Heaving in some mountain tunnels after completion occurs in roadbeds that were built without an inverted arch structure because the ground was relatively stable during construction. The stress-release by excavation, along with subsequent swelling by groundwater, cause the roadbed to lose strength, allowing upheaval of the road surface. In this study, we analysed geological degradation characteristics and the state of past tunnels in cases where inverts were installed. We also analysed measurement data for axial forces of inverts. The necessary performance was discussed through the result obtained by numerical analysis to compare support effectiveness of the invert shape and consider a shallow structure.
As its nationwide renewal project, NEXCO (Nippon Expressway Company Ltd.) is examining the countermeasures against heaving in mountain tunnels on highways that occurred after they were put in service. Its basic policy for the countermeasure to install inverted-arch concrete, so-called ‘invert’, but in and around the metropolitan area, the installment has been carried out not with road closure but with lane regulation, in order to alleviate traffic jam. Since the construction speed is slower with lane regulation than with road closure, design rationalization and construction efficiency are becoming increasingly important.
This paper studies the cases of countermeasures against heaving phenomenon, identifies the state and causes of the deformations, finds out the current situations of the countermeasure works and outlines the challenges. Then, it proposes the shallow structure of the invert based on the numerical analysis of the structure and measured stress after the installment.
Table 1 shows the cases of the countermeasures and figure 1 shows the cross-section of the tunnels and their invert structures. tunnel A, tunnel B, tunnel C, tunnel D and tunnel E were all constructed without inverts. For tunnel F and tunnel G, inverts were installed at the time of construction but later damaged by plastic pressure. Most of the cases are the former.