Climate Change Factor of Safety Assessment, Eglinton Crosstown Subway

Client: Crosslinx Transit Solutions

Ultimate Client: Metrolinx and Toronto Transit Commission

Key Personnel: D. Kassenaar, E.J. Wexler

The Eglinton Crosstown Subway is a $7B subway project spanning a 22 km route through central Toronto, Ontario, Canada. The central portion of the route is being constructed using earth pressure balance tunnelling technology while the subway stations are constructed with large scale temporary dewatering.

Earthfx was contracted to assess the long-term effects of climate change on the “factor of safety” thresholds used in the subway station buoyancy calculations. The station box designs must include sufficient concrete mass to counter buoyancy effects associated with the potentiometric heads in the underlying aquifers. The effects of future climate change are expected to result in warmer and wetter climate conditions, resulting in a rise in groundwater recharge, groundwater levels, and buoyancy effects.

Building on earlier work and local expertise, Earthfx constructed a fully-integrated surface water/groundwater model of the City of Toronto using the USGS GSFLOW numerical model code. The model represents urban land use, soil zone process, streamflow and groundwater flow in multiple shallow and deep aquifer systems.
Once calibrated, the “change field” method was used to statistically downscale global climate model results and modify local climate records to reflect climate conditions 100 years from present. The process was repeated for 10 different global climate models to evaluate the uncertainty in future climate predictions.

Long-term baseline simulations provided useful insight into the normal range of seasonal and interannual fluctuations in water levels along the subway line. The baseline insights helped the design engineers understand, interpret, and extrapolate the very limited short-term groundwater monitoring data that were available from site investigations. Baseline model simulations also helped identify gaps in the data set. The tunnels were advanced with earth pressure balance tunnelling technologies, so there were less hydrogeologic data available between stations (which will be constructed using dewatering techniques).

Future simulations indicated that the magnitude of the effect of climate change will vary along the subway line. In general, water levels will be higher in the future due to the increase in precipitation and change in winter processes, as shown below:

Seasonal and interannual fluctuations in water levels become more extreme under future climate conditions. The effects of climate change vary locally due topography, urbanization, recharge, and aquifer storage.

At some locations, the maximum rise in water levels is constrained by aquifer structure (geometric controls) that will cap the water level rise and limit climate change effects. The hydrograph shown below illustrates that even under a wetter future, groundwater levels are naturally capped at this station location due to local groundwater discharge.

Model results were used to assess and revise the water level “factor of safety” thresholds for the station designs. The ability to separately identify the effects of seasonal processes, interannual (wet year/dry year) and climate change affects, all while accounting for aquifer storage and response lag, was particularly valuable to this major infrastructure project.