Walking around the stuccoed streets of Kensington today, you might be in for a surreal sight. The marching rows of Doric columns, pedimented porticoes and dentilled cornices that define these imposing ranks of wedding-cake mansions have been joined by an unlikely addition to the classical architectural vocabulary.


Poking up at regular intervals, thrusting outwards from their molded openings as if performing a salute to passers-by, are lines of angled conveyor belts. Slowly rumbling away, they reach high above the trees, pouring a continuous stream of rubble into the cradles of awaiting skips. You would be forgiven for thinking that the residents of the royal borough have established a kind of coal-mining cottage industry. Or maybe they're digging for gold?


The reason for all this quarrying is not the discovery of a coal-rich seam beneath the *renaissance streets, but the local enthusiasm for subterranean development. Over the past four years, this local authority alone has granted planning applications for more than 800 basement extensions, refused 90, and has a further 20 outstanding. It is the most densely populated borough in the country, with no room to build outwards, and no permission to build upwards – so the only way is down.


The past five years have seen sprawling underground leisure lairs excavated across west London, from Knightsbridge to Belgravia, Fulham to *Nottinghill. They contain playrooms and cinemas, bowling alleys and spas, wine cellars and gun rooms – and even a two-storey climbing wall. It is leading to a kind of iceberg architecture, a humble mansion on the surface just the visible peak of a gargantuan underworld, with subterranean possibilities only limited by the client's imagination.

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This is about a project in Kensington, which is now complete. It has a two-car garage and three main bedrooms with en-suite facilities above ground, together with living areas, guest bedroom, swimming pool, cinema and various other rooms below.

The use of Sika® Watertight Concrete has enabled A P Arcon Construction Ltd to construct an exciting new, partly subterranean mansion, on a strip of previously vacant land between two existing buildings in Kensington. The main bedrooms, entrance and garage space are built above ground, but the main living, entertainment and swimming pool areas are underground, so total water tightness was an essential requirement on the project.


For this reason, A P Arcon chose Sika® Watertight Concrete to construct the below-ground shell of the building. Close to 1,000 m³ of Sika® Watertight Concrete were supplied by Hanson Concrete to create a watertight structure that conforms to BS8102 Grade 3 for habitable space. Around 1,200 skips of soil weighing around 10 tonnes each had to be excavated to create the working space.


With access being extremely limited, virtually the whole site had to be excavated by hand. Main piles up to 30 m with soft piles up to 7 m in length had to be constructed on the project before excavation could begin. Particular care had to be taken to *stabilise the adjoining properties as excavation progressed.


Sika® Watertight Concrete is a high performance, cost effective solution, saving time at both the design and construction stages, and it also delivers the maximum usable area to the developer, as the overdig requirement is much reduced. This is an important feature on projects such as this in cities where space is at a premium. Arek Palka, Director at A P Arcon commented: “We *choose Sika® Watertight Concrete for a number of reasons, including a 50-year track record, a competitive price and warranty. Importantly, Sika supplied good technical backup throughout which gave us added re-assurance on such a complex and difficult project”.


Indeed projects like this are very difficult. In other cases, houses began to subside after initial excavations, pulling their *neighbours’ *property down with them. Cracks appeared within the adjacent basements, and the facades sunk to such an extent that door frames shifted and people were trapped inside. That is why good construction materials, which guarantee long-term safety and stability, are essential and *Sika’s long track record of proven success.