710 freeway – proposed tunnels and related earthquake issues
Ken Hudnut, Ph.D. Geophysicist MISCONCEPTION Underground structures are ‘safe’ against earthquakes Deep structures are ‘safer’ than shallow structures Tunnels do not collapse or suffer other damage in earthquakes
Note: So much information is easily available to disprove these myths, it is hard to understand how such misinformation persists these days… even among practitioners
TRUTH Underground structures are safer than above‐ground structures during a seismic event (because of lateral confinement), but… Tunnels have collapsed and suffered damage in many earthquakes, so seismic countermeasure designs are essential for these new tunnels Summary of Occurrences (Prior to Japan 2004 and China 2008)
From Lanzano et al., 2008 presentation Japan (1923, 1978, 1995 & 2004), Taiwan (1999), Turkey (1999) and China (2008)
Proximity to large earthquakes with severe ground motion results in damage or even collapse of tunnels
Damage and collapse of What an tunnels has occurred earthquake even in newly built tunnel and very strong tunnels collapse looks like in advanced nations (Bolu, Turkey) with many extremely well‐documented cases Portal and internal collapses, Longxi Tunnel Types of tunnel damage from earthquakes
Portals and shallow sections are vulnerable Examples of problematic designs Design Countermeasures
With enough money, things that work well in theory may be tried to strengthen tunnels at points of likely seismic vulnerability. For example, where faults and material contrasts occur, a strong jacket or isolation layer can harden the tunnel in such places. 710 tunnel alternatives & faults 710 tunnel alternatives & faults 710 tunnel alternatives & faults 710 tunnel alternatives & faults 710 tunnel alternatives & faults
890 feet 830 feet Two 680 feet
crossings
410 feet