LuxHow fiber is built underground
Written by
Lux Team
Category
Guides
Date
Share this article
The pathway comes first
Underground fiber is usually built in two stages: first a pathway, most often plastic conduit or a bundle of microducts placed by trenching, plowing, directional boring or microtrenching, then the cable, pulled or blown through it from one handhole to the next. Which method a crew uses on a given street depends on the surface, the space in the right-of-way, the other utilities and what the road authority allows. This guide walks through the pathway, the four ways of placing it, and how the cable goes in.
Before any method starts, existing utilities are located. In the United States the contractor calls 811, and locators mark buried lines with paint or flags by color, orange for communications and red for electric among them, under the APWA Uniform Color Code. Crews expose a marked line by hand or by vacuum excavation where the new path crosses it.
The pathway is what lasts. Cable can be pulled out and replaced; a conduit in the ground is expensive to replace. So most networks place conduit with room to spare, often with several ducts or a microduct bundle so a second cable can be added later without digging again. A tracer wire or a metallic locate tape runs with non-metallic conduit so locators can find it later, and a warning tape is often buried above it to alert the next excavator.
Four ways to place it
Open trench
An excavator or trencher opens a trench, the conduit is laid in the bottom, and the trench is backfilled and compacted in layers, with the warning tape near the top. It is simple and lets the crew see every crossing, but it disturbs the most ground and the most surface, and restoring asphalt, concrete or landscaping is often the largest cost of the job.
Plowing
A vibratory or static plow cuts a narrow slot and feeds conduit or cable into it in one pass, and the slot closes behind it. It is fast where the ground is open and soft, such as rural road verges and fields. It does not work well through rock, dense utilities or paved areas.
Directional boring
Horizontal directional drilling places conduit under roads, driveways, rivers and yards without opening the surface between two pits. As Vermeer describes the process, the drill first steers a small pilot bore along the planned path while a locator above ground tracks the head, then a reamer enlarges the hole if needed, and finally the conduit is pulled back through it, with drilling fluid keeping the hole open and carrying out cuttings. Boring is slower per foot than trenching but avoids most surface restoration, which is why it is common in finished neighborhoods.
Microtrenching
A microtrench is a narrow saw cut in the pavement, shallower than a conventional trench, with microducts laid in the bottom and the cut filled and sealed. Cities define it in their own rules. Paris, Texas, for example, defines a micro-trench as between 0.5 and 2.5 inches wide and 16 to 24 inches deep, per its city code, and Round Rock, Texas, allows microtrenches only along the lip of the gutter with at least 10 inches of cover on local streets and 12 inches on arterials, per its microtrench ordinance. Rules differ widely from city to city, and some do not allow it at all, so check the local standard before designing.
Handholes, and getting the cable in
Conduit runs end in handholes or vaults: buried boxes with lids at grade, set at street corners, at splice points, at terminals and at intervals along long runs. They give access to pull the cable, store slack, and house closures and terminals. Their lids must be rated for where they sit, since a lid in a driveway carries traffic a lid in a lawn never sees.
Cable goes into conduit in one of two ways. It can be pulled: a pull line or rope already in the duct is attached to the cable end with a swivel and a grip, and a winch draws the cable through while a crew feeds it at the other end, keeping the pulling tension within the cable's rated limit and the bends within its minimum bend radius from the datasheet. Or it can be blown: compressed air is forced into the duct with the cable, and the moving air carries it along with very little tension. Blowing, also called jetting, is used in standard ducts as well as microducts and can cover long runs between handholes.
At each handhole the crew leaves a coil of slack, enough to bring the cable up to a splicing truck or tent and back, and labels the cable with its ID. Once the cable is in, the conduit ends are sealed against water and dirt. For what is inside the cable, see inside a fiber optic cable.
Worked example, key terms and questions
Worked example: one street, three methods
Here is an illustrative case. A 1,960 ft distribution run follows a residential street with a grass strip on one side, two side-street crossings and one 230 ft stretch where the grass strip gives way to a new concrete sidewalk the city will not let anyone cut.
Grass strip, 1,600 ft: open trench or a small trencher, with conduit, tracer wire and warning tape, then sod. It is the cheapest method here and the crew can see every service line it crosses.
Two side streets, 65 ft each: directional bores from pit to pit, so the roads stay open apart from a short lane closure for the rig.
New sidewalk, 230 ft: a third bore runs under it from the end of the trench to the next handhole, avoiding the cut the city has ruled out.
That is 1,600 + 65 + 65 + 230 = 1,960 ft. Four handholes go at the near corner of each side street, where each bore begins, and at each end of the sidewalk bore, which are also where the closures and terminals for the street sit. Each method and surface appears separately in the permit package and in the bill of quantities, because each is priced and restored differently. See how a permit package is assembled.
Key terms
Conduit: plastic pipe that forms the underground pathway for cable.
Microduct: small, thin-walled duct, often in bundles, for blown fiber.
Handhole: a buried access box with a lid at grade, where cable is pulled, stored and spliced.
Directional boring: placing conduit under the surface between two pits with a steerable drill.
Microtrench: a narrow, shallow saw cut in pavement holding microducts.
Tracer wire: a wire laid with non-metallic conduit so it can be located later.
Blowing: placing cable in duct with compressed air instead of pulling it.
How Lux does this work
Lux routes underground runs in the design and prices them from your rate card, and its permitting agent drafts a package for each street and crossing from the same approved design; see the design platform.
Frequently asked questions
How deep is underground fiber buried?
It depends on the method and on the road authority's rules. City microtrench rules run from about 10 to 24 inches, as the Paris and Round Rock examples above show. State highway rules ask for more: Texas requires 42 inches of cover for fiber placed along its highways, reducible to 36 inches with an indemnity, per the TxDOT Utility Manual. The local permit standard and your construction standard set the depth.
Is microtrenching better than directional boring?
Neither in general. Microtrenching is fast in pavement where the city allows it, but some cities restrict or ban it. Directional boring avoids cutting the surface and goes under obstacles, but it is slower and needs room for the rig and pits.
What is the difference between pulling and blowing fiber?
Pulling draws the cable through the duct with a rope and a winch, which puts tension on the cable. Blowing uses compressed air to carry the cable along the duct with little tension, which suits small cables in microduct over long runs.
Why lay empty conduit or extra ducts?
The pathway is the expensive part to build. Spare ducts let an operator add a cable later, or replace a damaged one, without digging the street again.
Share Blog
