1714
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 3. Mechanical parameters of rock mass for the case tunnel
σ
c
(MPa)
Unit weight
(t/m3)
m
b
s
a
Deformation
modulus
(MPa)
80
2.7
4.406 0.1084 0.5
7000
Table 4. Effect of overburden depth and post-peak strength degradation
on tunnel excavation behavior by using numerical analyses
Overburden=500m Overburden=1000m
HB model SD model HB model SD model
Roof
settlement
1.7 cm 3.7 cm 3.8 cm 9.8 cm
Tunnel
deformation Horizontal
convergence 1.9 cm 4.4 cm 4.2 cm 9.0 cm
Crown
0.5 m 1.5 m 1.5 m 3.5 m
Loosening
zone of rock
mass
Sidewall
0.5 m 1.5 m 1.5 m 2.5 m
＊
HB model means the Hoek-Brown model
；
SD model means the
post-peak strength degradation model
Figure 5. Correlation between roof settlement and overburden depth in
the case tunnel
5 CONCLUSIONS
Relatively small deformation would occur prior to failure
during loading, and under continued loading, the integrity of
rock would be destroyed with degrading rock strength. Except
for violent rockburst, the post-peak strength degradation may
endanger the stability of deep underground excavation.
However, there is still not a commonly acceptable method in
estimating the post-peak strength up to now. The post-peak
form of the Hoek-Brown failure criterion by introduced a
strength loss parameter may be a feasible method currently. A
serial of tri-axial compressive tests of marble, which is the
typical metamorphic rock in eastern Taiwan, were carried out in
this paper. The results of the tests show that the post-peak
strength is strongly affected by the confining stress. Therefore,
the relationship between strength loss parameter and confining
stress was established by using the strength loss experiment
method. It should be noticed that the estimation measure for
post-peak strength used in this paper is limited to the massive
unjointed brittle rock (such as GSI >70) because it was
established on the basis of intact rock tests. For jointed rock or
other rock type (such as schist, andesite, and so on), the
applicability of the method should be subjected to further
investigation and re-adjustment.
According to the analyses of actual tunnel case in eastern
Taiwan, the effect of post-peak strength degradation would not
be disregarded. The tunnel deformation would exceed the
design value in deep overburden. Tunnel requires re-mining due
to the inadequacy of inner section and the instability will be
anticipated. Actually, various studies and experiences revealed
that brittle failure initiating is extremely difficult to prevent in
highly stressed rock. The aim of the design of reinforcement or
support is to control the propagation of the brittle failure and to
retain the profile of the tunnel. The issue of brittle failure was
seldom studied in Taiwan in the past. Therefore, cautious design
and systematic monitoring should be fulfilled to overcome the
possible problems caused by brittle failure in deep underground
excavation in Taiwan.
6 REFERENCES
Crowder J.J., Coulson A.L. and Bawden W.F. 2006. The field-scale
rock mechanics laboratory: estimation of post-peak parameters and
behavior of fractured rock masses,
The 41st U.S. Symposium on
Rock Mechanics (USRMS)
, 17-21.
Cundall P., Carranza-Torres C. and Hart R. 2003. A new constitutive
model based on the Hoek-Brown criterion,
Proceeding of the Third
International Symposium on FLAC and FLAC3D Numerical
Modeling in Geomechanics
, Sudbury, Canada, Balkema, Ed.
Brummer et al.,17-25.
Hoek E., Carranza-Torres C., Corkum B. 2002. Hoek-Brown failure
criterion-2002 edition,
Proceeding of the 5th North American Rock
Mechanics Symposium
, Toronto, Canada, 267-273.
Hsiao, F.Y., Chern, J.C. and Chi, S.Y. 2012. Post-Peak strength
estimation for tunneling in marble.
ARMS7 - 7th Asian Rock
Mechanics Symposium
, Seoul, South Korea, 201-217.
Hsiao, F.Y., Wang, C.L. and Shao, H.J. 2011. Mechanical parameters
estimation and tunnel deformation study for brittle rock under high
overburden condition.
Rock and Soil Mechanics
, 32(Supp.2), 109-
114.
Hsiao F.Y., Chang Y.L., Tzeng C.E. and Tzai C.H. 2006. Application of
in-situ overcoring measurement technique in Taiwan,
2006 Taiwan
Rock Engineering Symposium
, Tainan, Taiwan, 621-630.
Kumar R., Sharma K.G. and Varadarajan A. 2008. Effect of strain
softening on tunnel behavior,
World Tunnel Congress 2008 -
Underground Facilities for Better Environment and Safety
, Delhi,
India, 541-550.
Ribacchi R. 2000. Mechanical tests on pervasively jointed rock
material: insight into rock mass behavior,
Rock Mechanics and
Rock Engineering
, 33, 243-266.
Sinotech Engineering Consultants, LTD. 2011. Report of geotechnical
survey for Suhua highway of provincial highway #9 mountain area
improvement project of Hochong-Qingshui section, Suhua
Improvement Engineering Office, Directorate General of
Highways, M.O.T.C., 6-2~6-6.
