2501
Improvement of a Clay Deposit using Prefabricated Vertical Drains and Pre-loading
- A Case Study
Amélioration d'un massif d'argile à l'aide de drains verticaux préfabriqués et de pré-chargement
- Une étude de cas
Islam M.S., Yasin S.J.M.
Bangladesh University of Engineering and Technology, Dhaka, Bangladesh
aBstract: construction of a container terminal covering an area of of 153,000 m
2
is underway at chittagong sea port in
Bangladesh, situated on the right bank of the Karnaphuli river at its confluence with the Bay of Bengal. the sub-soil at the site
consisted of a 4 to 6 m thick clay layer with random zones of soft to stiff clay (cl). this was underlain by a 10 m thick loose to
medium dense silt/fine sand (sm) layer below which a loose clayey silt layer existed beyond 30 m depth. the unconfined
compressive strength, void ratio and compression index of the soft clay zones varied in the ranges of 12~16 kpa, 0.8~1.2 and 0.2~0.3,
respectively. the targeted use of the land required improvement of the sub-soil. this paper presents the design considerations,
comparison of required time and cost of alternative options, effectiveness of the adopted measure and the achieved improvement of
the engineering properties. actual consolidation settlements were up to 600 mm over a period of about 30 days with pVd and pre-
load. the effectiveness of the available theories of consolidation settlement under vertical and radial drainage, in the design of the
ground improvement measures, has been demonstrated.
rÉsUmÉ : . la construction d'un terminal à conteneurs d'une superficie de 153.000 m
2
est en cours en ville portuaire de chittagong
au Bangladesh, située sur la rive droite de la rivière Karnaphuli à sa confluence avec le golfe du Bengale. le sous-sol sur le site se
composait d'une couche de 4 à 6 m d'épaisseur d'argile avec des zones aléatoires d'argile molle à raide (cl). cela a été superposée à
une épaisseur de 10 m limon / sable fin (sm) lâche à dense, une couche en dessous de laquelle une couche limon argileux lâche
existait au-delà de 30 m de profondeur. la résistance à la compression, l
’indice des
vides et l'indice de compression des zones d'argile
molle varient dans les plages de 12 à 16 kpa, 0.8 ~ 1.2 ~ 0.3 et 0.2, respectivement. l'utilisation obligatoire de ces terres requiert
l'amélioration du sous-sol. cet article présente la conception, la comparaison du temps nécessaire et le coût des options alternatives,
l'efficacité de la mesure adoptée et de l'amélioration obtenue des propriétés mécaniques. les tassements de consolidation réels ont été
de 600 mm sur une période d'environ 30 jours avec pVd et pré-chargement. dans la conception de l'amélioration des sols, l'efficacité
des théories existantes de tassement de consolidation en vertu de drainage vertical et radial a été démontré.
KeYWords: clay, Ground improvement, pre-loading, pVd
1 introdUction
chittagong sea port, the largest sea port of Bangladesh is
situated on the right bank of the Karnaphuli river at its
confluence with the Bay of Bengal. the port, that once handled
mostly bulk cargo is gradually shifting its operational mode to
handle increasing volume of container traffic. in this regard,
chittagong port authority (cpa) is implementing a project for
construction of backup facilities at new mooring behind berths
4 and 5. the site is locally known as
‘
nct
’
(new-mooring
container terminal). the project area is about 153,000 m
2
which is planned to accommodate stacking yard for containers,
passage for truck and trailer movement, tracks for Gantry
crane, electrical substation etc.
a comprehensive geotechnical investigation was carried out
at the site to assess the sub-soil condition, decide on the
necessity of improvement and determine relevant design
parameters for the envisaged improvement methodology. the
soil profile in the project area, consisted of a 4 to 6 m thick soft
to medium stiff clay layer, underlain by a 10 m thick loose to
medium dense silt/fine sand layer below which a loose clayey
silt layer existed to more than 30 m depth.
to keep conformity with the earlier constructed adjacent
yard, it is considered that the area will be paved with
interlocking block (ilB) except the rmG (rail mounted
Gantry) and rtG (rubber tyred Gantry) tracks which will be
pile founded. on the basis of design requirements and
geotechnical characteristics, improvement of the upper soft clay
layer was considered essential to eliminate the possibility of
differential settlement within the yard as well as between pile
founded structures (i.e. jetty and rmG, rtG tracks) and yard.
from an study of several alternatives, prefabricated Vertical
drain (pVd) with pre-loading was adopted as the ground
improvement measure for the site. improvement measures have
been completed on a part of the project area. the settlement
under preloading with pVd has been monitored using
settlement plates. field and laboratory tests have also been
conducted to evaluate the effectiveness of the adopted measures
in terms of change of soil properties. this paper presents the
geotechnical characteristics of the sub-soil in the area, the
design considerations and a comparison of cost of several
alternative improvement methods. from the limited data, that
has so far been available, comparison of some engineering
properties before and after preloading has also been made.
2 site location and topoGraphY
the site for the container yard is in a tidal plain at a narrow strip
between chittagong hilly uplands and the Bay of Bengal.
Geologically it is a recent alluvium formed by the material
carried by the river Karnaphuli and its tributaries from the upper
tertiary hills. figure 1 shows the site map with grid lines. about
half of the land, the eastern side (segments marked 1, 2, 3 and 4)
had been used as jetty yard for more than 50 years and housed
storage sheds for general cargo, road and railway tracks. the
other half (western part, segments marked 1a, 2a, 3a and 4a)
contained a city road, a residential area of cpa containing
one/two storey building, ponds, play ground, open land, village
dwellings etc. different parts of this western side had different
elevations with 1~3 m ditches. Because of earlier diverse use of
the land, there was little possibility of homogeneity of the upper
soil layer in the area.
Improvement of a Clay Deposit using Prefabricated Vertical Drains and Pre-loading.
A Case Study
Amélioration d'un massif d'argile à l'aide de drains verticaux préfabriqués et de pré-chargement.
Une étude de cas
Islam M.S., Yasin S.J.M.
Bangladesh Univ rsity of Engineering and Technology, Dhaka, Bangladesh
