
s solution. and the
consolidation settlement predicted by the modified solution is
comprared with measured settlement data in the field.
2 modified analYtical solUtion
hansbo
’
s (1981) solution has been widely used to evaluate the
consolidation behavior with vertical drain. it is simple and
accurate as compared with other rigorous solutions and
numerical analysis (onoue 1988, lo 1991). according to
hansbo (1981), the average degree of radial consolidation (
r
U
)
by vertical drain is
) / 8 exp(
1
s h
r
T
U
(1)
where, µ
s
= ln(d
e
/d
w
)+(k
h
/k
s
-1)ln(d
s
/d
w
)+
π
·
z(2l-z)(k
h
/q
w
)-0.75,
t
h
is a time factor (= c
h
·
t/d
e
2
), c
h
is the coefficient of horizontal
consolidation in the field, d
e
is the circular diameter influenced
by the drain, d
w
is the drain diameter, k
h
is the coefficient of
horizontal permeability in the undisturbed zone, k
s
is the
coefficient of horizontal permeability in the disturbed zone, d
s
is
the disturbed zone diameter, l is the drainage path length, and
q
w
is the drain discharge capacity.
hansbo
’
s analysis is based on the horizontal flow
characteristics of the undisturbed zone (k
h
or c
h
) to evaluate the
radial consolidation. however, it is difficult to obtain due to the
anisotropy of permeability and difference between laboratory
and field measurement values (Bergardo et al. 1991, chai and
miura 1999).
the consolidation characteristics of the disturbed zone are
homogeneous and isotropic due to the disturbance (lo 1991).
to analyze the radial consolidation based on the consolidation
characteristics of disturbed zone, hans
bo’
s solution is modified
in this study. since hansbo
’
s solution assumes an equal vertical
strain, the ratio of horizontal permeability between the
undisturbed and disturbed zones (k
h
/k
s
).
