The sudden road collapse that left a gaping hole in the middle of a street in downtown Ottawa Wednesday morning occurred over a vein of sand, silt and fractured rock so unstable that workers completing the light rail tunnel below had to inch along cautiously for fear of causing a cave-in.

The sinkhole occurred in the same place engineers conducting a geotechnical survey for the light rail project in 2011 discovered a 120-metre wide "bedrock valley" stretching under Rideau Street, just east of Sussex Drive.

The updated geotechnical data report submitted by Golder Associates to Capital Transit Partners, and then presented to the city, describes the valley as a natural trench in the bedrock filled with between 15 and 37 metres of loose fill, silty clay and "glacial till," or broken, weathered rock.

At its deepest point, the Rideau LRT station sits 27 metres below street level.

Samples taken from boreholes in the vicinity of the sinkhole revealed the bedrock itself was also fractured, unlike the solid limestone found along the rest of the light rail tunnel's route, according to the report, which was obtained by CBC News.

'A bad area'

"Everybody knew that that was going to be a bad area, that special precautions would have to be taken," said a source familiar with the light rail project.

People close to the project simply referred to the area as "rock valley," the source said.

Everybody knew that that was going to be a bad area, that special precautions would have to be taken. - A source familiar with Ottawa's LRT project

At a news conference Wednesday afternoon, Steve Cripps, director of Ottawa's rail implementation office, said special precautions were taken on that section of the tunnel, where workers were carefully excavating the final 50 metres between the future Rideau station and Colonel By Drive when the sinkhole formed.

Workers used lighter, more agile tunnelling equipment and carefully reinforced the tunnel as they dug, Cripps said. The city and consortium Rideau Transit Group met regularly with a mining and tunnelling consultant brought in to provide advice on that stretch of tunnel, Cripps said.

"Certainly the city's been aware, and RTG's been aware for a number of years, about the soil conditions in that area, and they have taken extensive precautions to deal with the soil in that area," Cripps said, adding nearby buildings have been equipped with seismic monitors to detect any shift in ground conditions.

Rail official won't draw link

However, Cripps was unwilling to draw a direct link between those soil conditions and the sinkhole.

Steve Cripps, director of Ottawa's light rail implementation office, speaks at a news conference Wednesday afternoon. (CBC) "We don't know if these soil conditions have caused this. It's really too early to speculate," Cripps said.

But another source familiar with the tunnelling project said everyone involved was well aware of the potential risk for failure near "rock valley."

"The worry was that there'd be pockets of disturbed material that didn't have the same strength as everything else," the source said. "That was the biggest challenge of the project, from my understanding … to construct the station underground, in soft soil."

Project officials haven't said whether shifting soil — or something else — caused the water main rupture that quickly filled the sinkhole with roiling, brown water Wednesday morning.

2014 sinkhole

Nor is it clear what sort of setback the sinkhole represents for the Confederation Line project, which is due to be completed in 2018.

"A lot of the focus was how tricky this would be, but it should be noted that they also almost finished it. This is the really sad part of the entire story," said the source familiar with the tunnelling project.

A sinkhole halted excavation near the LRT tunnel's east portal in 2014. It was filled with 700 cubic metres of concrete, then re-excavated, but the source said the void near Rideau Centre is much larger, and poses a greater challenge to fix.

The geotechnical report by Golder Associates only analyzed ground conditions, and didn't draw conclusions about the safety or viability of the tunnel in any one area.