The Matahina power station is a hydroelectric power facility in Bay of Plenty in New Zealand on the Rangitaiki River downstream of the Aniwhenua Power Station. The river was dammed to form Lake Matahina from which water is drawn and diverted through the power station before being discharged back into the river. The Matahina dam is the largest earth embankment dam in the North Island of New Zealand.
History Development
The Ministry of Works (MOW) investigated all possible sources of hydro-electric power in New Zealand. With the last of the power stations on the Waitako River nearing completion and wishing to find work for itself and its large workforce, investigations were undertaken for four years in the late 1950s on the hydro-electric potential of the Bay of Plenty. As it held the most promise, the Kaituna River was the first to be considered for development. However, the Kaituna proposal was abandoned after detailed investigations found that all of the possible sites on the river would be expensive to develop due to their geological complexity and would only be able to generate a small amount of electricity. Instead, a site on the northward flowing Rangitaiki River at the mouth of the Rangitaiki Gorge was selected, where the river had eroded through ignimbrite of andesitic composition to underlying tertiary age sediments of weak conglomerate, sandstone, and siltstone.
In the mid-1950s, the MOW was short of experienced investigating engineers and senior design staff, which meant that without a full in-depth study, work on the scheme was started before Matahina was authorised under an Order in Council dated 14 January 1959.
In 1959, the Power Planning Committee recommended approval of Matahina and this was given by the government later in that same year. The intention was that the first electricity would be generated by April 1965, when there was expected to be a shortfall in generation capacity in the North Island. The estimated cost was £8 million.
When the National Party was returned to power in 1960, they stopped work on the Maraetai II power station and ordered that its two turbines be allocated to Matahina. The use of these turbines required some redesign work, which slowed down construction.
In 1961, the Power Planning Committee decided that Matahina was no longer urgent and could be delayed for two years, because of the priority that was being given to the HVDC Inter-Island link.
Construction
Work began in 1960 on access roading and the construction of Te Mahoe village.
As preliminary construction work began on site, detailed site investigations found that the site's geological conditions were more difficult than expected, with complicated ground faults, and that the river had worn through the solid ignimbrite rock in its bed, exposing loose gravel and rock. There were also pumice beds under the rock at one of the abutments.
Because Matahina was some distance from the support facilities that had been built up at Mangakino to build the Waikato River stations this required the establishment of a substantial number of temporary facilities on site.
After the Audit Department criticised the MOW's cooperative contract system and its method of reimbursing skilled tradesmen, the MOW introduced at Matahina a “daily project construction allowance”. However, the Public Service Commission refused to authorise the payment. The Cabinet refused the Commissioner of Works' argument that without the allowance it would be difficult to attract workers, as they believed these allowances would contribute to inflation and as well there was plenty of employment available in the Bay of Plenty, especially at Kawerau. As a result, the Cabinet ordered that the project to be closed down. However, the Commissioner gained the permission of the Minister of Works to undertake only a “slowing down”. This allowed excavation works to continue and all existing contracts to be completed. As a result, completion was expected to be delayed by 12 months.
Diversion tunnel
In February 1960, the MOW called tenders for the excavation and the driving of the diversion tunnel, which was long by diameter through a rock abutment on the west side of the river.
The MOW had estimated that the work would cost £510,000, but the three lowest tenders were all under, that with the lowest being from the French company Etudes et Enterprises (£257,150), followed by Dominion Earthmovers and Morrison-Knudson/Downer/Ferguson. The MOW considered voiding the lowest two, and giving the work to highest bidder as they were judged to be financially sounder and to have a workforce coming available, as they were completing Meremere Power Station. The MOW instead declined all offers and proposed to ask them to reprice with the performance bond increased from £20,000 to £100,000. Instead a revised price of £287,124 was accepted from Etudes et Enterprises. The company had undertaken work in Australia and in New Zealand to an acceptable standard, but was persona non grata on the Snowy Mountain because it had the habit of bidding low and then claiming excessive variations. They had also recently bid low for the contract to construct the Victoria Park motorway bridge in Auckland.
Etudes et Enterprises commenced work on what was a planned 18 month long project by cutting a pilot tunnel at the top of the arch of proposed tunnel. This would provide ventilation while constructing the main tunnel. The company did a poor job of supporting the ground which caused cavities to be created outside of the required tunnel dimensions and therefore more back filling. They also did a poor job of excavating the overburden that covered the downstream portal area, which was 50% higher than stated in the contract and after it did a poor job of excavating the material, things were not helped by MOW being slow in clearing the trees in the area. Until it could obtain access to the downstream portal, and so undertake a full excavation, Etudes et Enterprises kept its tunnellers busy by widening the pilot drive. It made a claim on the MOW for the delay and was paid half of the amount and granted an extension of six months. By early 1962 most of the tunnel had been driven and it was more than half lined with concrete. Despite this extension, Etudies still completed the tunnel 15 months later and was awarded £25,611 of additional costs which meant that the overall contract price was still cheaper than the other tenderers.
When the river was initially diverted through the diversion tunnel, the water scoured a large hole at the outlet of the tunnel. The cause was put down to hasty design. This required the construction of a stilling basin, which delayed full diversion by several months.
Work begins on the foundations
In 1962 the pace of construction work began to increase due to the river having been diverted and the work-force doubling as resources were released from elsewhere. Work began on the dam foundations, which was made more complex as in a reversal of normal conditions, the rock was on top of the sediments. The river had removed all of the rock along the bottom of the gorge, and had deposited a mixture of boulders, gravels, pumice and other debris that was unsuitable for use as a foundation.
Removal of this debris required excavation of a long by wide trench to a depth of . A deep earthquake-resistant concrete cut-off wall was then constructed within it to control seepage. A specially imported machine was imported to make the concrete for this wall. It was a type of plastic concrete, designed to be pliable enough to withstand earthquakes. To further manage water seepage which was estimated to be , the engineers drilled 60 drainage deep wells below foundation level and filled them with gravel.
Dewatering
As detailed site investigations were undertaken, it soon became clear that the site of the power station was next to an active earthquake faultline, offered poor foundations and the surrounding area lacked suitable material for an earth dam. As problems with the material that was being used in the construction of the dam would require extensive grouting to control potential seepage, the MOW decided to install as a safety measure a dewatering tunnel to provide a means of partially lowering the water behind the dam, as well as a number of small drainage and grouting tunnels.
Four tenderers were received from Fletcher Construction, Downers, Wilkins & Davies, Green & McCahill to undertake the required work. As Fletcher Construction was the lowest at £326,993, it was awarded the contract, and completed the work six months early and £676 below the original contract price.
Construction ramps up
The intake and spillway structures were almost complete by 1963, and progress had been made on the powerhouse. By 1964, installation of the dam's foundations had been completed and the powerhouse was being prepared for installation of the turbines. In that same year the workforce reached its peak of nearly 700. Beginning in September 1964 the dam's core was formed. By 1965 nearly of material had been installed, the powerhouse was almost complete, all concrete structures had been completed and progress had been made on installing the generating units.
The dam was constructed with a sloping earth core flanked by inner and outer transition zones. Sandy clayey silt and silty clay derived from weathered greywacke was used to form the core of the dam. It was placed dry of optimum to high densities. It was however stiff and susceptible to cracking under shear at low confining pressures and is also readily erodible. On each face of the core an inner transition was formed of unwelded ignimbrite, which was compacted to such a high density and had a high unconfined compressive strength that the subsequent inspection gallery was tunnelled through this material without the need for any excavation support. The faces of the inner transition were then covered with an outer transition constructed of a mix of the fine grades of hard ignimbrite, sandy gravel and gravelly silt. Lastly, rockfill formed hard ignimbrite blocks ranging in size from to were added and compacted by heavy tractor track rolling to form the exterior upstream and downstream faces of the dam.
Commissioning
As a result of issues with obtaining suitable materials for the earth dam, modifying the design to take account of the adverse site ground conditions and extensive grouting and installing additional drainage tunnels, filling of the lake was delayed.
During the filling of the lake a large temporary increase in flow from the drainage blanket was observed, followed shortly afterwards by the finding of an erosion cavity downstream of the core in the east (right) abutment area. The lake was lowered and exploratory pits revealed that shear deformations induced by benches in the abutment had led to cracking of the core and inner transition of the dam. This needed extensive remedial work on the core, which combined with the previous delays meant that completion of the power station was delayed by two years, by which time the cost had doubled.
Generator 1 was commissioned on 25 January 1967 and generator 2 in April 1967.
Service
In 1978, the New Zealand Department of Electricity which had been operating the power station since its commissioning was restructured into the Ministry of Energy. In 1987, the government established a state-owned enterprise called the Electricity Corporation of New Zealand (Electricorp) to take over the trading operations of the Ministry of Energy.
1987 Edgecumbe earthquake