17th February 2025

The hidden engineering challenges behind London’s skyline boom

London’s skyline is rapidly evolving, and with nearly 600 new skyscrapers potentially planned over the next decade, the city is being reshaped into what some are calling a “Manhattan-on-Thames”. Recent approvals include a stylish trio of buildings at Blackfriars and the 74-storey One Undershaft, rivalling the Shard’s height. Beyond architectural ambition, the engineering complexities of building such high-rises in one of the world’s oldest cities are often overlooked.

Unlike many newer cities with straightforward ground conditions, London’s developers must navigate a tangle of historic infrastructure, and logistical headaches from hidden tunnels to unexploded World War II ordnance. So, how do we approach these projects without causing catastrophe?

The forgotten underground
We need to start from the bottom and work up. Skyscrapers require deep foundations, but London’s subterranean world presents unique challenges. Many historic rivers that have been culverted and covered run beneath the city, and they are often poorly documented. Obstructing them can create new flooding risks for basements, tunnels and surrounding structures, as well as the risk of environmental impacts.

Beyond natural obstacles, the city is home to forgotten tunnels, including abandoned train stations, old postal railways and Cold War-era bunkers, some of which were classified until very recently, meaning they’re often discovered only after construction begins. Since this infrastructure is so deep underground, standard surveying methods, such as ground-penetrating radar, may not detect them, adding to the complexity of foundation planning. Often, by the time you discover these challenges, you’ve already picked a site, so it’s a case of carefully planning and working around them.

Don’t upset the balance
If there are existing buildings on a prospective site, removing them changes ground conditions, potentially leading to complications. Some buildings have compressed the clay soil beneath them for decades, and when they are demolished, that soil can expand, causing unintended movement in neighbouring structures. If you then put a much larger weight back on that clay soil, it can start moving in other ways.

To mitigate these risks, you can design around not impacting the settlement to the ground. Failing that, teams on site can use real-time monitoring systems to track settlement and structural movement. If shifts do occur, engineers can strengthen neighbouring buildings or adjust construction techniques, but these interventions add time and cost to projects.

Construction’s congestion charge
Project teams also have to contend with the busy streets of London. Construction sites are wedged between narrow streets, historic landmarks and active businesses. With little space for storing materials, most projects rely on a just-in-time delivery model, bringing in materials like steel, concrete and other supplies as needed.

Any delays, whether from traffic congestion or road restrictions, can disrupt processes like concrete pouring, which require a steady stream of deliveries to maintain structural integrity. Above ground level, London’s skyline is also increasingly crowded with cranes, raising safety concerns. Lifting materials over existing buildings requires meticulous coordination between construction teams, air traffic controllers and city authorities to prevent accidents and minimise disruption.

Resource-heavy projects
Sustainability is another growing priority, but high-rise construction in London comes with limitations, particularly in light of tightening fire safety regulations, which impose rigorous restrictions on combustible materials in tall structures. This leaves steel and concrete as the primary materials for many projects, both of which can have high carbon footprints. The good news is that increasingly sustainable products and construction methods are being developed.

Once built, skyscrapers will also continue to put demand on resources for utilities, immensely straining London’s dated infrastructure. Power, water and sewage systems, many dating back to the Victorian era, were not designed to handle the increased load from modern high-rises.

The sky is the limit
London’s skyline is set for dramatic change, but behind every new tower lies a complex web of engineering challenges that demand foresight and precision.

As the skyline rises, so too must the industry’s approach to problem-solving. This means not only embracing cutting-edge construction techniques but also rethinking how skyscrapers interact with the city around them. Successfully navigating these challenges will ensure the viability of ambitious future projects, ensuring that its iconic skyline remains both a symbol of progress and a testament to the ingenuity of the engineers who make it possible.

– Stuart Neal, Associate, Perega