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Assessment of Damages in Mountain Tunnels due to the …-what are dumbs tunnels

Assessment of Damages in Mountain Tunnels due to the
Taiwan Chi-Chi Earthquake
W.L. Wang1, T.T. Wang2, J.J. Su1, C.H. Lin1, C.R. Seng1, and T.H. Huang2
1 United Geotech Inc., Taipei, Taiwan
2 Department of Civil Engineering, National Taiwan University, Taipei, Taiwan
Corresponding author
T.T. Wang
Department of Civil Engineering
National Taiwan University
1 Section 4 Roosevelt Road
Taipei, Taiwan
Tel: +886 2 2363 0231
Fax: +886 2 2364 5734
Email: wangseeu@ms11.hinet.net
Tunnels, being underground structures, have long been assumed to have the ability to sustain
earthquake with little damage. However, investigations of mountain tunnels after the Chi-Chi
Earthquake in central Taiwan revealed that many tunnels suffered significant damage to various extents.
This work describes the findings of a systematic assessment of damages in the mountain tunnels in
Taiwan after the earthquake. It was found that among the 57 tunnels investigated 49 of them were
damaged. The damage patterns are summarized based on the characteristics and the distribution of the
lining cracks. This systematic investigation, involving geological conditions, design documents,
construction and maintenance records of these tunnels, has been conducted to assess the potential factor
that may have influences on the various damage patterns and the earthquake loading on tunnel
engineering. The results show that the degree of damage is associated with the geological condition and
structural arrangement of the tunnel. A tunnel passing through displaced fault zone will definitely
suffer damage. Extent of geological weak zone, distance from the epicenter, and existence of slope face
are also significant influencing factors. The seismic capacity of the tunnel is influence by its structural
arrangement, type if lining, invert setup, lining reinforcement, and others.
Keywords: tunnel damage, earthquake, lining cracks.
1. Introduction
Mountain tunnels, being situated deep within rock layers, have generally been assumed to be
sustainable against damage from earthquakes. Previous studies have found earthquake damage in
tunnels to be localosed at sections with two important characteristics: those running through displaced
faults, which were damaged by shear forces that developed during the earthquake, and those near
surface slopes (especially at portal sections), which were damaged owing to slope failures (Dowding
and Rozen 1978, Yoshikawa 1981, Sharma and Judd 1991, Asakura and Sato 1996 and 1998). Most of
the design codes relating to earthquake mitigation for mountain tunnels are currently designed for use at
portals and sections near slope surfaces, and seldom for other sections, including deeper mined parts
and areas near intersections. Nevertheless, the Chi-Chi Earthquake incurred much damage on many
tunnels in central Taiwan, such as cracking, spalling of concrete lining and deformation of steel
reinforcement. These damages provide sufficient evidences to suggest that the effects of earthquakes
on tunnels should be further studied.
To study the damage influencing factors, the results of investigations of 57 mountain tunnels
affected by the Chi-Chi Earthquake were presented in this paper. For each tunnel the damage patterns
are examined on the basis of crack mapping results, and the degree of damage was assessed based on its
functionality after the earthquake. Tunnel damage, geological and geotechnical conditions and tunnel
structural characteristics are systematically investigated to evaluate the factors influencing tunnel
damage in an earthquake. This paper also discusses the seismic effects on tunnel engineering.
2. Brief description of the Chi-Chi Earthquake
On September 21, 1999, at 1:47 AM local time, a strong earthquake with a magnitude of 7.3 on the
Richter scale occurred near the town of Chi-Chi in central Taiwan (N23.78, E120.84), at a depth of