Photos emerged on Tuesday (November 28) showing workmen on a crane alongside the Shoreham Flyover.
Ahead of the works, which began on Monday, a spokesperson for National Highways said: “We’ll soon be carrying out essential maintenance work on the A27 Adur Viaduct at Shoreham.
"This includes repairing and carrying out maintenance on more than 150 bearings, which are the joints that attach the road deck to the supports below. This work is critical to maintaining and improving safety.
“We’ll do as much work as we can at night using lane closures or with the road open, but there will be occasions when full closures are needed. We’ll provide more information here as the work progresses."
Works will continue until the summer of 2024, National Highways said.
All road closures will be between 8pm and 6am, ‘unless stated’, the UK Government agency added.
From Monday this week until Monday, December 18, there will be weeknight closures of the eastbound exit slip, with lane closures on the A27 eastbound where it passes over the Adur Viaduct.
National Highways said a lane will remain open for traffic ‘on the main carriageway’.
A diversion route is in place, with motorists asked to continue on the eastbound A27 and travel to the Holmbush junction, ‘where you can join the A27 westbound and return’.
"What looks like usual overnight lane closures on the #A27 where it crosses the River Adur near Shoreham, are actually more than they might appear,” a National Highways spokesperson said.
"Underneath the road we’re working to repair more than 150 bearings. Bearings are the joints that attach the road deck to the supports below.
“The A27 Adur Viaduct runs high over the river and valley below, so to carry out these essential repairs, we’re working on raised platforms under the road. And to help keep those on the platforms safe, there’s a support team in boats on the river.”
The agency said each bearing ‘needs cleaning and checking’. Repairs are then completed, before the bearing is greased.
"The grease makes sure the joint can flex with the movement, expansion and contraction of the road and concrete supports.
"Without the ability to move, the road or supports would crack and need to be replaced, so this work is critical to maintaining and improving safety.
“The lane closures are there to keep those working under the carriageway safe, and stop vehicles passing over the bearings as they are being inspected or worked on.”