Underground cable routes
Underground telecommunications infrastructure, commonly called the Outside Plant (OSP), routes and protects copper and fiber-optic cables. It includes conduits, microducts, and access points needed for installation, replacement, and maintenance.
This planned system of underground pathways, often referred to as a , provides long-term mechanical protection, simplifies cable installation and future upgrades, and minimizes the disruption caused by repeatedly excavating public spaces.
Conduits, ducts, and microducts
Conduits form a continuous route for pulling, replacing, and adding telecommunication cables. The system must provide mechanical protection, access to the route, and sealing against water and contamination where required.
Buried systems use metallic, non-metallic, or composite conduits and fittings. Requirements apply to the complete system, including resistance to loading, soil conditions, and joint sealing.
Materials and types
- HDPE (High-Density Polyethylene): Flexible conduit used for routes that require long continuous sections, corrosion resistance, and low pulling friction. Its color or marking identifies the service according to the project.
- PVC (Polyvinyl Chloride) and PP (Polypropylene): Rigid or semi-rigid conduit options. Their suitability depends on the mechanical load, temperature, and installation method, so the material choice does not replace requirements for the complete conduit system.
- Split Ducts (Split Conduits): These conduits are designed in two longitudinal halves that can be snapped or locked together over an existing, already laid cable. They add protection when a directly buried cable needs a conduit during later work.
Specialized conduit systems
- Microducts: These are small, flexible tubes, usually with an outside diameter below 16 mm, a uniform cross-section, and a smooth inner surface. They can be bundled inside a larger conduit, and small fiber cables can be installed by blowing them with compressed air. Design checks include blowing efficiency, friction, route length, and change in attenuation.
Route planning and installation
An underground conduit design specifies the route, mechanical protection, bend radii, pulling or blowing method, sealing, and access chambers. Planning and installation outside buildings follow outside-plant cabling practices, while depth and separation also depend on site conditions and the infrastructure owner's rules.
Depth, loading, and separation
Burial depth and conduit protection are selected for surface loading, soil conditions, and local requirements. Project values are referenced to the top of the conduit or cable.
- Depth and loading: A route under a road or paved area requires protection and parameters matched to traffic loads.
- Separation: Clearance from power cables and other utilities follows local rules, the installation type, and any protective barrier.
- Route protection: Additional cover, marking, or mechanical protection is used where the route is exposed to damage.
Separation from other utilities limits damage and interference risks. When communication conduits share a trench with power lines, the required clearance or barrier is selected for the installation and local rules. Before cable placement, the conduit is checked for deformation or blockage with a test mandrel and, where required, visual inspection.
Warning and identification
- Warning tape: A warning ribbon is installed above the route according to the project and local requirements. It can include a metallic tracer or locating wire for detecting the conduit path.
- Cable slack: Cable is installed with project-defined reserve to limit stress caused by ground settlement and temperature changes. The bend radius and installation method must remain within the cable limits.
- Slack loops: Extra cable loops are left in manholes or at splice points. They make repairs and splicing easier by allowing cable ends to be brought to the work area without stressing the route.
Access points: manholes, handholes, and vaults
Manholes and handholes provide access to the conduit system. They allow crews to pull or blow cables, install splices, make branch connections, and perform maintenance. Cable entries are sealed according to the route design to limit water and contamination at the access point.
Classification by function
- Manholes: Large vaults, typically made of precast concrete, deep enough for a person to enter. They are used on main backbone routes where large cables are spliced or where multiple ducts intersect.
- Handholes (Vaults):Smaller, shallower boxes, often made of polymer concrete or high-density plastic. They do not permit personnel entry; work is performed from the surface by reaching down ("hand-hole"). They are used in distribution and access parts of the network.
- Classification by Function:
- In-line (Straight): Used along straight sections of a conduit run for cable pulling.
- Branch (Splice): Designed for T-intersections, allowing cables to branch off from the main run. They provide space for splicing equipment.
- Corner: Used where the conduit system changes direction.
The selection of a manhole or handhole depends on the number of conduits, cable diameter, bend radii, slack storage, and space needed for splicing and connection work.