The last of the project's tunnel boring machines (TBM), named "Grace", broke through on, having tunnelled over 11km eastwards from National Grid's New Cross substation in Southwark.
All 32.5km of the project's underground route are now complete, with the installation of 200km worth of high voltage cable - enough to stretch from London to Cardiff - already underway between substations at Wimbledon and Crayford.
Construction of the tunnels began in March 2020 with the tunnelling works undertaken by National Grid's delivery partner HOCHTIEF-Murphy joint venture (HMJV).
Alice Delahunty, president of National Grid Electricity Transmission, said: "Our London Power Tunnels project has achieved a lot since it kicked off in 2020, but the final tunnelling breakthrough at Eltham is a particularly remarkable moment.
"This complex engineering endeavour is now really taking shape, with completion of tunnelling now physically linking our sites across South London for the first time and meaning we can move on to the next chapter to progress our vital cabling work."
Santiago Daniele, HMJV project director, said: "It's been an incredible journey so far, from taking over our tunnel drive sites a week after the Covid lockdown, to sinking eight shafts and undertaking five tunnel drives with four TBM over the past three and half years.
"With our final TBM drive breaking through at Eltham ahead of the original baseline programme, it's testament to the collaboration, ingenuity and fantastic joint culture of all involved - despite all of the challenges faced. "
Tunnelling on the LPT project was completed in three sections between existing National Grid substations - Wimbledon-New Cross (12km); New Cross-Hurst (18km); and Hurst-Crayford (2.5km) - with the help of four TBM named Christine, Caroline, Edith and Grace.
Breakthroughs previously took place at Eltham in June 2022 (TBM Christine tunnelling from Hurst), Wimbledon in July 2022 (TBM Caroline tunnelling from Kings Avenue), Crayford in January 2023 (Christine again tunnelling from Hurst) and Kings Avenue in April 2023 (TBM Edith tunnelling from New Cross).
Vertical shafts ranging between 9-15m in diameter and up to 55m in depth were constructed along the route, serving as waypoints for the TBM, and helping project teams safely access the tunnels for maintenance now and when operational in the future (headhouses will be built to cover the shafts for safe future use).
The Hurst substation site recently saw a world record-breaking pour of cement-free concrete to fill the base of its 55m deep shaft following Christine's two tunnel drives to Eltham and Crayford.
In another first for LPT, National Grid's new Bengeworth Road substation on the tunnel route in Lambeth is being built free from greenhouse gas SF - the only one of its kind to date in Britain and part of National Grid's ambition for its infrastructure to be SF-free by 2050.
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