Shani Wallis and Peter Kenyon, TunnelTalk
- Roadheader excavation of the competent limestone beneath the streets of Ottawa began in late 2013 on the program to create the 2.5km long central subsurface reach of the city's new LRT line and its three underground stations.
- Design-build-finance-and-maintain construction of the 12.5km long Confederation Line was awarded in December 2012 to the Rideau Transit Group (RTG) consortium for a fixed-price delivery contract of Can$1.2 billion. The international consortium is led by ACS Infrastructure of Spain with Dragados of Spain and Montreal-based SNC-Lavalin Constructors as the main construction partners with a team of consultant and design engineers that includes SNC-Lavalin design division, Hatch Mott MacDonald, Dr Sauer & Partners; and Veolia Transportation.
Fig 1. Excavation sites for the underground LRT route
- The design of the 2.5km of underground works is based on single-tube, double-track running tunnels connecting three underground open-cut stations and between two transition portals to surface and elevated guideways for the remainder of the 12.5km route to the north west and south east of the downtown section (Fig 1). Excavation of Ottawa's competent limestone is an open-face SEM on a top heading and bench sequence, excavated using roadheaders and support with shotcrete and other elements of the SEM toolbox of techniques including pipe-roof arch and shotcreting of the face where necessary.
- Within the confines of the downtown urban environment, excavation advance working sites were established at the west and east transition portal zones and from a central mining shaft between Lyon and Parliament Stations (Fig 1). Three roadheaders, named for identity Jawbreaker, Chewrocka and Crocodile Rouge, and their tunnelling crews, were making good progress until circumstances and conditions collided at the early stages of the East Portal advance and a ground run in the crown of the top heading excavation advanced to create a sinkhole on the intersection of Waller Street and Laurier Avenue above.
- Tunnelling at the East Portal had started on 12 December 2013 and had progressed about 28m under the second round of overlapping pipe roof arch pre-support and was at the start of 24-hour mining operations at the time of the sinkhole incident.
- The incident started to occur at 10pm on Thursday night (20 February) when "our tunnel excavator and inspectors on site noticed some material coming through the crown of the tunnel at this point," explained Tim Stewart, LRT Construction Director, at a media brief after the event. By 1am Friday morning (21 February) the section under had collapsed, creating a sinkhole measuring 8m wide and 12m deep. By then the crew and roadheader had been pulled back so no-one was hurt and there is no damage to buildings on the streets either side.
- This early part of the drive westwards from the East Portal construction site passes through particularly soft/sandy soil before reaching the relatively shallow limestone bedrock that characterizes most of the alignment.
- Nancy Schepers, Deputy City Manager, told a media briefing at City Hall the day after the incident, that: "As they continued to observe material entering the tunnel, and the rates increased, the crew suspended tunnelling operations and immediately notified Rideau Transit Group's management to discuss next steps for implementing the stabilization strategy. We were aware of different geotechnical conditions in this vicinity and some of you will recall that before we went out to tender we actually shortened the length of the tunnel [which was to have been 3.2km long originally], recognizing that this type of material would be challenging. Monitoring equipment has confirmed that the impact is localized, and the geotechnical team has not identified any safety concerns at this point. Tunnelling will resume when both ourselves and RTG [the construction consortium] are comfortable that it is safe to do so."
- Excavation at the western and central jobsites continued as normal while the situation at the eastern portal was being stabilized ready for a restart.
- Kathryn Keyes, Communications Director for Rideau Transit Group (RTG) said of the incident: "It is because this site is particular in its geotechnical composition and that is something that we will look at in our analysis of what happened."
- Further investigations of the incident by TunnelTalk, and after speaking on the phone and connecting via email with several managers associated with the project for the owner and the construction consortium, reveals that there is evidence that the incident was caused by the combination of a shallow cover top heading as it gains depth from the portal transition zone, a zone of soft clay and sandy clay in the upper left corner of the top heading face, and a set of earlier and undocumented utility works in the intersection of the street.
Breakthrough of non-virgin material
Sinkhole above the utility corridor
Broken utilities and evidence of earlier sheet piling
- An overlapping pipe roof arch of pre-support was being installed through the mixed face, shallow cover start of excavation from the portal launch heading to the point where a full face of limestone could be confirmed by the geological investigations.
- When the start of a ground run began, it forced out a pocket of virgin clay and sandy clay deposits and what followed was break in of evident rubble material indicating a trench or surface excavation backfill operation. It was accompanied with an inrush of water and "a distinct sanitary smell".
- The sewer utility buried pipeline had broken and further evidence indicated that this pipeline or the accompanying water supply pipes in the same utility corridor or both had been leaking for some time. The concrete backfill of the sinkhole took more than the collapse material in the tunnel, indicating a pre-existing void under the street.
- Further examination of the walls of the sinkhole itself from the surface provides evidence of a rectangular excavation supported by sheet piling around the utility corridor that was later removed, once the access excavation was backfilled with un-compacted rubble.
- While investigations and recovery works continued, the emergency stabilization measures implemented by the construction crews, including creation of a bulkhead in the tunnel for concrete backfill of the sinkhole from the surface and delivery and pouring of an initial 345m3 of concrete into the void and a subsequent second stage concrete fill of about 280m3, "was completed within about 36 hours from the start of the sinkhole event. This immediate reaction certainly avoided any further deterioration and damages to the surrounding area and structures," reported a construction JV manager.
Progress on three headings to date
The project had just celebrated a 5 February milestone of 10% completion of mining when the incident happened. Excavation on the project started in early November and had advanced 250m to late February 2014, while at the central shaft controlled drill+blast excavation in October had completed the first 12m ahead of assembly and commissioning of the site's roadheader.
- Joint funding of the $2.1 billion Confederation Line is Can$600 million from the Government of Canada, another $600 million from the Province of Ontario and $161.5 million from the City of Ottawa with a $287 million City of Ottawa allocation from its Federal Gas Tax Fund. The City of Ottawa will also allocate Provincial Gas Tax receipts to the capital infrastructure. The remaining project budget funds will come from development charge and transit reserves.
- The 12.5km electric light rail system is the first stage in Ottawa's future rail network and is designed to replace existing diesel powered buses on the route between Blair Station in the east and Tunney's Pasture in the west. The route includes 13 stations and the 2.5km route under central Ottawa will alleviate congestion through the downtown core.
- As tunnelling advances uninterrupted from the West Portal and Central shaft headings, work is expected to resume at the East Portal "without extended delay" was the construction JV report from Ottawa.
Ottawa awards $2.1 billion Confederation Line - TunnelTalk, December 2012
Ottawa announces shortlist for LRT - TunnelTalk, November 2011
Plan for LRT under downtown Ottawa - TunnelTalk, January 2010
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