A large sinkhole formed on Waller Street when tunnelling for the new LRT line passed beneath a previously excavated construction pit, according to a technical memo released by the City of Ottawa on Friday.

Ottawa Light Rail Transit Constructors wrote in the memo that the mining crew noticed "excessive loose and wet material start to fall" around 9:40 p.m. on February 20. By 1 a.m. on February 21, the sinkhole reached the surface on Waller Street near Laurier Avenue, leaving an eight-metre wide opening in the street.

The OLRTC wrote it had "no indication of concern" prior to the "sinkhole anomaly," and that mining operations were "proceeding according to approved plans and drawings."

"The sinkhole event was, as could be surmised, unexpected," said the memo, signed by OLRTC technical director Roger Schmidt.

A technical analysis found that the previous excavation was filled with "poor quality, un-compacted backfill," according to the memo.

Waller Street sinkhole (Feb. 21, 2014)

This sinkhole formed on Feb. 21, 2014 after a road collapse in downtown Ottawa. (Photos courtesy of John Holtby and Brian Bancroft)

"The report does identify that the tunnel crossed beneath a previously excavated construction pit that was deeper than the known utilities and likely extended from bedrock to the surface. The shape of the old pit closely matched the shape of the sinkhole that opened up," the memo detailed.

Moving forward with tunnelling

The OLRTC wrote that the City of Ottawa's "extensive geotechnical investigation," followed by its own investigation after being awarded the construction contract, remains sound.

The company expects to resume tunnelling after some investigative drilling, including additional boreholes.

"The sinkhole anomaly was localized enough to not be caught by the borehole investigations. We do not expect another such incident, nevertheless we are currently reviewing our borehole logs and drawings to see where any other such an anomaly could potentially occur," the memo said.

For mobile users, click here to view the technical memo.