In August 2004, Kenaidan began the first phase of seismic safety retrofit, and the second phase in the southbound tube and west air duct started in December 2005. Both phases used continuous steel plates, rebar, and concrete slabs to stiffen the concrete tunnel structure.

The seismic restoration approach used an engineered method of strapping the tunnel sections together to act as one unit if subjected to seismic forces.

Phase II used three work areas throughout the tube to separate the simultaneous activities, such as soffit wall and road joints, curbs, scaffolding, and soffit work in the air duct. The work took place between 11:00 p.m. and 5:00 a.m. during the closure of the southbound tube.