By Susanna McLeod
Special to Ontario Construction News
A solution to the pressing traffic problem on St. Clair River in the late 1800s was long in coming. Ferries and boats jammed the waterway, loaded with passengers, cargo, and grain destined for American locations. The first subaqueous railway tunnel in the world opened in 1891 in southern Ontario, a miracle of construction engineering. Prominent engineer Joseph Hobson developed specialized machinery for the massive undertaking.
A surveyor, Hobson (b. Guelph Township 1834) worked his way up to rail line planning and bridge developments, and o to large railway projects. His reputation as a proficient engineer was cemented in the 1870s with his work on the railway bridge over Niagara River between Buffalo and Fort Erie. Promoted to Grand Trunk Railway’s chief engineer, Hobson led multiple large construction projects. When St. Clair Tunnel was in planning stages, “Joseph Hickson, the Grand Trunk [Railways] manager, insisted on Hobson, and eventually he prevailed,” stated Steven Thorning in “Mosborough engineer was considered best in Canada,” Wellington Advertiser, September 21, 2022.
Bolstered by years of experience, Hobson formulated solutions for successful drilling under the riverbed’s soft blue clay. “Several original tunnelling techniques were devised by Hobson for this project including a compressed air shield for the tunnel excavation and the installation of the cast iron liners,” said Canadian Civil Engineering History & Heritage.
Hobson studied valuable reference material from British engineers who built similar but smaller tunnels. He included “a diaphragm with closable doors” introduced by James Greathead and modified “sharpened horizontal shelves” from Alfred Beach’s design, according to Robert M. Vogel in “Tunnel Engineering, A Museum Treatment.” Smithsonian Institution 2012. Hobson’s enormous shield was about to dig the largest tunnel in the world, capable of handling full-scale railway traffic.
Tunnelling began simultaneously at Sarnia and at Port Huron, Michigan in 1888, but “before reaching proper depth the almost fluid clay and silt flowed up faster then it could be excavated,” said Vogel. Redesigned, tunnelling restarted. “Long open approach cuts were made,” and “the portals were established in the cuts, several thousand feet back from each bank and there the tunnelling began.”
On August 30, 1890, the shafts met. Remarkably, the tunnels were less than half a centimetre off.
Hobson’s shield design was made of steel cylinders that were 6.4 m in diameter. “Hydraulic jacks pushed the shields forward into the clay beneath the river like a cookie cutter,” noted Thorning. Workers loaded carts with materials to haul up to the surface, “then the shield was pushed ahead another two feet, and the digging process was repeated.” Workers following behind the shield installed 2.5 cm-thick steel plates, bolting the pieces together for the tunnel lining.
Using compressed air, Hobson pressurized the tunnel to keep the soft clay and water out of the working area. His technique had substantial problems. “At one point he raised the pressure to 28 pounds,” said Thorning. The method was unproven, and “rapid decompression produced severe cases of bends in his workers. Three died and several were crippled from the effects.” In other aspects, working conditions were better for the crews, with 8-hour shifts, a flow of fresh air, and electric lights. Over 600 workers were employed on tunnel construction.
Rainfall presented the potential for flooding, an alarming prospect that could damage machinery and threaten the project. Rainfall was controlled by steam-operated drainage pumps, powered by boiler plants at both entrances. Workers were assigned to the station around the clock to prevent disaster.
Preparing for tunnel completion, four specially designed steam locomotives were commissioned, “designed to handle the high tractive effort required to operate the trains over the grades in the tunnel [about 2%] and on the approaches,” said F.A. Sager in Electrification of the St. Clair Tunnel, Grand Trunk Railway System, Montreal 1908. The engines were “arranged to burn anthracite coal, in order to minimize the inconvenience due to excessive smoke in the tunnel.”
The completed subaqueous tunnel section was nearly 1131 metres in length, and on September 19, 1891, the groundbreaking St. Clair Tunnel opened for rail traffic.
Within a few years, electric locomotives were suggested for better air quality, along with installation of electrical services for roundhouse operations, lighting throughout, and pumps. Examining several options, a “decision was finally made in favor of alternating current, using a 3-phase system for the distribution of power required for pumping and for shop motors with single phase distribution for locomotives and lighting,” according to Sager. Electrical services were completed in 1908. Electric locomotives were used for several decades until the introduction of diesel train engines.
A historical plaque with a segment of cast-iron tunnel lining was installed in 1990, recognizing the tunnel as a National Historic Civil Engineering Site. In operation until 1994 when the nearby Paul M. Tellier Tunnel was constructed, the original St. Clair Tunnel was closed and sealed.
© 2024 Susanna McLeod. McLeod is a Kingston-based freelance writer who specializes in Canadian History.
As a senior in Ontario I feel stupid not knowing about this tunnel.
I blame our education system that wasted time with curriculum that offered little knowledge.