Dongao tunnel is adjacent to the collision zone between the Eurasian plate and the Philippine Sea plate. The geological structure and occurrence of tunnel excavation are affected by plate and gravitational slope movement, resulting in different tunneling features. Understanding the different geological characteristics, rock mass composition and rock mass behavior contribute to the establishment of relationships with ground behavior.

During the initial excavation stage, different degrees of displacement occurs in the rock wedge around the tunnel. The crew suffered from multiple severe instabilities and squeezing problems. This manuscript studied the response of surrounding rock after tunnel excavation, and the failure characteristics are discussed based on the images of tunnel sections, records of advancing surfaces and field investigation.

The results showed that: 1) When passing through the strata with the main lithology as slate, the tunnel section is generally stable during excavation, although it may collapse due to rock wedge falling. 2) The most difficult sections in the construction of the Dongao tunnel is between Xiaomaoshan Fault zone and Houyishan Fault zone. 3) The characteristics of rock mass deformed by gravity show that the open cracks between rock blocks inclined in the direction of slope. With the advancing of tunnel excavation, crack openings can be observed within the sections affected by gravitational deformation.

1. Introduction

Since the service of Suhua Highway, an coastal highway in eastern Taiwan, begun during Japanese colonial rule, it has never been able to maintain a long-term stability. The Directorate General of Highways, Ministry of Transportation and Communications (MOTC) proposed an improvement plan to fundamentally improve the serviceability of Suhua Highway. The study area is Dongao Tunnel, located near Suao Township, Yilan County, in northeastern Taiwan. Dongao Tunnel is a part of the improvement project of Suhua Highway. The total length of the tunnel is about 3.4 kilometers, and the alignment of the tunnel is in NNE-SSW.

Dongao tunnel is adjacent to the collision zone between the Eurasian plate and the Philippine Sea plate. The strata in the area had undergone several times of orogenic regional metamorphism and tectonic deformation, as well as subsequent complex folds and shear deformation structures formed by continuous plate compression. The tectonic stress is mainly compressive in the ENE-WSW direction and tensile in the NNW-SSE direction. Dominated by foliation and other discontinuities, engineering properties of the rock masses exhibit anisotropy.

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