Some expressway tunnels in Japan have suffered from heaving of the invert or roadbed after several decades in service. It is thought that these heaving are caused by increases in the volume of rock surrounding the tunnel, as occurs when rocks containing clay come into contact with water. Initially, when the amount of heaving is small, road performance can be maintained by cutting or overlay. However, as the heaving increases, cutting and overlay are no longer effective. In such cases, countermeasures include rockbolts and micropiles and, eventually, invert works. The invert works require digging under the road and closing entire lanes of the roadway, which results in social and economic losses.

We developed a new invert structure and an associated construction method as an alternative to the ordinary invert structure. The proposed invert structure, which we call the hybrid invert structure, comprises concrete-filled steel pipes and concrete abutments. Construction of the hybrid invert structure does not require lane closures. The purpose of this study is to identify the optimal structure and construction process for the proposed invert structure based on two-dimensional and three-dimensional finite element analysis.

The results indicate that

  1. the optimal invert geometry is an inverted arch-shaped concrete abutment,

  2. the optimal steel pipe shape is linear, and

  3. the most suitable construction procedure is as follows: the foot of the tunnel lining is excavated and filled with concrete, a relatively short longitudinal section under the lane of each side is excavated before steel pipes are situated, and a series of steel pipes are situated before the excavated area is filled with concrete. The procedure is repeated until the entire length is completed.

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