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Underground Cable Plant Construction

Underground cables are pulled in conduit that is buried underground, usually 1-1.2 meters (3-4 feet) deep to reduce the likelihood of accidentally being dug up. In extreme cold climates, cables may need to be buried at greater depths where there temperatures are colder and frost penetrates to greater depths. The process usually begins with digging a trench to bury the conduit which is generally PVC plastic pipe, sometimes with pre-installed innerduct (also called duct liner) with a pulling tape to facilitate the actual cable pulling process. Directional boring can also be used to avoid digging up the surface, for example in crossing streets or sidewalks.

If the conduit and cables are all dielectric, as they usually are, a conductive marker tape should be buried above the conduit to assist in future cable location and as a warning to anyone digging in the vicinity of the cable. Conductive tape to be detectable should be no more than 300mm (12 inches) below the surface.

This chapter covers many topics of relevance to OSP construction that should be considered as part of the overall project planning. For additional detail on the design, refer to the FOA online Guide material or the textbook on fiber optic network design.

Dig Once

Due to the disruptive nature of burying conduit, especially under roadways, many governments which grant permits for burying cable require the contractor to install extra conduits along the route to prevent having to dig again for any future cable installations. Since many cities have extensive conduits already buried for other services or may have required extra conduit to be buried during prior installations, conduit may be available for pulling new fiber optic cables.

The old story about the most likely fiber optic communications system failure being caused by "backhoe fade" is not a joke – it happens every day. But it reminds us that digging safely is vitally important. The risk is not just interrupting communications, but the life-threatening risk of digging up high voltage cables or gas lines. There are several services that maintain databases of the location of underground services that must be contacted before any digging occurs, but mapping these should be done during the design phase and double-checked before digging to ensure having the latest data.
At the same time as the cable is installed, markers like these indicating its location and ownership can also be installed.

Trenching

It’s immensely important for trenches to be excavated to such a depth that the crown of the duct has at least 800mm (32 inches) of backfill cover, in all soil conditions, except for where hard rock conditions are encountered. Where it is not possible to obtain the specified minimum trench depth, the client must be consulted.

The trench depth in hard rock conditions can be relaxed (i.e. apply for a concession) to a minimum depth of 300mm (12 inches) backfill cover over the uppermost duct. But it then requires protection in the form of a concrete slab (either pre-cast, or cast in situ) placed on top of the padding material before backfilling. This concrete slab shall have a strength of 20 Mpa reinforced with high tensile wires and measure; 75mm (3 inches) thick by 300mm (12 inches) wide, and 900mm (36 inches) in length.

Concrete encasing is not endorsed enthusiastically by everyone; some argue that it turns a previously flexible duct into a long unreinforced concrete beam of little strength, prone to fracture with ground movement. And, this in turn could potentially damage an encased duct. A view not shared by everyone.

Before pouring concrete, a slump test must be performed (take photos of this procedure). How do we measure the ideal slump? A slump of 10 cm or less is typically deemed acceptable (must not shear-off or collapse) - or as per client spec. Concrete that is poured too wet will be weak, regardless of how it is cured.

One other obvious consideration is the width of the trench, which of course, is dependent on the duct diameter.

Trenches that are too narrow will not allow for proper duct installation, whereas trenches that are overly wide are unnecessarily costly. On top of this, a too wide a trench will allow for too much duct snaking from the reel memory.

Areas where work is to be performed shall be cleared of all trees, shrubs, rubbish, and other objectionable material of any kind, which, if left in place, would interfere with the proper performance or completion of the contemplated work.

When the trench has been set out, pilot holes needs to be dug at 25 – 30 m (80-100 feet) intervals, particularly at points where the new trench crosses existing services.

Pilot holes are one of the most effective methods utilized not only for the location of services, but also to determine the position of a trench, relative to other services.