30 Grosvenor Square - Enabling Works

We created a new enlarged basement perimeter, including capping beam and part top-down construction elements i.e. propped slabs at ground floor, B1 and B2 levels; retaining the historic façade and diagrid floor; completed a refurbishment and demolition asbestos survey and removed all asbestos identified (over £1m of the project value); and undertook structural alterations, protecting historic assets.

For the removal of all historic items to be retained - we used specialist heritage subcontractor PAYE, while geotechnical specialist GEA was novated to us by the client to undertake site investigation works. Bauer and Martello were our piling partners, while Intersteel supplied and installed the steelwork and Mabey managed the complex jacking operation involved.

Overall, this challenging project involved multiple concurrent activities in close proximity, including; highways works, extensive services diversions and utility works, heritage removals, mechanical and engineering decommissioning, demolition, and piling. With the overall temporary works designed by Careys’ in-house Design Team, our groundworks team completed the 340m capping beam and ground floor perimeter slabs to allow for the facade retention system; the demolition team had to facilitate the truss installation and waler ties on completion; and finally, the engineering team managed multiple complex technicalities, including the jacking operations. In our role as principal contractor we held responsibility for the discharge of planning pre-commencement conditions and satisfying the pre-commencement requirements of the Section 106 agreement and worked closely with Westminster City Council (WCC) to ensure compliance.

The finished hotel will have 137 rooms and suites for guests, alongside dining and entertainment spaces, a spa, six retail spaces and a grand ballroom. Renowned British architect, Sir David Chipperfield, oversaw the design.

Careys Design Team’s façade retention scheme was designed to comply with strict AKT criteria. We created a BIM Execution Plan. We began by scanning the existing structure and creating a Revit 3D model, which was linked with the construction programme using Synchro 4D software. The scan captured the existing condition of the building (and surrounding buildings) and we then compared them to the historical information to identify any discrepancies. This was critical to the temporary works design and structural alteration sequence of the extended basement.

Once the scans and the existing structure final model was complete, we detailed the temporary works model in Tekla Structures to LOD 400 and the permanent RC formation structure in Revit to LOD 400, including rebar detailing. These models were shared with the project team in the common data environment (Aconex) The steelwork models were then sent in native Tekla format straight to the fabricator without the need for drawings. This reduced drafting time, RFI’s and questions, resulting in a 5% saving to the steelwork and a 3-week reduction in critical path activities. We continued with this level of fabrication by detailing all the reinforcing in Tekla and having the models approved by the client’s representative. This approach significantly reduced the cutting and bending of bar on site, and in turn the risk of RFIs and NCRs.

The 660 tonne system comprised both vertical and horizontal trusses to support a four-storey retained facade and a diagrid floor at first floor level. The horizontal trusses support the diagrid floor and form a ‘platform’ off which the vertical trusses are anchored to support the retained façade. Each horizontal truss is in turn supported at three locations; externally on secant piled perimeter to the new enlarged basement, at an existing building perimeter on plunge column piles, and internally on reinforced columns bearing onto ‘internal’ piles.

The façade was constructed using precast concrete columns, supporting precast concrete backing panels, all of which is clad in Portland Stone. It is made up of alternating O-rings with infill panels. The diagrid is a cast in-situ beam and slab design, waffle-like in its appearance, albeit at 45 degrees to the building grid. Both façade and diagrid are delicate in their construction, having been designed to tight parameters and leaving little residual strength to rely on during demolition. The temporary works system, took this fragility into account in terms of truss design, spacing, detailing and connection design. Most notably, the timber waler detail used at each O-ring anchored the façade to the steel system.

We designed the trusses to be prefabricated largely off site and installed using WK 355 cranes so as to minimise crane time and maximise on-site production. By using slightly heavier chord sections, we were able to reduce the size of the trusses so that normal lorry sizes could be used. Our bespoke trolley system allowed the main trusses (21m , 25t) to be installed in three five-tonne segments bolted together in situ. So effective was the design that these segments were simply positioned by two steel workers and a crane driver. The tender programme allowance of 26 weeks was reduced to 20 weeks on site, despite multiple challenges and downtime due to wind. We divided the building into three zones approximately aligned with the three ‘wings’ so as to allow phased completion of the steelwork installation and the introduction of load ( jacking) to the trusses. Our Design Team provided a load calculation for each jack location (120 no. in total), together with an expected jack displacement. The accuracy was such that both load and displacement were almost exactly as expected - remarkable given the variability of the building and the complexity of the load calculations. The movement experienced by the trusses, once the supporting columns to the existing structure were cut free, was 1-2 mm. Meanwhile, splitting the building’s support system into three had allowed the demolition to progress concurrently, both above and below the diagrid. We designed and detailed all waterproof concrete elements and completed all reinforcement detailing to the permanent works.

The project featured a significant amount of asbestos in unexpected places, for example, used as a cast-in socket to secure the screws to all window mullions. All was safely removed. A large number of heritage items - both internal and external - are being retained, including the structure’s façade with the window mullions and frames, the aluminium birdsmouth cornice on the roof, a large golden eagle structure on the front elevation, the Berlin Wall memorial, various statues, cruciform aluminium cladding, stonework on the entranceway walls and floors, the diagrid infill boards, slab lighting and roof lights, balustrades and handrail, and stonework cladding.

The project is pursuing a BREEAM ‘Outstanding’ rating and we achieved 100% diversion from landfill and recovered a total of 120,000m3 materials for reuse, We had to protect multiple ‘London plane’ trees surrounding the structure under Section 211 and in consultation with an arboriculturalist, requiring us to dig trenches between the tree-line and our works, excavating part of the exposed root ball and trimming it to distance the remainder from our utility works. Our Section 61 Agreement gave 78 dB as an action level - challenging given the acoustic mapping supplied and the demolition techniques we had agreed with Westminster at the outset. However, we met requirements and kept neighbours happy in spite of their increased perception of noise in the far quieter streets of Covid-19 lockdown.