IN Brief:
- Tunnel boring machine Caroline has passed 1.1km on the 2.2km electricity tunnel between Tilbury and Gravesend.
- More than 40,000 tonnes of material have been excavated as the project replaces an ageing 1960s high-voltage cable crossing.
- The new tunnel forms part of wider Grain to Tilbury reinforcement intended to increase transmission capability across the Thames Estuary.
National Grid has passed the halfway point in constructing its new electricity tunnel beneath the River Thames, with tunnel boring machine Caroline having travelled more than 1.1km along the 2.2km route between Tilbury in Essex and Gravesend in Kent.
The crossing forms part of National Grid’s Grain to Tilbury programme, which will replace an electricity cable tunnel dating from the 1960s and reinforce the wider transmission network between Tilbury, Kingsnorth, and the Isle of Grain.
More than 40,000 tonnes of excavated material have been removed since the drive began, while more than 5,100 precast concrete lining segments have been installed to create over 840 tunnel rings. The project team has achieved a peak production rate of 29 completed rings within 24 hours.
Caroline uses a Mixshield tunnelling configuration suited to working beneath the river, where groundwater pressure and changing ground conditions have to be controlled at the excavation face. The machine itself was launched in March at 271.5 tonnes before being connected to the trailing equipment required for spoil handling, lining installation, power, ventilation, guidance, and continuous tunnelling operations.
The complete operating train now represents substantially more equipment than the cutting machine alone. Maintaining progress beneath an active waterway requires excavation, material removal, segment delivery, alignment control, and lining construction to operate as a continuous process rather than a sequence of isolated civil-engineering tasks.
The new tunnel is being delivered with Ferrovial BEMO JV, while Herrenknecht supplied the tunnelling machinery. Access works on both sides of the Thames included deep shafts constructed before the horizontal drive could begin, creating the launch and reception infrastructure required for Caroline.
Once tunnelling is complete, the project moves from heavy civil engineering into an equally substantial electrical phase. High-voltage cable circuits must be installed through the completed crossing before being terminated, tested, protected, and integrated with the surrounding transmission system and its associated headhouse and connection equipment.
The existing Thames Cable Tunnel has carried high-voltage circuits for more than six decades and is approaching the end of its operational life. Replacing it allows National Grid to renew the physical crossing without relying indefinitely on ageing tunnel and cable infrastructure whose maintenance becomes more difficult as equipment and civil structures deteriorate.
The wider Grain to Tilbury programme also includes overhead-line upgrades intended to increase network capacity. Reinforcing the corridor will allow larger power flows to move between generation and demand centres as the pattern of electricity production changes across southern and eastern England.
The engineering challenge is particularly acute at river crossings because route choice is limited. Overhead circuits cannot simply continue through a major navigable channel, while a dedicated bored tunnel provides a controlled environment in which transmission cables can be installed, inspected, and maintained without direct exposure to the riverbed.
That controlled environment brings its own design constraints. High-current transmission cables produce heat, and thermal conditions within an enclosed tunnel influence the continuous current that the circuits can carry. Cable spacing, ventilation, ambient temperature, conductor losses, and emergency operating conditions therefore contribute directly to the eventual transmission rating.
Long high-voltage cable systems also behave electrically differently from equivalent overhead lines. Cable capacitance can generate reactive power, particularly when circuits are lightly loaded, while switching and fault conditions place specific demands on protection and voltage-control equipment at either end of the route.
Protection has to isolate a genuine cable fault rapidly without unnecessarily disconnecting healthy parts of the network. Relay zones, communications, circuit breakers, and fault-location systems must therefore be coordinated across the new crossing and its interfaces with the existing 400kV system.
The tunnel itself needs supporting systems throughout its operating life. Ventilation, drainage, lighting, access, fire detection, communications, and monitoring equipment become part of the electrical asset even though they do not directly transmit power, because maintenance teams have to be able to enter and work safely around energised high-voltage infrastructure.
National Grid’s current programme shows the main tunnel drive completing in November 2026, with the wider Grain to Tilbury works continuing afterwards. Overhead-line activity is already under way, while completion of the overall project is expected later in the decade.
Passing the halfway point therefore removes only one part of the delivery risk. More than a kilometre of tunnelling remains before the cable installation, testing, connection, and commissioning work can turn the new underground structure into an operational transmission route.
The milestone is nevertheless significant because the crossing is one of the elements that cannot be replicated incrementally. National Grid either completes a viable route beneath the Thames or it does not; at more than 1.1km, that work has now progressed firmly into the second half.