The third in a series of three reports from events exploring underground construction, held by Architecture Today in partnership with waterproofing specialist RIW

Subterranean City 1: Waterproofing Complex Basements
Alex Massingham, UK Technical Lead at RIW, explains the bespoke nature of waterproofing basement structures
Subterranean City 2: Expanding Institutions
In recent projects the British Museum, the V&A and UCL have dug deep to make the most of their campuses
Subterranean City 3: Dealing with Neighbours
Encountering restrictions both below and above ground at the Francis Crick Institute and the House in a Garden

Encountering restrictions both below and above ground at the Francis Crick Institute and the House in a Garden

Wayne McKiernan of PLP and Rob Partridge of AKTII on the Francis Crick Institute, completed in 2016

ph: Nick Guttridge

“The institute has five times the amount of plant space that would normally be required for an office building of a similar size – more than 4600 plant modules – and its curved roof was designed to hide some of this plant. Very deep ceiling voids accommodate both services and data cabling and the basement comprises four floors, including interstitial plant floors. In the Crick building the basement also provides an optimal environment for certain kinds of research work, including the use of MRI equipment.”

West-east section; piles were driven through the London clay to Thanet sands

“Floors 2 to 5 above ground are laboratory floors, each typically housing a central hub with administrative support and breakout space and four quadrants, with primary laboratories, write-up spaces and shared secondary laboratories.

There are four main parameters to consider in creating a big basement: adjacencies – which third party assets could potentially be damaged; programme, or how long it will take to build; cost; and logistics – how it is going to be built. The challenge for the engineer and contractor is to find the optimum way to deal with the four parameters for a particular location.”

The site, behind the stepped roofs of the British Library

“The site was once the northern part of the Somers Town Goods Yard. It was subsequently used as a work site for the Foster shed at St Pancras, then for the Thameslink ‘box’, a concrete structure housing Thameslink rail tunnels beneath St Pancras. There were foundations left from the goods yard – some of the mass concrete pad footings were six metres deep, which had to be excavated. On the east is the Thameslink Box, but most significantly to the north are two parallel gas mains, 120 years old, 900mm in diameter that serve the whole of Camden Town, just one foot away from the retaining walls. Large diameter piles were driven through London clay to the Thanet sands.”

Horizontal and vertical displacements

“Laing O’Rourke’s expanding piling was chosen early on and AKT II worked closely with the contractor to find a solution. It was a very bespoke way of building a basement, starting with a vast site, putting in a retaining wall and digging a massive hole, eight metres deep. Temporary 1.5-metre-diameter piles used during the High Speed 1 station construction had to be extracted, using a very large augur.”

Sonic echo tubes were cast into the diaphragm wall to monitor its construction

The solid concrete retaining wall, or diaphragm wall, is one-metre-thick and uses 25-metre reinforcement cages. A diaphragm wall, rather than a piled wall, was chosen for its stiffness, and without diagonal propping there was freedom for the contractor to work fully across the site. Sonic echo tubes were cast into the concrete for monitoring its construction.

Inserting a plunge column

“Stage 2 was to dig down a further eight metres. With top-down construction you can build a superstructure at the same time as excavating further. The pile is driven into the ground then its upper part filled with concrete and a steel column is cast in. Where MRI equipment was to be installed in the basement a precast concrete plunge pile with stainless steel reinforcements was used, which would have no magnetic impact.”

“The cast-iron pipe of the gas main moved by about 40mm when the hole was dug. Absolute movement is not necessarily a problem, but the development of a curvature would be, so as a mitigation measure the pipe’s leaded joints were wrapped in a polymer sleeve to protect them.

A mole hole was left to take soil out as excavation continued. Prefabricated interstitial or intermediate floors were hung from the floors above, so that plant could very quickly follow.”

Gianni Botsford of Gianni Botsford Architects on House in a Garden, completed in 2018

ph: Edmund Sumner

“This project, while much smaller than the others talked about, shares similar problems in its construction, comprising two storeys below ground and one above.

The practice calls what it does ‘local adaptation’. In nature that is when a population of organisms has evolved to be more well suited to its environment than members of the same species; in architecture local adaptation is to climate, culture and context.”

The sun’s movement and penetration through gaps between the neighbouring properties influenced the form of the building

“Simple rules can dictate the evolution of vernacular architecture, such as in Marrakech – where courtyards and very narrow alleys deal with heat and serve to prevent the penetration of the sun as much as possible. As a student at the AA my diploma project was a computer programme that I had developed which generated form based on solar movement.”

The existing site, flanked by tall houses

“The House in a Garden site had a 1960s bungalow built in the garden of an 1840s villa. It’s an overshadowed north-facing site, surrounded by four-storey buildings. Gaps between the buildings became really important. Analysis of the site suggested three different places where light collected. Two of the these were subsequently used for gardens and one for an oculus. Through that process the roof form developed, curving down on all edges in order not to impact on the neighbours.”

“The strategy was to use the external areas as much as possible. The ground floor houses the kitchen and living space. The two lightwells also serve the floors below. To enter the house one crosses a steel grating; this lightwell serves the guest bedroom, guest bathroom and hall; below is a small courtyard and a skylight to the pool. The larger lightwell also has a skylight at its foot and serves the master bedroom and the master bathroom. At the second basement level this skylight serves a yoga room and the main gallery space. ”

Sections: of the house’s three storeys, two are underground

“The basement was built top down, the largest reason for this being a Thames Water ring main 40 metres below ground. Thames Water was concerned that the ring main would weaken if a basement was built above it, but was eventually persuaded that by using top-down construction the ground below would remain unmoved during excavation. Casting floors as you go reassures people more than seeing a big hole in the ground. Very deep anchoring piles stop the building from popping up like a duck in the bath – it wants to float.”

“The house has the traditional context of a pavilion in a garden but the context of its surroundings, rather than being one of similar material or form, is one that you can read quite logically by understanding how it relates to the movement of the sun.”

ph: Edmund Sumner

“The gardens are built where the light is and much of the house where the shadow is. Despite going nine metres underground every space is naturally light and that’s how it relates to the outside. The marble lining of the lightwells reflects the light around. Basements combined with lightwells create their own privacy.”