The saw that could save us $2b

Diamond-encrusted circular saws could shave billions of dollars off the cost of the National Party's proposed home fibre network, a key plank in the party's election campaign, but have some roading engineers breaking out in a cold sweat.

Micro-trenching is a relatively new technique for laying communications cables that involves cutting a slit in the road surface, about half a centimetre wide and 15-20cm deep, into which a flexible conduit containing fibre cable is laid.

Compared with the alternative – scraping off the road surface with machines, handing digging around utilities and burying cables nearly a metre deep underground – micro-trenching seems a no-brainer.

According to some estimates, cable crews can lay 1600 feet of fibre a day at a cost of $20 a foot – less than half the cost of conventional trenching techniques.

When the National Party's communications spokesman Maurice Williamson committed a future National government to buying a half share in a $3 billion network that would reach 75 per cent of homes in New Zealand's 22 largest cities, he was pinning his faith in micro-trenching technology, which he has viewed first hand in the United States.

Most other estimates of the cost of the network proposed by National – including advice received by Communications Minister David Cunliffe – suggest the bill would top $5 billion.

But there are drawbacks. Wellington City Council roading engineers warn the saws could prove a false economy as they could shorten the life of roads and slice through stormwater drains, gas pipes and electricity cables before operators even knew they were there.

Laying the fibre in the "sweet spot" in the road surface just above the road base is harder than it might seem because the depth of the road surface can vary by as much as 20cm over time as roads are built up during repairs, and then milled down and resurfaced.

If, during this process, fibre cables become too exposed, they risk being damaged by the weight of buses and trucks which send a "bow wave" through the first few centimetres of the road surface. But cut the micro-trench too deep, and cable gangs risk slicing into the road base, allowing water to permeate, causing pot holes or – at worst – breaking apart the road.

The council signalled in April that it would attempt a trial of micro or shallow trenching, but infrastructure manager Jon Visser says he is not even certain a suitable location can be found for that.

"Yes, it is a technology that has a cheaper price to do a job," he says. "But what we have found, in the cbds, especially, is that there are many utility services that run very close to the surface.

"They are not shown in the drawings at that height, but in reality they have been put in too shallow. Recently in the Lambton Quay upgrade project we hit quite a large gas main and a 33 kilovolt cable, both in a region just below the surface."

Some pipes appear to have risen to the surface by themselves, he says, as a result of liquefaction and earthquakes.

Shallow-trenching may have greater potential in cities such as Palmerston North and Christchurch where there are often wide grass verges between roads and footpaths where cables can be laid, he says.

"Here in Wellington, we have got building frontages, then the footpath and then the carriage way and there is nothing in between along which services could be run, so we have got a very constrained environment, with services running very close together."

Mr Visser also warns of the risk that cables in micro-trenches may be damaged when the top road surface is removed to carry out repairs to deep-trenched utilities or on roads themselves.

Lisa Payne, president of Canadian micro-trenching specialist Teraspan, concedes micro- trenching is not right for every situation, but plays down some of the council's concerns.

"If cities have a good system to locate existing utilities, then 'fantastic'. Some of the cities in countries we deal with don't have any maps, but what they usually do know is that the utilities are below 30cm."

She argues it is generally possible to tell the depth of the road surface and says Teraspan has micro-trenched more than 50km of cable in Vancouver, which is on a fault line, and 50km in Winnipeg, where extreme frost heaves cause constant land movements. The company is now expanding worldwide, having undertaken projects in the United States, Saudi Arabia and Scandinavia.

Ms Payne says cables laid by Teraspan have been pulled out of the micro-trenches and laid by the side of the road during construction works, and then replaced in the micro-trench, all without any disruption to the service.

If they are accidentally cut, they can be repaired cheaply in two or three hours.

"We really are suitable for densely populated city areas," she says.

In rural areas, she suggests stringing fibre above ground and she says it may be worth using conventional trenching techniques alongside highways and across open ground where "you have got some pretty sophisticated equipment that can rip through the ground without hitting any utilities.

"You cannot do that in the city centres, where you have got to consider the disruption to the community."

Mark Ratcliffe, chief executive of Telecom's network arm Chorus, says that when it comes to laying cable "every kilometre is different".

"It would be foolish to assume you could micro-trench everywhere. But clearly, micro- trenching in some areas will work very effectively," he says.

The cost of replacing fibre is another factor that may come into play.

Scientific journals suggest fibre cable has a life of about 40 years.

New fibre can be blown through empty channels left in cables when they are laid, but replacing the entire cable in a conventional duct would be cheaper than cutting a new micro-trench, if that was required.

For that reason, it is hard to predict the appropriate scale of micro-trenching, even if it was clear what skeletons lay under our shifting roads.

"If we knew what we knew now, we would not have a roading system that looks like it currently does," cautions Mr Ratcliffe.

"We would have dual carriageways almost everywhere with the ability for them to be three lanes in one direction and one lane in the other, whenever it suited us. But we have not done that."

- © Fairfax NZ News