The tunnel is undergoing the second stage of works this week (beginning 27 January), meaning it will be closed overnight for much of the next month.
Smoke tests were carried out last week, to work out how quickly the tunnel fills with smoke in the event of a fire.
From this week, work is beginning on a structural survey, the first since 2015. This involves removing the steel panelling and allowing experts to assess the concrete lining.
To reduce disruption, the work is being done overnight 27 January - 21 February.
The tunnel will be closed Mondays-Thursdays from 7pm, reopening at 6am the following morning.
It's be open as normal Fridays, Saturdays and Sundays and during the day from Monday - Thursday.
Infrastructure Minister Andy Jehan
Infrastructure Minister Andy Jehan says the work is essential:
"It's a vital piece of island infrastructure. 20,000 vehicles pass through the tunnel each day, so it is important we maintain that part of our infrastructure.
"This is a part of that work - preventative maintenance."
The government will use the results from last week's smoke test and these surveys to decide what potential upgrades are needed.
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