2161
The Challenge of the Slope Failure Problem and Its Remedial Considerations at
Mileage 39km, Mt. Ali Road, Taiwan
Le défi du problème du glissement de pente et des dispositions correctives apportées, au kilomètre
39, de la route Mt. Ali Road, àTaïwan
Chang M., Huang R.
National Yunlin University of Sci. & Tech., Taiwan
ABSTRACT: Mt. Ali Road is a main scenic route to Alishan National Scenic Area and Yushan National Park in the middle of
Taiwan. At Mileage 39Km, the downslope side of the road consists of a 200m deep 60° angle bare cliff with a creek passing around
the slope’s toe. Despite complex geologic conditions, the underlying slope materials appeared competent for about 20 years after the
road opening. In 1999, however, a severe earthquake (Chi-Chi,
M
w
= 7.6) hit central Taiwan and caused numerous landslips in the
mountain range of the island. Although no obvious damages to the road were observed after the time of shaking, the geologic setting
of the site was deteriorating. During the attack of Typhoon Nari in 2001, the first serious effect on the road appeared and caused
several parallel cracks along the alignment. Remedial work was initiated. Nonetheless, a loss of road base during the monsoon season
next year had called for a stop of the work. The remedial plan was revised and implemented, but failed again due to Typhoons Talim
and Longwang in 2005. The current repair works have been completed and generally show no major signs of slope distress. In view of
repair histories and the geologic setting, this paper discusses the concerns and challenges for long-term stability of slopes at the site.
RÉSUMÉ : Mt. Ali Road est une route principale pittoresque qui mène à l’Alishan National Scenic Area et au parc national de
Yushan, au centre de Taiwan. Au kilomètre 39, le côté aval de la route est formé d’une falaise nue de 200m de haut, inclinée à 60°
avec un ruisseau la bordant en pied de pente. En dépit des conditions géologiques complexes, les matériaux de la pente se sont bien
comportés au cours des 20 années l’ouverture de la route. Cependant, un violent tremblement de terre (Chi-Chi,
Mw
= 7,6) a frappé le
centre de Taiwan en 1999, et a causé de nombreux glissements de terrain dans la partie montagneuse de l'île. Bien qu'aucun dommage
apparent n’ait été détecté après la secousse, la situation géologique du site s’est détériorée. Les premiers effets conséquents ont pu être
observés lors du typhon Nari en 2001, plusieurs fissures parallèles étant apparues le long du tracé. Un projet de confortement fut
entrepris. Malgré cela, une rupture de la couche de fondation, de la route l'année suivante pendant la saison des moussons a necesssité
un arrêt temporaire des travaux. Le projet fut révisé et remis en œuvre, mais a du être à nouveau arrêté en raison des typhons Talim
et Longwang en 2005. Les réparations sont aujourd’hui terminées, et la pente ne montre en général aucun signe de désordre. Compte
tenu de l'historique des réparations et de la situation géologique, ce document aborde les préoccupations et les défis concernant la
stabilité à long terme des pentes du site.
KEYWORDS: slope failure, repair works, landslip prevention and mitigation, slope stability, case histories, Mt. Ali Road.
1 GENERAL
Mt. Ali Road is a main scenic route to Alishan National Scenic
Area and Yushan National Park in the middle of Taiwan. As
entering into the hilly terrain of central mountain range of the
island, the road starts climbing up along the side of slopes for a
distance of about 5Km until it reaches a hill top with an
elevation of 460m MSL at the mileage of 39Km, where the road
levels off and makes a loop around the hill. The downslope side
of the road at this location consists of a 200m deep 60° angle
bare cliff with a creek passing around the slope’s toe.
Although both regional and local geology generally show a
complex geologic structure at the site, the subgrade materials
and underlying rock appeared competent for about 20 years
since the road’s opening. Figure 1 shows the regional geology
of the area, and Figure 2 illustrates the geologic profile of the
road section and its adjacent slopes.
On September 21, 1999, a severe earthquake (Chi-Chi,
M
w
=7.6)
hit central Taiwan and caused numerous landslips or slope
failures in the mountain range of the island, as indicated in
Figure 3. Based on studies by National Center for Researches
on Earthquake Engineering (NCREE 2000), approximately
1500-2000 landslides were directly induced by the earthquake.
Although the shaking did not have immediate impacts at the
site, the condition of the road and its adjacent side slopes
appears to be deteriorating.
Figure 1. Regional geology of the area (Liu & Tseng 2000).
The monsoon season normally starts from early April to the end
of September in the island. Annual precipitation is around
2150mm (Sinica 1999). Figure 4 shows accumulated rainfalls of
the site in 2001 & 2005 were 4700mm & 4600mm, respectively.
As will be discussed later, the abundance of rainfalls appeared
to have contributed to the failures of slopes along the Mileage
39Km in 2001 & 2005.
