2513
Improvement of soft fat clay using rigid inclusions and vertical drains
Amélioration d’une argile plastique molle par inclusions rigides et drains verticaux
Kirstein J.F.
BVT DYNIV GmbH; Germany
Wittorf N.
Ingenieurbüro Dr. Lehners und Wittorf; Germany
ABSTRACT: In the case of a new road crossing in Germany with 1.5 to 7.0 m high embankments nearby the Danish border
particularly soft clays were found 13 to 20 m deep below sea level. The undrained shear strength of the clay varied between 7 and 20
kN/m². The water content was almost 100 % and the organic matter below 6 %. The consolidation coefficient C
v
< 0.3 m²/year is
characteristics of a fat clay which requires a long time or tight spacing of vertical drains to consolidate. Due to stability risks, vertical
wick drains were installed at a 0.5 m spacing in the part of the highest embankments, which were built in three load steps, each time
waiting for 60 to 80 % consolidation degree before loading the next step. Even using 600 kN/m woven geotextiles, a total vertical
settlement of around 1.5 m and up to 27 cm horizontal deformation were measured throughout one year of monitoring. These
deformations were too high for the existing and running highway in the middle of the new projects. Therefore, full displacement
concrete columns (rigid inclusions system CMC) were installed up to 22 m deep with load transfer platforms installed on top the
inlcusions. In order to improve the installation process of the rigid inclusions, additional vertical drains were installed in the soft soil
before the inclusions. Within the first two years, the area supported by the rigid inclusion experienced less than 2 cm of deformation, a
proportionally small amount compared to the deformations recorded in the wick drain consolidation parts of the project.
RÉSUMÉ : Pour un projet d’une nouvelle route sur des remblais de 1,5 à 7,0 m de hauteur en Allemagne près de la frontière danoise,
des argiles particulièrement molles ont été trouvés de 13 à 20 m de profondeur sous le niveau de la mer. La résistance au cisaillement
de l'argile varie entre 7 et 20 kN / m². La teneur en eau est proche de 100% et la matière organique inférieure à 6%. Le coefficient de
consolidation Cv <0,3 m² / an montre une argile plastique qui nécessite un long temps ou un réseau de drains verticaux très serrée
pour la consolidation. En raison de calculs de stabilité, les drains verticaux ont été installés avec un espacement de 50 cm dans la
partie des remblais les plus hauts, qui ont été construits en trois étapes de chargement, avec pour chaque étape des périodes d'attente
de 60 à 80% degré de consolidation avant de la prochaine étape de chargement. Même avec l’utilisation de geotextiles de 600 kN/m,
des tassements verticaux de 1.5 m et des déformations horizontales jusqu’à 27 cm ont été mesurés pendant une année de surveillance.
Ces déformations sont trop importantes pour l'autoroute existante en exploitation près du nouveau projet. Des inclusions rigides
(systeme CMC) ont été installées jusqu'à 22 m de profondeur avec différents matelas de répartition placés au dessus des colonnes.
Afin d’améliorer le processus d'installation des inclusions rigides supplémentaires, des drains verticaux ont été installés dans le sol
mou avant l’installation des colonnes. Au cours des deux premières années de construction, la zone supportée par les inclusions
rigides a eu moins de 2 cm de déformation, une déformation relativement petite comparé avec celles enregistrées dans des zones du
projet consolidées par des drains verticaux.
KEYWORDS: soil improvement, Controlled modulus columns (CMC), vertical drains
1 INTRODUCTION AND DESCRIPTION OF THE
PROJECT
Large areas nearby the northern sea are nearly flat with
elevations slightly above or under the sea level. Soft soil of silt,
clay, mud and peat reach between five and twenty meters from
the surface, before glacial sands are encountered.
The existing west coast highway B5 near the German city of
Husum will be widened from two to three lanes in the future in
order to improve traffic. The crossing between B5 and B202
was designed as a bridge project with high embankments
located on the unconsolidated soft soils, typical at the flat costal
region near the North Sea.
All traffic constructions bring new loads in form of dead-
and live-loads to these soft soils. Without soil improvement
methods large long-time settlements will occur, which often
causes damages to the road during the construction or later on.
The traffic on the highways B5 and B202 in the site had to
be maintained during the construction period and the existing
road could not tolerate additional stability risks or settlements,
especially when the 1.5 to 7 m high embankments are built
directly beside the traffic. There are different stages to look at,
but we will focus only on the western part with the highest dam
nearby the bridge.
Figure 1
.
detail of the highest embankment west with the bridge
abutment over the highway B5 (CMC close to bridge and coloured areas
with vertical drains and preloading)
