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Seismic Design of Tunnels - Parsons Brinckerhoff

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3.3 Free-Field Axial and Curvature Deformations 31<br />

Background 31<br />

A Practical Approach to Describing Ground Behavior 31<br />

Simplified Equations for Axial Strains and Curvature 33<br />

3.4 <strong>Design</strong> Conforming to Free-Field Axial and Curvature Deformations 35<br />

Background and Assumptions 35<br />

<strong>Design</strong> Example 1: The Los Angeles Metro 35<br />

Applicability <strong>of</strong> the Free-Field Deformation Approach 37<br />

3.5 Tunnel-Ground Interaction 37<br />

Simplified Interaction Equations 38<br />

<strong>Design</strong> Example 2: A Linear Tunnel in S<strong>of</strong>t Ground 43<br />

3.6 Special Considerations 48<br />

Unstable Ground 48<br />

Faulting 48<br />

Abrupt Changes in Structural Stiffness or Ground Conditions 49<br />

4.0 Ovaling Effect on Circular <strong>Tunnels</strong> 53<br />

4.1 Ovaling Effect 55<br />

4.2 Free-Field Shear Deformations 55<br />

Simplified Equation for Shear Deformations 56<br />

4.3 Lining Conforming to Free-Field Shear Deformations 58<br />

4.4 Importance <strong>of</strong> Lining Stiffness 60<br />

Compressibility and Flexibility Ratios 60<br />

Example 1 61<br />

Example 2 62<br />

Summary and Conclusions 63<br />

4.5 Lining-Ground Interaction 64<br />

Closed Form Solutions 64<br />

Numerical Analysis 76<br />

Results and Recommendations 76<br />

5.0 Racking Effect on Rectangular <strong>Tunnels</strong> 83<br />

5.1 General 85<br />

5.2 Racking Effect 86<br />

5.3 Dynamic Earth Pressure Methods 87<br />

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