The Reasonable Layout of Cross Passages for Qianjiang River Tunnel

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Nov 16, 2013 (3 years and 11 months ago)

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1








The Reasonable Layout of Cross Passages for Qianjiang River Tunnel


based on seismic analysis

Qizhu Jiao
1
,

Di Zhang
2
,

Shao
m
ing Liao
3
,

Yi
z
hi Xu
4

1
China Railway No.4 Survey and Design Group Co., Ltd., 745 Peace Avenue, Wuhan 430063, P. R.
China;
jfocus@163.com

2
China Railway No.4 Survey and Design Group Co., Ltd., 745 Peace Avenue, Wuhan 430063, P. R.
China;
zdiok@126.com

3
Departme
nt of Geotechnical Engineering, Tongji University, 1
239 Siping Road,
Shanghai 200092,
P.
R.
China
;
engcent@tongji.edu.cn

4
Departme
nt of Geotechnical Engineering, Tongji University, 1239 Siping Road,
Shanghai 200092,
P.
R.
China
;

xyz.61533@yahoo.com.cn


ABSTRACT
:
Num
erical modeling and Response Displacement Method are applied
to assess the effects of the presence of cross passages and geology formations on the
seismic responses of the Qianjiang River tunnel, in order to determine the
proper

locations of cross passages
. The numerical analysis shows that the presence of cross
passages affects the longitudinal seismic response of the main tunnel significantly
within 25m distance from the intersections of cross passages with main tunnels. So
the possible local failure of t
he intersections
will be a major concern. This is
largely attributed to the
complicated geologic formations and irregular ground
conditions.

The analysis based on the Response Displacement Method indicates that
cross passages in
flat and uniform ground la
yers can
mitigate

the local failure
at

the
intersections.
Optimal l
ocation for cross passages are proposed
based on

the detailed
analysis of the geologic profiles of Qianjiang River Tunnel.

INTRODUCTION

There has been a
serious dispute among the tunnel eng
ineers in China since 2003
over

whether the cross passages should be
constructed

for
Qianjiang

R
iver
T
unnel,
especially the risks
associated with

constructing cross passages in
saturated soft

ground.
The

catastroph
ic failure of the No. 4 route

in Shanghai
happened during
the
last step
excavating

a cross passage under Huangpu river

taught a major lesson
.
A
nother great concern is the effect of cross passages on the structural safety of
tunnels
during

earthquake
s. T
he local failure at the intersections of
unde
r river
cross
passages in soft ground
can cause it to
collapse,
which can result in
severe

2

catastroph
ic failure of
the whole tunnel. These made the layout of the cross passages
a

crucial
task in design.

D
amages of tunnels during
recent
earthquake
s, i.e.,
t
he 1999 Chi
-
Chi, Taiwan
earthquake (Wang 2002)
, h
ave revived the interest in
studying the safety of

tunnel
structures subjected to earthquake shaking.
Significant amount of

researches have
been
conducted

to estimate the
seismic responses of
tunnel lining

a
long the
transverse direction over the past decades (Hashash et al. 2005, Park 2009, Chen
2011). However, the longitudinal responses (bending, axial compression and
extension) are also essential to the
safety

of tunnel

during seismic excit
ation

(He and
Koi
zumi 1999)
.

Regarding the longitudinal seismic response of shield tunnel, Yukio SHIBA,

Kazuhiko KAVASHIMA, Naomi OBINATA and Takashi KANO evaluated
the
axial
force and bending moment in linings subjected to ground displacement with
sinusoidal distribution
along the tunnel axis by assuming the linings to be uniform
beams (Yukio et al. 1989). He and
Koizum
i (1999)

studied the dynamic
responses in
the

longitudinal direction of shield tunnel in non
-
uniform ground under

axial and
transverse seismic
loads. The st
udy revealed

the adverse consequences for shield
tunnel located in irregular ground.

In the
previous
studies, the Response Displacement Method were usually
used as a

simplified approximation

with
and the inertia of the structure ignored. As some
studies ha
ve pointed out, tunnels were designed to accommodate the free
-
field
ground deformations for tunnels constructed in stiff ground. But for tunnels in soft
soils, the soil
-
structure interaction
s

should be
considered (Shahrour et al. 2010). The
criteria of whe
ther soil
-
structure interaction should be
considered

can be checked via
the flexibility index,
F
,
which measures the ability of the lining to resist distortion
from the ground
(Wang 1993)
:


2 3
3
2 (1 )
(1 )



m l
l m s
E v R
F
E v t


(1)

where
E
m
is Yong’s modulus of the surrounding soil,
E
l

is Yong’s modulus of the
structure material,
v
m

is Poisson’s ratio of the surrounding soil,
v
l

is Poisson’s ratio of
the structure material,
t
s

is the thickness of the cross
-
section, and
R

is the structure
radi
us
(Kouretzis et al. 2006)
.


A sound engineering approach is
to
ignor
e

overall the soil
-
structure interaction
(SSI) only if values of the
calculated
flexibility index
is
higher than 20
(Hendron and
Fernandez 1983, O’Rourke and Liu 1999).


In this researc
h, the impact of the longitudinal seismic response of
the
main
tunnels by the cross passages is
firstly
analyzed with a simplified numerical
simulation, then the Response Displacement Method is employed to
study

the
stresses of the intersection
s

under diff
erent geology conditions and seismic forces.

Finally
optimal
locations for cross passages of the tunnel

are
proposed
.


3

PROJECT BACKGROUND

T
he ground formations that the tunnel passes through are generally silty clay,
sandy silt, silty sand, mucky silty clay
, clayed silt and silty clay, according to the
geological investigation reports. The physical and mechanical properties of these
formations are summarized in Table 1. And the longitudinal and typical transverse
geology profiles are shown in F
IG
.
1

and F
IG
.
2
.

Table 1. The properties of soil layers

Soil
layer
No.

Soil
description

Cohesion

(kPa)

Angle of
friction
(°)

Shear
velocity
V
s

(m/s)

Categories of
soil layers

1

Plain fill

/

/

92.7

soft Topsoil

2
-
2

Silty clay

15

19.8

94.0

3
-
1

Sandy silt



130.5

Soft
soil

3
-
2

Silty sand

15.2

19.1

155.0

4
-
1

Mucky silty
clay

16.3

16.4

139.3

5
-
1

Silty clay



181.0

Medium
-
soft
soil

5
-
2

Silty clay

24.8

20.1

240.0

6
-
3

Silty clay

/

/

313.7

Medium
-
hard
soil

205
150
148
146
150
154
145
154
151
149
150
150
150
150
150
150
151
190
171
282
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Section 8
Section 9
Section 10
Section 11
Section 12
Section 13
Section 14
Section 15
Section 16
Section 17
Section 18
Section 19
topsoil
v
s

(90-94m/s)
soft soil
v
s

(130-155m/s)
medium-soft soil
v
s

(180-260m/s)
medium-hard soil
v
s

(310-356m/s)
3246m
0
-10
-20
-30
-40
-50
10
-60
North bank
South bank
working shaft
working shaft
approach road
approach road

F
IG
.
1
. Geologic formation along the longitudinal direction
of the tunnel

T
HE EFFECT OF THE PRESENCE OF CROSS PASSAGES

The dynamic analysis module of FLAC
3D
program is employed to evaluate the
effects of cross passages to the longitudinal seismic response of the main tunnels.
The calculation is based on the explici
t finite difference scheme to solve the full

4

equation of motion, using lumped grid point masses derived from the real density of
surrounding zones (rather than fictitious masses used for static solution). This
formulation can be coupled to the structural e
lement model, thus permitting dynamic
analysis of soil
-
structure interaction. No slip is considered between the tunnel
structure and the surrounding soil mass, and the tunnel lining are simulated as
continuum structure for simplified analysis in this study
.

Section 1
Section 2
Section 3
Section 4
Section 5
+4.13
+1.67
+4.13
+1.43
-25.98
-26.37
-44.37
-47.37
-44.37
-47.37
12.89
+4.41
+1.21
+2.85
+1.51
-17.19
-36.59
-18.07
-43.42
15.04
-44.37
17.22
+0.53
-3.97
-15.64
-42.75
-59.87
+0.53
-3.97
-44.47
-59.87
-22.37
+0.30
+0.57
-4.10
-15.70
-40.20
-49.75
-4.83
-15.43
-45.43
-49.63
18.97
+0.29
-4.86
-22.61
-39.99
-60.06
+0.29
-5.91
-42.99
-60.06
-22.61
21.62
Section 6
Section 7
Section 8
Section 9
Section 10
+0.29
-4.98
-41.86
-54.25
-22.16
-4.70
-44.73
-49.93
-23.33
23.25
+0.77
+0.87
-4.53
-43.94
-55.23
-26.26
+0.87
-5.36
-44.82
-55.23
-26.83
23.25
+2.47
-3.23
-42.24
-48.43
-15.03
23.25
+0.93
-6.22
-43.62
-49.53
-19.07
+0.85
-2.61
-45.81
-55.45
-20.13
+0.85
-3.26
-43.92
-55.45
-16.77
23.25
+1.82
-3.27
-44.18
-48.64
-17.38
+2.09
-3.51
-45.41
-53.07
-16.41
23.25
Section 11
Section 12
Section 13
Section 14
Section 15
+1.70
-2.50
-48.70
-18.20
+2.20
-1.40
-49.70
-17.50
-53.63
23.25
+2.00
-3.60
-46.10
-18.70
-55.63
+1.62
-1.68
-46.58
-19.68
-55.63
23.25
+0.54
-2.76
-47.06
-17.76
-55.06
+0.54
-2.29
-47.06
-18.99
-55.06
23.25
-0.45
-3.95
-47.15
-21.85
-51.55
+0.93
-2.47
-47.47
-18.97
-50.07
23.25
+0.24
-3.92
-48.16
-18.65
-54.96
+0.24
-4.11
-48.16
-20.56
-54.96
23.25
Section 16
Section 17
Section 18
Section 19
Legend
+1.29
-3.62
-49.52
-23.21
-54.96
-0.81
-4.51
-47.81
-25.21
-45.58
23.25
+0.63
-2.87
-46.82
-24.37
-55.47
+0.63
-2.49
-46.27
-24.37
-55.47
23.25
+6.95
+3.45
-25.19
-48.48
+6.86
+2.86
-25.64
-48.84
23.25
+6.31
+1.85
-25.17
-44.47
+6.31
+2.92
-24.81
-44.47
23.25
topsoil
v
s
(90-94m/s)
soft soil
v
s
(130-155m/s)
medium-soft soil
v
s
(180-260m/s)
medium-hard soil
v
s
(310-356m/s)

F
IG
.
2
. Typical sections of geologic formations along the Qianjiang River tunnel

For convenience, the nodes and the elements of the numerical model is created
using FEM program ANSYS, which is extremely powerful in pre
-
processing, and
then be imported i
nto FLAC
3D
. F
IG
.
3
, F
IG
.
4
, F
IG
.
5

and F
IG
.
6

show the detailed
information about the model. The global model is excited at grid points at the base
by a uniform Cosine curve type of acceleration along the axis of cross passage (to
simulate the shear wave excit
ation).


5

North
South
90
East Line
West Line
20
25
M
x
y
z
N



F
IG
.
3
. cross passage

F
IG
.
4
. Meshed model


F
IG
.
5
. Meshed tunnel lining

The soil mass is considered to be mohr
-
coulomb material with specific weight
γ
=
19 KN/m
3
, friction angle
φ
= 15°, and cohesion
c

= 20 KN. The tunnel linings are
simulated as isotropic linear elastic material with Young’s modulus
E

= 30 GPa and
Possion’s ratio of
υ

= 0.2.
The damping of the whole model is neglected, so the effect
o
f the presence of cross passage is analyzed conservatively in th
is

study.

Point M, shown in F
IG
.
3
, 25m (4.17 times the diameter of cross passage) away
from the intersection, was chosen for analysis. F
IG
.
6

compares the acceleration
-
time
responses of the mai
n tunnel under two conditions (i.e., “tunnel with cross passage”
and “tunnel without cross passage”)

at point M.
The curves show that

the presence of
cross passage even
reduce

the response longitudinal accelerations of the main tunnel
compared with “tunnel

without cross passage”, despite that the effect is not so
significant (within 10% at the largest gap). However, as shown in F
IG
.
7
, the response
shear stresses at the intersection is much bigger due to the stress concentration effect.
The result indicates
that the presence of cross passage even make
s

the main tunnel
safer
at
a certain distance away from the cross passage, and the local failure of the
intersection may be the primary problem rather than the overall seismic response.



F
IG
.
6
.
Seismic
response(point M)
F
IG
.
7
. stress concentration effect

THE EFFECTS OF GEOLOGICAL FORMATIONS

Response Displacement Method was employed here to assess the effects of
geological formations on the response of
the
main tunnels. In Response
Displacement
Method, a “bed rock” layer whose shear wave velocity
V
s

is larger than
300m/s, usually need
s

to be determined first. Vibration of the soil layers above the

6

“bed rock” is assumed as a Cosine curve which
culmulates

at the ground surface

(shown as F
IG
.
8
a).

If

a twin
-
tunnel with cross passages locates on a declining “bed rock” plane at an
angle
α
from

the horizontal plane along the transverse direction, the displacement
response of the two tunnels will not be
synchronous due to the diversities of their
distance
s to the “bed rock”
(shown as F
IG
.
8
a)
. Then extra tensions/shears for the
cross passage will be induced, aggravating the stress states at the junction
.

Because the distance of the main tunnel to
“bed rock”

(C·tan(α)) increased with
the increase of inclinat
ion of the “bed rock” plane, it is advisable to locate the cross
passage at sections of the tunnel with small inclination.

ground surface
axial seismic force
H
D
β
B
C
β
= 0
β

= 0
β
≠ 0
β

≠ 0
A(
l
)
A(0)
L
l
o
α
o
H
D
B
ground surface
transverse seismic force
C
no phase difference between the two lines
phase difference
φ
=
π
between the two lines
phase difference
φ
=
π
between the two lines
bed rock
bed rock
bed rock plane
bed rock plane
(a) axial seismic force
(b) transverse seismic force

F
IG
.
8
. Tunnels acted by seismic forces(After He and Koizumi, 1999)

Similarly, as shown in F
IG
.
8b
, in case of an inclined “bed rock”

plane, the peak
magnitude of axial displacements will be somewhat different at different points along
the longitudinal direction
.

It is also that the smaller of the
β
, the safer of the
intersection of cross passage and local junction.

205
150
148
146
150
154
145
154
151
149
150
150
150
150
150
150
151
190
171
282
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Section 8
Section 9
Section 10
Section 11
Section 12
Section 13
Section 14
Section 15
Section 16
Section 17
Section 18
Section 19
topsoil
v
s

(90-94m/s)
soft soil
v
s

(130-155m/s)
medium-soft soil
v
s

(180-260m/s)
medium-hard soil
v
s

(310-356m/s)
3246m
0
-10
-20
-30
-40
-50
10
-60
North bank
South bank
working shaft
working shaft
approach road
approach road
reasonable location
for cross passage

F
IG
.
9
. Reasonable locations for cross passage based on seismic analysis

Extending the above key ideas to a more general sense, reasonable locations for
cross passages should be “unifor
m” and “regular”

ground layers
. In this regard, for
Qianjiang River Tunnel
,

Section 7, Section 11, Section 12 and Section 17
(FIG.10)

are
good choices both for the longitudinal and transverse direction according to F
IG
.
1

and F
IG
.
2.
Additionally, the distanc
e
between adjacent
cross passage
s

needs to

7

consider factors such as emergency
evacuation.

C
ONCLUSIONS AND DISCUSSIONS

The aim of this research was to study the seismic response of parallel twin
-
tunnel
with cross passages along the longitudinal direction un
der earthquake shaking, and to
provide advices on reasonable locations of cross passages for Qianjiang River Tunnel.

The following conclusions are obtained based on computational and conceptual
analyses:

Firstly, preventing the local failure of the inters
ections at cross passages
should be

the primary concern in design.

Secondly, the complicated formations of geology impose significant effects on
stability and dynamic responses of the intersections at cross passages.

R
easonable
locations of cross passages
can be decided on according to the inclinations of the
“bed rock”. Another concern about the geological condition is the thick layer of silty
sand at the south bank of the Qianjiang River, which is very prone to liquefaction
under seismic action.
It is the
refore adviced to avoid locating

the cross passage in
this area. Otherwise, measures should be taken to
improve the ground to prevent

the liquefaction.

Genrally speaking, the regular space between cross passages required by evacution
in fire has a
large w
eight

in determing the
distance
of cross passages, but this
factor
alone
is insufficient for
determining the location of
cross passages.
Many
factors
should be considered in determining the location of cross passages.
The optimal
location needs to be set b
ased on a comprehensive evaluation of all the relevant
factors
.

ACKNOWLEDGMENTS

The research funds provided by National Science Foundation of China (NSFC
Grant No. 51078292), Ministry of Transport of China (Grant No. 2009
-
353
-
333
-
340),
Kwang
-
hua Funds for
College of Civil Engineering at Tongji University are
acknowledged.

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