A Second Tunnel Proposed in Ottawa's Downtown

Post date: Feb 12, 2013 3:22:36 PM

In December 2012 the City of Ottawa released a detailed environmental assessment for a downtown tunnel that is proposed to be constructed for reducing the overflows of sewage into the Ottawa River. The plan is to dig a 3m diameter tunnel across downtown and then through Sandy Hill/Lowertown which will be used to store excess run-off from the combined sanitary and storm sewers during high rainfall events. Apparently there are no unreasonable technical challenges that stand in the way of such a project. It should be noted that a significant portion of the proposed tunnel is to be routed through the same basic area of Sandy Hill/Lowertown as a vehicle access tunnel to the Macdonald-Cartier bridge would be.

The Study proposes using a tunnel boring machine to dig a 4.4km long tunnel which is 3m in diameter.

The tunnel basically follows a route that:

• starts at Lebreton flats,
• goes under Laurier Ave (approximately)
• under the canal,
• crosses the LRT tunnel
• turns north around UofO's Taberet Hall,
• follows Cumberland St north to around St Patrick
• continues east to Stanley Park in New Edinburgh

The cost for just the tunneling work is estimated at 62m$.

It appears that the main cost of making tunnels is digging out and removing the rock. The volume of rock to be moved is 4.4km (long) * 3.14 (pi) * 1.5m * 1.5m (radius squared) = 31,000 m3,

Now compare to a vehicle tunnel that could be considered to join the Macdonald-Cartier bridge to Hwy 417 via Nicholas St south of Laurier Ave. This would be virtually parallel to much of the route of the proposed sewage storage tunnel.

Volume of rock = 1.7km (long) * 13m wide * 8m high = 150,000 m3

150,000/31,000 (sewage tunnel rock to be removed) = about 5x more rock * 62m$ = 289m$ = approximate cost for basic tunneling of the downtown truck tunnel.

Remember that the approximate cost of a new east end bridge has been quoted as 500m$ (in 2008). These estimates were updated in 2013 to a projected cost of approximately 1.1B$

There are additional costs to add for a downtown vehicle tunnel.

• cost of the boring machine
• cost of paving, emergency response/safety measures
• ventilation

However, after adding an allowance for these, the results could still be in line with what is being proposed for spending on an east end bridge. This is another data point that suggests that there is nothing unreasonable about considering a downtown tunnel which could carry all 2500 trucks per day currently plaguing the downtown, in addition to up to 20,000 vehicles which make the journey between the bridge and the 417 through downtown each day.