PURPOSE: To obtain a segment having higher strength by connecting a tunnel bedrock side steel plate to tunnel inside space side steel plate with a lattice-like reinforcing steel material, providing projections to a concrete side of the steel plates and an opposite side, and providing segment connecting joint boxes.
CONSTITUTION: Segment joint boxes 5, ring joint boxes 6 and a grouting hole 7 are formed at specific positions of steel plates 2 and 3 of a composite segment 1 provided with a tunnel bedrock side steel plate 2 and tunnel inside space side steel plate 3. After that, the steel plates 2 and 3 are connected to each other with a lattice-like reinforcing steel material 8, and a part of concrete 4 is formed so that it is held between the steel plates 2 and 3. The composite segments 1 are combined by making use of an erector, etc., of a shield machine, and they are clamped with bolts to complete a tunnel. According to the constitution, load resistance force of the tunnel is promoted, the thickness of the composite segment 1 can be reduced, and the lightweight segment can be obtained.