Tunnel Headwall Softeyes: Fiberglass Design Applications and Constructability Challenges-Paper

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The paper reviews more than a decade of using fiber-reinforced polymer (FRP) components in tunnel headwall “softeyes” for Greater Toronto Area transit projects. Softeyes are sections of shaft walls designed to be penetrated by tunnel boring machines (TBMs), enabling launch, reception, access, or mining operations.<br /><br />FRP is suitable because TBM cutterheads can break it apart, unlike steel reinforcement. Applications include FRP beams spliced to steel, FRP-reinforced cages, and fiberglass anchors. These systems have accommodated varied ground conditions, shaft geometries, and wall heights. Anchors can reduce bending and connection shear, simplify layouts, and allow more open or sequential excavations.<br /><br />Design must account for FRP’s anisotropic properties, low stiffness and shear capacity, and brittle failure behavior. These differ substantially from steel and can affect load distribution and connections. Concrete strength may need to be increased to help resist shear, and manufacturer-specific material properties should inform design.<br /><br />Construction experience revealed challenges with lifting and installing flexible FRP elements, accurately positioning the softeye, trimming secant-pile walls without damaging FRP, and accounting for high TBM pressures before breakthrough. Monitoring with surveys, inclinometers, and anchor load cells was found useful. Long FRP-reinforced cages also suffered failures during installation and casing extraction. Recommended improvements include stronger splice restraint, more clearance around cages, additional shear ties, and careful control of concrete mix and construction loads.<br /><br />Overall, FRP systems have proved an effective solution for large-diameter tunnel softeyes in the GTA, but successful use depends on integrating design and construction planning and addressing FRP’s distinct material behavior.

Keywords

fiber-reinforced polymer

FRP

tunnel headwall

softeyes

TBM

Greater Toronto Area

shaft walls

fiberglass anchors

anisotropic properties

construction monitoring