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  Reconstruction of a heaved invert without closing expressway

  The Nikkureyama Tunnel is a two-lane traffic tunnel 2,000 m long, located on the Joshinetsu Expressway that extends from the Tokyo metropolitan area to Nagano. The section in the second phase was completed in 2002 by rerouting, maintaining the distance from the super-squeezing zone in the section of the first phase in order to avoid interference. However, eight years after the opening of service, road surface heaving of 5 cm was observed. In this zone, since the weak alternation layer of mudstone and tuff had emerged during excavation earlier than expected, the section was closed early by placing an invert to control the sudden increase of displacement. This paper reports the first example of invert reconstruction as a countermeasure against heaved road surfaces in Japan. It was carried out safely and effectively by controlling one lane without closing traffic on the expressway. First, we analyze the data obtained during first construction and the countermeasure project, and summarize the characteristics of the squeezing ground. Secondly, after discussing the design and construction method of the countermeasures, the effect and evaluation of the countermeasure are discussed based on the conditions and monitoring results taken during the project. In addition, the paper discusses the cause of the deformation clarified by this study and also refers to the issues when the impact of water inflow continues for a long time.

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