10th Asian Regional Conference of IAEG (2015)

Analysis on deformation and failure of rock mass in deeply buried Guanshan railway
tunnel under high in situ stress
Ning LIANG(1,2), Faquan WU(2), Yunfeng WANG(3), Yang MI(4), Han BAO(1,2)(1) University of Chinese Academy of Sciences, Beijing 100049, China
E-mail: liangning320@qq.com(2) Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing 100029, China,(3) Tianping Railway Corporation Limited, Gansu 741020, China(4) China University of Geosciences, Beijing 100083, ChinaAbstract
Guanshan railway tunnel, located in Gansu Province, China, is a deeply buried dioriterock mass tunnel under high in situ stress. On the contrary of the good rock mass classification
predicted in investigation stage, the real situation after excavation shows a great change of rockmass parameters and structure, revealing not only the degradation of rock mass quality also thesignificant anisotropy of rock mass. The degradation and anisotropy of rock mass is describedand analyzed based on the statistic analysis of the rock structural plane in field. Meanwhile,borehole camera is adopted to further observe the progressive deformation and failure of rockmass in an interval after excavation. As one of the hard and brittle plutonite, the uniaxial
compressive strength (UCS) of diorite is a relatively high index on investigation stage, which isobviously larger than the in situ stress. However, the deformation and failure of diorite rockmass happened prevailingly. As the evidence obtained by borehole camera, the micro fracture
or closed rock structural plane under high in situ stress will re-open and develop quickly after
the excavation, which degraded the stability and quality of rock mass structure. To figure out
the deformation and failure process, an angle rotation of uniaxial compressive test is designed.
The uniaxial pressure is lower than the uniaxial compressive strength (UCS) of diorite. This
test fully presents the feasibility of the progressive deformation and failure process of dioriterock mass during the strongly stress field change after excavation. It proves that thedeformation and failure of rock mass is the result of the degraded rock mass structure, induced
by stress field change after excavation. Besides the rock mass structure, stress field change isalways a significant factor in deeply buried excavation project under high in situ stress.

Keywords: rock mass, deformation and failure, stress field, diorite
1.

Introduction

Tianshui-Pingliang railway under construction is an
important National Class I electrified railway, for boththe passengers and freight service in the remote andundeveloped area in Gansu Province ， EW China.
Tianshui-Pingliang railway stretches about 99 km in
main line, including about 78 km of bridges andtunnels. This project aims to built a connection
betweenTianshuistationon
existing
Lanzhou-Lianyungang railway and Huating station on
existing Baoji-Zhongwei railway towards Pingliang
city (see Fig. 1). The challenges of this project focus on
the deeply buried Guanshan tunneling project，with themaximum buried depth 831m, from Gongmen town toFig. 1 Location and route chart of the project1

10th Asian Regional Conference of IAEG (2015)

Qinglin, acrossing the section of local Guanshan
Mountain within the elevation range from 1000m to
3500m, belongs to Liupanshan Mountains area, one of
the youngest mountains in China.
2.

With the help of microstructure test and ScanningElectron Microscope (SEM) on samples at
DIK76+558 in a buried depth of 795m (see Photo. 1and Photo.2), it is observed that the diorite surrounding
the tunnel is mainly consist of plagioclase, amphibole,chlorite and mica. Silicate minerals account for a great
proportion and make the rock mass hard. In addition,
after the hydrothermal process in magmatic period,
amphibole and black mica altered into chlorite, whilefeldspar altered into mica, kaolinite and so on. In someplaces, the rich layer silicate minerals such as mica andchlorite gathered and created membrane or thin layer,
which will weaken the strength of structural planedramatically, especially enhanced by the watersoftening. Meanwhile, on the account of a large amountof plagioclase, mica, chlorite and other minerals,
diorite became the direction-sense and developed sheet
structure, revealing significant anisotropy. When theexternal force changes, it is easy to occur inter layer
deformation and shear dislocation, spalling or bulking
failure, especially the structure is weaken by mica andchlorite after encountering water.

Geological settings

2.1

Topography
The direction of the Guanshan railway tunnel
axial is mostly N46°E. As the result of changed terrain
fluctuation and developed gullies, the natural slope isusually 20°~ 35°and some of them are steeper withvisible bedrock scarps. The surface elevation is around
1695m~2608m with a maximum buried depth of tunnel
831m. The whole area is in a state forestry centre
covered by afforestation over 10 years. Therefore thisarea is forested and the vegetation coverage rate isnearly 90%.
2.2

Lithology

The region is a large area of diorite intrusion in
Variscan with huge batholite and associated small rockbody. The rock mass consists mainly of amphibolite,
biotitegranodiorite,
Biotitequartzdiorite,biotite adamellite, hornblende diorite, quartzite,
andesite, diorite and granite. The area near Guanshan
tunnel is mainly diorite(δ4) , in the color of steel gray.
The diorite particle size is fine grain to coarse grain
structure and structure is massive. Some of rock mass
are with developed structural planes.

plagioclase

mica

amphibole+ chlorite

Photo.1 Composition analysis of diorite

Photo.2 Microstructure analysis of diorite
2.3 Tectonic structure and in situ stress

Fig.2

Geological map of Guanshan Tunnel

In the aspect of tectonic structure, Guanshan
mountains where the Guanshan railway tunnel located210th Asian Regional Conference of IAEG (2015)

is in place within the Guanshan fold belt, which is asecondary structural unit of Liupanshan fold belt.
Guanshan fold belt is a geosyncline uplift belt among
Tongwei-Qingshui fault (F3) and Zhuanglang-Guguan
fault (F4) (see Fig. 3). Except a large area of diorite
intrusion in Variscan at the ridges of Guanshanmountains, there is no other evident of fracture
structure nearby Guanshan railway tunnel. Based on
the tectonic structures, we could infer that theorientation of maximum principal compressive stress isnear NE direction. And it is basically parallel to thetunnel axial orientation, beneficial for releasing therock mass pressure.

2000

horizontal convergence
MP-B
MP-A
upper bench basement

2500

9000

4500

MP-C

7000

Fig.4 Measuring points for displacement(unit: mm)
the adverse effect of excavation . In many section ofGuanshan tunnel, rock mass wave velocity ahead theworking face, obtained by advanced prediction ofgeological radar, is usually nearly 5000 m/s. However,after excavation, the p-wave velocity of rock block
dramatically drops down to 2300 m/s in many cases.
As Equation 1 shown, elastic modulus is sensitive to
p-wave velocity. The decrease in p-wave velocity ofGuanshan tunnel rock mass describes the reductive
elastic modulus and the weakening ability of rock mass
to resist deformation and failure in the changed stressfield during the tunnel excavation.

Fig 3 Tectonic structure map
By the hydro-jacking test at DIK76+070, it isanalyzed that, in Guanshan railway tunnel, themaximum principal compressive stress is about 31.35
MPa, the in situ tensile strength is about 1.32 MPa ~15.0 MPa and the predominant direction of maximum
principal compressive stress nearby the tunnel is N55ºE(1)

Where E is elastic modulus,  is rock density and is p-wave velocity and μis Poisson' s ratio.

~ N59ºE. As a whole, deeply buried Guanshan railway
tunnel is under high~very high in situ stress.
3.

E1 -   1   1 - 2 

Deformation and failure
phenomenon of rock mass

According the displacement monitoring of upper
bench near DIK76+558 in a buried depth of 795m, thedisplacement of support is still large at initial stage inthe tunnel. After the ring closure of inverted arch, thedisplacement will end in stability, the maximum crownsettlement (MP-C) is about 80mm and the maximum
horizontal convergence is about 200mm. The
horizontal convergence is far above the crownsettlement (MP-C). Displacements of MP-A are greaterthan MP-B both in horizontal and vertical directions.
Corresponding with the severe horizontal
convergence, it frequently appears that the failure rockmass and steel arches in side walls (see photo.3),
indicating the strong deformation and failure afterexcavation. Also, the wave velocity inversion proves

Photo.3 Failure rock mass and steel arches in sidewall at DIK76+558
After excavation in deeply buried depth and highin situ stress, the rock mass is easy to occur
deformation, especially towards the free faces. The310th Asian Regional Conference of IAEG (2015)

decreased elastic modulus worsens this process. Morecracks in different direction inside the rock mass willdevelop, weaken the integrity of rock mass andhighlight the anisotropy of rock mass.
4.

5.

For a more intuitive and comparative analysis on
the degradation of rock mass after tunnel excavation,
we adopted the borehole camera to observe the rockmass situation twice in 1# and 2# observation holes
ahead the working face on DIK76+543 (buried depth795m) respectively, given 6 hours apart (see photo. 4).
The two observation holes are almost perpendicular to
the working face, with a small inclination.

RQD anisotropy of rock mass

After field survey and structural plane statistics
around the excavation surface of rock mass in thetunnel, it is found that Guanshan tunnel develops threesets of preferred structural plane (see Table 2).
Assuming the intervals of intersection points obey
negative exponential distribution along any direction,
we could predict the RQD, Deere (1989), in any
direction by Equation 2 (Wu Faquan, 1993; Wines D
R et al., 2002, Hu Xiuhong, 2009).

RQD  1  0.1s e 0.2s 100%Borehole camera observation

Table 2 Preferred structural plane parameters
Dip direction

Dip angle

Normal density

(°)

(°)

(m-1)

J1

6080

8.1

J2251

34

6.2

J321070

10ID(2)Where s is structural plane density in any direction,is given by:ns   i cos si

(3)

i 1

Where n is the total amount of the structural planes andcos si is the cosine of included angle between any
direction and normal of each structural plane.
As shown in Fig.5, when the tunnel axial ismostly N46°E, the RQD in all directions on tunnel
cross section proves the apparent anisotropy, ranges
from 9.1% to 50.5%. Besides the structure, the strengthof rock mass is anisotropic as well. The rock massquality nearby the excavation surface is very bad,especially in the side walls and crown of the tunnel.
However, the RQD will gradually increase apart fromthe excavation damage zone.

Photo.4 1# (left) and 2# (right) borehole camera
observation holes at DIK76+543

Fig.6 1# borehole camera observation hole
As shown in Fig.6 and Fig.7, we could observethat after 6 hours the rock mass structure ahead theworking face is changing strongly with the evidence ofsoaring new crack amount. With the effect of stressfield adjustment induced by excavation in a deeply

Fig.5 RQD in all directions on tunnel cross section

4

10th Asian Regional Conference of IAEG (2015)

Table 3 Stress rotation test result of Sample No.2
No.

1st round

Surfaces

Fracturing description

1-1a

No significant changes

2-2a

No significant changes

3-3a

No significant changes

No.

Surfaces
1-1a

3rd round

2-2a
3-3a

Fracturing description
Through cracks within surface 1 and 2
respectively and cracks develop andintersect in surface 3.
Cracks develop rapidly in surface 2and 2a. A crossing crack between
surface 1 and 2a.
A crossing crack and wedge damage
in surface 3 and 2a
Through cracks in surface 1, 2a and 3
respectively.

Inclined cracks at the corners in
1-1a
surface 1,1a,2a and 3a.
New cracks in surface 1 and 3 andFailure. Cracks develop rapidly in all
2-2awedge damage at the border of
2nd round
4th roundsurfaces and significant tensile cracks
surface 1 and 3.
2-2a along the direction of unaxial
Existed cracks develop in surface 1
compressive pressure in the border of
3-3a and 1a. New cracks and wedge
surface 1,2 and 3.
damage in surface 2 and 3a.
buried 795m tunnel in high in situ stress, the new crack
6. Stress rotation test
along with the reopened close cracks develop furtherand degrade the structure and strength of rock mass.
As one of the hard and brittle plutonite, the uniaxial
The rate of new crack increase could be 200% andcompressive strength (UCS) of diorite, nearly 100 MPa,
400% averagely in two observation holes but exceeds
is a relatively high index on investigation stage, which
600% in the advanced depth 110cm~130cm in 2#
is obviously larger than the in situ stress of Guanshan
tunnel as well. However, as the evidences obtained by
RQD anisotropy analyses and borehole camera
observation of rock mass after excavation, thedeformation and failure of the rock mass happened
largely. To figure out the degradation on strength andstructure of rock mass under the changing stress field
induced by excavation disturbance, an angle rotation ofuniaxial compressive test is designed.
In the stress rotation test, among several dioriterock blocks samples after excavation at DIK76+543 in
the buried depth of 795m, we were successful to cut
one of the rock blocks into 3 cubic rock samples(Sample No. 1, 2 and 3) in the size of 100mm x100mm x 100mm and labeled the 6 surfaces by 3 pairsof parallel surfaces (1-1a, 2-2a and 3-3a).
Sample No.1 is taken to the uniaxial compressiveFig.7 2# borehole camera observation hole result
test and the UCS is 82 MPa, which will be taken as thereference UCS for all the samples cut from the sameobservation hole. Importantly, analyzing thediorite rock block. Then the Sample No.2 and No.3
slope of the curves of the second time of observation in
were applied by an uniaxial compressive pressure on
both 1# and 2# holes, we could find the crack amountthe 3 pairs of parallel surfaces in turns until failure. The
increase more quickly both in the range of 1m
cycling order is first 1-1a and then 2-2a and finallyadvanced depth. It is also the severe damage zone of
3-3a. The uniaxial compressive pressure is 49 MPa,rock mass. It suggests that we should pay abundant
60% of the reference UCS and about 1.5 times of theattention to this key and very nearby zone, easy tomaximum principal compressive of in situ stress (about
impose engineering measures to mitigate the the bad31.35 MPa) in the Guanshan tunnel. The two samples
effect of stress field adjustment induced by excavation,
finally failed in the second stage 2-2a and the firstto control the degradation of strength and structure ofstage 1-1a of 4th round of stress rotation test
deeper rock mass.
respectively.
1-1a5

10th Asian Regional Conference of IAEG (2015)

We take the detailed test result of Sample No.2 asexample, listed in Table 3. During the 4 rounds ofrepeating compressive pressure rotation, no obivious
cracks or wedges happen in first round and then thecracks and wedge damages begin to occur in secondround. In third round, wedge damages occur severely
and new cracks develop greatly. Finally, Sample No.2
failed with a significant tensile cracks along thedirection of unaxial compressive pressure in the second
stage of fourth round.
It implies that, under the influence of stressrotation, the existed and new developing cracks could
more fully and easily reactivate and grow in lower
compressive pressure than UCS, resulting in theanisotropy and whole degradation of rock sample.
Some of the cracks show preferred orientation andinduced the significant through cracks to failure. The
stress rotation test fully presents the feasibility of
progressive deformation and failure process of dioriterock mass nearby the surface during the stress fieldadjustment after tunnel excavation.

Press.
WINES D R, LILLY P A (2002): Measurement andanalysis of rock mass discontinuity spacing andfrequency in part of the Fimiston Open Pit operationin Kalgoorlie，Western Australia: a case study,
International Journal of Rock Mechanics & Mining
Sciences, Vol. 39, pp. 589-602
HU Xiuhong, WU Faquan (2009): Research on
two-parameter negative exponential distribution ofdiscontinuity spacings in rock mass, Rock and Soil
Mechanics, Vol.30, No.8, pp. 2353-2358

Photo.5 Wedge and crack damages of Sample No. 2
after the first stage 1-1a, 4th round of stress rotation test
7.

Conclusions

The deformation and failure of rock mass inGuanshan tunnel is the result of the degraded strengthand structure of rock mass, induced by stress fieldchange after excavation disturbance. In this case, theanistropy and integrity of rock mass is much more
important than the strength of rock mass. The disturbed
stress field change is always a significant factor onrock mass quality in deeply buried excavation project
under high in situ stress.
References
Deere, D.U. (1989): Rock quality designation (RQD)
after 20 years, U.S. Army Corps Engrs Contract
Report GL-89-1. Vicksburg, MS: Waterways
Experimental Station
WU Faquan (1993): Principle of statistic rock massmechanics, Wuhan. China University of Geosciences6