Today a Bus Rapid Transit system services downtown Ottawa. Approximately 180 buses, per hour, per direction, work their way through 14 lighted intersections. Meanwhile, vehicles, pedestrians and an ever-growing number of cyclists swarm through the same downtown corridor.

Throw in a special event, car accident, bad weather or an ambulance — or any combination of these — and buses are plagued by delays; transit downtown is unreliable and the disruption ripples through the system.

Not in 2017, though, when the City will operate a light rail transit (LRT) system in this core where ridership volumes are at their the highest. More than 50 per cent of buses currently running will be deployed in suburban communities.

"In addition, the use of LRT within the core will allow Ottawa to accommodate ridership increases over the coming decades much less expensively than by continuing to acquire buses and push them through already congested downtown streets," says John Jensen, director for rail implementation with the City of Ottawa.

He describes the future of downtown Ottawa, "For the 10km of existing grade separated Bus Rapid Transitway, there will be no bus operations, those will be entirely replaced by light rail vehicles. Downtown, where the system currently operates at-grade in a bus only lane, bus traffic will be signi-ficantly reduced. Cross town buses will be replaced by LRT operations in the 2.5km underneath downtown, only local bus service will continue on surface streets."

LRT will run from Tunney’s Pasture in the west to Blair Road in the east. For those not local to Ottawa, Tunney’s Pasture is home to the city’s federal government buildings and Blair Road is a major north-south artery.

Running 12.5km, the 13-station line includes a 2.5km tunnel through the city’s downtown to address the transit bottleneck (see Figure 1). The tunnel’s west portal will be located east of LeBreton Station, and there will be three underground stations (Downtown West, Downtown East, and Rideau) before the east portal located north of Campus Station. Each underground station will have platforms 150m long on each side of the tracks.

Planning
Ottawa’s Light Rail Transit (OLRT) project began in January 2007 with the mayor’s Task Force on Transportation, which recommended an east-west rail tunnel bored through downtown.

Planning and development continued with a preliminary geotechnical program by Delcan Corporation as part of the environmental assessment. In September 2010 the city hired Capital Transit Partners (CTP, a joint venture of Morrison Hersh¬ eld Limited, Jacobs Associates, STV Canada Consulting Inc, and URS Canada Incorporated) for the preliminary engineering program.

The city issued a request for quotation (RFG) in July 2011 and later in October announced a shortlist of three consortia – Ottawa Transit Partners, Rideau Transit Group and Rideau Transit Partners – that are competing in the project’s RFP phase. The City of Ottawa and Infrastructure Ontario will evaluate each submission and present a winning proposal to the city council for ratification later this year. With Federal environmental assessment approval announced this July, construction is expected to start in early 2013.

The entire light rail project is estimated at CAD 2.1bn (USD 2.08bn) and expected to complete in December 2017. OLRT is being procured as a Design-Build-Finance-Maintain (DBFM) project. The city is requiring the winning consortium to provide private financing of up to CAD 400M (USD 407M) of the construction costs to stand behind overall contract performance. DBFM proposals are asked to provide a fixed-price to complete construction and maintain the system for a minimum of 15 years.

Depth decisions
In 2010, the city hired Golder Associates to do a detailed geotechnical and hydrogeological investigation as part of the preliminary engineering program.

Boreholes were advanced using truck-mounted or track-mounted conventional drill rigs (operated by George Downing Estate Drilling of Grenville-sur-larouge, Quebec, and Marathon Drilling Company of Ottawa), or using a sonic drill rig operated by Boart Longyear of Marysville, Ontario. Stanton Drilling of Pakenham, Ontario, used cable tools or percussion drilling techniques to drive the boreholes for installation of pumping test wells, according to Golder’s December 2011 report.

After the final depth was reached, the boreholes requiring geophysical logging were left open (with a flush mount cover in place) for up to approximately one month until logging within the boreholes was carried out. On completion of the drilling operations, the soil and rock samples from the boreholes were transported to various laboratories for testing and then to an off-site storage facility.

Initially, the tunnel was designed to have four underground stations and run for 3.2km. A report released in December 2010 by Golder, after the first phase of its geotechnical investigations, recommended the City shorten the tunnel by 600m to 900m.

Rock samples from 34 boreholes drilled between May and August 2010 show the ground to be as expected except for in one area of the alignment where bedrock is 2m to 10m deeper than anticipated. Golder’s recommended design modification would have Campus Station on the surface – where it now is planned to be.

The December 2011 final report finds that for most of the downtown area, "the bedrock formations are anticipated to be flat lying to gently dipping, although zones of local folding are evident. In general, the limestone is medium bedded, but occasionally thinly to thickly bedded horizons exist with thin shale partings and interbeds."

Generally, the overburden in the area of the tunnel ranges in thickness from about 2m to 8m, except in two zones. The overburden is locally thicker at Rideau Street, "where an infilled valley in the surface of the bedrock is indicated to extend to depths in excess of 30m below ground surface." The overburden is also locally thicker at the east end of the tunnel alignment, where the surface of the bedrock is at depths ranging from about 7m to 15m.

The overburden generally consists of fill, related to underground services and or previous uses in the area, underlain by glacial till, containing sand and or gravel layers at some locations. Localised deposits of silt and silty clay/clayey silt were encountered along Rideau Street at the valley in the surface of the bedrock. A more extensive deposit of silty clay to clayey silt exists at the east end of the tunnel alignment.

Originally the alignment would be approximately 40m. During preliminary engineering, CTP identified a shallower tunnel by moving the alignment north by one block for the western portion of the tunnel (see Figure 2).

"The original alignment was chosen based on the understanding of the limited available geotechnical data at that time," Jensen says. He adds, "The adjusted alignment runs underneath Queen Street instead of tunnelling deep underneath building foundations with extensive underground parking structures. The result is a far shallower tunnel that is substantially less expensive to build and service."

The Ottawa City Council approved this change in design, among others in July 2011. "Prior to these design changes our project estimate was CAD 2.1bn [USD 2.14bn] in 2009 dollars; it is now CAD 2.1bn but in construction dollars — equivalent to CAD 1.74bn [USD 1.77bn] in 2009 dollars, a CAD 360M [USD 366M] savings," says Jensen.

Rideau riddle
One of the three underground stations, Rideau, is being relocated. Its original location underneath the Rideau Canal with an entrance on either side showed that 93 per cent of riders would be using the east entrance, says Jensen. "This would create an imbalanced station. As a result, the decision was taken to locate the station further east underneath Rideau Street, balancing out the station’s ridership in and outflows and bringing an additional 6,000 residents and employees within a 600m catchment (walking distance) of the station."

Flexibility regarding the precise location underneath Rideau Street has been provided to the consortia bidding on the project. Figures 1 and 2 show the zones of innovation for Rideau Station.

Most of the tunnel alignment is anticipated to pass through hard-rock ground conditions predominantly of limestone with lesser amounts of shale. As mentioned previously, the exception to this is in the area along Rideau Street where there is a "valley" in the bedrock surface. Here, the tunnel will be above the bedrock surface and will pass through glacial till.

The procurement process will finish this September when proposals are submitted and at the end of the year when the winning consortia is announced