The Nothlink Railway is constructed to link northern and eastern Taiwan. It runs along a stretch of coast facing the Pacific Ocean, which has been renowned for its grandeur of cliffs and for its breathtaking scenery. Tectonically, the stretch of cliffs concerned is supposed to be a faultline scarp and forms the northern part of the East Taiwan Rift, which has been regarded as an important tectonic boundary between the Asian Continental Plate and the Pacific Oceanic Plate(Wu et al.,1976).
Difficulties in constructing the railway concerned could be well envisaged from the above-mentioned topographical and geological features of the terrain. The fact is, the work was delayed one Year to complete by unexpected engineering geological problems, encountered mainly in digging the tunnels, which form nearly one-third of the total length of the very railway. The failure of KU-Yin Tunnel is one of the cases and is to be described in the present Paper.
In the present paper the Ku-Yin Tunnel refers to that part of the railway from kilometer point L32+530 to L32+800, Which is about halfway of the railway. The terrain where the tunnel was located is amphitheater, or armchair, in form with the open side facing the Pacific Ocean (figure 1).
The central lower part slopes gently down and protrudes to the ocean and assumes a hummocky appearance. It is made up of cliff debris of various schists, mainly black schist, and ranging in size from huge blocks, some of them being no less than a big house, to clay. All these materials are mixed together in irregular way. Geological drillings made in the investigation stage and the subsequent excavation revealed that the thickness of this colluvial deposit is up to 30 meters at the place where the tunnel was located(see figure 3).
(Figure in full paper)
The central upper part as well as the spurs on both sides has much steeper gradient. It is made up of interbeds of black schist, quartz schist, and green schist. They provided those materials accumulating in the central lower part by way of block slide, rock slide, debris slide, and rock fall as well.(figure 2).
(Figure in full paper)
The schists forming the southern half of the terrain concerned show rather regular foliation attitude generally N84 W24°-33° NE, namely, The strike of the foliation is roughly perpendicular to the slope face. The rocks are rather sound lithologically and with very few discontinuities. This lithological characteristics along with the favourable disposition of the strata makes it a stable slope.
On the contrary, the schists on the northern side are structurally disturbed, hence with varied foliation attitudes and closely spaced joints and fractures in the rock mass. These make it an unstable slope and did give rise to some stability problems during and after grading at the stage of correction. Slidings mostly have taken place from this part, with the upper surface of, the sound rock mass as its lower limit.
