Mohaka job doubled in size because of underslip: 9-metre drop a challenge for workers

Work continues on cyclone recovery road works at various sites on State Highway 2 south including a challenging “underslip” fix at Mohaka. 

Transport Rebuild East Coast Alliance (Trec) crews have repaired the slip site alongside the Mohaka River, one of many sites being worked on along the highway. 

Underslips occur on the downhill side of the road. They don’t always directly affect the road’s surface but pose a risk if unaddressed. 

“Underslips can have a devastating effect on the land below the road,” a NZ Transport Agency Waka Kotahi spokesman said. 

“Extremely steep terrain on the bank made this a difficult site to fix. 

“The steep drop seen in the image (provided) is 9 metres deep from the road level, so it was a challenge to get the heavy machinery in the right position to make repairs.” 

The spokesman said excess water from a damaged culvert meant the road surface had started to crack and an underslip had developed. 

“To reduce further deterioration to the underslip and the road, the water was temporarily diverted while we repaired the slip and culvert. 

“Heavy rain during repair work made the underslip worse and exposed another damaged culvert. The worksite nearly doubled in size.” 

To minimise scouring (erosion), crews built a dam-like structure in the northern part of the stream bank to slow water flow when there’s heavy rain. 

“On the southern end they excavated and repaired the bank. Channels have been installed on all the culverts to redirect the water flow on the hillside. These repairs will make this section more resilient to heavy rain.” 

This spring crews will work on five other recovery sites between Tūtira and Wairoa on SH2.