The continued rapid growth of many Canadian cities is dependent on development of essential municipal infrastructure, including underground infrastructure for transit expansion, water, wastewater, and flood protection to meet future needs. 

Ongoing transit projects in the Greater Toronto Area (GTA) include the Eglinton Crosstown West Extension, Ontario Line, Scarborough Subway Extension, and Yonge North Subway Extension. The City of Toronto is also investing in significant water infrastructure tunnel projects to protect communities against basement flooding – such as the Fairbanks Silverthorne Storm Trunk Sewer System Project – and to eliminate the release of combined sewer overflows to Lake Ontario, including a 22-kilometre tunnel for the Don River and Central Waterfront Wet Weather Flow System and a 3.5-kilometre tunnel for the Ashbridges Bay Treatment Plant Outfall. 

(Photos by 30 Forensic Engineering) 

In the Greater Toronto Area (GTA), the designers and constructors of underground infrastructure must consider a wide array of ground conditions. The geology in the GTA includes glacial lake deposits consisting of gravel, sand, silt, and clay, glacial ice deposits mostly consisting of young clayey silt till, and bedrock consisting of shale, interbedded siltstone, and minor limestone. Each of these materials requires specific considerations at the design and construction stages of a project. 

Shored excavations for subway stations, tunnel shafts, and shallow tunnels excavated in glacial deposits need to account for heterogeneity of the soil deposits and the resulting complex groundwater conditions which may be controlled by localized stratigraphic features such as thin low permeability clay layers. 

For excavations and tunnels in the deeper shales bedrock, the mechanical behaviour and groundwater flow patterns are controlled by rock discontinuities that may convey significant groundwater rates of flow.  

Such localized features in soils and bedrock may be difficult to capture in conventional geotechnical investigation programs and may lead to unforeseen problems during construction, including undesired effects in aboveground and underground structures near the construction zone.    

Another significant issue in deep excavation/tunnelling in shales is the effect of construction itself. Drilling, excavation, and tunnelling may cause changes in stress conditions in the rock and in the expansion and propagation of existing joints, or formation of new fractures in deep shales, altering the rock macroscopic behaviour with respect to the “design conditions.” 

These issues are often the causes of multi-milliondollar insurance claims and legal disputes. Forensic analyses must investigate the impact of such complex issues.  

Post-loss forensic geotechnical investigations are often aided by the use of novel smart technologies that allow the attainment of large amounts of data about large masses of soils and bedrock with relatively low efforts, timescales, and costs. These include penetration testing instrumented with sensors, geophysical surveys, and innovative remote sensing approaches. 

The perception that such technologies are still experimental and costly is, in this writer’s experience, generally unfounded. Rather, geotechnical investigations that include the application of appropriate novel technologies often succeed in identifying problems, minimizing construction and post-construction issues and delays and, ultimately, saving money.  

In the forensic analysis of underground infrastructure projects in the GTA, in addition to addressing the technical issues related to complex subsurface geology and hydrogeology, it is also important to consider “standard of care” (i.e., level of skill and diligence that a reasonably competent engineer would exercise under similar circumstances) in the selection of investigation methods. Novel technologies can no longer be dismissed in favour of conventional approaches, as they elevate the sophistication of investigations, while delivering significant advantages through the project lifecycle.