Why Cast-Iron Segments Remain a Practical Choice for Tunnel Eyes and Cross-Passages

In underground construction, standard tunnel rings are only part of the picture. Some of the most demanding areas on a project are tunnel eyes, cross-passages, junctions and other transition zones where geometry changes, interfaces become more complicated and ground conditions are often less forgiving.

These are the areas where the lining choice matters most. The right solution can help reduce excavation size, improve stability, limit temporary works and provide dependable long-term watertightness. That is why cast-iron segments are still widely used for tunnel eyes, cross-passages and junctions in difficult ground.

Why these areas are more demanding

Tunnel eyes, cross-passages and junctions are very different from standard running tunnel rings. They often involve enlarged or irregular openings, non-standard geometry and interfaces with shafts or existing tunnels. They are also frequently built in poor or variable ground, where risk is already higher.

In these situations, excavation size has a direct effect on construction risk. A larger opening usually means more disturbed ground, more temporary support, more ground treatment and less control during installation.

This is one of the reasons cast-iron segments still make sense.

Smaller excavations, better control

A key benefit of cast-iron tunnel eye and cross-passage solutions is that they allow for a smaller excavation envelope. Because the segmented ring acts as the permanent lining, there is no need to leave extra space for a secondary sprayed or in-situ concrete lining.

That can make a real difference on site. A smaller excavation can improve face stability, reduce the amount of disturbed ground and lower the need for temporary support or treatment works. In difficult ground, that often means a safer and more controlled construction process.

This is particularly important in sensitive areas such as under-river sections, shaft connections and constrained urban works.

A permanent lining from the start

Another practical advantage is that cast-iron segments form a permanent bolted lining from the outset. This can simplify the construction sequence compared with solutions that rely on temporary support followed by secondary lining works later.

In practice, that can reduce follow-on activities, limit delays between stages and make critical interfaces easier to manage. For tunnel eyes and cross-passages, where one difficult area can affect the wider programme, that added predictability is important.

Watertightness also matters

Water control is a major issue in many underground structures, especially at openings, junctions and passage connections where geometry is more complex and sealing becomes more critical.

A bolted and machined segmented lining system offers repeatable sealing interfaces and consistent fit-up, which helps support reliable watertight performance. Compared with more site-dependent lining methods, this gives engineers and contractors better control in areas where access is difficult and conditions are less predictable.

DTS has long experience with shaft and tunnel lining systems using traditional sealing methods such as lead and rubber gaskets, helping achieve dependable watertightness in demanding underground applications.

A proven approach

Cast-iron tunnel eye and cross-passage systems are not just a theoretical option. They have been used successfully on major underground projects where excavation control, sealing performance and installation certainty were all important.

One example is the London Underground Northern Line Extension, where DTS supplied cast-iron lining components for the step plate junction at Kennington Loop, along with cross-passages and opening sets for the twin running tunnels.

Projects like this show where cast-iron segments work well: non-standard underground structures where fit, stiffness, watertightness and constructability all matter at the same time.

Why manufacturing quality is important

The material alone is not enough. The success of tunnel eyes, cross-passages and junctions also depends on dimensional accuracy, machining quality and proper QA throughout production.

For these applications, bolt alignment, sealing surfaces and interchangeability are critical. The more complex the geometry, the less room there is for inconsistency.

That is why DTS focuses on engineering support, manufacturing drawings, casting, machining, inspection and QA/QC, with all casting and machining carried out in South Korea. This level of control is important for critical underground work, where fit-up problems quickly become site problems.

Conclusion

As underground projects move into more constrained sites and more difficult ground conditions, cast-iron tunnel eyes, cross-passages and junctions remain a practical and proven solution.

They help reduce excavation size, improve control during installation, provide a permanent lining from the start and support dependable watertightness in some of the most difficult parts of a project.

That is why cast-iron segments still make sense - not because they are traditional, but because in the right applications they still offer clear technical and construction advantages.