Ottawa Trillium Line – Caissons

The Project

The Stage 2 Trillium Line South Extension project includes eight new stations and sixteen additional kilometres of new track. This project will extend the benefits of the O-Train from Greenboro Station to Limebank Road in Riverside South, bringing southern communities such as Manotick, Findlay Creek, Greely and Osgoode much closer to this high-quality transit service.

As part of the project, numerous rail-over-road structures are needed to be built to take the tracks over existing infrastructure and roadways. Caissons are being used to provide the base for these structures.

The Solution

Caissons, also known as drilled shafts, offer a higher capacity and larger lateral resistance than H-Piles or Pipe Piles. As a result, they decrease the number of foundation elements needed to meet seismic requirements for new structures.

Caissons for this project were between 1500mm & 1800mm in diameter and up to 30 meters deep. The geology in this area presented a challenge, as there was the potential for boulders in the sandy ground. As well, artesian conditions were present, which further impacted ground stability and added the potential for sediment in our casing or washout of our cement when filling with concrete. Finally, these caissons were all being installed in close proximity to major roads and live traffic, requiring close monitoring to prevent settlement from vibration and excavation.

Our inhouse engineering team designed a road side protection system using sheet piles, which we monitored to prevent any settlement issues. Our engineering team then recommended the proper tooling based on their review of the geotechnical reporting to deal with potential boulders. The artesian conditions were countered by our experienced production team maintaining proper head pressure while advancing the casing.

This solution enabled the project to be completed within the schedule requirement.