How does transit tunnel boring technology work?
Kevin William Grant
Published on
December 31, 2020

A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for microtunneling. They can bore through anything from hard rock to sand.

Tunnel boring machines can operates 24 hours a day, 7 days a week. The advantage of this approach is that tunnelling takes place deep underground with minimal disruption to the ground above

Tunnel Boring Technology

The tunnel boring machines are manufactured by Caterpillar (formerly Lovat Inc.) and are manufactured in Etobicoke.


Once it finishes a "tunnel drive", the tunnel boring machine is broken down into several parts at the extraction shaft and transported by truck to the next launch shaft and prepared for its next tunnel drive.  Two tunnel boring machines will drill in parallel about a month apart, in the same direction, to create the twin tunnels for the subway.

how to build twin tu

A construction staging area, or worksite, is prepared for the launch shaft in advance of the arrival of the tunnel boring machine and the tunnel liners.  A smaller extraction shaft area will also be prepared before the tunnel boring machine reaches its destination.

Concrete Technology

Munro Concrete Products Ltd, located in Essa Townships, Simcoe County will provide the concrete tunnel liners for The Crosstown.

As the tunnel boring machine advances, pre-cast concrete tunnel segments called  "liners" will be set in place behind the machine to form the tunnel.  These concrete liners are pre-made off-site and trucked into the tunel construction site on a regular basis.

How to build subway

Launch Shaft Construction for Tunnelling

A massive launch shaft – 60 metres long, 20 metres wide and 16m below the ground will be excavated adjacent to Keelesdale Park between Black Creek Drive and Keele Street. The shaft will enable the tunnelling machines to be safely lowered and launched eastward in 2012. The tunnelling machines are so large that the parts will be lowered by crane and assembled on site.

Tunnel Boring Machines In-Action in Toronto

TBMs were used in Toronto recently to complete the Toronto–York Spadina Subway Extension which extended Line 1 rom Sheppard West station to Vaughan Metro Centre.

The subway line extension opened on December 17, 2017. Here are a few diagrams that show how this infrastructure work was completed in Toronto.

Keelesdale Launch Area  bridge with Eglinton and Black Creek Labels2

Keelesdale Park – Launch Area and rendering of future Bridge and York Community Centre at Black Creek and Eglinton.

Launch Shaft Excavation  1a

Launch Shaft Excavation Stage One.

Launch Shaft Excavation 2 a

Launch Shaft Excavation Stage Two.

Launch Shaft Excavation 3 with Decking a

Launch Shaft Excavation Stage Three.

Launch Box .00004 a

Launch Box and Tunnelling Machines lowered by crane.

Tunnelscale a

Tunnel From above with beams removed to visualize scale.

Launch Shaft and Tunnel a