Crews recently started tunnelling the final set of tunnels for the Eglinton Crosstown West Extension, which will connect the extension to future TTC Line 5 at Mount Dennis Station. 

The extension will make the Eglinton Crosstown LRT (to be known as Line 5 Eglinton when it opens) another expansive east-west rapid transit line for the Greater Toronto Area (GTA), connecting commuters all the way from Scarborough, through midtown Toronto and into Mississauga. The project will add seven new stations over nine kilometers to Line 5 Eglinton’s 25 stations and stops.  

Unlike the 6.3-kilometre stretch of tunnels on the western end of the line which were excavated using tunnel boring machines, the 500-metre twin tunnels under construction are being built using the sequential excavation method, also referred to as SEM. This is a method often used for smaller sections of tunnels and is preferred for projects in urban areas where space is constrained.  

Unlike other methods of tunnelling, which fight the pressure of the surrounding soil and rock, SEM works with the ground, using the surrounding geology as part of the tunnel’s structural support. SEM relies on something called observational engineering. The tunnel is monitored during excavation and engineers can make real-time adjustments during tunnelling. 

How sequential excavation works 

Workers excavate sections of each tunnel bit by bit. The area to be tunnelled is first divided into small sections, which are then dug out in sequence using excavators, starting with the top sections. The tunnels are carved out in small sections, called rounds – sort of like slicing a loaf of bread. tunnels are carved out in small sections, called rounds – sort of like slicing a loaf of bread.  

The next step is mucking, or removing the materials that have been excavated, as each section is dug out. Mucking involves hauling away rock, soil and other debris that would otherwise block the tunnel.  

Once the mucking is complete, crews apply shotcrete, a special type of concrete sprayed onto the tunnel walls. This is like building a tunnel with spray paint. Shotcrete helps reinforce the tunnel structure and provides the first layer of support. It’s a quick-hardening material, which means it can stabilize the excavated area almost immediately, making sure the tunnel remains secure before moving on to further digging. 

After reinforcing each section with shotcrete, workers then add a final lining made of cast concrete. This provides the tunnel with its permanent structure, adding strength and durability. The final lining also protects the tunnel from any wear and tear that might occur over time. 

With major construction in full swing on the tunnels, the extension is now one step closer to bringing 37,500 more people and 23,600 more jobs within walking distance to transit.

To keep up with progress on the tunnels, follow the Eglinton Crosstown West Extension on Instagram, X, and Facebook.