Audley Square Redevelopment

Caudwell Properties received planning permission from Westminster City Council to create a world-class new residential scheme compatible with the historic character of Mayfair. It comprises a new building of eight storeys, plus a lower ground level and five levels of basement, to provide: 29 private residential units; a gym, swimming pools and spa; a courtyard garden and terraces; and car and cycle parking.

The eight-storey superstructure uses post-tensioned and RC slabs, pre-cast concrete walls, perimeter blade walls and RC columns. Two main RC cores to the east and west provide superstructure slab support, as well as a transfer structure in the form of post-tensioned concrete beams at Level 01. The five-storey basement covers the whole site footprint and is constructed of RC flat slabs and garden level transfer slabs, all supported on piled foundations. The site is split into two zones, with a and a hybrid method of top-down construction (Zone 1 to the west) and open-cut construction (Zone 2 to the east).

Demolition of the existing buildings had already been completed by a demolition contractor when Careys was appointed to undertake the design and execution of the piling, excavation, installation of ground source heat pumps, drainage and the Grade 3 waterproof reinforced concrete basement and superstructure. With over 50 years of construction experience, we have delivered some of the UK’s most complex and challenging basements and structures, including elsewhere in Mayfair.

Careys is adhering to ‘Best Practicable Means’ principles to achieve the required world-class standard in super-prime residential property development - from initial concept and construction methodology through to the design and construction phases. This has required continuous evaluation of performance and real-time monitoring to respond quickly with any necessary changes in approach.

Careys Design Team’s extensive engineering and BIM expertise was intrinsic to overcoming the many challenges posed by the city-based site. 3D and 4D modelling techniques and clash detection ensured the stability of the cores, with tolerances monitored via point-cloud technology.

Summary scope of works

Design, construction, installation of the following:

• Pile installation

• Ground source heat pump scheme installation borehole installation

• Bulk excavation for five levels of basement (top-down and open-cut zones)

• Waterproofing system installation to basement

• Drainage installation

• Construction of reinforced concrete basement

• Construction of reinforced concrete and post-tensioned concrete superstructure

Technical detail

A key benefit of our hybrid construction model is in shortening the programme through the simultaneous top-down construction of the basement levels and the upper floor levels and the open-cut construction, which excavates to formation level and installs the required temporary propping as works progress to secure the boundary and surrounding properties, some of which are Grade 2 listed. It will provide a uniform rate of excavation and concrete works that will allow us to hand over half the building early to the follow-on fit-out contractor.

We will also be able to control ground movements to surrounding properties within acceptable movement criteria and install a logistics slab to enable on-site management of wagons, minimising disruption to the surrounding streets and our neighbours. The basement area in the top-down zone will have in situ columns and walls throughout. The basement area in the open-cut zone will have a combination of pre-cast in situ columns and walls.

The courtyard area was programmed as an independent structure to allow the early installation of hoists to feed the superstructure fit-out above the top-down zone. The courtyard basement slab relied on the completion of the top-down excavation works due to the proximity of the work interfaces and health and safety risks. To optimise the programme, our design team has reviewed two scenarios for the excavation sequence: (1) top-down dig first – this would deliver the courtyard two weeks earlier or (2) open and cut dig to B1 first – the latter gave a two-week improvement on programme.

The RC structure could then be constructed from formation level upwards - Core 1 using a jumpform method, with Core 2 being constructed traditionally from floor to floor. The superstructure has a combination of twin wall panels, pre-cast in situ columns and walls, and other in situ elements. Floors are post-tensioned slabs, with a RC roof structure.

Careys Plant and Fleet division supplied the jumpform rig and telescopic clamshell excavators, the latter enabling us to create the five-level basement within extremely limited time and space constraints.

Traffic management

Careys has a well-established relationship with Westminster City Council through various other projects. A detailed Traffic Management and Logistics Plan for the new project and technical approval document (AIP) were submitted to and approved by their Highways Team.

The main criterion for site logistics was to keep more construction vehicles on site and off the surrounding roads and highways. The hybrid scheme allows for safe access to facilitate and secure the construction programme, maximising on-site capacity to hold wagons for loading/off-loading and the garden level logistics slab and a continuous outward flow of the excavated material. Once complete, the site could be split into Zone 1 to the west (top-down area) and Zone 2 to the east (open-cut area), with excavation taking place concurrently in both.

Two 36m Wolff 355B tower cranes supported our excavation and concreting works. Extensive consultation with the supplier ensured that their delivery, set-up and dismantling optimised health and safety while minimising traffic disruption. Early-stage large plant deliveries on articulated lorries such as piling rigs and excavators, have also been carefully planned. Our swept-path analysis revealed that certain bollards obstructed the turning radius of articulated vehicles at one junction and we agreed with Westminster City Council that our trained traffic marshals could temporarily remove and reinstate them.

We use Biosite technology to efficiently manage deliveries to site. Our supply chain was issued with a logistics pack, including clear CGIs, showing what routes to take, which access gates and what contact numbers to use to ensure ‘just in time’ delivery without vehicle stacking on the highway.

Construction sequence logistics

Our in-house design team’s challenge was to unlock the most efficient construction sequence and programme, optimising site resources to enable the earliest possible handover.

Temporary works were planned as follows: temporary and vertical support of the top-down structure; temporary and lateral support of the secant piled wall utilising the top-down slabs, logistics slabs, and temporary steel shoring (waler beams and props); piling enabling works and the temporary support of the existing support system; tower crane support structure; and safe removal of the temporary structure or its integration into the permanent conditions.

The crane-lifting capacities are heavily influenced by the installation of the B3 transfer trusses. The tower cranes have the capacity to off-load the truss into the open excavation, where they will then be positioned by the excavators beneath the top-down slabs. Hook-time analysis is coordinated with the Principal Contractor, facilitating crane time where possible for other trades.

A static trailer pump located in Zone 1 serviced the top-down slab construction, relocated to the east traffic slab to cater for early-stage concrete. A mini spider boom was placed on top of the jumpform formwork to pour the core and materials were crane-lifted down through openings in the garden slab, as well as at the open face beside the long-reach excavator.

Designing our excavation sequence in the top-down zone to skip the construction of the new B1 slab and go straight to B2 allowed us to undermine the existing car park slab during excavation for easier removal. The existing B1 carpark slab ran underneath the top-down work area and this, together with the existing contiguous walls, was removed at the same time as the excavated material. We used Brokks to break the full existing slab, and our excavators to remove broken sections. We constructed an acoustic enclosure over the slab and undertook a validation reporting process to ensure we were fully compliant with noise and vibration limits prior to commencing. Once the slab was removed, excavation progressed to the B2 slab formation.

In Zone 1, once the slab removal was complete and the B2 slab constructed, the vertical core was built back up to B1, at which point the B1 slab was cast in the traditional manner. Excavation then progressed to the B3 formation in Zone 1. The perimeter of the five-storey basement was constructed with a 900mm load-bearing firm secant-piled wall. The internal columns are supported by pile caps on load bearing piles. A thick RC base slab incorporated sumps and pits, pile cap thickenings, building drainage, external waterproof system, and heave void. The pile locations were coordinated with the GHSP piles which were to be installed in advance of the bearing piles.

In Zone 2 of the open excavation, another level of props was installed at B4 prior to the excavation progressing down to the B5 formation. In the top-down area, the B3 slab was constructed, together with transfer columns and the steel truss. The excavation progress to B5. In the open-cut work area, the excavation will be complete, and the final B5 raft slab was formed and cast.

Prior to this, the access ramp on South Audley Street was removed. The long-reach excavator and concrete pump were relocated to the south logistics slab until the top-down excavation was complete to B5. In the open-cut area, the core jumpform was established. As the superstructure progressed, the upper level basement slabs in Zone 2 were cast in the traditional manner. We provided access for UKPN to install their equipment in the basement.

We liaised in a timely manner with all statutory authorities, including Thames Water (as regards drainage and the diversion of the existing ring main around the perimeter of the project); National Grid; Openreach; and UKPN.

Piling strategy

This was based on using large capacity machines at existing ground level to optimise logistics. Although we also modelled carrying out the bearing pile works from the B5 level, due to the logistics of removing the pile arisings from a much lower level, this would have been less efficient. Works were carried out by our subcontractor, Bauer, in two working zones, with both bearing and secant piling works being undertaken concurrently. The piling works were all carried out from the existing pile mat level, with a time allowance for coring through the existing car park base slab. Capping beams were then integral with the G0 slabs and being cast monolithically, with capping beams at B1 to be cast independently.

HSSQ and the community

In addition to our ISO 9001, ISO 14001 and ISO 45001-certified policies and procedures, Careys continuously reviews and updates our ways of working to ensure we adhere to government guidelines and BuildUK Site Operating Procedures for Covid-19.

The site is signed up to Westminster City Council’s Code of Construction Practice, which provides guidance on best practice in construction site management, and with the Considerate Constructors Scheme. 100% of waste is being diverted from landfill. We engaged early with Westminster City Council’s Environmental Sciences Department: a detailed Construction Phase Plan was to be produced and submitted for approval 4-6 weeks in advance, including a detailed Site Environmental Management Plan. A WCC Environmental Health Officer was assigned to the project and has regular monthly meetings with the site team to ensure compliance with the WCC Code. Promotion and communication of the site’s environmental credentials with the workforce includes regular environmental toolbox talks and information on usage displayed around the site.

We used second-hand plywood from our another of our large projects in the Borough to fix slab stop ends and voids to help reduce cost and waste of materials. Practical measures to help reduce the usage of electricity on site included PIRs in the ecotype cabins, LED lamps throughout etc. Site waste is all segregated.

A number of large items of plant that require NRMM certification and all of these comply. All plant is turned off when not in use. We do not allow vehicle stacking on Hill Street: the traffic marshals either send the wagons in a loop round the block or dismiss them if they have not arrived at the prescribed time. The neighbours surrounding this site – including seven party wall and several Grade 2 listed buildings - having already experienced the demolition works carried out by an earlier contractor, were understandably sensitive to these new works. We have extensive experience of working with neighbours in dense city environments to ensure good relations and Audley Square was no exception. A dedicated Neighbour Liaison Manager (NLM) was deployed to oversee open and regular neighbour engagement, including updates on site activities and timelines. All feedback channels are managed and responded to swiftly and effectively, under the charge of the NLM and Project Director.

Regular communications include monthly neighbour liaison meetings using MS Teams, and a monthly newsletter to keep all project neighbours and stakeholders informed of current and impending activities, with specific e-mails when site changes occurred. We also use CGIs from the Carey Visual Standards to describe methodologies and constraints, answer questions and allay fears. All newsletters are uploaded to the project construction website.

Careys’ dedicated NLM attends the site regularly. The NLM’s direct mobile phone number, a 24-hour security number and dedicated email address are published for stakeholder communications. A log of compliments and complaints is kept and any issues raised are promptly acted upon.

As previously described, the hybrid scheme provides the quickest overall programme to minimise the period of inconvenience, while allowing us to keep more construction vehicles on site and off the surrounding roads. A full top-down construction method would require large, noisy ventilation units running full time. The open-faced top-down zone reduces this disturbance by providing natural ventilation to the work area. It also conceals the breaking of most of the existing basement slab and suppresses overall noise impact.

All BPM analyses carried out by the project team are recorded on a tracker spreadsheet and a live BPM register is being maintained throughout the project. Noise Prediction Software is utilised to understand noise outputs and allow development of mitigation measures. Neighbours are informed in advance if specific works are taking place beside their building. An example is where we were installing secant piles directly against the party wall property.

Noise and vibration tracking is being undertaken by various means, including receptors installed at locations chosen following a technical analysis of neighbouring properties and in consultation with WCC. The number installed far exceeds the legally required minimum. A number of goodwill gestures have been made to the neighbours, such as the provision of alternative office space for a neighbour who required a quieter work environment.

Careys uses ultra-modern and well-maintained equipment which not only increases safety and productivity but reduces noise and vibration. Local acoustic screens are used around plant, for instance around both GSHP mini drilling rigs on site, and machines are turned off when not in use, with designated access routes to minimise vehicle noise. No reversing alarms are allowed on site. Ratchet guns were used instead of manual spanners when building the PERI stairs so that noise was made for a shorter period.

Our piling subcontractor, Bauer, used ultra-modern and efficient equipment, including piling rigs manufactured for low noise emission. One noise and vibration mitigation method has been the use of rotary piling instead of pile driving, with dampened Kelly bars to reduce high-frequency sound. To reduce noise during the discharge of spoil from the auger during the rig’s automated spin-off movement, Bauer has changed to a manual stop and start by the driver. They have also introduced longer auger heads for the secant piles, also lubricated with mould oil, to allow the clay to be removed more easily. A hydraulic pile cropper was used rather than traditional breaking methods to reduce noise. Bauer also established a hold point schedule to prevent any overrun past 18.00.

Numerous other smaller measures are being taken to help neighbourhood relations, such as Monarflex wrapped round the PERI stairs to the welfare units, to reduce the visual impact for our neighbours.

Social value

Various initiatives are taking place to help meet WCC’s social value commitments under Section 106. Careys is writing has written an Employment and Skills Plan, and is working with WCC which includes targeting work placements and apprenticeships. This plan is now in place and we are working with WCC to get apprentices and work placements to site. Careys has increased our existing pledge with the 5% Club, with a commitment to having 30% of our workforce in ‘Earn and Learn’ roles. We sit on the Construction Academy West London Employee Advisory Board, which means we are working with all the training providers based in west London to ensure training provision is aligned to our projects. In Westminster Borough, we are involved in multiple initiatives that help us to meet social value targets at Audley Square. Our starting point was the WCC City for all Strategy and local needs analysis. This led us to working closely with West London College and the University of West London and with Beam (which supports homeless people with work placements and apprenticeships). As appropriate to the length of the project, we have committed to two 12-month Groundworker apprenticeships (Groundworkers) and two Surveying or Civil Technician apprenticeships. Both of these pathways can then lead onto further apprenticeship pathways.

Working with the overall project team, we have committed to raise a target £1m for the Caudwell Children charity for disabled children across the lifetime of the project. Our registered charity, Careys Foundation, is donating £350K, with the project team set to raise the rest.