London’s Victoria and Albert Museum may be best known for its fabulous collections of art and design, but it is also currently showcasing some impressive engineering.
The 150-year old museum is spending £49M on an extension to its home in South Kensington, and as a result, there is currently a 17m deep excavation nestled between the Grade I-listed Victorian buildings.
The extension has been designed by Amanda Levete Architects, which won an international design competition in 2011. The competing designers were asked to create a new subterranean gallery for temporary exhibitions, a public courtyard set within the existing buildings, and a new entrance on the west side of the museum.
The new gallery will be below street level, with a courtyard above, so it is being created by digging a massive box that fills almost the entire footprint of the available space, bounded on three sides by museum buildings and on the other by the busy Exhibition Road.
The £28M contract to turn Amanda Levete Architects’ vision into reality was won by Wates Construction, which in turn appointed London-based Toureen Group to undertake the demolition of existing structures; remove spoil for the piling subcontractor; carry out excavation work; prop the main box and two additional subterranean structures; and construct the internal concrete substructures for the basement galleries.
Toureen Group was formed 24 years ago, and operates in a range of specialist sectors that include demolition, groundworks and civil engineering, multi-level basement construction and high-rise reinforced concrete frames.
“This project drew in the skill set and talent of all disciplines within the Toureen Group, we felt like custodians of the Grade I-listed building during the works,” explains Toureen Group managing director Denis Nolan.
Client: Victoria and Albert Museum
Main contractor: Wates Construction
Specialist contractor: Toureen Group
Structural engineer: Arup
Project manager LendLease
Architect Amanda Levete Architects
Cost consultant Aecom
He says that the success of the project was down to team work between Toureen, main contractor Wates, structural engineer Arup and client the Victoria and Albert Museum.
Nolan says Wates initially approached Toureen because of its expertise in deep level basements. The group includes City Basements, which specialises in complex, multi-level basements in the capital; and Tilley & Barrett which specialises in demolition.
The firm’s first task was to demolish a series of old boilerhouses in the existing courtyard. They were up to 8.5m deep and surrounded by very sensitive Grade I-listed buildings. “We had to do it in a metic ulous fashion to split the old substructure from the foundations of the adjoining structures, and then demolish them,” explains Eddie McDaid, operations manager of Toureen Group subsidiary Tilley & Barrett Demolition. “Understandably, we had to work within very strict vibration limits..”
Having demolished the boilerhouses and their extensive foundations, Toureen backfilled the area to lower ground level – 2m below the level of the adjoining Exhibition Road – and left a flat surface for the piling contractors Keller and Martello Piling to come in and install a secant piled wall –formed from 600mm and 900mm piles in the main box around the entire perimeter of the new subterranean building and this included a temporary retaining wall 4m high, which supported an operating 70t piling rig.
Toureen stayed on site while the piling was going on, providing wagons to dispose of the arisings that were displaced as the piles went in. Then, when the piled basement walls were complete, Toureen installed a capping beam on top of the piles, and then started to excavate the massive underground box.
As the work progressed, Toureen installed a temporary steel platform at street level, supported on three sides by the secant piled wall and on the third by temporary piles. This heavy steel structure was key to the success of the excavation as it provided a working platform for the 30t long reach excavator that removed excavated material as it was dug. It was also used for parking wagons while they were loaded with the excavated material – ensuring they were kept off Exhibition Road.
The project involves three areas of excavation:
- the main box, measuring 35m by 40m in plan, and predominantly 17m deep
- the “dog leg” – a 30m by 9m by 17m deep corridor that extends from the north east corner of the main box between two existing buildings
- a link structure beneath the existing “Western Range” of buildings on the north side of the box, which is where the new galleries will connect with the existing museum space.
The three areas were dug using excavators ranging in size from 1.5t to 13t, with the material having to be double- and triple-handled to get it within reach of the long reach excavator perched on top of the platform. On average, 35 tipper trucks a day were removing spoil from the site. The top 7m of the dig is through gravels, with London clay below.
As the excavations progressed, Toureen installed some very intricate temporary works throughout the dig, but especially around and below the Western Range.
The Western Range excavation involved supporting the rear façade and parts of the internal structure with a system of temporary plunge columns in piles, and needles, before digging 10m below ground to form the link between the old and new parts of the museum.
These temporary works were designed by Wates and Toureen whose in-house temporary works department used stiffness and loading parameters specified by the project’s structural engineer, Arup.
“Engineering a solution to support the Western Range façade required a very coordinated approach,” explains Wates Construction’s senior temporary works engineer Colin Luckhurst
The carefully coordinated scheme meant Toureen was able to strategically place the piled plunge columns, needle the sensitive structure, jack it up by 1mm to mobilise its static load, partially excavate under it, install the permanent new support beams, then excavate to depth, bracing the columns as it went.
Temporary works solution
The main contractor also engineered a temporary works solution to enable the piling in the Western Ranges to be carried out in one visit as opposed to the two proposed on the original Arup sequence drawings.
This involved utilising parts of the permanent rectangular steel frames that were to be installed to allow the removal of internal cross-walls, to act in the temporary condition. This created the clear space required for Martello’s mini-piling rig to install the 600mm diameter piles in one visit. Wates worked closely with Arup to develop an acceptable solution that did not exert any undue stresses on the structure.
All props and associated walings were positioned such that the permanent works could be constructed without the need to reposition the props.
Three types of beam
Walings took the form of capping beams, steel universal beams or concrete beams formed within the permanent works lining walls, the latter being used to avoid any re-propping during the top down construction where the lining walls had structural requirements.
Meanwhile, in the “dog leg” section of the new building – constructed using conventional “bottom up” methods – Toureen propped the excavation as it went down, installing four layers of temporary props as the excavation proceeded.
The internal walls have since been constructed using insitu reinforced concrete, with shutters and reinforcement threaded through the main box excavation and the props.
“We had to line up all the temporary props to create enough clear space to thread the formwork and reinforcement through,” explains Toureen Contractors site engineer Donal O’Shea.
“The positioning of the temporary works was meticulously planned to make sure we didn’t have any clashes and that we had as many apertures as possible to get materials through,” says O’Shea.
Three intermediate concrete floors have since been formed within the dog leg using insitu concrete.
Unlike the dog leg, the main box has been constructed using the “top down” method. Toureen excavated down to B2 basement level, installing props as the excavation progressed. It then cast the concrete slab that will eventually form the floor at this level.
The new galleries, courtyard and entrance are due to open to the public in spring 2017.
A 3D federated model of the building was compiled in-house by Wates from different sources including Arup, Amanda Levene Architects, Bourne Steel, Wates, and Toureen via a number of different 3D platforms.
This allowed the temporary works, gantry and façade support to be accurately positioned while not fouling the very complex permanent works structure.
Screen shot 2016 07 07 at 16.15.17
Details were taken right down to the modelling of the flanges which connected the tubular props as these were positioned to fall between permanent steel beams which were to be installed later.
This model provided a vital role in coordinating structural components, some of which were engineered with minimal clearance between permanent and temporary works. Several passes of clash detection were carried out as the scheme developed including incorporating as-built positions of props once they were installed.
Numerous design workshops between Toureen, Arup, Wates and the V&A Museum were held, so that the best possible workable temporary works solutions were develop to satisfy the constraints and concerns of all parties.
Sound and vibration monitoring
All buildings surrounding the site have vibration, sound and movement monitoring with strict trigger levels.
The vibration sensors are very sensitive and when acceptable levels are exceeded (this happened typically during the demolition stage), a text message is sent to selected personnel to make them aware of the issue.
Works in the vicinity of the triggered sensor then cease while the cause is investigated, evaluated, and a solution agreed. The basement excavation was carried out without exceeding any of the trigger levels for ground movement, which were set by Arup after it carried out a finite element analysis of the dig to understand how the surrounding buildings and ground would react.
This analysis was also pivotal in rationalising the propping scheme Arup proposed at tender stage, as it meant that fewer props could be installed without compromising the structural ability of the secant piles to support the ground and prevent the subsequent reaction of the surrounding buildings to any movement induced.
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