Gaskets for tunnel segments

Tunnel segments are assembled into a cylindrical tunnel lining in similar fashion as Lego pieces. The segments themselves are made of concrete which is virtually waterproof as long as not cracked; it is the joints between segments that need to be sealed against groundwater, if a dry environment is needed in the tunnel. This is typically achieved by gaskets. Segment gaskets come in pairs attached to two adjoining segments in contact. They run parallel to each other along the entire circumference of each segment to achieve water tightness.

Fundamentally, gaskets for tunnel segments are designed to resist certain water pressure from penetration, thus maintaining water tightness in a tunnel. Generally speaking, the higher water pressure a pair of gaskets can resist when:

• Gaskets are squeezed tighter. All gaskets must be in compression to function. The smaller the gap is between the gaskets, the higher compression is established. Every single joint in a segmental tunnel must be in a sufficient amount of compression to ensure water tightness. In the longitudinal joints, joint rotation could possibly reduce the gasket compression

• The wider the gasket profile is. Generally speaking, larger diameter tunnels require wider gasket profiles, not just because of a higher water pressure they need to resist, but also because of a higher build tolerance.

• Gaskets are better aligned with each other. The more offset there is, the less water pressure they can resist. Misalignment relative to the theoretical position can come from manufacturing and build tolerances, joint

• The younger the gaskets are. Gaskets have stress relaxation over time reducing the highest water pressure they can resist. The relaxation happens quicker upfront and gradually tails down over time.

In addition, the durability of the gaskets need to be considered in relation to the surroundings that the tunnel is situated within. A common material for tunnel gaskets is EPDM, which is a type of rubber. This material is generally vulnerable to diesel, UV (sun light) and chloride, which make it degrade. These are relatively rare situations to encounter, but when it is the case, alternative materials such as nitrile are available as substitution.

Some practical consideration is also needed. Nowadays gaskets are usually cast-in rather than glued-on, reducing health hazard from exposure to toxic chemicals. Gaskets need to be lubricated to ease the sliding actions during segment installation by a TBM. Without lubrication, gaskets can be damaged or fall out of position resulting in loss of water tightness.

Sometimes hydrophilic materials are used either in combination with gaskets or in isolation to gaskets as another barrier. Note that hydrophilic materials work in a completely different mechanism – they rely on the swelling from getting in contact with water, hence can not provide a fully dry environment from day 1.