New details of issues encountered by Crossrail when tunnelling under central London have emerged. They include an incident which sent bricks from a Victorian well “dramatically flying through the excavation”.
Transport for London (TfL) principal geotechnical engineer Mike Black revealed the dramatic incident at the Railway Civil Engineers’ Association lecture on the engineering challenges faced by Crossrail 2.
He added that the Crossrail 2 team was learning from issues encountered by its predecessor during planning for the line which is to run from Surrey to Hertfordshire.
During Crossrail tunnelling, Black revealed that bricks were sent flying after tunnelling passed by a Victorian well. He said that the tunnel passed close to a full well and that the water pressure was enough to burst the side of the tunnel. A huge grouting programme was then needed to stabilise the ground.
“Some bricks flew dramatically through the excavation, no one was hurt and we backfilled it and continued, but we had no idea that was there before hand,” he said.
“Obstructions are a very big issue and we spend a huge amount of time on desk studies trying to work out where everything is or where it might be. For some of the older buildings the records don’t exist.”
The risk of hitting unknown variable ground conditions when tunnelling was another example of the risks the project faced, he said.
On Crossrail, Black said due to restricted access to carry out ground investigations in the area around Moorgate, the team did not know about a scour feature – an area of highly variable ground – until later in the day.
When it was found to be filled with water, he said a “massive” phase of ground treatment grouting was required to stabilised the ground.
“This was extremely variable conditions and we didn’t expect it to be quite as variable as it turned out. A lack of access meant we that we couldn’t drill in the areas we wanted to for ground investigations. Moorgate itself is a major highway so we couldn’t close that,” he said.
“The area where the [station] box was constructed had buildings on it so until they were demolished we couldn’t do any boreholes there either so we didn’t know about this particular feature until later in the day.”
Despite the recently announced delay of up to a year, London transport bosses have vowed to press ahead with Crossrail 2, with Transport for London (TfL) commissioner Mike Brown saying he was “very confident” when asked whether TfL still planned to pay for half of Crossrail 2.
The new line is planned to run from Surrey and South West London to North London and Hertfordshire via twin bored tunnels.
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