HS2's Massive Tunnelling Operation in Chilterns Unveils Engineering Marvels and Challenges

Posted on 14 June 2021

From the elevated viewing platform, the Chilterns construction site, with its colossal tunnel boring machines (TBMs), resembles a futuristic launch pad straight out of a Ridley Scott film. Bathed in the early summer sunlight, accompanied by the gentle clinking of flint below ground, the scene exudes an unexpected tranquillity, far from the typical hustle and bustle of a construction site.

Among the machinery, the first TBM, named Florence, weighing a staggering 2,000 tons, is already well into its subterranean journey, passing beneath the M25 and forging ahead. Only the rear section of the 170-meter-long machine, forming the final gantry, remains visible, as the majority of Florence is nestled beneath the earth. Florence is just one of ten TBMs assigned to excavate 102 kilometres of tunnels for the high-speed rail line.

Soon, Florence's sister TBM, Cecilia, will commence parallel drilling alongside her. The monumental size of the machines becomes unmistakable from the vantage point of the viewing platform. These two identical TBMs are meticulously designed for the challenging composition of chalk and flints beneath the Chilterns, each responsible for digging separate 16-kilometre-long tunnels to accommodate north and southbound trains.

Behind the cutter head of each TBM are six gantries mounted on enormous rollers, facilitating a range of functions. These gantries house a variety of features, including an automated system for erecting tunnel lining segments, as well as facilities like a canteen and toilets for the tunnellers on board. Functioning as self-contained underground factories, these machines dig the tunnels, line them with concrete wall segments, and secure them in place with grouting at a remarkable pace of approximately 15 meters per day.

The fifth and sixth gantries accommodate colossal air compressors and ventilation cassettes, ensuring ample air supply for the workers toiling underground. Meanwhile, the middle gantry houses a refuge chamber, a stark reminder of the inherent risks faced by those involved in the ambitious HS2 project. The operation of the TBMs falls under the responsibility of Align, the main works contractor for HS2, comprising a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Each TBM requires a crew of up to 17 individuals, working in shifts around the clock to maintain continuous progress. On the surface, over 100 personnel manage logistics and oversee the smooth operation of the tunnelling endeavour. Standing beside these immense machines, one gains a genuine appreciation for the challenges faced by the men and women working here. In full personal protective equipment (PPE) and helmets, on what happens to be the hottest day thus far, the demanding nature of their work is evident, leaving little desire to repeat the experience.

When asked about the temperatures inside the tunnels, senior project manager Mark Clapp simply states, "It gets pretty, bloody hot in there." He explains that as the TBMs advance further, the temperatures are likely to rise. Efforts are made to cool the crew using the ventilation cassettes, but the conditions underground can be sweltering. The crew members on each TBM rotate in 12-hour shifts to ensure uninterrupted progress, except when tunnelling beneath the M25, where Highways England mandates the use of a single TBM at a time to minimise the impact on the road above.

Once the TBMs have moved past their launch chamber, workers are transported in people carriers to the machines. As the TBMs near the end of their 16-kilometre journey, it can take up to an hour for them to travel from the on-site accommodation, humorously referred to as the "HS2 Hotel," to the TBMs. HS2 Ltd and its contractors are exploring alternative options to the people carriers to ensure social distancing can be maintained, thereby minimising the risk of a COVID-19 outbreak.

The on-site accommodation block itself is a remarkable sight. Stacked containers resembling Shoreditch's Box Park, but on a grander scale, make up the "HS2 Hotel." Housing a total of 140 rooms, each containing a bed and bathroom, the accommodations are functional, albeit somewhat cosy. Additionally, a separate container serves as the canteen, described as more reminiscent of a French motorway service station than a fine dining establishment.

The construction of each tunnel necessitates 56,000 precisely engineered, fibre-reinforced segments, all produced on-site. A purpose-built factory situated between the HS2 Hotel and the TBM launch chamber is responsible for this task. From the outside, the factory resembles a colossal aircraft hangar, while inside, it takes on the appearance of a car factory showcased in an artistic Toyota advertisement, with human workers and robots working in harmony. Conveyor belts and robotic arms seamlessly transport, flip, and position each segment along a carousel, with a dedicated crew of 14 working diligently.

In a single operation, 49 segments, equivalent to seven tunnel rings, are manufactured, with the segments specially designed to interlock securely. The entire casting process takes seven hours, producing an impressive daily output of 196 segments. Since February, over 16,043 segments have been cast, sufficient for constructing more than 290 tunnel rings. Completed segments are stacked in groups of three or four, extending rows upon rows across the storage yard, ensuring an uninterrupted supply for the tunnelling process. At any given time, a minimum of 160 rings are stored, guaranteeing seamless progress.

Adjacent to the precast tunnel factory, an even larger hangar-like facility is under construction. This future site will house the precast factory for the Colne Valley Viaduct, planned for the southern part of the site. During the visit, workers, donned in protective hazmat-like overalls over their PPE, are pouring the concrete foundation for the viaduct segment storage yard. Once completed, the viaduct will span 3.4 kilometres, carrying the high-speed rail line across a series of lakes and waterways on the northwestern outskirts of London. This ambitious structure will be almost a kilometre longer than the Forth Rail Bridge, accommodating trains travelling at speeds of up to 200 miles per hour.

Undoubtedly, this colossal undertaking poses significant challenges for the men and women working at HS2's largest and potentially most demanding site.

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