Calamity on the coast

As slips go it may not have been major league, but one year on and the skilful remedial work is still being carried out.   BY GAVIN RILEY

Every wet winter slips become an unwelcome feature of the undulating New Zealand landscape. Thankfully, none come anywhere near the monster that occurred in Fiordland 13,000 years ago when a whole chunk of mountain range collapsed into the valley below – 27 cubic kilometres of hard-rock material covering an area of 45 square kilometres.

The most famous New Zealand landslip in living memory, and the largest in an urban area, happened 30 years ago in the Dunedin suburb of Abbotsford. It resulted in the evacuation of more than 600 people and left 70 homes uninhabitable.

Compared to Abbotsford, the Turei Hill slip at Kawakawa Bay on Auckland’s southeast coast in August last year is hardly major league. But it caused disruption to the lives of nearby residents, and painstaking remedial work by Downer EDI Works that is not due to be completed until November.

The first warning of what was to occur at Turei Hill came during a very wet spell in late July last year when a series of slips forced Manukau City Council to erect warning signs on the Clevedon-Kawakawa coastal road at the foot of the hill. Downer EDI Works, which holds the council’s Clevedon-Botany maintenance contract, cleaned up after each slip.

At 11pm on August 1 a large slip blocked the road and Works crews worked through the night to get one lane open by morning. This marked the beginning of eight weeks of intensive activity by the council, Works and Opus International Consultants to maintain local residents’ road access to the outside world. The only alternative access was a 100 kilometre detour south via Kaiaua and State Highway 2, rejoining State Highway 1 at the bottom of the Bombay Hills.

A heavy rainstorm in late August, by which time Works and Opus were carrying out 24-hour monitoring, resulted in a decision being made at noon on August 24 to close the road as an increasing amount of debris was falling down the slip face. A contingency plan immediately swung into operation, which included setting up detours and variable-message boards.

About an hour later a significant failure of the slip face saw some 500 cubic metres of soil and debris engulf the road. Closure points were manned around the clock to control the site and advise residents and motorists.

Geological investigations, carried out by Opus and Tonkin & Taylor on behalf of the Earthquake Commission, revealed the slip showed signs of increasing movement. This prompted the council to evacuate, temporarily, five houses at the base of the slip. The landowner’s house at the top of the slip was later demolished during the earthworks construction phase. 

With access to the coastal settlements at Kawakawa Bay and Orere Point now closed, Works engaged local subcontractors to construct a four-wheel-drive access over hilly farmland track to enable a shuttle service to operate. Staff also metalled a walking track, constructed car park areas at each closure point for motorists to leave their vehicles under security watch, and installed information and safety signs at short notice.

In mid-September a helicopter with a 1300-litre monsoon bucket was employed to saturate unstable material above the slip in an attempt to dislodge it so the road could be re-opened. Following this, the council decided it was safe to open a single lane of road. Over the next few days a large silt-removal and clean-up task was carried out, road-surface repairs were completed, concrete barriers for a single-lane operation were installed along with temporary traffic lights, and the road was re-opened at midday on September 26.

Further investigations by Opus and Tonkin & Taylor revealed that the landslip involved some 150,000-200,000 cubic metres of material on the hillside and the failure plane was up to 25 metres deep.

Manukau City Council invited four contractors to tender for the slip-repair contract. Downer EDI Works was successful with its tender of $4.235 million. Opus was appointed the main consultant and Tonkin & Taylor the main designer.

The work, which began in mid-January, called for soil to be removed or used elsewhere on site; for a 25-metre-high reinforced earth embankment to be constructed to buttress the toe of the slip; for bored horizontal drains to be installed to reduce pore pressure in the hill; and for the hill to be permanently retained by the use of lower and upper anchors. All three activities are required to ensure the long-term safety of the hill.

Since the beginning of the year a total of about 100,000 cubic metres of earth has been moved from Turei Hill with the help of subcontractor Hopper Construction. Some 55,000 cubic metres has gone to a landfill, 20,000 cubic metres has been used as buttress fill, 15,000 cubic metres as general fill, and 5000 cubic metres as hard fill under the buttress fill.

The hard-fill placement began in February and occupied three weeks. It was an important aspect of the project as it provided a safe and stable platform at the toe of the slip. The subsequent buttress fill was overlaid with one-metre layers of geogrid and the fill compacted to 140kPa.

Boring the horizontal drains is being carried out in four stages. The first stage was on the south side of the buttress and was done in March. The second stage was completed last month, the third is underway, and the final stage should be completed by the end of July. This task is having to be done piecemeal because localised earthworks have to be completed before the drilling rig is positioned. In all there will be 51 drains, ranging from 65 to 100 metres long and totalling 4000 metres.

Works project manager Robert Mackie says the bored drains and a temporary pump have released a lot of water from the hill. “We’re getting water coming out constantly from within the hill. The more water can be reduced from the hill the better.”

A permanent pump will be installed towards the end of the contract and will have a float switch to keep the water table low at the buttress.

Although the Turei Hill slip repair is on target for a November finish, the project has had its difficulties and challenges, mainly around co-ordination of the earthworks and drilling on a steep and confined site.

An unusually wet May severely impeded the earthworks operation and impacted generally on productivity. But by the beginning of June Moxy trucks were able to move soil again along the access roads, productivity was back to normal, and testing by Tonkin & Taylor revealed minimal slip-stability problems. Earlier, sediment-discharge and erosion-control measures had been strengthened.

In early June drilling was being carried out round the clock as part of the catch-up process following the wet spell, and this was completed by the middle of the month.

Some hydroseeding has been carried out on the upper slopes of the hill and other areas have received topsoil and straw mulch.

Ahead lies re-opening the Clevedon-Kawakawa road to two lanes in August with the aid of a long-reach excavator to clear away remaining debris, and the installation of 21 lower and 25 upper anchors on the slope. The latter will be a time-consuming task, starting in late July and continuing till the end of the project. Each anchor will go in 40 metres and will have a five-tonne, 2.4-metre-square concrete face plate. Subcontractor Geovert will carry out the drilling and Works will do the excavation and placement.

Safety has been paramount throughout the Turei Hill project, particularly around access and truck movements, due to the difficulties and risks attached to having to deal with an unstable slope.

Mackie says it is by no means a run-of-the-mill slip repair.

He describes it as “an interesting project” because it has involved not only earthworks and anchors but building a huge reinforced-earth retaining structure. “It’s a challenge because of the health and safety aspects and methodologies employed,” he adds.