Fiberglass reinforcements for concrete structures, Soft-Eye technique

Sireg Geotech designs and manufactures temporary fiberglass reinforcements for concrete structures.

DURGLASS reinforced structures can be easily bored by the TBM (Tunnel Boring Machines). This technique is known as “Soft-Eye”.

Tunnel excavation using Tunnelling Boring Machines (TBM, EPB, Mixshield etc) has significantly expanded over the last decade.

One of the most serious challenges faced by TBMs is cutting through the reinforced concrete structures of the launching, recovery shafts and stations built along the TBM route. Such structures, often built at great depth, are usually reinforced with large amounts of steel rebar in order to stand strong ground and water pressure.

To break these walls is impossible for the TBM ,because it would damage its cutting tools. For this reason, the reinforcement has to be cut manually, by breaking the concrete wall and cutting the reinforcing steel bars inside it. This operation requires a long time and it could be dangerous, specially in presence of groundwater pressures.

The more the thickness of the wall is reduced, the more the water counter-pressure can suddenly break into the launching shafts, causing water flooding inside it. To avoid this risk, nowadays Glass Fibre Reinforced Polymer rebars (Soft Eye Technique) are more and more adopted as tunnel front reinforcement.

The Soft-Eye technique consists in substituting the internal steel reinforcing bars of the wall with GFRP bars and stirrups, which have a high tensile strength but low shear strength, therefore allowing the TBM to bore through the wall section easily and without damaging the machine’s cutting tools.

Usually, a section (cage) of the wall corresponding to the dimension of the TBM shield is reinforced with GFRP, leaving the upper and the lower sections of the cage reinforced with steel rebars.

The two steel sections are than connected to the GFRP cage by overlapping steel and glass fibre bars. Nowadays the soft-eye technique is used in several projects around the world and applied not only for diaphragm walls but also as reinforcement for piles and for complex concrete structures.