The Pros and Cons of Microtrenching

The cost of laying fiber is still the most expensive part of bringing broadband to new places. There is a relatively new construction technique that is worth considering for fiber construction. While it’s been around for a few years, microtrenching is gaining in acceptance as a less expensive way to lay fiber.

Microtrenching involves digging a narrow trench of one to two inches wide and up to two foot deep. These trenches can then hold multiple conduits for fiber. The technique can be used on open highways and is most easily explained by this short video from Ditch Witch. There are also smaller units that can be used to cross sidewalks, parking lots and driveways.

There are a few cities that are now encouraging microtrenching. The first was New York City that adopted the technique in 2013 and which now requires it unless there is a good reason to use some other technique. San Francisco just proposed requiring the technique a few weeks ago for all future fiber construction there. Google used microtrenching in some of their city fiber builds like in Austin and Charlotte, and is trying to get cities like San Antonio to approve the technique.

The pros for this technique are significant, mostly dealing with cost. The alternative to microtrenching for traversing sidewalks, driveways and parking lots is boring. The boring technique involves digging a somewhat deep hole of 3 – 5 feet and then using equipment to bore sideways underneath the concrete. There is significant labor involved in the process and there is always a danger of hitting other utilities, particularly when boring away from public rights-of-ways. Google says that microtrenching is vastly more efficient in areas with urban sidewalks and that they can microtrench as many as 50 customers in the time it used to take to bore to one customer.

There are places where this might be the only sensible technique. For example, I have a client that wanted to pass through national forest roads to connect two parts of a fiber network. The Forest Service would not let them use any technique that would disturb the soil off of the paved asphalt. But perhaps they would have allowed microtrenching.

But there are certainly downsides. Probably the biggest downside is that the microtrenching is a lot shallower than other kinds of fiber construction. That certainly is going to present a problem in later years when it’s time to repave streets. Most city streets around the country are on 30 – 40 year replacement cycles. During that time they usually get repaved with asphalt a few times, but at the end of the cycle the old paving must be excavated and the process started over. That replacement process generally digs down anywhere from 18 inches to three feet depending upon local soil and substrate conditions. And that could mean digging up all of the microtrenching. The replacement lifecycle for streets is so long that it would be easy for future road construction crews to not understand the unusually narrow depths of the microtrenched fibers.

The same concerns apply to parking lots and sidewalks, although there is often less chance of these being completely excavated to any great depth for replacement purposes. But there is often more localized construction done to replace or bring gas, water or electric utilities and future work could more easily cut microtrenched fiber.

Most cities have specific expectations of various utilities. They expect different utilities to be at specified depths, and anything that falls outside these expectations is likely to cause problems in the long run. This means that any locality that allows microtrenching needs to make sure to create ordinances that allows for it and to change the rules for other utility work and road work to account for it.

Another practical issue for many ISPs considering the technique is that there are not a lot of contractors doing microtrenching today – and that might make it harder to find somebody who can do the work for you. But the potential cost savings for microtrenching are so large that it’s something that should be more regularly considered as part of any major fiber construction plan.

 

 

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