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Effect of pre-ground improvement method during shallow NATM tunnel excavations
under unconsolidated conditions
Effets de la méthode d’amélioration préalable des sols durant l’excavation de tunnel peu profond
utilisant la nouvelle méthode autrichienne (NATM) dans un sol non-consolidé
Cui Y.
Meijo University, Japan
Kishida K.
Kyoto University, Japan
ABSTRACT: During the construction of the Bullet Train lines in Japan, several shallow tunnels were excavated in unconsolidated
ground using the New Austrian Tunnelling Method (NATM). However, ground and tunnel settlements frequently occurred due to the
low stiffness of the ground and the shallow overburdens. In order to prevent such settlements and to ensure the stabilization of the
cutting face of the tunnels, a pr-ground improvement method was adopted in the above fields. Various combinations of improved
areas and levels of strength were tried in the fields, and the tunnels were excavated successfully. However, the mechanism of the
effect of the pre-ground improvement method, the influence of the strengh of the improved ground, and the influence of the depth and
the width of the improved areas are not clearly understood. Therefore, 2D elasto-plastic FE analyses are carried out here to clarify the
effect of the pre-ground improvement method on the prevention of ground and tunnel settlements.
RÉSUMÉ : Durant la construction des lignes du train rapide japonais, plusieurs tunnels peu profonds ont été excavés dans des sols
non-consolidés en utilisant la nouvelle méthode autrichienne (NATM). Cependant, à cause de la faible cohésion du sol et du mort-
terrain peu profond, des affaissements du sol et du tunnel arrivaient fréquemment. Dans le but de prévenir ces affaissements, et pour
s’assurer de la stabilité des parois d
u tunnel, une méthode supplémentaire d amélioration préalable du sol a été adoptée dans les
terrains mentionnes ci-dessus. Divers combinaisons des surfaces améliorées et des niveaux de résistance ont été essayées sur place, et
les tunnels ont été creusés a
vec succès. Cependant, l’effet de la méthode d’amélioration préalable des sols et l’influence de la
résistance des terrains améliorés, de leur profondeur et de leur largeur n’est pas clairement comprise. Dans cette étude, des analyses
2D par éléments finis élasto-
plastique sont effectuées afin de clarifier l’effet de la méthode d’amélioration préalable des sols sur la
prévention des affaissements du sol et du tunnel.
KEYWORDS: shallow tunnel, unconsolidated ground, NATM, pre-ground improvement method, surface settlement
1 INTRODUCTION
Up to now, the open-cut method has been the main tunneling
method when excavating shallow tunnels in unconsolidated
ground. And, the New Austrian Tunnelling Method (NATM)
has been thought to be suitable when excavating tunnels in
mountainous areas. Recently, however, not only because of
advances in construction and measurement techniques, but also
because it is more economical than either the shield tunneling
method or the open-cut method, NATM has also become
popular for shallow tunnel excavations. For example, during the
construction of the Bullet Train lines in Japan, several shallow
tunnels were excavated in unconsolidated ground using NATM
(Kitagawa et al., 2005, 2009). However, ground and tunnel
settlements frequently occurred due to the low stiffness of the
unconsolidated ground and the significant reduction in the
arching effect arising from the shallow overburdens. In order to
prevent such settlements and to ensure the stabilization of the
cutting face of the tunnels, a pre-ground improvement method
was adopted in the above-mentioned fields.
Figure 1 shows the construction process of the pre-ground
improvement method. First of all, the ground is excavated to the
upper part of the tunnel crown. Then, cement is mixed with the
natural ground around the side wall of the tunnel using the
shallow or deep mixing stabilization method. Thereafter,
spreading and rolling compaction of the premixed soils are
performed over the tunnel crown area. Finally, backfilling and
rolling compaction of the excavated soils are performed to the
ground surface. Thereafter, the tunnel is excavated using
NATM. Various combinations of improved areas and levels of
strength of the improved ground were tried in the fields, and the
tunnels were excavated successfully. Morover, the effect of the
pre-ground improvement method was confirmed in a previous
analytical study (Cui et al., 2012). However, the mechanical
behavior of the ground and the tunnel during the tunnel
excavation process has not been discussed sufficiently. The
strength of the improved ground, and the depth and the width of
the improved areas, have been determined through practical
construction works .
In this study, therefore, 2D elasto-plastic finite element
analyses, that simulate the tunnel excavation process, are carried
out to clarify the effect of the pre-ground improvement method.
Figure 1. Construction process of pre-ground improvement method
