KEEPING ELECTRICITY PYLONS ABOVE WATER

Campbell Robertson1

SUMMARY

The current upgrade to the electricity grid has presented suppliers with a number of challenges. One of the most interesting has been the necessity to supply concrete to pylons carrying 220kV lines over in the . These pylons were constructed in 1944 and their concrete bases have suffered from soft water attack. With the lake unable to be drained the supply of concrete required barging a pump and the concrete trucks into the middle of the lake and having the concrete pumped into formwork attached to the existing structure. Doing this safely with minimal environmental impact was a challenge that was relished by all, and the job was completed successfully on 7 May 2010.

1 Technical Services Manager, Holcim () Ltd.

INTRODUCTION:

In 2007 New Zealanders consumed 9622 kWh/capita of electricity, down from a high of 9721 kWh/capita in 2005. Statistics New Zealand estimated our population at 4.25 million2 at 31 December 2007, meaning a total electricity usage of almost 41 million MWh. At that time greater Auckland accounted for approximately 1.25 million people, using approximately 12 million MWh. With that level of use it is essential that the security of electricity supply is strong. This was identified by Transpower and various projects have resulted from reports generated as a result of those investigations. In particular, the stability of the pylons carrying 220kV lines from Whakamaru in the central North Island to the Auckland area was identified as a problem to be addressed. At Lake Karapiro, a 7.7 square km lake 6 km south of Cambridge3, the line crosses the water and is supported by a total of four pylons. Two of these were considered to be the more important to upgrade and planning was carried out to ensure this could be carried out safely, in time for the World Rowing Championships in late October 2010 and with no environmental impact. When an initial inspection dive was carried out in 2007 the deterioration of the columns was noted, however a further dive in late 2009 noted a major change in the level of degradation over that time. It is considered possible that some of the increase in degradation resulted from the extra movement caused by increased windage on the towers after public notices were erected on the towers to try to ensure the safety of the public around the towers and the overhead lines. The upgrade of the Karapiro Towers was part of the total contract awarded to Electrix Ltd, Waikato branch as part of a bigger project which involved upgrading up to 300 pylon bases. Whilst the total project was interesting enough in itself, requiring steel fibre reinforced concrete, 70mm slump on site and in locations often distant Launching from plants, the Karapiro Pylons provided ramp a particular challenge for all concerned. As is common with a challenge such as this, the goal posts shifted often during the planning stages as more information became available to each of the parties Pylons to be repaired. involved. Some of these were minor; others had a wider effect on all involved. In the end the project within a project was successfully completed for all concerned. Figure 1: Lake Karapiro showing pylons and launching point. THE CONCRETE:

The concrete parameters were set by a number of requirements. From a project perspective, strength was only a very small part of the total concrete need. An important parameter was the necessity to mitigate the effects of the water conditions the concrete was to live in. Others factors influencing the concrete design included the placement method chosen and the assumed time from batching until final placement. The water of the is soft, caused by the heavy load of ash that it carries from its origins in the volcanic highlands of the central plateau of New Zealand. It is now well known that soft water will attack concrete, an effect that was not well understood at the time of construction of the pylon bases. The original pylons were constructed prior to the lake being filled.

2 Statistics New Zealand National Population Estimates December 2007 3 http://www.waipadc.govt.nz/District/Lake+Karapiro/

To mitigate the effect of the soft water it was agreed that amorphous silica would be used as a densifier to minimise water penetration and take advantage of its other high performance advantages such as improved chemical resistance and increased strength development. As always with amorphous silica a superplasticiser was used to ensure the concrete retained workability. As it was to be pumped to its underwater position a dose of pumping aid was also added for both pumpability and to help minimise cement leaching underwater. As part of the planning around pumping the concrete into place without any ability to consolidate it, a design workability of 180mm slump was opted for. The pumpability of the concrete had to be assured as the ability to adjust the concrete at the pylons was not available, and the added difficulties of a blocked pump would put the continuity of each pour at risk. Further, during planning for the placement it became clear that there would be a need to retard the concrete as there could be considerable time between batching and final placement, given an estimated one hour tow time for the approximately one kilometre journey on the lake once trucks were loaded onto the barge. In all, a reasonably chemical concrete. A triple blend to include flyash was considered, but as supply was coming from a small rural based plant, silo space was not available, and the low volume required meant it was uneconomic to add silo space specifically for this project.

DELIVERY:

Delivery of the concrete to its final position was a moving target initially, but as each of the parties clarified their particular requirements it became clear that the safest method was going to be to barge the concrete trucks out to the pylons with a trailer mounted concrete pump on the same barge. Control of the pump delivery hose was handled by a barge mounted hiab type crane, with divers in the water to look after the underwater end. The principal contractor engaged a specialist underwater contractor to provide their above and below water services and equipment. From a concrete suppliers viewpoint it was essential that we were confident with the safety of everything to do with delivering the concrete once it left solid ground. As an industry we are all very conscious of the fatalities in Picton on 19 August 2005 and all wished to learn from that tragedy and avoid the possibility of it being repeated. We studied the report4 published by Maritime Zealand covering the circumstances around the incident, and our operations team discussed our requirements with the suppliers of the barge. As it transpired, it appears that the barge used on this project was the same vessel that was involved in the earlier incident. In the report the stability and seaworthiness of that barge, known as a dumb barge in maritime terms, the Mac III, was not implicated as part of the cause of the vehicles falling into the Picton harbour. The Holcim operations team and the Electrix team, including their barge/underwater services suppliers, agreed a process whereby the concrete trucks would be delivered to the top of the slip that was to be used, and the trucks would be reversed onto the barge by the barge suppliers. To assist this, the centreline of the barge was marked in a bright fluorescent colour. Once on the barge the truck drivers would be transported to the discharge site on the tug, and the tug/barge combination would be shadowed by two small vessels that had divers on board ready to enter the water immediately if there were any problems once moving. All staff were fully kitted with necessary safety equipment for operating on the water and under live power wires. On arrival at the Keeley’s Landing with the first load it was found that when the barge was presented against the slip to be used the ramp was not long enough to allow the trucks to be loaded as planned. Instead it was placed at right angles to the slipway, and the truck approach became more difficult. At that first pour the loading process was reassessed and it was agreed that our drivers would reverse their trucks onto the barge as the manoeuvre was more suited to their skills. Prior to placing his truck on to the barge each driver put on his life jacket as well as his normal safety equipment. The driver would then leave his truck and the previous plan for transport to the site was used.

4 www.maritimenz.govt.nz/Publications-and-forms/Accidents-and-investigations/Accident- reports/Rakanui-053885-mnz-accident-report2005.pdf

As noted previously the distance to the pylons was approximately one kilometre, and the actual transport timing was not known as the process was untried on this lake in these conditions. The first load took most of the hour allowed for in the original planning, and as the project progressed this time reduced and the total turnaround time was closer to half an hour. Once at the pylon the barge was manoeuvred into position to allow the pump hose to be lowered into place at the bottom of the leg being repaired and the concrete was pumped tremmie style to minimise the leaching of cement from the concrete. To ensure that there was no contamination of the lake the water being displaced from each construction element was captured and pumped to the surface where it was mechanically filtered on the barge before being discharged into the lake Figure 2: Loading at right angles to original plan. as clean water.

CONSTRUCTION:

As always with work in sensitive areas there was extensive consultation required to ensure that all interested parties had their interests taken into account. These included local Iwi, Mighty River Power as the organisation that controls the lake for power generation, Council as the Local Body that administers the use of the lake as a recreational facility, Environment Waikato as the Regional Authority and others who were likely to be affected by the work. Due to the importance of the project the preparation by all the above allowed for a fast tracking of the process as everyone co- operated to allow the work to take place in the best possible weather. The construction techniques required excellent engineering as all the work took place underwater. The original structure consisted of 460mm X 480mm columns on reinforced concrete piles with the steel lattice towers above. There was a mid height beam between each column of approximately 500mm square, and similarly at the top. The upgrade provided for the Figure 3: Truck and barge at pylon use of a 1.5 metre diameter concrete filled steel column encasement with externally bolted steel bracing to reinforce the existing concrete structure. The casings were made as a ¾ circle and ¼ circle to improve rigidity of the casing. The casing was bolted down to the foundation pads after they had gained sufficient strength. Each tower was poured in 4 sections, being the 3 metre square by 1 metre deep foundation pads, the lower segments column lift to the underside of the mid height beams, the mid segments to encase the mid height beam column joints, then the top segments of the columns up to the top beams and including the beam column joint. Each foundation pad had L bars that protruded 800mm into the new concrete to provide the necessary tie in of the new work. At the top of the columns there was extensive reinforcing to again tie the new work to the existing reinforced concrete. Prior to concrete being poured the lake floor in the vicinity of the pour was covered with silt matting that was later lifted and the silt removed via the barge. Each pylon was surrounded by a floating silt curtain to ensure that no cement slurry escaped into the lake. Divers were in the water with bell type helmets provided with air from the surface, and with communications and cameras so that the above water team could monitor what was happening

underwater. All operations were photographed as they took place, and were inspected independently to be able to provide the required Producer Statement at the completion of the job. With the successful completion of the strengthening of these two pylons investigation work is taking place to determine when the other pylons in the lake will be strengthened. These pylons are closer to the Karapiro Dam end of the lake.

ACKNOWLEDGEMENTS:

This paper has been prepared with the help of many people. In particular I would like to thank Steve Reading from Electrix Waikato for his help, and the Sales and Production people at Holcim Concrete and Aggregates who are based in the Waikato.