National Grid completes tunnelling beneath Thames 49 days early

National Grid has completed the tunnelling beneath the River Thames 49 days ahead of schedule, clearing a major construction hurdle in its Grain to Tilbury electricity transmission upgrade.

The breakthrough took place on September 27, when the tunnel boring machine, named Caroline, reached Gravesend after travelling 2.2km beneath the Thames from Tilbury in Essex.

The new crossing will replace an existing cable tunnel built in the 1960s, which is approaching the end of its operational life and carries part of two 400kV transmission circuits connecting Tilbury with Grain and Kingsnorth.

While some will be wondering why National Grid went through the expense of boring a new tunnel when there was an existing tunnel – it has quite a simple explanation. That’s because there simply wasn’t enough room to accommodate the 12 new cables, while carrying out the work alongside live high-voltage equipment would create safety risks.

Replacing the cables would also require lengthy outages that National Grid couldn't accommodate without affecting the wider network. Building a separate crossing allows most of the construction work to take place while the existing circuits remain in service.

The tunnel forms part of a wider upgrade that also includes refurbishing overhead power lines between Tilbury, Kingsnorth and the Isle of Grain. Together, the improvements are intended to increase the amount of electricity the network can carry.

How the tunnel finished ahead of schedule

National Grid has attributed the early breakthrough to favourable ground conditions, reliable equipment and careful coordination of construction materials.

Keeping the boring machine supplied with concrete lining segments was particularly important, alongside the performance of the plant used to process material excavated from beneath the river.

At its fastest, Caroline advanced 926m in a single month, while its best daily performance saw the machine excavate 41m and install 31 tunnel lining rings.

National Grid also says digital monitoring helped engineers track performance, check the quality of the work and make decisions during construction. An underwater camera system allowed some inspections to be carried out remotely, reducing the need for workers to intervene directly.

For the final breakthrough, engineers deliberately flooded the reception shaft at Gravesend to balance the water pressure between the shaft and the surrounding ground. This reduced the risk of groundwater entering as the machine broke through.

Cable installation still to come

Despite finishing the tunnelling early, there's still work to complete before the new transmission circuits can carry electricity.

The remaining tunnel lining must first be finished, while work continues to remove water from the Gravesend shaft. Once those activities are complete, the boring machine can be lifted out and dismantled.

The tunnel will then be prepared for its new high-voltage cables, which will need to be installed, connected to the wider transmission network, tested and brought into service.

While the early breakthrough removes a significant construction hurdle, National Grid has not confirmed whether the 49-day saving will bring forward the overall project's completion or energisation date.