Outside plant pathways: underground and direct buried

The geotechnical report for the Harbourview site arrives on a Friday. Daniel skims it for the words he has learned to look for: fill, clay, rock, groundwater. The parking lot sits on old fill over clay, with bedrock about four metres down and the water table well below the depth of any trench. 'That's good news,' Roz says. 'No rock to blast, no water in the trench. Now the hard part is deciding exactly what goes in the ground, because once it is buried, nobody will ever see it again until something goes wrong.'

Underground pathways carry most campus backbones. They protect cables from weather, vehicles and vandals, they last for decades and, when they are built with spare ducts, they let the owner add or replace cables without digging again. This lesson covers the two main kinds of underground plant: conduit systems, including concrete-encased duct banks, and direct buried cable. It also covers how cables are pulled into ducts, because pulling forces often decide where maintenance holes and handholes must go.

Conduit systems and duct banks

An underground conduit system is a set of ducts buried between structures such as maintenance holes, handholes and building entrances. Cables are pulled into the ducts after construction. The ducts may be laid directly in a trench and backfilled, or they may be encased in concrete to form a duct bank, which protects them from crushing and from damage by later digging. Concrete encasement costs more but is common under roads, parking lots and other areas with heavy loads or a high chance of future excavation.

Ducts for telecommunications are typically 100 mm trade size, sometimes 50 or 75 mm for small branches. The most common materials are rigid PVC, either thick-walled for direct burial or thinner-walled types meant for concrete encasement, and high-density polyethylene, which comes in long coils and suits directional boring and plowing. Duct materials must be approved for the use under the standards adopted locally, such as the CSA standards for underground conduit, and joints must be glued or fused so that silt cannot enter.

A duct bank is designed as a cross-section. The designer chooses the number of ducts, their arrangement in rows and columns, the spacing between them, the thickness of concrete around them and the depth of cover from the top of the concrete to the finished grade. Typical designs place ducts in a grid held by plastic spacers every metre or two, with a few centimetres of concrete between ducts and at least 75 mm around the outside. The required depth of cover depends on the owner's standard, the road authority and the location, but cover of 600 to 900 mm to the top of the bank is common, with more under roads.

Innerduct

Pulling a single small fibre cable into a large duct wastes space, because a second cable pulled in later may wrap around and damage the first. Innerduct solves this. It is smaller, flexible plastic tubing, often corrugated or ribbed inside to reduce friction, pulled into the main duct before any cables. A 100 mm duct commonly holds three innerducts of about 32 mm, and each innerduct takes one fibre cable. Fabric innerduct, made of woven textile cells, packs even more cables into one duct. Each innerduct should have a pull line left in it for the next cable.

How full can a duct be?

Cables must fit loosely enough to be pulled without damage and, for heat-producing power cables, to dissipate heat. Telecommunications cables do not produce much heat, but friction and jamming still limit fill. The common guidance for three or more cables in a conduit, adopted from electrical practice, is to use no more than about four tenths of the conduit's internal area. For a single cable the practical limit is set by pulling tension and jamming rather than area.

Pulling cables

Every cable has a maximum rated pulling tension, given by the manufacturer. Outside plant fibre cables commonly have ratings in the range of 2,700 newtons, but the actual value must come from the data sheet. If the force needed to pull a cable through a duct section exceeds the rating, the fibres can be strained, causing loss now or failure years later. Tension builds up along straight runs because of friction with the duct wall and multiplies at every bend, so long sections with many bends are the main risk.

In these formulas μ is the coefficient of friction between cable and duct, typically 0.2 to 0.5 depending on materials and lubricant, w is the cable's weight per metre in newtons, L is the length in metres and θ is the bend angle in radians. A 90 degree bend is π/2 or about 1.57 radians. The bend formula, often called the capstan equation, shows why bends matter: each one multiplies the tension rather than adding to it.

Because bends multiply tension, outside plant standards and owners limit the total bend between pull points. A common rule is no more than 180 degrees of bends, for example two 90 degree sweeps, in any section between maintenance holes, handholes or other pull points. Sweeps should have large radii, much larger than the minimum bend radius of the cable, which for fibre is typically twenty times its diameter while being pulled and ten times when installed. Pulls should use lubricant approved by the cable maker, a swivel between the pull rope and the cable, and a breakaway link or tension meter that stops the pull if the rated tension is reached.

Pull direction matters too. Because bends multiply whatever tension already exists, pulling from the end nearest the bends usually needs less force than pulling from the far end. For very long runs, cable can be pulled from a midpoint in both directions, with the cable laid out in a figure eight on the ground so that it does not twist, or pulled by a blowing machine that uses air to carry the cable through the duct with very little tension.

Directional boring

Under the municipal street, the college does not want an open trench, and neither does the city. Horizontal directional drilling, often called directional boring, drills a pilot hole under the obstacle from a pit on one side, enlarges it and pulls a coil of polyethylene duct back through. It avoids cutting the road and disturbing traffic. The bore path must be planned around existing utilities, which must be located and, where they cross the bore, exposed by careful digging, as Lesson 12 explains. The depth under the road is set by the road authority's permit.

Direct buried cable

Direct buried cable is laid in the ground with no duct. It is cheaper to install than a conduit system, especially over long distances in open country where a plow can cut a slot and lay the cable in one pass. But it cannot be replaced or added to without digging again, and it is more exposed to damage by later digging or by rodents. Cables for direct burial are designed for it, usually with a metallic armour or a heavy jacket, and they must be laid at a depth required by the owner's standard and local rules, with a marker tape or warning ribbon above and, for dielectric cables, a tracer wire so that the cable can be found later.

On most campuses, direct buried cable is used only where future changes are unlikely and the route is easy to locate: a long run across a field to a remote building, for instance. Where the route crosses roads, parking lots or areas likely to be dug up later, conduit is the safer choice. Daniel considers direct burial for a future connection to the college's sports field lights but recommends a single duct with innerduct instead, because the field is due to be regraded in five years.

Separation from other utilities

Underground telecommunications plant must be kept apart from other utilities. Separation from power cables protects people working on either system and limits interference. Separation from gas, water and sewer lines protects all of them during repairs. The required distances come from the electrical and safety codes adopted locally, the overhead and underground systems standards in the CSA C22.3 series, utility standards and the owner's or road authority's rules. Where telecommunications and power share a trench by agreement, called joint trench construction, the agreement sets the separation and the order in which ducts are laid.

Working with the civil engineer

On most projects the ICT designer does not draw the trench details alone. A civil engineer designs grading, drainage, paving and the structural parts of maintenance holes, and the ICT designer provides the duct count, arrangement, sweep radii, pull points and cable requirements. The two must agree on the route, the depth, the crossings with storm sewers and water mains and the restoration. Daniel meets the civil engineer on site with the survey drawing. They move the duct bank half a metre north to clear a storm sewer manhole and add a low point drain in one maintenance hole, because water will find its way into any duct system eventually.