Investigating the Performance of Ground Anchor Through The Failure Slope Disaster in Taiwan

At milestone 3.1 km of the Formosa Freeway in northern Taiwan, a landslide occurred on April 25, 2000, causing nearly 200,000 m3 of earth and rock to slump down onto the freeway below. Four people trapped in cars beneath the collapsed slope died. How such a tremendous slope failure could happen in dry weather without advanced warning is attributed to two key factors: (1) Long-term groundwater infiltration resulting in the softening of thin interlayer between sandstone and shale; (2) Ground anchor corrosion resulting in a decrease in slope stability. Together these two factors caused the slope to reach a critical limit resulting in a collapse. In Taiwan ground anchors have been widely used to improve slope stability along roadways for more than 40 years. After the Formosa Freeway slope collapse the government began a comprehensive survey to examine anchors on the slopes along all freeways. This paper uses finding from this survey as well as information from other slope failure investigations to examine the performance of ground anchors in Taiwan. The factors contributing to the failures of the permanent ground anchors and the required inspections/maintenances are discussed in addition to recommendations for improving design and construction.