Construction of the largest waste water tunnel in Scotland has been completed with the help of a new tunnel segment sealing gasket developed by VIP Polymers. The 4.9km long Shieldhall tunnel beneath Glasgow has been one of the first projects to make use of the rubber gaskets manufactured by VIP, designed to reduce the risk of segment cracking during installation.

VIP gaskets contributed to zero segment cracking from high corner contact pressure

The TBM broke through in October 2017 after 15 months excavating the 4.7m-diameter tunnel for Scottish Water. Its journey from Craigton to Queen's Park, across the south of Glasgow, has created the largest-diameter bored tunnel in Scotland.

Construction JV Costain Vinci Construction Grands Projects chose VIP Polymers to supply cast-in tunnel segment gaskets with compliant compressible corner technology to ensure consistent load performance along the entire gasket perimeter. This eliminates the risk of a build-up of pressure at one or more of the gasket's corners during installation, which could contribute to segment cracking.

"Our new cast-in gaskets have performed very well throughout the Shieldhall tunnel construction process, demonstrating reliability,” said Matthew Levitt, VIP Technical Business Development Manager.

Gasket being installed in a segment mould

VIP cast-in gaskets at FP McCann's segment production centre

"The steel fibre reinforced concrete segment manufacturing process was achieved within the tightest of specifications and the VIP gaskets contributed to an exceptionally well-built tunnel, with no reported segment cracking arising from high corner contact pressure, or segment ram loading transfer, cracks or leakage, meeting the tunnel lining specification without compromise,” said Sam Simons, Tunnel Lining Supply Manager for Costain.

Prior to manufacturing any gaskets, members of the Costain waterproofing team witnessed corner loading (T and Cruciform) and pressure testing of the gaskets in VIP's testing facilities in Huntingdon.

Over the last 18 months, VIP has manufactured more than 19,500 individual TSGs for the project which have been cast into each tunnel segment at FP McCann's site in Drakelow, Derbyshire, England.

"The geometry of the segments meant that the gaskets required intricately formed corners to fit the steel moulds correctly and provide a draft angle to the sealing face of the key and adjacent segments along the Z axis,” said Dave Derbyshire, Operations Manager for Underground Products at FP McCann. "Technical representatives from VIP visited our facility at Drakelow prior to segment production, to ensure the correct fit was achieved on all six segment types.”

The £100 million Shieldhall tunnel is one of the most important wastewater infrastructure projects in Glasgow since Victorian times. It will help tackle flooding and improve river water quality across the city and will provide 90,000m³ of extra storm water storage.

The cast-in rubber gasket compressible corner joint system designed by VIP Polymers to reduce the risk of tunnel segment cracking during installation has been awarded a UK patent.

Tunnel under construction with VIP patented gaskets

The new compliant compressible corner system ensures a consistent load performance along the perimeter of the tunnel segment gasket to avoid build-up of pressure at one or more of the gasket corners during installation which can lead to segment cracking.

“The design and manufacture of our new compressible corner joint, which is used across our range of cast-in gaskets, has been recognised with a UK patent and an international patent pending,” said Steve Casey, VIP Sales and Technical Director.

“Segment cracking during tunnelling operations is a major cause for concern for tunnel designers and tunnelling contractors as it can be costly and we sought to address this when designing the new corner joint. Our new design has been favourably received, used in a number of tunnel-building projects, and been chosen for use in future projects.”

Most cast-in segment gaskets are made with conventional shot-joint corners. During the moulding process, the rubber in these corners fills the compression cavities in the extruded gasket sections, creating a solid rubber corner. When these tunnel segments are installed, the compression of solid corners is limited and can lead to corner point loading, increasing the risk of segment cracking.

Close-up of VIP Polymers cast-in tunnel segment gasket

The VIP patented corner joint is created with a particular joining process that maintains the gasket profile right to the corner edge. This makes the complete gasket compliant with all specified load requirements and provides greater assurance to designers, builders, and clients on the quality of the segmental ring build.

Rigorous comparative tests have shown that solid corner joints are harder to close, resulting in increasing loads. In contrast, the VIPcorner joint maintains a uniform closure performance around the full cast-in gasket.

The new corner joint can also be manufactured to a variety of angles from90 degrees, to obtuse or acute allowing the gasket to be moulded to the exact shape and angle of each segment in the design.

The profile orientation can also vary along the Z axis of the gasket which helps with the precise installation of final segments, or non-uniform segment systems, to again reduce the risk of segment damage or partial sealing of joints.

“A multi-disciplinary team of sales, laboratory, and product development departments has contributed to a new product that is now being used on international tunnelling projects, and can save the tunnelling industry both time and money,” said Casey. “Test results and positive feedback from within the industry are testament to the collaborative effort from staff at VIP.”

VIP gaskets are manufactured from the company headquarters in Huntingdon, Cambridgeshire in the UK.