For several decades, TBM’s have been used for the construction of tunnels. Depending on the local situation, the TBM may be placed at the start or at the end of its drive; for example maybe in a precut in the open terrain or maybe by lowering it into an excavation shaft down to the tunnel level. This latter technique is used mostly in congested city areas. A few years ago, starting and receiving a TBM in an excavation shaft required extensive measures such as breaking through the walls of the shaft, which are secured out of steel reinforced concrete. This preparation work needed time and has been expensive. In recent years however the use of Soft-Eyes in these areas are becoming more and more popular. A Soft-Eye may for example be a diaphragm wall or bore piles reinforced with Glass Fiber Reinforced Polymer bars (GFRP) instead of reinforcement out of steel. Also an anchored tunnel face with GFRP anchors will not obstruct the TBM head driving through. The use of GFRP products in tunnelling is getting more and more common in Southeast Asia and is widely applied in Europe and Japan nowadays.
no preparation works are needed for TBM brekthrough
no obstruction of the TBM's cutter head driving through the diaphragm wall
Soft-Eyes consist usually of bore piles or diaphragm walls, which are locally reinforced with GFRP bars. The sections below and above the tunnel are reinforced conventionally. Depending on the designer and contractors preferences, full rectangular sections are built out of GFRP bars and the fiber reinforcement follows more closely the tunnel section resulting in a circular arrangement of the GFRP links or may be a circular sections.
Both possibilities have their advantages. While a rectangular arrangement saves time during the design and assembly of the cages, following more closely the tunnel section thus reducing the material costs for the GFRP bars. Often applied as a compromise, where the vertical bars cover a rectangular section, while the shear links follow the circular layout. Experience shows that this approach decreases the material costs for the GFRP material by less than 5% still maintaining the detailed design and managing the assembly of the cage to be efficient. Building the corresponding reinforcement cages out of GFRP bars on site requires the same working procedures as for an equal steel cage.
The necessary bars are tailor made and delivered to site where the assembly takes place. The bars are fixed together with binding wire, cable binders or similar products. U-bolts are used for clamping bars together when high loads have to be transferred over a connection.
This is a connection between the vertical GFRP bars and the corresponding steel bars, which have to carry the dead load of the reinforcement cage during the lifting process and lowering of the cage into the trench. Welding as is commonly done with steel reinforcement but not possible with GFRP bars.
The reinforcing bar consists of a multitude of continuous bers, oriented in the direction of the load, bonded in a resin matrix. The production process guarantees the complete impregnation of the glass bers and an extremely high degree of curing. The bers provide the longitudinal strength and stiffness of the material. The resin matrix holds the bers in place, distributes the load and protects the bers against damaging influences. The geometry of the ribs and the fact that the ribs are ground into the hardened bar guarantee bond properties which are analogous to those of steel rebar.
Bent bars and stirrups are produced by bending a bundle of glass bers impregnated with resin. Afterwards these raw bars are thermally cured. This procedure allows for a high ber content and nearly parallel alignment of bers in the bent portion of the bars resulting in high strength and a modulus of elasticity similar to that of straight bars. Bent bars can be produced in all 2D shapes (e.g. z shapes and hat shapes) and 3D bars like spirals can be produced on demand.