Seismic design of tunnel

The seismic design techniques of surface structures consider inertial effect of structure itself, however this is not the case for underground structure.

Seismic response of structure is dominated by surrounding ground response.

Historically underground facilities experienced a lower rate of damage than surface structure when exposed to seismic load. Regardless underground structures should be designed to withstand seismic and static loading. 

Research shows that in surface structure, acceleration is high, but moment is low, but in case of underground structure acceleration is low but moment is high.

Types of tunnels

a. Bored tunnel

b. Cut and cover tunnel

c. Immersed tube tunnel

Impact of earthquake on underground structure

a. Ground shaking-----------Ground deformation----

b. Ground failure (Liquefaction, slope instability, fault displacement)

 

Types of deformation

1. Axial compression and extension

Generated by component of seismic waves that move parallel to the axis of tunnel

2. Longitudinal Bending

      Generated by component of seismic waves that produce motion perpendicular to the axis of tunnel.

3. Ovaling/Racking

Takes place where, shear wave propagates normal or nearly normal to tunnel axis, distorting cross section of tunnel lining.

 

1. Seismic Hazard Analysis

According to extent of active fault in region, potential for fault motion, frequency with which fault releases stored energy.

Using deterministic SHA (seismic hazard analysis)- certain factors are considered

Using Probabilistic SHA- Uncertainties in size, location, recurrence factors are considered.

2. Design of earthquake criteria

Maximum design earthquake (priority is that public safety be guaranteed during and after design event)

Operational design earthquake (Priority is that system continues operation during and after ODE and system experiences little to no damage with minimum inelastic deformation)

3. Ground Motion Parameters

a. Acceleration, velocity and displacement amplitudes (to determine MDE and ODE, maximum of peak acceleration (PGA and PGV) peak ground velocity are considered. However, effective rather than peak ground motion indicator is considered better.

b. Target response spectra and motion time history (usually expressed through acceleration response spectra). Not application in case where soil structure response is highly non-linear or very long structure.

4. Ground Shaking Deformation

5. Seismic design loading criteria

Obtained for MDE and ODE using load factor design method.

Uses the different combination of load for bored tunnel and cut & cover method.

6. Underground structure response to ground deformation

a. Free field deformation approach

b. Soil-structure interaction approach

There are other methods like dynamic earth pressure method, simplified frame analysis model

7. Special seismic design issues

The provision for seismic design are:

a. Tunnel and underground structures should be primarily designed to accommodate the transient and permanent deformation since these are more important than transient seismic load due to structure’s inertia.

b. Consider spatial variability of the ground motion and the associated phenomenon.

c. Special design consideration of joints and intersection is necessary.

d. Soil structure interaction effects should be considered accordingly.

e. When required, seismic earth pressures may be estimated according to current seismic codes.