The road from Greymouth to Kaiata is closed until at least Monday as the slip at Omoto continues to move significantly, the New Zealand Transport Agency says.
The agency's contractor Fulton Hogan dumped multiple truckloads of crushed gravel into the slumping roadway on Thursday afternoon.
It had slumped by more than a metre over the previous 24 hours.
It continued to move on Thursday and showed "no signs of slowing down", NZTA West Coast maintenance contract manager Moira Whinham said.
State Highway 7 was reduced to a single lane with traffic management and close monitoring since mid-August when the historic slip began to move following heavy rain.
"With more rain forecast it will remain closed through the weekend and reassessed Monday afternoon," Ms Whinham said.
Wet weather and saturated land at the slip site in recent weeks had contributed.
Until this week, NZTA crews were able to maintain a single lane of traffic between Greymouth and Kaiata to keep the highway open.
"Ground investigations have now been completed," Ms Whinham said yesterday.
"We are considering a number of medium- and long-term remedial options, working closely with Kiwi Rail and our geotechnical advisers on a solution for both the rail and the road. But, we need the ground to stop moving before these can be put in place."
Ms Whinham said at this stage the best thing to be done was to continue to monitor the rate of movement, keep the highway closed and the detour in place "for everyone's safety".
"We thank local people who are inconvenienced by this closure for their patience on the detour and we will be doing whatever short-term measures we can to manage ground movements and saturation at the site."
These included reinstating the swale-depression at the road to prevent additional stormwater entering the slip and the head of the slope.
NZTA would also investigate whether any changes could be made to the culvert east of the slip site to better cut off stormwater at this point.
Meanwhile, State-rail operator KiwiRail says it is in the same position until early next week at least.
The Tranz Alpine is being terminated at Arthur's Pass with passengers transferred to buses to travel to Greymouth via Moana.
KiwiRail South Island operations manager Janine Benson, said the historic slip area needed to stabilise before work could begin on the line at that point.
"We will be reassessing the situation on Monday."
She declined to say which freight customers were affected but confirmed there were three.
"Their identities are commercially sensitive."
However, Westland Milk Products said on Thursday it had contingencies in place with the daily train from Hokitika to Christchurch not able to run.
Rail for Mainfreight's Greymouth depot and the recently instituted log trans-shipping operation in the Elmer Lane yard previously undertaken at Stillwater would also be affected.
- By Brendon McMahon
