Installing fibre-optic cables underground

By Neil Bradley, Fibre Options

Analysis shows that between 60% and 80% of the capital costs of a fibre project are due to civil work, ducts and cables. In other words, the cost of digging holes and filling them in again.

There are ways of getting round these costs, such as wireless transmission, overhead poles, and so on, but in the main if a future-proof network is to be employed then only fibre will do the right job.

Costs for digging can vary enormously, from £5 per metre to £100 per metre depending on where you are going and what disruption you are incurring. If permissions have to be granted they can depend on the traffic control of diversion of that area, which is costly. If the digging is in soft areas and reinstatement is not a problem then costs are low. If the dig can be achieved by slot cut with a very narrow channel then costs are about £25/metre. Costs then escalate up to £35 to £50 per metre for cutting into the pavement and could be £100 per metre in the main carriageway.

Over the past few years, lower cost alternatives to traditional trenching have emerged.  Here we will introduce some of these methods.

Micro-Trenching

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Micro-trenching is particularly suited to roadways and sidewalks where utilities are already present beneath the road surface.  It requires only a shallow trench, typically about 15 cms deep, which does not penetrate beyond the surface layer of the road.

Advantages: Significantly faster and less expensive to deploy than traditional trenching - approx. 35% less. There is less damaging to existing roadways.  Less depth also means that cables are closer to the surface, easier to get to and fix if there is a problem.

Moleploughing

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This installation method is suitable for burying cable or sub-duct in rural verges or across farmland. Specialist machines ‘plough’ a slot directly into the ground and lay the cable or sub-duct into the slot immediately, in one continuous operation.  The ground then closes over the slot and needs no re-instatement.  

Advantages: Significantly faster and less expensive to deploy than traditional trenching - typically 40% cheaper.
Disadvantages: Moleplough products are generally somewhat tougher than standard designs in order to match the heavy duty installation method.

Directional Drilling

Directional drills are relatively compact, allowing them to get into tight spaces and to be placed at the side of a road without impeding traffic.  A small crew is required: a drill operator and locating equipment operator.  The locator operator electronically tracks the progress of the drill head beneath the surface using a hand-held locator.  He also gathers data from the sonde located in the drill head behind the drill bit.
The sonde gathers data such as location, depth, roll angle, pitch, and temperature to help the driller adjust the direction of the head and control the bore path.

Advantages: Clean, trenchless solution without disturbing the surface above, leading to cost savings in excavations, reinstatement costs.  No more need to apply for road-opening permits for public road works.

Impact Moling

Unlike horizontal directional drilling – which can be guided – impact moling works in straight lines, and requires both a launch pit and a reception/catch pit. In the launch pit, the mole is lined up with the catch pit and then set in motion. Impact moling has many applications including the renewal of lead water pipes, the installation of utility pipework and cable-laying.

Advantages: Suitable for all soil conditions except rock. Minimal or no excavation beyond the necessary connection pits, and minimal disruption to the customer and customer’s property.

Comments

You don't mention usage of

You don't mention usage of existing pipes, e.g. sewerage.

Possible future article

Good point.  This article is about about digging, but it would be great to have another to look at how to exploit existing underground infrastructure such as sewers.  I'll add that to a list of possible future articles.

Collateral damage

Mole ploughing and similar techniques (giant chainsaw ripping through the ground in fast trenching) often leave damage behind to drains and the like that require substantial repair costs. Farmers and locals need to be vigilant and ensure faults are rectified before the contractor disappears over the hill.Pasture land is never ever the same after crossing it with any form of trenching technology, whatever they promise.

community digs...

If the farmer is digging his own trench like in the b4rn project he will presumably know where his drains are. We didn't cause any damage whatsoever when we dug our fibre in.

Collateral damage

I agree entirely with Phil T's comments, which is why thorough site survying should be taken in advance, plus a study of any infrastructures that are already in situ. Unfortunately these plans are not always up to date. Even in urban areas digging has been known to disturb or break vital ducts and cables.Re crossing land I do not advocate direct crossings but to stick to bridal paths and tow paths. I am not a farmer, but I do know that some ploughs can cut through deep channels, thus disturbing ducts.

site surveys

The site surveys for B4RN are done with the farmer and landowners, and their advice is crucial to getting the route right. They know the land better than anyone, so together with the contractors/farmers doing the digging a plan of least impact is decided upon, and because all the diggers are local people there is no escape over the hill if there is damage, the land has to be left in the same order it was before the dig. End of. No excuses. This model is replicable.