After completion, a structure enters its operational stage, and maintenance initiates. A standard procedure of maintenance and management includes monitoring and safety evaluation based on interpretation of monitoring results. Ideal tunnel maintenance procedures are hard to deliver because of the difficulties in connecting monitoring results and the causes. In recent years, novel technology has helped the administration collect massive monitoring data of operational tunnels. The crucial obstacle lies ahead now is how to define the integrity of a tunnel according to the monitoring results.

Since 2009, a mountain tunnel in southeast Taiwan have been monitored through self-developed tunnel lining micro-displacement monitoring and image-mosaic technology. By the end of 2011, the lining displacement and crack monitoring showed that the degradation started to accelerate. Reinforcement is then conducted on some seriously deformed area in 2012. This manuscript introduces results analysed from tunnel displacement through characteristic matrix method. Characteristic matrix method is an approach established to analyse displacement of tunnel profiles. The method disassembles displacement of a tunnel profile into the sum of several displacement modes. There are two categories of displacement modes, rigid-body-motion type that includes translation and rotation, and deformation-type that comprises uniform, elliptic, triangular, square deformation, and other polygonal deformation. Distinct differences were found between the lining displacements before and after reinforcement. Before the reinforcement, the tunnel degrades as time goes by. In the meanwhile, the composition of higher degree deformation-type modes, square and pentagonal deformation, increases. After the reinforcement, lower degree deformation-type modes dominate, and higher degree deformation-type modes diminish.

1 Introduction

Two decades ago, most of the studies about "tunnel displacement" were about the deformation of primary support induced by stress relief after tunnelling (Brown et al. 1983; Pelli et al. 1991; Kaiser and Mccreath 1994). While in recent years, the subjects of tunnel displacement monitoring have no longer be limited to tunnels in construction, but also include tunnels in operation (Japan Society of Civil Engineers 2003; He et al. 2009; Chen et al. 2016). Researches about how exterior factors such as environmental changes and interior factors like structural defects thrive. The needs to prevent operational tunnels from deformed under multiple causes is booming (Japan Road Association 2009; Lee and Wang 2014; Chiu et al 2017; Wang et al. 2018).

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