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Effect of brittle failure on deep underground excavation in eastern Taiwan
Effet de la rupture fragile sur l'excavation souterraine profonde dans l'est de Taiwan
Hsiao F.Y., Chi S.Y.
Sinotech Engineeering Consultant, INC., Taipei, Taiwan, Chinese Taipei
ABSTRACT: The excavation depth is increasing rapidly in Taiwan. The effect of brittle failure for hard rock was seldom studied in
Taiwan in the past. A serial of tri-axial compressive tests of marble were performed in this paper. Relatively small deformation occurs
prior to failure during loading, and under continued loading, the integrity of rock is destroyed with degrading rock strength. To
reasonably estimate the post-peak strength, the post-peak form of the Hoek-Brown failure criterion and the strength loss experiment
method was adopted to establish the relationship between strength loss parameter and confining stress. Furthermore, the numerical
modeling of an actual tunnel in eastern Taiwan was conducted to examine the impact of the post-peak strength degradation. The
analyses show that the effect of post-peak strength degradation on excavation deformation is progressively significant with increasing
the depth. Severe tunnel deformation may endanger the excavation stability in deep overburden. The strength degradation beyond
brittle failure plays an important role in the stability of deep underground excavation.
RÉSUMÉ : La profondeur d'excavation augmente rapidement à Taiwan. L'effet de la rupture fragile pour le hard rock a été rarement
étudié à Taiwan dans le passé. Série TA de tri-axiales essais de compression de marbre ont été réalisés dans ce document.
Déformation relativement faible se produit avant la panne pendant le chargement, et sous une charge continue, l'intégrité de la roche
est détruit avec résistance de la roche dégradants. Pour estimer raisonnablement la force post-pic, la forme post-pic du critère de
défaillance Hoek-Brown et la méthode de perte de force expérimentation a été adoptée pour établir la relation entre le paramètre de la
perte de force et de la contrainte de confinement. En outre, la modélisation numérique d'un tunnel réelle dans l'est de Taiwan a été
menée pour examiner l'effet de la dégradation de la résistance post-pic. Les analyses montrent que l'effet de la dégradation de la
résistance post-pic sur la déformation de l'excavation est progressivement significative avec l'augmentation de la profondeur.
Déformation du tunnel de grande taille peut mettre en danger la stabilité de l'excavation dans les profondeurs. La dégradation de la
résistance au-delà de la rupture fragile joue un rôle important dans la stabilité des excavations souterraines profondes.
KEYWORDS: Brittle failure, post-peak strength, numerical modeling , deep underground excavation
1 INTRODUCTION
Metamorphic hard rock extensively exists in eastern Taiwan.
The uniaxial compressive strength of the metamorphic rock in
this region is mainly in the range of 50-200 MPa. In general,
underground excavation in hard rock is normally stable at
shallow depth except for wedge failure. However, the
excavation depth is increasing in Taiwan, e.g. the maximum
overburden of 1188 m for Chungren Tunnel in Suhua highway
is being constructed. In high overburden stress condition, the
integrity of rock would be destroyed with degrading rock
strength. The post-peak strength degradation of brittle failure
would significantly affect tunnel behavior, including tunnel
deformation and the support pressure required, as portrayed in
Figure 1. Therefore, understanding both the peak and the post-
peak strength are necessary for the stability assessment of deep
underground excavation.
Although many researches have been proposed on the
determination of peak strength of rock, only few attempts have
been made to estimate the post-peak strength of rock. Actually,
experiences from case studies or results from material tests are
the usually used method. For instance, the relations of
s
r
=0.04
s
and
m
r
=0.65
m
b
were suggested to estimate the post-peak
strength of jointed rock masses by Ribacchi (2000), where
s
and
m
b
are the Hoek-Brown peak strength parameters and the
subscript
”
r
”
indicates residual values. Furthermore, the
relations of
s
r
=0.04
s
and
m
r
=0.65
m
b
were presented to portray
the post-peak strength of muscovite schist by Crowder et al.
(2006), which is obtained from the back analysis of
underground mining. The Mohr-Coulomb parameters of c
r
=0.1c
and
r
=0.9
b
were adopted to simulate the post-peak strength of
sandstone in a hydraulic tunnel by Kumar et al. (2008).
However, these suggestions for the post-peak strength
estimation are the individual case and the appropriateness on
different rock type or in different area needs to be re-examined.
Figure 1. Effect of post-peak strength variation of rock on tunnel
behavior
2 LABORATORY TESTS
Marble is one of major metamorphic rock types in eastern
Taiwan. To understand the post-peak characteristics of marble,
six sets of laboratory tests on marble, including uniaxial
compressive test and triaxial compressive test, were carried out.
