A short section of the road, just south of Seaford College, has been closed since Saturday, October 2 for safety reasons.
West Sussex Highways said signs were still in place to aid with the diversion.
A West Sussex County Council spokesperson said: “Every effort is being made to find a lasting solution so that the section of the A285 at Duncton can be reopened as soon as possible.”
The council said this is ‘not a straightforward road repair situation’.
Issues include road subsidence and the challenge of a damaged carriageway which has a culvert spanning beneath it and carrying water from the adjacent large pond.
The spokesman added: “We have pulled together specialist teams and contractors, including geotechnical engineers, topographical experts and a diver for surveying beneath water level – they will all be working with our experienced highways engineers so the best solution can be designed.
"Timescales will only become clearer once that solution has been identified but the section of road is likely to remain closed for some time.
“Meanwhile, work over the next few days will include vegetation clearance, trial pits and CCTV investigations. Extensive signage, including electronic variable message signs, has been placed at key locations to pre-warn road users of the closure.”
Businesses have been able to open as usual and the council is working with Seaford College to ‘minimise the impact as much as possible’.
“We apologise again for any inconvenience,” the spokesman said.