Innovative repair for slips
Residents living alongside a walkway reserve at O¯ mokoroa can sleep easier at night now landslips below their homes have been repaired.
The walkway reserve between Bramley Drive and Ruamoana Place has been closed since the slips occurred during the two cyclones in March and April 2017.
A ribbon-cutting event on Friday to re-open the walkway was attended by the residents whose properties were affected by the 2017 slip, along with Western Bay of Plenty Mayor Garry Webber, elected members, project staff and contractors.
The slip-damaged section was repaired using a trial method of drilling vertical chimney drains down the damaged cliff faces, linked with directional drilling, to channel groundwater out to sea from the multiple layers of the cliff.
Council commissioned specialist engineering consultants Tonkin and Taylor who created the innovative chimney drain method. This aimed to reduce the groundwater pressure in the cliff faces by channelling it to the bottom and out to sea.
While this method provided the best potential solution, there was no guarantee of its success. Council decided to trial the approach based on sound risk analysis. The work was funded on a cost share basis between Western Bay of Plenty District Council and affected residents at 23, 24 and 25 Ruamoana Place. Each property owner contributed
$100,000 toward the $600,000 cost and Council funded the 50 per cent balance.
The initiative to encourage Council to fund the project with
50 per cent coming from the most affected residents was led by John Roper, who spoke on behalf of his neighbours thanking Council staff and representatives.
“This is 40 years since Bramley in 1979.
“There’s been a lot of work done on seawall reticulation but it was Peter Clark who had the idea of horizontal drainage which proved 99 per cent successful.
“This is an experiment and we’re absolutely hoping for the best,” he said.
“We want this place to be enjoyed and with the reopening of the walkway it’s a win-win.”
Mayor Garry Webber said the success of the project justified Council’s faith in attempting what was an unproven method of reinstatement.
“There were are no guarantees, but we understood the risks we were taking.
“I commend the property owners for taking this step of faith with Council and we are very grateful to Tonkin and Taylor for coming up with such an innovative solution — and to the residents for their generous contribution.
“This could be a method we may be able to consider in other parts of the District where coastal erosion leading to cliffedge failure is becoming an increasing challenge.”
Project Manager Peter Clark was delighted with the outcome.
“The contractors have performed both individually and as a team to produce an excellent outcome in particularly difficult conditions. The shape of the slope is great and the vegetation is now showing. It has been a rewarding project to lead.”
Tonkin and Taylor’s civil engineer Tom Charleson said what they have installed will help pull groundwater out of the slope.
“It is somewhat experimental but it’s commonly known that groundwater contributes to landslides. All we’ve done is provide a way for groundwater to come out of the slope and hopefully reduce the buildup.
He said it would take time to see how effective it is.
“We’ve got some bore holes through here which tell what the groundwater levels are so we can monitor how they respond in a big rainfall event.
“It’s difficult to get more than that, from what we know, you can’t conclusively say it will make it less likely there will be a landslip. All we can say is it’s reducing the risk by pulling out the groundwater.”
The project was completed within budget, and the property owners were delighted their assets were secure and that the walkway has been stabilised.