- Colossal column-free space
- Value engineering rejected
- Resisting uplift
- Hard-working columns
- 3D modelling to the rescue
- Visitors to appreciate clever engineering
- Nightmare logistics and red alerts
A bold new entrance courtyard and colossal underground gallery calls for sophisticated engineering at London’s Victoria and Albert Museum.
Client: The Victoria and Albert Museum
Contract value: £28m
Main contractor: Wates
Architect: Amanda Levete Architects
Structural engineer: Arup
Groundworks subcontractor: Toureen Mangan
Completion date: December 2016
Exhibition Road in London’s South Kensington is home to three of the world’s greatest museums: the Natural History, Science and Victoria and Albert.
All founded in the 19th century, their grand facades and great halls packed with dinosaur skeletons, steam engines and art and design objects are famous worldwide.
However, since they date from the Victorian era, the buildings also contain much smaller proportioned rooms that struggle to show off their exhibits to best effect.
As a consequence, over the past few decades, each establishment has had to expand in different ways to offer a visitor experience in keeping with modern expectations.
Colossal column-free space
At the V&A, a colossal 1,100 sq m column-free exhibition space with a head height of 10.5 m is currently being built beneath an underused courtyard (known as the boilerhouse yard) which fronts Exhibition Road.
It will be the museum’s most significant addition in 100 years.
Bounded by the V&A’s Henry Cole Wing to the north, Aston Webb building to the south and Western Range galleries to the east, the roof of this new subterranean gallery will also form a new entrance courtyard.
The courtyard will be clad in handmade ceramic tiles – claimed to be the first of their kind in the UK – and the gallery will be accessed via a sinuous staircase located in the Western Range.
A café and glazed openings will also punctuate the courtyard, giving it sculptural qualities.
The original stone-pillared wall that screened the yard from Exhibition Road (also known as the Aston Webb Screen) has also been painstakingly dismantled during construction and will be reinstated, minus the wall, to form a colonnade through which the public will enter.
On completion, the public will be able to view the decorative facades of the courtyard buildings for the first time.
Creating a vast cavity in the ground for the new gallery adjacent to a Grade I-listed building represents bold engineering, particularly as the excavation extends beneath the existing building in one location (see box).
This, combined with working on a tight site in an operational museum with the requirement for the highest quality finish, appealed to main contractor Wates Construction, admits project director Neil Lock.
“These challenges are the great attraction,” he says.
The contractor is currently excavating and installing props to reach final basement level across the site.
“These challenges are the great attraction… We had to buy into their vision”
Neil Lock, Wates Construction
The £28m contract was awarded to Wates, Mr Lock says, due to its commitment to understanding the needs of the build.
The firm visited suppliers in Spain to understand first-hand the lengthy process of producing handmade ceramic tiles and the preparation required to lay them on the new gallery roof and courtyard.
“Normal granite tiles can be laid relatively quickly, but those on this project will need more careful alignment and bedding – we were able to plan realistically how long this would take,” he says.
The contractor also has a long history of working on heritage buildings and was recently involved in refurbishing the east wing of Somerset House in London.
Value engineering rejected
Unusually, many suggestions made by Wates to value engineer details prior to starting on site were rejected by the V&A and architect Amanda Levete Architects.
But the process, Mr Lock explains, brought about a greater understanding of what the scheme was about: high-quality materials and workmanship reflecting the splendour of the objects housed in the original museum.
“We had to buy into their vision,” he says.
“The whole thing wants to lift up because there is no load being applied above ground”
Alice Blair, Arup
The boilerhouse yard area had been previously cleared of services by subcontractor Mitie and from January 2014, over a period of 16 weeks, the low-level buildings were demolished and a piling mat established.
From this level 2 m below the pavement, rigs installed secant piles to create a watertight perimeter basement wall.
The site is split into two areas: the main courtyard and a spur to the north along the Royal College Building.
The Royal College basement will be divided into four levels (L0, B1, B2 and B3), while the main courtyard basement features just two (B2 and B3).
The main courtyard and Royal College basements therefore share a common ground, B2 and B3 levels (B3 is the lowest basement level).
B2 is the main gallery level beneath the courtyard and has a room height of 10.5 m.
Reinforced concrete piles around the main courtyard area are 900 mm in diameter and 20-30 m deep.
With the development requiring 22,500 cu m of earth to be removed and with relatively little weight being applied to the ground from the new structure, one of the main design challenges was to resist heave from the ground below, Arup senior engineer Alice Blair explains.
“On this project, the whole thing wants to lift up because there is no load being applied above ground.”
As such, 13 tension piles 1,200 mm in diameter and 50 m in length had to be installed within the main courtyard gallery footprint.
In addition, 254 UC plunge columns were embedded into these piles to provide the future structure between levels B2 and B3 and to facilitate top-down construction from this level.
Watch the construction
The courtyard piles took 17 weeks to install – one week longer than planned.
Within the Royal College area, secant piles are 20-30 m long and 600 mm in diameter, but took more than double the time than expected to complete due to harder-than-expected ground, as well as congestion of construction vehicles in the courtyard area and the sole entrance and exit gates (see box).
With London Clay underlying 7.4 m of made ground, all piles had to be sleeved 2 m into the clay layer.
After piling was complete and capping beams installed, work on excavating the gallery space and installing massive temporary props could proceed within the safe confines of a watertight enclosure, structurally distinct from the original gallery buildings.
With the main courtyard gallery space eventually measuring about 30 m by 40 m and being column-free, the props here are working extremely hard.
They will be removed as site workers erect the main roof structure and floors.
At the time of CN’s June visit, excavation had reached B2 level under the main courtyard, where a 300 mm-thick slab with 500 mm-thick column heads will soon be cast.
Two rectangular openings will be cast in the slab to allow excavation to continue to B3.
“Nothing is straight or square on this project so we combined Arup’s 3D structural model and their propping model to check for clashes”
Colin Luckhurst, Wates Construction
“The slab is needed to provide sufficient restraint to the perimeter secant piled wall,” Wates senior temporary works engineer Colin Luckhurst explains.
Plunge columns embedded in the tension piles provide the vertical supports for this slab, which is due to be cast by mid-June.
As with top-down construction, 30-tonne excavators will work from the openings in the B2 slab to remove soil beneath.
When this is complete, the 1,200-1,500 mm-thick reinforced concrete B3 slab can be cast.
In the Royal College basement area, excavation has now reached B3 level, with more frequent propping with depth at every floor level.
Across the site, the propping regime has also had to take into account the locations of permanent steelwork.
“Nothing is straight or square on this project, so we combined Arup’s 3D structural model and their propping model to check for clashes,” Mr Luckhurst says.
“Every single prop has had to be adjusted to some degree to work around the permanent steelwork.”
3D modelling to the rescue
In fact, a 3D model was essential to visualise the structure, since the courtyard floor/gallery roof slopes to accommodate a 6 m height difference between the bases of each of the surrounding buildings and Exhibition Road.
A single primary truss 30 m long bisects the roof and spans between Exhibition Road and the Western Range galleries.
Thirteen 3 m-wide, 3 m-deep secondary trusses, made up of a twin top chord and a single bottom chord, transfer loads from the primary truss to fin walls.
These project out from a wall cast on top of the secant piling along the Aston Webb Building.
The bottom chord is a square section and back-to-back parallel flange sections make up each top chord.
UC and UB sections have been used for all other components in the truss.
These trusses are between 5.5 m and 9 m in height and have been designed to allow for a minimum room height, but also to meet the needs of the sloping courtyard floor.
“We’ve been very conscious of keeping the weight of the structure down. The courtyard is 30 m by 40 m, so any increase in blanket load has a big effect on the structure”
Alice Blair, Arup
Trusses have not been used in the roof spanning between the primary truss and the Henry Cole building, nor the spur along the Royal College building.
Instead, beam sections have been used, which support the café and shop that sit at courtyard level.
Within the Royal College basement area, B1 level is a 300 mm-thick RC slab while L0 is a 200 mm-thick composite slab in the café and shop locations and 300 mm-thick RC slab in others.
“We’ve been very conscious of keeping the weight of the structure down,” Ms Blair explains.
“The courtyard is 30 m by 40 m, so any increase in blanket load has a big effect on the structure.”
The length of the secondary trusses have also been limited by the length of the delivery lorry (25 m), while the primary truss will arrive in four sections with splices located according to structural requirements and the lifting limit of the tower crane (15 tonnes at 40 m).
Wates is due to complete the project by the end of 2016.
An exhibition describing the design and construction of the basement gallery extension is currently open to the public at the main museum and will remain until October 2016, just before the grand unveiling of the real deal in 2017.
Visitors to appreciate clever engineering
A single stretch of Western Range Galleries façade about 10 m long forms the most daring engineering component of the V&A extension.
This will also be expressed in the final design to remind visitors to the underground gallery of its sheer ingenuity.
Each pier on this façade used to transfer between 90 and 160 tonnes of load onto pad foundations.
Since the new basement gallery extends under these piers, their loads have had to be carefully transferred to a temporary steel structure that rises up through the basement and sits on new piled foundations.
To knit this stretch of façade into the new structure, 14 steel beam ‘needles’ pierce the stonework and then connect back to two braced steelwork towers.
“The needles pass into pre-cored holes in the stonework and had to be located so that they did not clash with the permanent steelwork,” Mr Lock says.
Jacking and monitoring equipment confirmed that the pier loads had been transferred from their original pad foundations to the steel towers.
“The façade was lifted 1 mm to demonstrate that the load was off the old foundations,” Mr Luckhurst explains.
“Square pad foundations measuring 600 mm deep by 2 m wide were then carefully cut from the structure and broken up.
“This part of the building used to be ground-bearing and now there is about 10 m of clear space beneath it.”
In the permanent case, the section of façade will be supported by an 1,100 mm-deep steel girder and steel columns.
The feature staircase will pass close to this junction and, on completion, will allow a unique view which takes in the basement gallery under the Western Range piers and the listed building façade through a window.
Gallery roof steelwork had to be carefully detailed, sized and located to ensure these views were preserved.
“We had to get the beams in the right place to keep those views – once that feature staircase has been constructed you’ll see those perspectives we worked so hard to achieve,” Ms Blair says.
Nightmare logistics and red alerts
With tourists constantly visiting Exhibition Road and a tenfold increase in traffic on bank holiday weekends, Wates employed logistics specialist MadiganGill to develop a traffic management plan for the project.
A heavy-duty temporary gantry was erected along the Exhibition Road site boundary to allow vehicles to enter and leave the site.
At the peak of excavation, 35 to 40 wagons a day removed spoil from site.
This required the pavement directly in front of the site to be closed off to pedestrians for just two minutes at a time as wagons were filled up.
In conjunction with the Kensington and Chelsea Borough Council and the Metropolitan Police, cycle safety events have also been organised for construction vehicle drivers and cyclists to be made aware of the challenges of sharing the same space.
Working so close to the listed buildings and the public, construction methods and equipment had to be selected to meet noise and vibration restrictions.
Specialist subcontractor ITM Soil is monitoring the site to check work stays within acceptable limits, with an amber alert indicating that the method of work must be re-evaluated and a red alert meaning work must stop and an alternative solution found.
A red alert occurred when the piling mat was being established on site, recalls Mr Lock: “There was a requirement for stiff compaction using a vibrating roller.
“Due to the compaction required, the machine specified produced too much vibration when it was close to the Aston Webb building.
“The red alert prompted work to stop until a better solution could be found that would minimise vibration.
“The piling mat was redesigned to be compacted in thinner layers with more geotextile mesh between the layers of crushed concrete.
“The consequence of this was that a smaller, lower-vibration rolling machine could be used for longer to gain the required compaction.”
Monitoring equipment has also been installed in specific piles and in the ground around the basement to provide information about the effects of the structure on the ground during construction and in the long term.
This will feed into work carried out by the Cambridge Centre for Smart Infrastructure and Construction, which aims to collate information about how structures respond to changes ground conditions in order to encourage more efficient designs.