Skanska faces complex challenges on Knightsbridge scheme
The site is at the junction of Brompton Road and Sloane Street (AJPMedia)
With piles driven within millimetres of Tube tunnels and a complex facade retention on London’s most exclusive shopping street, Skanska has its work cut out on the £150m Knightsbridge Estate scheme. Stephen Cousins reports.
Construction management is all about keeping multiple plates spinning, but with 14 sectional handovers to deliver by 2021 Skanska must focus hearts and minds to deliver the huge Knightsbridge Estate mixed-use development in this well-to-do area of central London.
The contractor is delivering major building works for the scheme, located in the Royal Borough of Kensington and Chelsea, a mere stone’s throw from Harrods, under a circa £150m lump sum contract with Carraig Investments, a wholly owned subsidiary of the Olayan Group, a private multinational enterprise based in Saudi Arabia.
John Birch, project director at Skanska, told CM: “I’ve been in the industry a long time and on most jobs there are two or three handovers in total, but this construction schedule is very detailed, with nine separate critical paths.”
Employer’s representative: Aecom
Cost consultant: Alinea
Main contractor: Skanska
Architect: Fletcher Priest Architects
Structural engineer: Waterman Structures
Building services engineer: Chapman BDSP
Steel frame: Severfield
Cladding and shopfronts: Permasteelisa
Facade repair and restoration: PAYE
Three retail units will hand over almost a year before Skanska finishes on site and the shops will be trading for six months while the rest of the building is completed.
The logistical imperative to hit programme milestones meant firstly tackling a number of technical challenges, including the complex substructure and above-ground temporary works designed to streamline and support work on the main build.
An ambitious facade retention scheme (FRS) was devised to prevent a patchwork of historic elevations, stretching almost the entire perimeter of the massive 3,800 sq m site, from collapsing onto the street, while the existing building interiors were demolished.
Elaborate foundation works involved the excavation of some 18,000 cu m of muck – 50 lorry loads a day for 10 weeks – to create an 18m-deep, three-storey basement, and the insertion of some 50 piles within a London Underground protection zone.
“At points we had piles less than 250mm away from London Underground assets, when projects are normally not permitted to go within 3m. That proximity required detailed method statements for the piling work, extra monitoring and London Underground had an engineer on site full time to oversee what we were doing,” says Birch.
Skanska’s involvement in Knightsbridge Estates dates back to July 2016, when it was appointed by Carraig Investments under a 34-week pre-construction services agreement (PCSA) to scope out work based on a design by Fletcher Priest Architects (FPA).
The PCSA was subsequently extended by a further 34 weeks and in early 2018 the main contractor was awarded the build contract. Construction got underway in May. Day-to-day progress on site is overseen by developer Chelsfield, which manages Olayan Group assets in the UK, and a client monitoring team comprising FPA, Waterman Structures and Chapman BDSP.
Above: The proposed final development. Below: Concrete is poured for the building’s frame (AJPMedia)
The project is located on a roughly rectangular plot of land at the junction of Brompton Road and Sloane Street in the heart of bustling Knightsbridge, where a plethora of upmarket shops and fine dining venues encourage the great and good to part with their cash.
The prime location dictated a retail-led scheme (65% by value) with shops spread across basement level 1, ground, first and parts of the second floor. Residential apartments and offices are split across levels 2 to 6 and residents have access to an open-air courtyard garden with an orangery at the centre of the scheme on the second floor. A restaurant is located at the western end of the development on floors 6 and 7.
The superstructure combines a concrete frame for the apartments, needed for acoustic insulation, and a steel frame. The project involves a total 1,000 tonnes of steel and circa 17,300 cu m of concrete (frame and basement).
Knightsbridge Estates integrates with access to Knightsbridge Underground station on the Piccadilly Line and, as part of a development agreement with London Underground, it involves major alterations to station access and circulation.
The main entrance to the tube, on the corner of Sloane Street and Brompton Road, will be converted into a retail unit and replaced by a new entrance in the Brompton Road elevation with a widened pavement for pedestrians. A second entrance will be created at Hooper’s Court, an alleyway on the western flank of the development, alongside the existing 1906 station entrance with its distinctive ‘oxblood’ glazed red tiling, which will be retained and restored.
A new step-free route to the platforms, including two 17-person lifts at Hooper’s Court, will reopen areas of the station closed in the early 1930s when escalators were introduced.
The site’s proximity to London Underground assets, including two tunnels running under Brompton Road, a disused tunnel that loops away from these at right angles, and two existing cast iron shafts (being converted to accommodate a new lift shaft and fire escape stair) dictated the need for stringent control over ground movement during the formation of the substructure.
Scott Newton, engineering manager at Skanska, says: “Dig a big hole and the ground is going to want to lift up and move inwards, so the temporary works basement propping scheme had to be designed to meet tough trigger limits set by London Underground.”
Enabling works behind the retained facade in January 2019
Location of the massive site close to Hyde Park
The massive escalator barrel to the north of the site was a particular concern, its horizontal movement had to be kept below 9mm at the top and 17mm at the bottom during excavation.
Some 578 structural piles were needed to support the new building, including 1,500mm-diameter piles positioned away from London Underground assets and 50 smaller piles of between 600mm and 750mm diameter positioned much closer in.
“If construction projects come within a 3m zone, London Underground has to get involved and a lot of our smaller piles, in the area of the disused tunnels and the cast iron shafts, are well within that – four are just over 200mm away,” says Newton.
Detailed method statements for piling in the sub-3m zone were drawn up and reviewed, in discussion with London Underground, the client, its structural engineer and the piling contractor. Work was then carried out in a stop/go process involving regular checks on pile verticality. A small camera was inserted inside the bore holes to check integrity.
Retaining perimeter frontage
If pedestrians were unaware of the challenging work going on beneath their feet, they will surely notice the facade retention scheme (FRS), a lattice of chunky steel towers and beams erected around the outside of the existing facade to prevent it from collapsing while the floors behind were demolished.
The decision to retain most of the existing perimeter frontage was unusual, given that just one of the buildings, a 1904-built four-storey property at the corner of Sloane Street and Basil Street, is listed. This is being retained on its existing foundations and refurbished to create apartments.
“The facade has undergone a lot of modernising to make it work while staying sensitive to the original construction. We’re adapting the existing, but in a way that looks like it is original.”
Tom Salmons, FPA
Tom Salmons, senior associate at FPA, told CM: “The project sits within the Hans Town Conservation Area so we wanted to improve the facade, rather than replace it, because it is such an iconic location and the planners didn’t want us to take down the entire city block.”
The reuse of the facade supports the project’s environmental credentials; the offices achieved a BREEAM Excellent rating and the retail Very Good.
Some sections of the frontage were demolished, including a 1950s building at corner of Brompton Road and Hooper’s Court, which did not fit visually with the rest of the scheme. This gap currently provides site access, but will be filled with a recreation of the building that originally stood on the plot before being bombed in the Second World War. The design was developed in collaboration with the project’s facade repair and restoration specialist PAYE, based on archive photographs and historic reports.
The Hooper’s Court elevation was demolished above ground floor level to make way for a unitised facade with white enamel tiling and a floral pattern inspired by John Hooper, a keen gardener who developed the masterplan for the existing city block.
The ornate stone facade of the 1920s building at the corner of Brompton Road and Sloane Street had sections of the first floor removed and reconfigured to create taller windows more suitable for residential use. The original architect’s plan for an impressive corner dome and turret was never fully realised but will be reinstated.
Above: Oxblood red tiling is being retained and restored on the Hooper’s Court facade. Below: Construction work takes place on the steel frame of the new build (AJPMedia)
“The facade has undergone a lot of modernising to make it work whilst staying sensitive to the original construction. We’re adapting the existing, but in a way that looks like it is original,” says Salmons.
Facade retention design
The FRS was designed by engineering design consultancy Wentworth House Partnership for Keltbray Demolition & Civils as part of the demolition contract. It was subsequently redesigned during Skanska’s PCSA period to meet the needs of main building works, including the requirement to control ground movements during basement excavation.
The steel columns and beams rise to the fourth floor and a height of about 25m and connect back to the facade with timber struts. The bases of the columns plunge through the pavement and are cast into existing vaults underneath, which absorbs loads generated by the movement of the facade. The vaults around the perimeter were all infilled with concrete and tied together with cast-in reinforcement to create a ‘ring beam’ that anchors the FRS and shores up support for the basement excavation.
“The biggest issue with installing an external retention scheme is the need to avoid services in the footpath and we were limited to a certain extent by where services are located,” says Newton. “It meant a lot more preliminary investigations and hand digging to avoid damage.”
The presence of London Under-ground assets close to the surface, including a ticket hall and escalator barrel at the junction of Brompton Road and Sloane Street, meant the FRS at this location could not connect to the vaults and instead was retained on strip foundations at pavement level. Additional support was provided by a support tower positioned behind the facade.
The project team’s diligent planning and execution for substructure works ultimately paid off and trigger levels for London Underground were never exceeded.
The FRS is now being dismantled as the building’s concrete and steel superstructure rises behind the temporary works structure. Construction work on the Knightsbridge Estate development will continue through 2020 with a scheduled completion date of April 2021.