Seismic Design of Tunnels - Parsons Brinckerhoff

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3.6 Special Considerations Through the design examples 1 and 2 presented above, it was demonstrated that under normal conditions the axial and curvature strains of the ground were not critical to the design of horizontally aligned linear tunnels. Special attention is required, however, in the following situations: • Unstable ground, including ground that is susceptible to landslide and/or liquefaction • Faulting, including tectonic uplift and subsidence • Abrupt changes in structural stiffness or ground conditions Unstable Ground It is generally not feasible to design a tunnel lining of sufficient strength to resist large permanent ground deformations resulting from an unstable ground. Therefore, the proper design measures in dealing with this problem should consider the following: • Ground stabilization (e.g., compaction, draining, reinforcement, grouting, and earth retaining systems) • Removal and replacement of problem soils • Reroute or deeper burial Faulting With regard to fault displacements, the best solution is to avoid any potential crossing of active faults. If this is not possible, the general design philosophy is to design a tunnel structure to accept and accommodate these fault displacements. For example, in the North Outfall Replacement Sewer (NORS, City of Los Angeles) project, the amount of fault displacement associated with an M=6.5 design earthquake on the Newport-Inglewood fault was estimated to be about 8 inches at the crossing. To accommodate this displacement, a design scheme using an oversized excavation and a compressible backpacking material was provided. The backpacking material was designed to withstand the static loads, yet be crushable under faulting movements to protect the pipe. 48
It is believed that the only transportation tunnel in the U.S. designed and constructed to take into consideration potential active fault displacements is the Berkeley Hills Tunnel, part of the San Francisco BART system. This horse-shoe-shaped tunnel was driven through the historically active creeping Hayward Fault with a one-foot oversized excavation. The purpose of the over-excavation was to provide adequate clearance for rail passage even when the tunnel was distorted by the creeping displacements. Thus rails in this section could be realigned and train services could be resumed quickly afterward. The tunnel was lined with concrete encased ductile steel ribs on two-foot centers. The concrete encased steel rib lining is particularly suitable for this design because it provides sufficient ductility to accommodate the lining distortions with little strength degradation. The two projects described above have several common design assumptions that allowed the special design to be feasible both technically and economically: • The locations of the faults at crossings can be identified with acceptable uncertainty, limiting the lengths of the structures that require such special design. • The design fault displacements are limited to be within one foot. The cost associated with special design may become excessively high when significant uncertainty exists in defining the activities and locations of the fault crossings, or when the design fault displacements become large (e.g., five feet). Faced with these situations, designers as well as owners should re-evaluate and determine the performance goals of the structures based on a risk-cost balanced consideration, and design should be carried out accordingly. Abrupt Changes in Structural Stiffness or Ground Conditions These conditions include, but are not limited to, the following: • When a regular tunnel section is connected to a station end wall or a rigid, massive structure such as a ventilation building • At the junctions of tunnels • When a tunnel traverses two distinct geological media with sharp contrast in stiffness • When tunnels are locally restrained from movements by any means (i.e., “hard spots”) 49
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1991 William Barclay Parsons Fellow
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CONTENTS Foreword ix 1.0 Introducti
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Mononobe-Okabe Method 87 Wood Metho
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Figure Title Page 20 Typical Free-F
Page 9 and 10: LIST OF TABLES Table Title Page 1 F
Page 11 and 12: FOREWORD For more than a century, P
Page 13: 1.0 INTRODUCTION 1
Page 16 and 17: 1.2 Scope of this Study The work pe
Page 18 and 19: Figure 1. Ground Response to Seismi
Page 20 and 21: Ground Failure Ground failure broad
Page 22 and 23: - Formation of plastic hinges at th
Page 24 and 25: • The effects of overburden depth
Page 27 and 28: 2.0 SEISMIC DESIGN PHILOSOPHY FOR T
Page 29 and 30: 2.3 Seismic Design Philosophies for
Page 31 and 32: 2.4 Proposed Seismic Design Philoso
Page 33 and 34: expressed in terms of internal mome
Page 35: Comments on Loading Combinations fo
Page 39 and 40: 3.0 RUNNING LINE TUNNEL DESIGN 3.1
Page 41 and 42: Ovaling or Racking Deformations The
Page 43 and 44: 3.3 Free-Field Axial and Curvature
Page 45 and 46: Simplified Equations for Axial Stra
Page 47 and 48: 3.4 Design Conforming to Free-Field
Page 49 and 50: Applicability of the Free-Field Def
Page 51 and 52: Figure 6. Sectional Forces Due to C
Page 53 and 54: - In the JSCE (Japanese Society of
Page 55 and 56: Design Example 2: A Linear Tunnel i
Page 57 and 58: 5. Derive the ground displacement a
Page 59: 10. Calculate the allowable shear s
Page 63: Based on Equations 3-7 and 3-8, the
Page 67 and 68: 4.0 OVALING EFFECT ON CIRCULAR TUNN
Page 69 and 70: Figure 7. Free-Field Shear Distorti
Page 71 and 72: Figure 8. Free-Field Shear Distorti
Page 73 and 74: R = radius of the tunnel lining t =
Page 75 and 76: Lining Properties Soil Properties R
Page 77 and 78: The expressions of these lining res
Page 79 and 80: Figure 11. Lining Response Coeffici
Page 81 and 82: Thrust Response Coefficient, K 2 Fi
Page 83 and 84: Thrust Response Coefficient, K 2 Fi
Page 85 and 86: Figure 15. Normalized Lining Deflec
Page 87 and 88: Figure 17. Finite Difference Mesh (
Page 89 and 90: Table 2. Cases Analyzed by Finite D
Page 91 and 92: maximum bending moment than the no-
Page 93 and 94: Table 3. Influence of Interface Con
Page 95: 5.0 RACKING EFFECT ON RECTANGULAR T
Page 98 and 99: Third, typically soil is backfilled
Page 100 and 101: thrust that is approximately 1.5 to
Page 102 and 103: San Francisco BART In his pioneerin
Page 104 and 105: general, flexibility can be achieve
Page 106 and 107: • 254 ft/sec for case I • 415 f
Page 108 and 109: locations of roof and invert are ap
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Figure 25. Structure Deformations v
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Factors Contributing to the Soil-St
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As Figures 28A and 28B show, earthq
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Figure 28A. West Coast Earthquake A
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Figure 29. Design Response Spectra
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Figure 31. Relative Stiffness Betwe
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developed for a one-barrel frame wi
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• For flexibility ratios less tha
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where g s = angular distortion of t
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Structure Deformation Free-Field De
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Because the Poisson’s Ratios of t
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Table 5. Cases Analyzed to Study th
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In comparison with the results show
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Structure Deformation Free-Field De
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• Pseudo-Concentrated Force Model
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deformations result. Section 2.4 in
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Figure 40. Moments at Invert-Wall C
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Figure 42. Moments at Invert-Wall C
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Table 7. Seismic Racking Design App
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136
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• The more frequently occurring e
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Use of this method will lead to a c
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142
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Duddeck, H. and Erdman, J., “Stru
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Tunnels, Report No. UMTA-MA-06-0100
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Seismic Design of Tunnels - Parsons Brinckerhoff

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