National Environmental Standards for Electricity Transmission Activities: technical (7 of 9)
The technical pages provide background and explanations of:
Work on support structures
Abrasive blasting (wet and dry), washing, painting
To prevent corrosion, the steel of transmission line towers is coated with a layer of zinc metal during manufacture (galvanizing). However, corrosion of tower steel eventually occurs. Dirt, rust and corrosion must be removed and surface coatings applied to protect the steel.
For tower foundations, the foundations are excavated to expose the rusted steel. Because the excavation pits would fill with water during the blasting, wet abrasive blasting cannot be used to remove the rust there. Dry abrasive blasting is therefore used for tower foundation work, before surface coatings are applied. Pressurised air is used to convey the abrasive medium, and screens to limit the effects of wind-blown material.
For towers, both wet and dry abrasive blasting can be undertaken under live line conditions. For wet abrasive blasting on towers while the circuits are live, very pure water is needed to convey the abrasive medium, so that it will not conduct electricity. Before the wet abrasive blasting operation, towers are water-blasted (no chemical additives are used) to remove dust, grime and bird droppings.
Once the tower steel has been prepared through abrasive blasting, painting must be done as soon as possible. Towers are hand-painted with polymer vinyl or moisture-cured urethane paint systems. Foundations are re-galvanised using hot-zinc spray.
Lead-based paint has been used in the past to control corrosion. This type of paint now creates problems where towers need to be abrasively blasted and re-painted. Special mitigation measures must be put in place to collect and control the blast detritus to the extent possible.
Foundation types and associated earthworks
A range of foundation types is in use on Transpower transmission support structures.
Poles are generally directly buried. The pole can be embedded in compacted backfill, stabilised cement or encased in concrete. Additional supports such as breast and toe blocks are sometimes required. Guying may also be incorporated into the structure's support.
Towers – older structures are generally supported on steel 'grillage' foundations (a steel grill pad is connected to the end of the tower leg below ground). More recent structures have bored concrete piles, others have driven steel and wooden foundations, concrete pad or chimney (similar to grillage), or concrete rafts.
Refurbishment or strengthening depends on the foundation type, and the amount of strengthening required.
For grillages, the common method is to excavate to about 2m depth and backfill with concrete. The type of pile foundations depends more on soil type. Methods include pressure grouting the surrounding soil, addition of ground anchors, mini piles, or mass concrete.
In some instances, new foundations may be required, which will generally involve a minor relocation of the tower.
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Tower leg before (left) and after strengthening (right)
Tower / pole strengthening
Strengthening of a transmission tower will include adding or replacing steel members on the tower as well as reinforcement work. An alternative to this is replacing the tower altogether. Transmission poles may also need strengthening. This can be done through pole replacement, or sometimes guy wires will be added.
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The pair of pictures shows the base of a tower, before (left) and after (right) strengthening has occurred: an extra strip of steel has been bolted onto the existing leg.
Relocation / replacement
If a structure needs to be relocated, it is usually preferable for it to remain as close to its existing location and alignment as possible. Otherwise the loadings on the structures on either side of it will increase. Transmission structures may need to be replaced if they are in poor condition, or for additional strength such as when upgrading a line with a larger conductor.
Tower extension
When increasing the capacity of a transmission line (or 'uprating'), the conductors will have more sag. This can cause the conductors to not meet the New Zealand Electrical Code of Practice for Electrical Safe Distances (NZECP:34) and Transpower’s clearance requirements. Electical safe distances are required primarily to protect persons, property, vehicles and mobile plant from harm or damage from electrical hazards.
To avoid potential clearance violations, conductors may need to be raised. This may take the form of re-tensioning, altering the towers. An alternative is to remove the reason why the violation is occurring (such as earthworks, or removal of a building, or vegetation clearance). Where the option required is to alter the tower by an extension, this may occur in two different ways. The first is by undertaking a “body extension” which is where an extension is put in to the middle of the tower; alternatively the entire tower may be lifted on to new bored piles and legs.
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Tower before (left) and after inserting a body extension (right)
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Tower before (left) and after bored piles (right)
Last updated: 18 January 2010
On this 'technical' page: Work on support structures
Abrasive blasting (wet and dry), washing, painting
Foundation types and associated earthworks
