No-one was injured in the incident on March 9 in west London when temporary concrete broke free from the tunnel portal face.
A spokesman for Crossrail confirmed that an investigation was under way.
The Crossrail spokesman said “On Friday 9 March, a small amount of temporary concrete came loose and broke free from the face of the tunnel portal at Royal Oak.
“No injuries resulted and repairs were quickly undertaken.
“Crossrail regards the safety of all its employees and contractors as paramount and is committed to delivering Crossrail in the safest manner possible.
“Construction of Crossrail has been underway for nearly three years and during that time the project has had a good safety record.”
Rail union RMT leader Bob Crow said: “Clearly this was a major incident which could have had serious repercussions and rather than the attempt to sweep it under the carpet there should be a full Health and Safety Executive investigation into the structural failure that caused this tunnel collapse to ensure that lessons are learnt and it doesn’t happen again.”