Unanticipated soil conditions were the cause of the collapse of a section of Waller Street where workers were digging the city's light rail tunnel, according to a memo sent to city councillors Tuesday.

The assessment comes after Rideau Transit Group (RTG) had completed their analysis of the collapse, according to deputy city manager Nancy Schepers in a note to Ottawa mayor Jim Watson and city councillors.

RTG — the consortium of companies building the city's light rail system — is now developing a plan for resuming tunnelling operations, Schepers wrote.

RTG will be making an insurance claim to cover the extra cost, as the consortium of companies building the city's light rail system has responsibility over geotechnical risk in any cost overruns.

Hole opened on Waller Street on Feb. 21

A hole eight metres wide and 12 metres deep had opened up at Waller Street just south of Laurier Avenue in the early morning of Feb. 21, after a digging crew first noticed dirt falling into the tunnel underneath hours earlier. The road was already closed to traffic because of the tunnelling work.

RTG on Tuesday will be removing the temporary structure placed into the tunnel to contain the concrete that had been poured into the hole to patch it, Schepers wrote.

This will allow workers to drill into the concrete and give them more information when determining how to restart excavation, she wrote.

Storm and sanitary sewer infrastructure has been reconnected since the incident first occurred, and additional borehole investigations began last week, according to Schepers.

Excavation continues at the western and central entrances to the tunnel.