A tale of twin cut slopes in Taiwan

Freeway No. 3 and Highway T-62 were cut through the same dip slope formation in northern Taiwan in the late 1990s. The former used an extensive tie-back system to hold back the cut slope; the latter adopted very limited slope stabilisation measures. On 25 April 2010, a catastrophic dip slope failure suddenly occurred at the cut slope of Freeway No. 3. Ironically, the cut slope of Highway T-62 remained stable. As found from the investigation, slope cutting and excavation of a bridge launching pit near the slope toe caused complete daylighting of the sliding plane of the freeway slope, whereas the sliding plane of the highway slope was not exposed by slope cutting. Owing to the poor corrosion protection of tie-back anchors installed in the freeway slope, a majority of tie-back anchors were seriously corroded and caused significant loss of tie-back load capacity. Although some sign of slope sliding was observed during the construction of the freeway slope, no proper measure was taken to effectively stop the dip slope from further sliding. The monitoring devices installed on the freeway slope also failed to identify the subsequent slope movement. Therefore, when the slope failed, the event came as a complete surprise to everyone.