Northolt tunnel. The Northolt Tunnel is a high-speed railway tunnel currently under construction in Greater London, England, and will upon completion carry the High Speed 2 (HS2) railway line under the West London suburbs. The twin-bore tunnels will run for  between Old Oak Common and Ruislip.

Plans for the construction of a tunnel were first unveiled in early 2013; it was hailed as being less disruptive and quicker to construct than a surface-based alignment. An option for the tunnel's extension along the Colne Valley to replace the proposed Colne Valley Viaduct was studied but rejected as unnecessarily costly during 2015. Extensive ground surveys along the intended route were conducted during the 2010s in advance of construction work. During April 2020, it was announced that a contract worth approximately £3.3billion had been awarded to a joint venture company, Skanska Costain Strabag, for the tunnel's construction.

History 
The construction of the High Speed 2 (HS2) railway is to involve numerous major civil engineering works along its intended route; it has been anticipated that around  of tunnels are to be bored along various parts of its route in order to accommodate it. During early 2013, it was confirmed that this section of HS2 would be constructed in a tunnel, rather than above ground. At the time, HS2 Ltd, the delivery company behind the line, stated that the use of a tunnel was the optimum solution, reducing the disruption caused by the railway's construction upon the local community while also accelerating the timetable for that portion of the route.

Prior to any construction activity commencing, numerous surveys and ground investigations were conducted along the intended route; the effort was described by Steve Reynolds, HS2’s ground investigation programme manager, as "the largest ground investigation programme that the UK has ever seen". During early 2018, a layer of black clay was discovered, which has been dubbed the "Ruislip Bed"; this material dates back 56millionyears and was formed from densely wooded marshes at the coast of a sub-tropical sea.

During the early-to-mid 2010s, the possibility of extending the tunnel to traverse the Colne Valley, as an alternative to the proposed Colne Valley Viaduct, was examined in detail. However, this option was formally dismissed in a report released in February 2015, primarily due to the increased costs and construction time that such a change would predicted to involve.

During April 2020, it was announced that a contract had been awarded to a joint venture company, Skanska Costain Strabag, for the tunnel's construction. In October 2020, it was revealed that Herrenknecht would be supplying the two tunnel boring machines (TBMs) that will be used in the boring of the western section.

Construction 
The tunnel is being bored in two sections; the western section will be  long and the eastern section will be  Of these,  has been designed as a twin bored tunnel, which will be excavated using a total of four earth pressure balance TBMs. Following the completion of boring, all four TBMs are to be retrieved at the point where the two sections meet; this site will then become the Green Park Way ventilation shaft.

The bored section of the tunnel ends at the Victoria Road Crossover Box, the first "caterpillar shaft" in the UK. This is an open air shaft where trains can switch running lines before entering the tunnel, connected by a 360m long sprayed concrete lined tunnel to Old Oak Common station itself. This crossover box will also be where the two TBMs boring the eastern end of the tunnel will be launched from.

The first two TBMs, with a cutter-head diameter of , are designed to bore through Lambeth Group rock, chalk and London Clay, and will assemble 4,220 tunnel segment rings each. They are named Caroline (after astronomer Caroline Herschel) and Sushila (after local school teacher Sushila Hirani, Lead for STEM at Greenford High School), arrived at the West Ruislip launch site to dig the western section of the tunnel in December 2021; with TBM Sushila launching on 6 October 2022  and Caroline following suit on 27 October. 

The second pair of TBMs named Emily (after midwife and former Ealing mayor Emily Sophie Taylor) and Anne (after education reformer and philanthropist Lady Anne Byron), arrived at the Victoria Road Crossover Box for assembly and launch in late October 2023. These TBMs, with a cutter-head diameter of , are designed to encounter London Clay. Emily launched on 25 February 2024, with Anne following suit in March. The tunnel is set to be complete by early 2025.

Progress 

Source: HS2 Ltd.

References 

Railway tunnels in London
High Speed 2