Precision Stitch Core Cutting for the Joka–Esplanade Metro Project

When a 600-tonne Tunnel Boring Machine needs a precise passage, every millimetre matters.

Read the Telegraph Article ↗

How Do You Cut an 8.6-Metre Hole in Concrete for a Tunnel Boring Machine? Inside Armatix's Work on Kolkata Metro

When a tunnel boring machine (TBM) needs to pass through a shaft wall or diaphragm wall on a metro project, the opening is too large and too heavily reinforced for a wall saw or a single core bit. The solution is stitch core cutting — drilling a continuous ring of overlapping diamond cores along the exact perimeter of the required opening, then breaking the ring into equal segments for controlled, vibration-free removal. That is exactly what Armatix's mobile diamond cutting team executed for an 8.6-metre diameter circular opening, cut in 8 equal segments, to let a TBM pass through — part of the underground works on Kolkata Metro's Joka–Esplanade corridor (Purple Line).

The Project: A Milestone for Kolkata's Purple Line

In August 2026, Kolkata Metro's Purple Line achieved a major construction milestone. Tunnel Boring Machine "Divya" completed its drive between Khidderpore and Victoria stations — a stretch of roughly 1.72 km through some of Kolkata's most challenging soil, dense with groundwater and packed with existing infrastructure overhead. Its sister machine, "Durga," had broken through the same section a few weeks earlier. Together, the two drives completed the twin-tunnel connection for this segment of the Joka–Esplanade corridor, one of the most closely watched underground rail projects in eastern India, executed by RVNL with L&T as the contracting agency.

A TBM breakthrough is a highly technical moment. Long before the machine reaches its target, a precise opening has to be cut into the receiving shaft's concrete wall — sized, positioned, and finished to exact tolerances so the machine can emerge without damaging the structure around it or compromising the surrounding soil. This is where specialized concrete cutting comes in — and where Armatix's team was engaged.

Armatix's Role: Stitch Core Cutting an 8.6m Opening

For this project, the requirement was to create a circular opening 8.6 metres in diameter through heavily reinforced concrete — far beyond what a conventional wall saw or single-pass core drill can handle. Armatix approached it using stitch core cutting, executed as follows:

  • The opening was divided into 8 equal segments, each planned around the full 8.6m circumference.
  • A series of closely spaced diamond core holes was drilled along the cut line of each segment, so adjacent holes overlapped enough to form a continuous, controlled separation line — essentially "stitching" through the concrete core by core.
  • Each segment was then broken out and removed individually, rather than attempting to free the entire 8.6m disc in one piece — reducing load, vibration, and risk to the surrounding shaft structure.
  • Reinforcement bars encountered along the cut line were cut clean by the diamond core bits themselves, avoiding the impact and vibration that a hydraulic breaker or conventional demolition method would introduce.

The result: a precisely sized opening, cut without cracking, over-break, or vibration damage to the shaft lining — clearing the exact path the TBM needed to pass through safely.

Why Stitch Core Cutting — and Not a Wall Saw or Breaker?

For an opening this large and this critical, three constraints rule out most conventional methods:

  1. Scale. No single wall saw blade or core bit can cut an 8.6m diameter opening in one pass. The geometry has to be broken down into manageable segments.
  2. Reinforcement density. Shaft walls on metro projects carry heavy rebar cages. Diamond core drilling cuts cleanly through steel and concrete alike, where impact tools would snag, deflect, or cause uncontrolled cracking.
  3. Zero tolerance for vibration. A TBM breakthrough point sits close to retained soil, groundwater, and adjacent structures. Vibration-free, non-destructive cutting protects the surrounding shaft and the tunnel lining already in place, and keeps the schedule from slipping due to unplanned repairs.

Segmenting the cut into 8 equal parts also gives the site team control over sequencing — each segment can be inspected, monitored for water ingress, and removed in a planned order, rather than releasing the full concrete disc at once.

The Bigger Picture: Precision Cutting for Infrastructure-Scale Projects

This isn't a typical core-cutting job. Most concrete cutting requests in Kolkata involve openings for ducts, pipes, staircases, or structural modifications — usually well under a metre in diameter. An 8.6m stitch-cut opening for a TBM breakthrough sits at the far end of what diamond cutting technology can achieve, and it demands the same engineering discipline as any structural intervention: pre-cutting concrete scanning to map rebar and services, a segment-by-segment cutting sequence, and coordination with the site's structural and tunnelling engineers throughout.

It's also a reminder that diamond cutting isn't limited to renovation and retrofit work. On infrastructure projects — metro corridors, flyovers, underground utilities — controlled concrete cutting is often the difference between a breakthrough that stays on schedule and one that doesn't.

Need Specialist Concrete Cutting?

Planning a large opening, structural modification, metro/infrastructure cutting or heavy RCC cutting project?

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