On August 8

2009

Typhoon Morakot hit southern Taiwan. It carried huge accumulated precipitation of 2

500 mm in 72 hours at Alishan area

and caused catastrophic erosion and landslide. Thus

the hundred-year historic Alishan Forest Railway was suspended due to serious embankment collapses. The rehabilitation principle for these two large embankment collapses followed the natural doctrine. Therefore

the D&B/Mechanical bypass tunnel was adopted to overcome the geological changes and groundwater problem in these two huge landslide areas. The lengths of west portal section colluviums of two tunnels were up to 99 m and 109.8 m

which were the crucial sections of tunneling engineering. The tunnel supporting system design was based on PCCR-system (Rock mass classification of Public Construction Commission) & NATM

and steel pipe roofing

lattice girders

and steel fiber reinforced shotcrete

as well as self-drilling anchors were adopted for increasing the safety and shortening the working days during construction. Currently

two tunnels have been driven through

and the amount of deformation was under control and in accord with safety level. This result indicates that implementation of pre-supporting system in drilling through the crushing colluviums section is effective

and can be used as a reference for the relevant engineering design.