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Stability analysis of a large-span and deep tunnel
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
STABILITY ANALYSIS OF A LARGE-SPAN AND DEEP TUNNEL S. H. Wang 1, J. X. Liu2, C. A. Tang 3, L.C. Li2, X.D. Zhao2 ¹)Center for Rockbursts and Induced Seismicity Research, Northeastern University Key Lab of Conrete Research, Tongji University, Shanghai, P. R. China [email protected] ²) Center for Rockbursts and Induced Seismicity Research, Northeastern University [email protected] 3 ) Center for Rockbursts and Induced Seismicity Research, Northeastern University Key Lab of Conrete Research, Tongji University, Shanghai, , P. R. China
Abstract: The first large-span tunnel in the northern area of Liaoning province is scheduled to be completed in 2003. This tunnel is 460 m in length, 21.242 m in width and 15.52m in height. However, The stability of large-span tunnel is affected by faults or joints located nearby. Hence, a better understanding of the mechanics of influence, especially regarding the risk assessment of faults is required. At the department of geotechnical engineering, the influence of faults on the stability of underground openings has been investigated using numerical methods. In this paper, especially the displacement behavior for different locations of a fault around the large-span opening is discussed. The deformation of the fault after the excavation is also briefly presented. It is hope that this case study will shed some light on future projects of similar nature, and a good reference in the design and construction of similar tunnel engineering projects. Keywords: numerical simulation, large-span and low deep, stability analysis, rock mechanics, design and construction, displacement behavior
1. INTRODUCTION During initial planning of the project of Hanjialing Tunnel, the priority that large-span and low-deep tunnel should be considered, its demolition should be avoided and environmental impact along the construction alignment should be minimized. However, the rock formations in the this area around the Hanjialing tunnel are mostly their lithology, thickness and geologic structure often vary within even a short distance, this fact became the main cause of many cases of design changes as a result of encountering difficult geological conditions. In this present paper, the construction procedure will be first presented, and the counter actions taken and related analyses on the failure mode as well as the construction method adopted following change of design are also presented case history, an insight into design and construction of tunnels that run through high slope with unbalanced ground pressure would be gained for future reference (Chen, 1997).
2 GENERAL DESCRIPTION OF PROJECT 2.1 Engineering layout and design The Hanjialing tunnel passes through the hilly areas around the Dalian City in Liaoning province of china. The tunnel was a four-lane highway tunnel . the ecavation span and height are 21.242 m and 15.52m respectively. The most long distance from the top of tunnel to the ground is 189m.The main primary supports for this tunnel included shotcrete, wire mesh, H-beams and rockbolts. This tunnel is one of the largest-span tunnel in china, due to the complexity of geological conditions and the efflorescent rock on the ground.
2.2 Topography and geology The formation of this tunnel is composed of thick-bedded muddy sandstore, thin alternation of sandstone and shale. The sandstone and shale alternation is less weathered and is not severely stained. However, as a result of squeezing effects from the Dalian thrust fault outside of the project area, shale beds in sandstone and shale alternation
1
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
are transformed into clay seams that may locally reach thickness in excess of 30cm, Furthermore ,the bedding planes are often open, and shear strength is thus low. The beds are overturned ,their attitudes are N42o -50 o E/70o -85o SE. The strike of the beds intersects the tunnel axis and the excavation slopes at a ansgle of 22o -25o . the data in situ is analysised by hemispherical projection, and groups of rock rupture and its major characters sen table1, the figure of Hemispherical projection and Equivalence line are given by Fig.1 and fig.2 respectively. Table1 grouing of rock rupture and its major characters Groups strike direction dip direction dip angle
1
2
3
44
307
305
134
217
35
40
50
78
4 80
5 30
6 338
7 63
8 35
9 359
10 338
350
300
248
153
125
89
68
75
84
86
71
72
74
40
Fig2 Equivalence line figure As a measure in reinforcing the rock mass over the crown of the tunnel, micro piles are constructed in the overburden to increase the self supporting ability of the rock mass. Tunnel excavation should be performed through excavation of side gallery to reduce the area of the cross section and to enhance stability of the excavation face. Primary support should be erected immediately after the excavation, followed by construction of reinforced rock lining. Through all these, deformation of rock mass around the tunnel would be effectively controlled by the rigidity of the lining, and any adverse effects on the stability of the slope would be avoided.
3 NUMERICAL ANALYSIS
Fig1 Hemispherical projection figure This tunnel will have to accommodate traffics from the interchange, and the excavation span of more than 21m, such excessive span under such thin cover are adverse for tunnel construction, as remedial measures, the following construction plans are adapted.
In the present engineering project, several attempts wee made to consolidate adjustments to correct the situation. And some adjustments measures is according numerical analysis results. For understanding the mechanical behavior of the tunnel and the long term stability of tunnel. Detailed numerical analysis was performed on the current construction condition and the forth coming construction procedure to validate the feasibility of the treatment plans. The analysis was performed using RFPA(Tang & Wang).
3.1 Mesoscopic mechanical model In order to simulate the process of excavation, the heterogeneity of mesoscopic structures of rock must be considered and included in the numerical model. Here the failure process simulation is attained when using FEM as the basic stress analysis tool, where the four-node isoparametric
2
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
element is used as the basic element in the finite element mesh, and the elastic damage constitutive relationship of meso-level elements is incorporated in it. In order to reflect the heterogeneity of rock at mesoscopic level, the rock is numerically described with many mesoscopic elements with same size, and the mechanical parameters of these elements in three phases of rock are assumed to be conformed to specific Weibull distribution. This kind of randomness used in the assignment of mechanical properties of elements is quite different from that of stochastic finite element method, because the mechanical parameters of an element are actually definite after the assignment is finished, no probability is incorporated in the finite element analysis (Napier, 1992). The mesoscopic elements also acted as element for finite element analysis, which are assumed homogeneous and isotropic, whose damage evolutions meet with the specific elastic damage constitutive law. In order to reflect the heterogeneity of quasibrittle materials at meso-level, the mechanical parameters of materials, including the Young’s modulus, strength and Poisson’s ratio are assumed to conform to Weibull distribution as defined in the following probability density function: m f (u ) = u0
u u0
m −1
u exp − u0
m
(1)
Where u is the parameter of element (such as strength or elastic modulus); the scale parameter u 0 is related to the average of element parameter and the parameter m defines the shape of the distribution function. The parameter m defines the degree of material homogeneity, is called homogeneity index. According to the definition of Weibull distribution, the value of parameter m must be larger than 1.0. In general, we assumed that Young’s modulus and strength conform to two individual distributions with the same heterogeneity index. The distribution of Poisson’s ratio is not very disperse in reality, therefore, a high homogeneity index of 100 is specified in the following numerical simulations. In previous paper how the homogeneity index affects the macroscopic mechanical response has been discussed and found that the homogeneity index is a very important Weibull distribution parameter to control the macroscopic response of numerical specimen.
Based on the above assumption, we can numerically produce the heterogeneous material using this model, and the material is composed of many mesoscopic elements. Here the mesoscopic element is also acted as the element of finite element analysis. The mesoscopic element is assumed to be isotropic and homogeneous. This heterogeneous material produced by computer is usually used to indicate the real specimen used in the laboratory, so it is called numerical specimen in this investigation. The mesoscopic elements in the specimen must be relatively small enough to reflect the mesoscopic mechanical properties of materials under the conditions that current computer can perform this analysis because the number of mesoscopic elements is substantially limited by the computer capacity. Continuum damage mechanics has proved to be an efficient tool for understanding and describing the structural evolutions, here we use it to describe the mechanical behavior of mesoscopic elements those in rock numerical specimen. In the paper, the material is analyzed at meso-scopic level. At the beginning, the element is considered to be elastic; its elastic properties can be defined by Young’s modulus and Poisson’s ratio. The stressstrain curve of element is considered linear elastic till the given damage threshold is attained, and then is followed by softening. We choose the maximum tensile strain criterion and Mohr-Coulomb criterion respectively as the damage threshold. At any event, the tensile strain criterion is preferential. If the maximum tensile strain criterion is met, the element damages in tensile mode, it will not decide whether the element will damage according to Mohr-Coulomb criterion. Contrariwise, if the element does not damage in tensile mode, we will use Mohr-Coulomb criterion to judge whether the damage of the element occurs in shear mode. It has been proved that the macroscopic mechanical response of rock at macroscopic level can be simulated effectively even if very simple constitutive law (such as elastic-brittle) of mesoscopic element is used. In the present analysis, the parameters used for the rock masses were derived through back calculation of in-situ monitored data. The following rock mechanical parameters were used in this analysis for cases. Intack rock: density, 2670Kg/m3; Elastic modulus, 5000MPa; Poisson’s tatio, 0.25; initialstress ratio, 0.12. The fault:cohesion,3 Kpa; Fiction angle,30; density,2670Kg/m3; Concrete materials:Elastic
3
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
modulus,3000MPa; Poisson’s ratio,0.15; initial stress ratio,0.12.
2.5M Pa
3.2 Numerical simulation result For a better understanding of the effect of faults on the stress distribution and displacement around excavations, a comparison is presented here. Fig. 3 is the stress result and elastic result. From the numerical results (Fig.3), we can conclude that though the stress is to be concentrated during the excavation, it does not cause the failure of wall rock of tunnel. That validates the excavation by step is reasonable. However, after the tunnel is excavated and before the support is applied, failure will occur at the top and bottom of the tunnel when the load reaches 7Mpa, seen more detailed in Fig.4; and when the load comes to 8Mpa, failure of wall rock will be more violent along with the failed area. However, if the corresponding support measure will be taken into account after the tunnel excavated, the strength of wall rock of tunnel will be enhanced. And though the stress will be concentrated at some extent after the support, it could not cause the failure of the support structure, since the confining pressure is not more than the strength of the concrete. Fig. 5 is the numerical result after the support applied. We can conclude that wall rock of the tunnel fails partly when the load reaches 52Mpa, however failure does not occur in the support structure, this validates that the support structure is reasonable. We also made the deformation measure of wall rock on the field before and after the support was applied. The roof subsidence value is 12mm, and becomes 0 under support. Fig. 6 is the displacement curves before and after the support applied obtained by numerical simulation. From Fig.6, the max value of roof subsidence is 8.5mm. The numerical results keep well accordance with the field results.
2MPa
3MPa
7MPa
8MPa
(a)Stress
(b)Elastic modulus
Fig.3 Destruction process analysis by step by step excavation
7MPa
8MPa
Fig.4 AE (Acoustic Emission) of tunnel excavation process
2MPa
4
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
5 ACKNOWLEDGEMENTS
52MP a
(a) Stress
(b) Elastic modulus
Fig.5 the stress analysis of supported tunnel m
8 7
displacement(
6
This research was support by the Chinese National Natural Science Foundation (Grant 59525408, 49974009 and 50174013), the Liaoning Natural Science Foundation of P. R. China (Grant 20021008) and the post-doctor Natural Science Foundation of china (Grant 2001-14-30). The authors wish to thank the colleagues of professor Fusheng Zhu, Shizhong Lu, Professor Bin Liu, for their help in collection of data in site and their support in fieldwork. We are also grateful to the reviewers for their careful reading of our manuscript and their many helpful comments.
5
6 REFERENCES
4 3 2 1 0 1
11 no
21
support
31
41 u n d e r
51
61
s u p p o r t
71
load step
Fig.6 the subsidence curves of roof (obtained by RFPA)
4 CONCLUSIONS AND RECOMMENDATIONS After stability analysis to the whole construction process of Hanjialing Tunnel, we draw conclusion and suggestions as follows: 1) The stability issue of large-span tunnel is the key point to the safe construction. And some helpful numerical simulation is necessary. 2) Preventive measure is always far better than remedial ones. Disasters in tunnel did not occur abruptly; therefore, assessing of the geological conditions and evaluating the failure mechanisms, precise monitoring, and detailed analysis would always unveil omens that harbinger a possible disaster that may be prevented in time. And 3) Tunnel engineering is an engineering project with higher risk factors. It is unavoidable to have mishaps in tunnel contraction. In this view, we should make the best effort in coordination and communication to prevent mishaps from occurring, especially in the large-span and low-deep tunnel. The present project had been confronted with several recurrence of rock instability. However, thanks to cooperation from all parties concerned, the project was completed despite all of the seemingly unsurpassable obstacles.
Chen, W. L. & Lee, S. C. 1997. Stability issues of excavation on slopes: A case study base on Hsintien No.1 Tunnel, Computer Methods and Advances in Geomechanics, Yuan (ed.), 1997 Balkema, Rotterdam, ISBN, 1709-1714 Robina H. C. Rong, Lin P., Chau K. T., Tang C. A. 2000, Experimental study on failure behavior of tunnel in jointed rock mass under excavation. Pacific rock 2000, Girard, Liebman, Breads & Doe (eds), 2000 Balkema, Rotterdam, ISBN, 1211-1216 Napier J. A. L. & Hildyard M. W. 1992. Simulation of fracture growth around openings in highly stressed, brittle rock. Journal of the South African Institute of mining and Metallurgy. 92(6) : 159-168 Tang C. A., Wang S.H., 2003. Numerical Test of Rock Failure, Beijing, Science press, (in Chinese) Wang S. H., 2000. Study of Mechanic al Behaviors in the contraction process of road tunnel [Dissertation], Shenyang, Northeastern University, (in Chinese) Wang S.H., Zhu F.S., Zhang K. and Liu B., 2002. Intelligent decision-making aided system for rock tunnel construction, Chinese Journal Of Rock Mechanics And Engineering, 21(4) : 590594(in Chinese) Wang S.H., Liu B., Liu Z.Y., 1999. Analysis of the self-sustaining structure of surrounding No.1 driving station and its stability, Chinese journal of Mining, Oct., 8(5):84 87(in Chinese) Wang S.H., Zhu W.C., Liu B., Pan K.M., 1999. Development of non-linear FEM program of tensile fracture and its calculation example ,
5
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
Journal of northeastern science), 20(1): 75 78
university(natural
Wang S.H., Tang C. A., Zhu W. C., etc. 2002. Numerical test of fracture characteristics of rock-brick blocks material failure subjected to
biaxial loading, Key Engineering Materials, Dunhuang, 364-371 Wang S. H., Tang C. A., Zhu W. C., etc, 2002. Modeling of mixed-crack propagation and coalescence in rock, New Development In Rock Mechanics and Rock Engineering, Rinton Press, 337-341
6
STABILITY ANALYSIS OF A LARGE-SPAN AND DEEP TUNNEL S. H. Wang 1, J. X. Liu2, C. A. Tang 3, L.C. Li2, X.D. Zhao2 ¹)Center for Rockbursts and Induced Seismicity Research, Northeastern University Key Lab of Conrete Research, Tongji University, Shanghai, P. R. China [email protected] ²) Center for Rockbursts and Induced Seismicity Research, Northeastern University [email protected] 3 ) Center for Rockbursts and Induced Seismicity Research, Northeastern University Key Lab of Conrete Research, Tongji University, Shanghai, , P. R. China
Abstract: The first large-span tunnel in the northern area of Liaoning province is scheduled to be completed in 2003. This tunnel is 460 m in length, 21.242 m in width and 15.52m in height. However, The stability of large-span tunnel is affected by faults or joints located nearby. Hence, a better understanding of the mechanics of influence, especially regarding the risk assessment of faults is required. At the department of geotechnical engineering, the influence of faults on the stability of underground openings has been investigated using numerical methods. In this paper, especially the displacement behavior for different locations of a fault around the large-span opening is discussed. The deformation of the fault after the excavation is also briefly presented. It is hope that this case study will shed some light on future projects of similar nature, and a good reference in the design and construction of similar tunnel engineering projects. Keywords: numerical simulation, large-span and low deep, stability analysis, rock mechanics, design and construction, displacement behavior
1. INTRODUCTION During initial planning of the project of Hanjialing Tunnel, the priority that large-span and low-deep tunnel should be considered, its demolition should be avoided and environmental impact along the construction alignment should be minimized. However, the rock formations in the this area around the Hanjialing tunnel are mostly their lithology, thickness and geologic structure often vary within even a short distance, this fact became the main cause of many cases of design changes as a result of encountering difficult geological conditions. In this present paper, the construction procedure will be first presented, and the counter actions taken and related analyses on the failure mode as well as the construction method adopted following change of design are also presented case history, an insight into design and construction of tunnels that run through high slope with unbalanced ground pressure would be gained for future reference (Chen, 1997).
2 GENERAL DESCRIPTION OF PROJECT 2.1 Engineering layout and design The Hanjialing tunnel passes through the hilly areas around the Dalian City in Liaoning province of china. The tunnel was a four-lane highway tunnel . the ecavation span and height are 21.242 m and 15.52m respectively. The most long distance from the top of tunnel to the ground is 189m.The main primary supports for this tunnel included shotcrete, wire mesh, H-beams and rockbolts. This tunnel is one of the largest-span tunnel in china, due to the complexity of geological conditions and the efflorescent rock on the ground.
2.2 Topography and geology The formation of this tunnel is composed of thick-bedded muddy sandstore, thin alternation of sandstone and shale. The sandstone and shale alternation is less weathered and is not severely stained. However, as a result of squeezing effects from the Dalian thrust fault outside of the project area, shale beds in sandstone and shale alternation
1
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
are transformed into clay seams that may locally reach thickness in excess of 30cm, Furthermore ,the bedding planes are often open, and shear strength is thus low. The beds are overturned ,their attitudes are N42o -50 o E/70o -85o SE. The strike of the beds intersects the tunnel axis and the excavation slopes at a ansgle of 22o -25o . the data in situ is analysised by hemispherical projection, and groups of rock rupture and its major characters sen table1, the figure of Hemispherical projection and Equivalence line are given by Fig.1 and fig.2 respectively. Table1 grouing of rock rupture and its major characters Groups strike direction dip direction dip angle
1
2
3
44
307
305
134
217
35
40
50
78
4 80
5 30
6 338
7 63
8 35
9 359
10 338
350
300
248
153
125
89
68
75
84
86
71
72
74
40
Fig2 Equivalence line figure As a measure in reinforcing the rock mass over the crown of the tunnel, micro piles are constructed in the overburden to increase the self supporting ability of the rock mass. Tunnel excavation should be performed through excavation of side gallery to reduce the area of the cross section and to enhance stability of the excavation face. Primary support should be erected immediately after the excavation, followed by construction of reinforced rock lining. Through all these, deformation of rock mass around the tunnel would be effectively controlled by the rigidity of the lining, and any adverse effects on the stability of the slope would be avoided.
3 NUMERICAL ANALYSIS
Fig1 Hemispherical projection figure This tunnel will have to accommodate traffics from the interchange, and the excavation span of more than 21m, such excessive span under such thin cover are adverse for tunnel construction, as remedial measures, the following construction plans are adapted.
In the present engineering project, several attempts wee made to consolidate adjustments to correct the situation. And some adjustments measures is according numerical analysis results. For understanding the mechanical behavior of the tunnel and the long term stability of tunnel. Detailed numerical analysis was performed on the current construction condition and the forth coming construction procedure to validate the feasibility of the treatment plans. The analysis was performed using RFPA(Tang & Wang).
3.1 Mesoscopic mechanical model In order to simulate the process of excavation, the heterogeneity of mesoscopic structures of rock must be considered and included in the numerical model. Here the failure process simulation is attained when using FEM as the basic stress analysis tool, where the four-node isoparametric
2
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
element is used as the basic element in the finite element mesh, and the elastic damage constitutive relationship of meso-level elements is incorporated in it. In order to reflect the heterogeneity of rock at mesoscopic level, the rock is numerically described with many mesoscopic elements with same size, and the mechanical parameters of these elements in three phases of rock are assumed to be conformed to specific Weibull distribution. This kind of randomness used in the assignment of mechanical properties of elements is quite different from that of stochastic finite element method, because the mechanical parameters of an element are actually definite after the assignment is finished, no probability is incorporated in the finite element analysis (Napier, 1992). The mesoscopic elements also acted as element for finite element analysis, which are assumed homogeneous and isotropic, whose damage evolutions meet with the specific elastic damage constitutive law. In order to reflect the heterogeneity of quasibrittle materials at meso-level, the mechanical parameters of materials, including the Young’s modulus, strength and Poisson’s ratio are assumed to conform to Weibull distribution as defined in the following probability density function: m f (u ) = u0
u u0
m −1
u exp − u0
m
(1)
Where u is the parameter of element (such as strength or elastic modulus); the scale parameter u 0 is related to the average of element parameter and the parameter m defines the shape of the distribution function. The parameter m defines the degree of material homogeneity, is called homogeneity index. According to the definition of Weibull distribution, the value of parameter m must be larger than 1.0. In general, we assumed that Young’s modulus and strength conform to two individual distributions with the same heterogeneity index. The distribution of Poisson’s ratio is not very disperse in reality, therefore, a high homogeneity index of 100 is specified in the following numerical simulations. In previous paper how the homogeneity index affects the macroscopic mechanical response has been discussed and found that the homogeneity index is a very important Weibull distribution parameter to control the macroscopic response of numerical specimen.
Based on the above assumption, we can numerically produce the heterogeneous material using this model, and the material is composed of many mesoscopic elements. Here the mesoscopic element is also acted as the element of finite element analysis. The mesoscopic element is assumed to be isotropic and homogeneous. This heterogeneous material produced by computer is usually used to indicate the real specimen used in the laboratory, so it is called numerical specimen in this investigation. The mesoscopic elements in the specimen must be relatively small enough to reflect the mesoscopic mechanical properties of materials under the conditions that current computer can perform this analysis because the number of mesoscopic elements is substantially limited by the computer capacity. Continuum damage mechanics has proved to be an efficient tool for understanding and describing the structural evolutions, here we use it to describe the mechanical behavior of mesoscopic elements those in rock numerical specimen. In the paper, the material is analyzed at meso-scopic level. At the beginning, the element is considered to be elastic; its elastic properties can be defined by Young’s modulus and Poisson’s ratio. The stressstrain curve of element is considered linear elastic till the given damage threshold is attained, and then is followed by softening. We choose the maximum tensile strain criterion and Mohr-Coulomb criterion respectively as the damage threshold. At any event, the tensile strain criterion is preferential. If the maximum tensile strain criterion is met, the element damages in tensile mode, it will not decide whether the element will damage according to Mohr-Coulomb criterion. Contrariwise, if the element does not damage in tensile mode, we will use Mohr-Coulomb criterion to judge whether the damage of the element occurs in shear mode. It has been proved that the macroscopic mechanical response of rock at macroscopic level can be simulated effectively even if very simple constitutive law (such as elastic-brittle) of mesoscopic element is used. In the present analysis, the parameters used for the rock masses were derived through back calculation of in-situ monitored data. The following rock mechanical parameters were used in this analysis for cases. Intack rock: density, 2670Kg/m3; Elastic modulus, 5000MPa; Poisson’s tatio, 0.25; initialstress ratio, 0.12. The fault:cohesion,3 Kpa; Fiction angle,30; density,2670Kg/m3; Concrete materials:Elastic
3
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
modulus,3000MPa; Poisson’s ratio,0.15; initial stress ratio,0.12.
2.5M Pa
3.2 Numerical simulation result For a better understanding of the effect of faults on the stress distribution and displacement around excavations, a comparison is presented here. Fig. 3 is the stress result and elastic result. From the numerical results (Fig.3), we can conclude that though the stress is to be concentrated during the excavation, it does not cause the failure of wall rock of tunnel. That validates the excavation by step is reasonable. However, after the tunnel is excavated and before the support is applied, failure will occur at the top and bottom of the tunnel when the load reaches 7Mpa, seen more detailed in Fig.4; and when the load comes to 8Mpa, failure of wall rock will be more violent along with the failed area. However, if the corresponding support measure will be taken into account after the tunnel excavated, the strength of wall rock of tunnel will be enhanced. And though the stress will be concentrated at some extent after the support, it could not cause the failure of the support structure, since the confining pressure is not more than the strength of the concrete. Fig. 5 is the numerical result after the support applied. We can conclude that wall rock of the tunnel fails partly when the load reaches 52Mpa, however failure does not occur in the support structure, this validates that the support structure is reasonable. We also made the deformation measure of wall rock on the field before and after the support was applied. The roof subsidence value is 12mm, and becomes 0 under support. Fig. 6 is the displacement curves before and after the support applied obtained by numerical simulation. From Fig.6, the max value of roof subsidence is 8.5mm. The numerical results keep well accordance with the field results.
2MPa
3MPa
7MPa
8MPa
(a)Stress
(b)Elastic modulus
Fig.3 Destruction process analysis by step by step excavation
7MPa
8MPa
Fig.4 AE (Acoustic Emission) of tunnel excavation process
2MPa
4
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
5 ACKNOWLEDGEMENTS
52MP a
(a) Stress
(b) Elastic modulus
Fig.5 the stress analysis of supported tunnel m
8 7
displacement(
6
This research was support by the Chinese National Natural Science Foundation (Grant 59525408, 49974009 and 50174013), the Liaoning Natural Science Foundation of P. R. China (Grant 20021008) and the post-doctor Natural Science Foundation of china (Grant 2001-14-30). The authors wish to thank the colleagues of professor Fusheng Zhu, Shizhong Lu, Professor Bin Liu, for their help in collection of data in site and their support in fieldwork. We are also grateful to the reviewers for their careful reading of our manuscript and their many helpful comments.
5
6 REFERENCES
4 3 2 1 0 1
11 no
21
support
31
41 u n d e r
51
61
s u p p o r t
71
load step
Fig.6 the subsidence curves of roof (obtained by RFPA)
4 CONCLUSIONS AND RECOMMENDATIONS After stability analysis to the whole construction process of Hanjialing Tunnel, we draw conclusion and suggestions as follows: 1) The stability issue of large-span tunnel is the key point to the safe construction. And some helpful numerical simulation is necessary. 2) Preventive measure is always far better than remedial ones. Disasters in tunnel did not occur abruptly; therefore, assessing of the geological conditions and evaluating the failure mechanisms, precise monitoring, and detailed analysis would always unveil omens that harbinger a possible disaster that may be prevented in time. And 3) Tunnel engineering is an engineering project with higher risk factors. It is unavoidable to have mishaps in tunnel contraction. In this view, we should make the best effort in coordination and communication to prevent mishaps from occurring, especially in the large-span and low-deep tunnel. The present project had been confronted with several recurrence of rock instability. However, thanks to cooperation from all parties concerned, the project was completed despite all of the seemingly unsurpassable obstacles.
Chen, W. L. & Lee, S. C. 1997. Stability issues of excavation on slopes: A case study base on Hsintien No.1 Tunnel, Computer Methods and Advances in Geomechanics, Yuan (ed.), 1997 Balkema, Rotterdam, ISBN, 1709-1714 Robina H. C. Rong, Lin P., Chau K. T., Tang C. A. 2000, Experimental study on failure behavior of tunnel in jointed rock mass under excavation. Pacific rock 2000, Girard, Liebman, Breads & Doe (eds), 2000 Balkema, Rotterdam, ISBN, 1211-1216 Napier J. A. L. & Hildyard M. W. 1992. Simulation of fracture growth around openings in highly stressed, brittle rock. Journal of the South African Institute of mining and Metallurgy. 92(6) : 159-168 Tang C. A., Wang S.H., 2003. Numerical Test of Rock Failure, Beijing, Science press, (in Chinese) Wang S. H., 2000. Study of Mechanic al Behaviors in the contraction process of road tunnel [Dissertation], Shenyang, Northeastern University, (in Chinese) Wang S.H., Zhu F.S., Zhang K. and Liu B., 2002. Intelligent decision-making aided system for rock tunnel construction, Chinese Journal Of Rock Mechanics And Engineering, 21(4) : 590594(in Chinese) Wang S.H., Liu B., Liu Z.Y., 1999. Analysis of the self-sustaining structure of surrounding No.1 driving station and its stability, Chinese journal of Mining, Oct., 8(5):84 87(in Chinese) Wang S.H., Zhu W.C., Liu B., Pan K.M., 1999. Development of non-linear FEM program of tensile fracture and its calculation example ,
5
Paper 3B 17 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.
Journal of northeastern science), 20(1): 75 78
university(natural
Wang S.H., Tang C. A., Zhu W. C., etc. 2002. Numerical test of fracture characteristics of rock-brick blocks material failure subjected to
biaxial loading, Key Engineering Materials, Dunhuang, 364-371 Wang S. H., Tang C. A., Zhu W. C., etc, 2002. Modeling of mixed-crack propagation and coalescence in rock, New Development In Rock Mechanics and Rock Engineering, Rinton Press, 337-341
6