Helicopters drop water to sluice slip above highway
MORE than 150,000 litres of water were used to control and flush a landslip near Cromwell Gorge which threatened to close State Highway 8 yesterday.
The slip, which measured roughly 30m from top to bottom, 20m wide and 15m deep, was made up of schist, earth, tussock and other vegetation and sat precariously above State Highway 8 near the intersection of State Highway 8B at the Cromwell Bridge.
NZ Transport Agency maintenance contract manager Mark Stewart said more than half a dozen crew and two helicopters tackled it — including road crews, engineers, a geotechnical adviser and traffic control using Stop/Go manual traffic management for about 100m in both directions from the intersection with SH8B.
Plans to put staff on the slope were put on hold as it was ‘‘too unstable’’.
Early fears the road would close overnight did not eventuate but traffic was reduced to one lane in the immediate path of the slip with close monitoring, to protect road users.
Mr Stewart said options were limited as the slip was ‘‘actively moving’’ and too unstable for them to use other methods.
The Queenstownbased helicopters tackled the slip with monsoon buckets from about 1pm.
Each carried roughly 1000 litres of water — scooped up from Lake Dunstan downstream from the bridge.
All traffic was stopped for brief periods while the helicopters worked in rotation to sluice debris.
‘‘They [the helicopters] use that water to drop on to the unstable slope . . . the weight of the water and the movement of the water washes down all the loose material up there.’’
He described it as a ‘‘substantial’’ slip which had the capability of causing significant damage.
‘‘I have calculated approximately 150,000 litres of water have been put on the slope.’’
The helicopters were stood down shortly before 4.30pm.
Mr Stewart said road user safety was the ‘‘primary concern’’.
‘‘If we were not able to get up there to do the sluicing because of fog — we would have closed the road.
‘‘Rocks and small amounts of material were continuing to fall. That sluicing brings down all that material that is likely to fall into the line of traffic.’’
A woman across the lake from the slip told the Otago Daily
she spotted it from her kitchen window early yesterday morning and said days of rain followed by ‘‘two good frosts’’ might have led to the earth moving.