Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels SHANG Kejian

Physical and Numerical Simulation of Geotechnical Engineering
2nd ISSUE, March 2011
Analysis of Influence of Deep Rock Excavation on Adjacent
Tunnels
SHANG Kejian1, JIANG Zhaohua2, ZHOU Lin3
1. Wuhan Polytechnic University, Wuhan 430023, Hubei, China
2. College of Civil Engineering, Chongqing University, Chongqing 400030, China
3. China Communications the Second Highway Consultants, Wuhan 430056, China
ABSTRACT：According to the practical engineering of rock deep excavation at depths about 30 m
adjacent to the large span light railway tunnel, the finite difference numerical model is established for
the foundation pit project. Reserved rock wall thickness and foundation pit supporting scheme by
stages have been presented to protect the adjacent existing tunnel. Different supporting construction
methods are simulated by the numerical method. The Fish program of point safety factor for tensile
criterion of rock mass is compiled. The results indicate that foundation pit supporting scheme method
by stages can effectively control the deformation of the adjacent existing tunnel.
KEYWORDS: tunnel, foundation excavation, two stages supporting, point safety factor, numerical
modelling
1 INTRODUCTION
With the development and utilization of underground
space, the number of deep excavation has become more and
more. There are structures around the foundation before the
excavation, and many of the deep pits are close to the light
rail tunnel [1-2].
The excavation would break the original stress
equilibrium, making the rock stress re-distribution and
leading to internal forces and deformation. Light rail tunnel
should meet the requirements of strength and deformation,
especially deformation control requirements so as to keep
safety of the Tunnel. Thus its impact on the adjacent tunnel
should be considered during excavation process. A
reasonable choice of foundation and excavation support
structure should be selected.
In recent years, many scholars at home and abroad has
studied this issue, mainly in the following aspects. A
method is presented for estimating the maximum bending
moment for continuous or rigidly jointed pipelines affected
by tunnel-induced ground movement[3-4]. The estimation
can be made based on the knowledge of tunnel and pipeline
geometries, the stiffness of soil and pipeline, and
tunnel-induced ground deformation at the pipeline level.
The method takes account of soil nonlinearity by an
equivalent linear approach. The approach is conservative
and promises that the bending moment is not
underestimated. The validity of the method as an upper
bound approximation is evaluated against centrifuge test
results
Combined with the practical engineering problem of
tunnel excavation of Shenzhen metro by non-water
lowering with adjacent pipeline, the construction scheme is
illustrated; and evaluation standards of buried pipelines are
given. Firstly, the effect of
interval tunnel
excavation on the buried pipelines is simulated
by means of centrifugal model test. Secondly, the
© St. PLUM-BLOSSOM PRESS PTY LTD
3-D FEM coupling analysis model of tunnel pipeline is
established.
Deformation
and internal force
of
buried
pipeline
are
analyzed
specially
by
numerical simulation of tunnel excavation and
also the safety of the pipeline is forecasted. The
rationality
and
reliability
are
proved
by
comparing the results of centrifugal model test,
numerical simulation and in-site measurement. Theoretical
base and guidance are provided for real engineering and
some meaningful results are achieved [5].
Engineering activities are inevitable in the urban region
adjacent to the current metro tunnel with the continuous
city development and increased needs for land of building.
But the deep excavation of the foundation pit adjacent to
the tunnel will change the stress state of the neighboring
soil. And the deformation of tunnel will be induced which
cause impair to the daily operation and the safety of tunnel.
According to the practice of deep excavation neighboring
the tunnel in Shanghai，the application of plastic finite
element method with Mohr-Coulomb model to simulate
different construction stage of deep pit excavation is
presented. The results show good agreement with the field
test data which show that the finite element method can
provide good simulation for this kind of engineering
activity and provide convincing computing results for the
design and on striation. Different construction method，
furthermore，is simulated by the numerical method and the
calculated results suggest that the rational excavation
method will reduce the negative influence of the tunnel and
ensure the success of the whole project [6].
Therefore, this paper is on deep excavation of a rock
near the tunnel. It’s different from other general excavation.
The main features of excavation are depth; large span
tunnels near the station, in addition to the excavation of
rock blasting ensure the integrity of rock itself, minimizing
damage or weakening the surrounding rock strength. For
these reasons, this paper establishes a three-dimensional
Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels
DOI: 10. 5503/J. PNSGE. 2010. 02.016
elastic-plastic model, proposes support structure foundation
program in phases, compares analysis of different
alternatives and calculate the displacement and the point
safety factor, in order to optimize the design and
construction of similar projects to provide useful
Reference.
pile reverse for the whole building for the underground
structure along , K pile section size is 2m × 3.5m and
length is designed to 36.5m, with 3 rows of 11φ15.2 cable,
cable tensile force is 820KN,it is shown in Figure 2.
Excavation scheme 1: In order to protect the tunnel and
facilitate the support piles, construction of retaining
structures can reduce the effect of excavation blasting on
rock. Supporting scheme in phases is put forward. Inverse
excavation, construction of underground structure along for
the last order with H piles, the upper reserved rock wall
thickness will set to 8m. That is to say, the distance is edges
foundation and the outer edge of the second tunnel lining.
Retaining pile section size is 1m × 1.8m, pile length is 22m.
The down stairs is the pile anchor, then the rock wall
thickness set aside 11m, I-pile section is 1m × 1m, pile
length is 17m, and 3 rows of the lower pile 11φ15.2 cable,
cable tensile force is 820KN, shown in Figure 3.
2 PROJECT OVERVIEW
Foundation is located in central area of southwest, the
upper part is for the square and the lower part is for the
underground structures, which the number of layers is for
the 5-6F.The excavation of foundation pit will form about
30m rock slope. The slope A1-A2 part is adjacent to the
tunnel, as shown in Figure 1, the width of the tunnels is
21.08m, and height is 20.2m. It is key issues for tunnel
safety to build a reasonable reserve rock wall thickness and
a reasonable support structure approach. Model calculation
parameters are shown in Table 1..
excavation
foundation pit
light railway
rock wall
lig
ht
rai
lwa
y
Fig.2 The first supporting scheme of excavation
Fig.1 Relative position of tunnel and excavation
excavation
Table 1 Rock mass parameters for computation
layer
thick
/m
γ
3
/kN/m
C

E
/ MPa
/°
/ MPa
ν
light railway
excavation
1
23
24.62
2.09
39.2
2800
0.08
2
32
25.66
0.8
33.4
1500
0.34
Fig.3 The second supporting scheme of excavation
3 EXCAVATION SUPPORT SCHEME
4 ANALYSIS OF THE TUNNEL EXCAVATION
IMPACT
Excavation scheme 1: The pile anchor program, the rock
wall thickness between the excavation outer edge and
adjacent buildings lining is set to 8m, excavation anchor
When there is the existing structure around foundation
pit, the initial stress field is calculated, and then excavation
79
Physical and Numerical Simulation of Geotechnical Engineering
2nd ISSUE, March 2011
and support is simulated. To simplify the analysis, the A-A
section of Figure 1, the thickness distance is between piles,
the bottom is all constraints, both ends is the horizontal
constraints, the computational domain is 70m × 55m × 4m,
a total of 14475 nodes, 11148 elements are shown in Figure
4-5. The thickness of secondary Lining is 800mm with a
C30 concrete, without considering the anchor .To monitor
the displacement of the tunnel, respectively, arch foot wall,
the bottom center of the observation points is monitored
during the calculation process, in Figure 5. Supporting piles
and anchor of the calculated parameters is shown in Table 2,
Table 3.
Table 2 prestressed anchor cable parameters
E
/Pa
A
2
/( m )
Y
gr_per
gr_coh
/ Pa
/m
/ N.m-1
2.1E
1.52E
1.86
11
-3
E9
0.53
2.5E4
Fig.5 Calculation monitoring point of tunnel liner
4.1 Supporting schemes displacement analysis
gr_k
Comparison of the two programs in the tunnel
excavation monitoring displacement, the larger horizontal
displacement is calculated in arch foot point 2, the midpoint
of the wall 4, the displacement are 3.5mm and 2.8mm.The
results can be seen that the horizontal displacement is the
larger point of the bottom wall of the center point of arch
foot 6 and 2, displacement, respectively 2.4mm,
2.0mm;crown point a displacement of 1.4mm; maximum
additional displacement of the dome is point 1, the
displacement is 0.3mm. Through the above analysis it is not
difficult to see the program in stages 2, the displacement of
supporting can control the tunnel well, and the maximum
horizontal displacement and vertical displacement is
smaller than scheme, the displacement gradient small. In
figure 6-7, point 4 displacement gradient is larger than flat
displacement scheme 2. In addition, pit blasting process,
due to the program reserved for rock wall thickness of 2
large, can reduce the damage or weaken the surrounding
rock. At process of excavation, the tunnel runs in good
condition.
3.5E9
Table 3 retaining pile parameters
pile
E
ν
A
Iy
2
Iz
/m
/ m4
0.2
0.15
0.486
1
0.2
0.0833
0.0833
7
0.2
2.333
7.146
/Pa
/( m )
H
3.0E10
1.8
I
3.0E10
K
3.0E10
4
4
3.5
点1
点5
点4
点6
点3
点2
displacement/mm
3
2.5
2
1.5
1
0.5
0
4
Fig.4 Flac3D model
7
12
16
19
Excavation depth/m
24
30
Fig.6 X-displacement of the first method monitoring
point
80
Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels
DOI: 10. 5503/J. PNSGE. 2010. 02.016
2.5
点1
点5
点4
点6
点3
点2
displacement/mm
2
1.5
1
0.5
0
4
7
12
16
19
Excavation depth/m
24
30
Fig.7 X-displacement of the second method monitoring
point
Fig.9 Contours of z-displacement of the model
In Figure 8-9, the crown point appear a larger horizontal
displacement gradient, that is to say, the depth of the tunnel
excavation are equal, when the excavation of the 12m or so.
Open pit depth of the tunnel is at the very arch, arch
horizontal displacement of point 2 feet is larger gradient,
when the Ministry of excavation in the end wall of the
central arch 2 and 4 points higher displacement gradient,
vertical displacement is relatively stable of the excavation
in the end. The crown displacement is large and more
sensitive by the excavation. Gradient conditions in the
location of the displacement are large and easy to
strengthen the tunnel displacement monitoring.
In figure 8-9, it can be seen that rock moved out after
excavation, the horizontal and vertical displacement appear.
Horizontal displacement is larger than vertical
displacement. The tunnel deformation appears mainly in
the central vertical wall lining, the horizontal displacement
of tunnel is greater than the additional vertical displacement,
and the deformation don’t exceed the required rail tunnel
control displacement. Foundation itself, the next steps of
the maximum horizontal displacement of additional rock is
at the middle, because of the vertical displacement of
unloading rebound.
Fig.10 Point safety factor
4.2 Point safety factor calculation
Hooke took the point safety concept safety in the slope
for the analysis of stability. Point safety factor can be
described stability of each unit, can quantitatively assess
the degree of cell close to the plastic yield, the slope factor
of safety usually only reflect the overall stability of the
slope[7-8]. However, the current forms generally consider
shear stress failure. Tensile failures often happen in lower
tensile strength of rock mass characteristics. Based on the
above view about the definition of safety factor, point
safety factor in Mohr-coulomb yield criterion can be
defined combined with Flac3D.
Fs 
c cos  
1   3
2
sin 
1   3
2
(1)
Where c,  ,  1 ,  3 , is the cohesion, internal friction angle,
maximum and minimum principal stress for the rock.
The point safety factor of tensile failures
Fig.8 Contours of x-displacement of the model
81
Physical and Numerical Simulation of Geotechnical Engineering
2nd ISSUE, March 2011
Fs 
t
3
REFERENCES
(2)
[1].
Where,  t is rock mass tensile strength.
By comparing the point safety factor of shear stress and
principal tensile stress, the smaller values is safety factor as
the point. The point safety factor is compiled by Flac3D
Fish.
It is shown that a program of two phases Tunnel
excavation in figure 10, the point safety factor is greater
than 1. The calculation results of plastic zone does not
appear, indicating that the two calculation methods are
consistent and rock slope is stable.
[2].
[3].
[4].
[5].
5 CONCLUSION
[6].
Analysis of influence of deep rock excavation on
adjacent tunnels and comparison of different support
schemes, the conclusions is as following:
(1) the horizontal displacement of the tunnel deformation
is relatively large. Phased Support scheme can effectively
control the tunnel displacement. Reserved rock wall by
stages have been presented to protect the tunnel.
(2) According to the definition of the point safety factor,
tensile strength of rock mass calculation Fish program is
compiled based on the modified Mohr-coulomb model.
[7].
[8].
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