How Is Fibre Optic Cable Installed Underground?

Underground installation is one of the most common methods used to lay fibre optic cable across the UK. It protects the cable from weather, physical damage, and interference, and it keeps networks out of sight in urban and rural environments alike. But it is also a process that involves careful planning, civil engineering, and compliance with street works legislation before a single metre of cable goes in the ground.

This guide explains how the process works, what each stage involves, and what contractors, developers, and project managers need to consider when planning underground fibre works.

Why Install Fibre Underground?

Above-ground infrastructure such as telegraph poles and overhead cables can be quicker to deploy in some situations, but underground routes offer clear long-term advantages.

Underground networks are less exposed to storm damage, vandalism, and accidental interference. They are preferred in built-up areas where overhead cables would be impractical or visually intrusive. For new housing developments, commercial sites, and public sector projects, underground installation is almost always the standard approach.

It is also worth noting that much of the UK’s existing telecom network already runs underground, meaning new fibre routes are often connecting into underground infrastructure from the start.

Stage 1: Survey and Route Planning

Every underground installation starts with a survey. The purpose is to identify the best route for the cable, map existing underground services, and understand the ground conditions along the proposed path.

This stage involves:

• Reviewing existing utility records and drawings
• Carrying out ground-penetrating radar (GPR) scans where needed
• Identifying potential conflicts with gas, water, electricity, and drainage assets
• Assessing ground type and likely excavation conditions
• Planning access points and chamber locations

A well-executed survey prevents costly surprises during excavation. Striking an uncharted utility cable or drainage pipe mid-dig is a serious safety risk and can cause significant delays.

The survey output feeds directly into the design of the network, including duct routes, chamber positions, cable splice points, and the overall installation method.

Stage 2: Permissions and Street Works Notices

Where installation crosses or runs along public highways, pavements, or public land, statutory permissions are required under the New Roads and Street Works Act 1991 (NRSWA).

A street works licence must be obtained, and the relevant notices submitted to the highway authority. The notice period depends on the scale of the work:

• Minor works (up to three days): minimum three days’ notice
• Standard works (up to ten days): minimum seven days’ notice
• Major works (over ten days): minimum three months’ notice

Traffic management plans must also be approved before any lane closures, road restrictions, or footpath diversions are put in place. Failing to comply with notice requirements can result in fixed penalty charges and enforcement action from the local authority.

For works on private land, wayleave agreements may be needed to secure permission from landowners.

Stage 3: Traffic Management

On any works affecting the public highway, traffic management must be in place before excavation begins. The type of management needed depends on the location and the nature of the works.

Common setups include:

• Traffic lights (stop and go)
• Lane closures with cones and signing
• Full road closures with signed diversion routes
• Footpath closures with safe pedestrian alternatives

Traffic management must comply with the Traffic Signs Regulations and General Directions and Chapter 8 of the Traffic Signs Manual. Operatives involved in setting out and maintaining traffic management must hold the relevant LANTRA or NRSWA accreditations.

Getting traffic management right is not just a legal requirement — it protects the workforce and the public throughout the duration of the works.

Stage 4: Excavation and Ducting

Once permissions are in place and traffic management is set up, excavation can begin. The method used depends on the ground conditions, the environment, and the depth required.

Open Cut Excavation

The most straightforward method. A trench is dug along the planned route using a combination of mechanical excavation (a tracked excavator or trenching machine) and hand digging near existing services.

Duct is then laid in the trench at the required depth. In the UK, telecoms ducting is typically installed at a minimum depth of 350mm under footways and 600mm under carriageways, though this can vary depending on the site conditions and network owner requirements.

The duct is bedded on sand or fine material to protect it, and marker tape is laid above to identify the route during any future excavations.

Directional Drilling (HDD)

Where open-cut excavation is not possible — under a road, river, railway, or existing structure — horizontal directional drilling (HDD) is used. A drill bores a path underground along the planned route without disturbing the surface.

HDD is more expensive than open-cut but avoids the disruption of a full excavation. It is commonly used for road crossings and crossings under sensitive infrastructure.

Moling

Moling uses a pneumatic tool to drive a steel probe through the ground, creating a bore without excavation. It works well in soft ground over short distances and is a quick, low-disruption method for short road or path crossings.

Stage 5: Cable Installation

Once ducting is in place, the fibre cable is installed through it. The two main methods are pulling and blowing.

Cable pulling uses a draw wire or pull cord already threaded through the duct. The cable is attached and pulled through by hand or with mechanical assistance. This works well for short runs.

Cable blowing (also called jetting) uses compressed air to propel the cable along the duct at speed. This is the preferred method for longer runs and allows cable to be installed quickly with minimal risk of damage. Cables can be blown over distances of several hundred metres in a single go.

Access chambers and manholes are used as working points for splicing and jointing. At each splice point, optical fibres are fused together using specialist fusion splicing equipment. The splice is then protected in a splice tray within a jointing enclosure.

Stage 6: Testing

Before the network is handed over or connected to live services, every link must be tested. The standard method is Optical Time Domain Reflectometer (OTDR) testing.

An OTDR sends a pulse of light along the cable and measures the return signal. It identifies the location and severity of any faults, losses, or reflections along the route. Every splice, connector, and section of cable is tested against the required performance specification.

Any issues found at this stage are resolved before the network is commissioned. Good testing documentation also forms part of the as-built records handed to the client.

Stage 7: Reinstatement

Once cable installation and testing are complete, all excavated areas must be reinstated to the standard required by the highway authority. Reinstatement is governed by the Specification for the Reinstatement of Openings in Highways (SROH), commonly known as the Pink Book.

Reinstatement standards vary depending on the road or footway type. Permanent reinstatement must match the existing surface and meet specified compaction and material requirements. In many cases, a temporary reinstatement is placed first, with a permanent one completed within a set timeframe.

Poor reinstatement is one of the most common causes of fixed penalty notices from highway authorities. Getting it right first time matters.