Blossom Street

September 22, 2020

Cantillon were initially appointed by British Land in March 2016 to undertake a strategic review of the overall demolition and construction strategy for the Enabling works package on a prestigious and highly sensitive Blossom Street development site situated on the outskirts of the City of London.

The strategy was defined to highlight the effective ‘sweet spot’ for overlap between the demolition, piling and substructure elements across three sites which would ultimately form the basis for the Enabling Works contract. The works include the careful demolition of existing site buildings immediately adjacent to or adjoining retained heritage structures, prior to installation of the permanent piled foundations, reduced level site wide excavation and the construction of the new permanent RC structure back up to ground level.

Following a presentation to the wider Client and Design Team, Cantillon continued to develop the strategy with British Land and were engaged to fulfil a significant pre-construction services appointment from July 2018 which involved early design team engagement to assist further develop design development from stage 3 onwards and to undertake significant lead in pre-commencement activities including discharge of planning conditions, Local Authority approvals and Network Rail asset protection submissions.

Cantillon took possession of the site in Aug 2018 and immediately began the process of securing the site and making safe the redundant buildings, some of which had been vacant for several years and required temporary support and the installation of life safety systems.

A significant number of surveys were commissioned and undertaken to aid the design teams understanding, including sewer and pavement services surveys, existing building fabric and ground investigation surveys, Ecological and wildlife surveys, deleterious and hazardous materials surveys, and extensive photographic condition surveys of existing heritage structures and highways amongst many other very specific survey requirements.

Network Rail and London Underground

Cantillon’s in house engineering team fulfilled the necessary contractor’s role of CEM and CRE to engage the Network Rail Asset Protection Team working initially under a Basic Asset Protection Agreement [BAPA] at an early stage to develop the necessary temporary works designs and Works Package Plans in line with the planned works owing to the proximity of the works immediately bounding the East Coast Greater Anglia line directly into Liverpool Street. A full Asset Protection Agreement [APA] was executed and superseded the BAPA during the contract period.

Examples of the approvals required, include the protection measures including hoardings and scaffolding directly on the boundary, designed to be erected in normal working hours, to reduce the requirement for track possessions. All piling works were subject to NWR asset protection agreement under the APA requiring Permanent Works F02 and F03 acceptance carried out by Cantillon and Bachy Soletanche.

Excavation and enabling works to expose the existing historic masonry retaining wall that bounds the northern rail cutting required a movement assessment on the structure in relation to the works, with respect to temporary construction loads including earthmoving plant, piling platforms, and associated rigs and tower cranes.

In addition, there were two London Underground Central Line tunnels running beneath the site, which required the works to be carried out sympathetically and with an ongoing dialogue regarding piling and excavation works.

Cantillon’s engineering team fulfilled and led the monitoring response action plan communication with TFL.

Grade II Listed Road

Key to the logistical strategy for the site was the formation of a temporary haul road through the site, which required the existing Grade II listed cobble-set road to be protected, by the careful lifting of the individual setts, transporting off-site to be safely stored until the highway is replaced upon completion of the works.

In order to satisfy the conditions of the listed building consent, Cantillon’s team successfully negotiated with the local authority and local conservation trustees to allay fears that the temporary removal of the highway would be undertaken with due care and consideration.

Cantillon commissioned a state of the art hi-definition photographic survey to record the authentic and original nature of the cobbled sett bond and material, which would be used by the Client to faithfully reinstate and restored the highway to the satisfaction of the Consent and trustees.

The site is also predominantly situated over a scheduled ancient monument, known as a below-ground archaeological feature of the historic Priory of St Mary Spital which required the enabling works to be carried out in line with a Scheduled Ancient Monument consent granted by Historic England.

Cantillon’s team worked closely with MOLA and were instrumental in satisfying Historic England that the conditions of the consent were met, devising an archaeological excavation sequence to ensure that the Museum of London archaeological unit would have sufficient time to excavate and record features, following demolition and ahead of the piling phase.

Neighbourly Relations and Environmental Considerations

Of significance to the Clients, concerns were the proximity of the development in relation to the local residential neighborhood, particularly in relation to managing and gaining the confidence of a community that was divided in terms of opinion on the scheme.

In response, Cantillon commissioned and undertook a detailed noise prediction study in advance of the Enabling works phase, identifying areas of potential noise emissions, which would be used to agree on the s61 Prior Consent with London Borough of Tower Hamlets.

Following various noise baseline trials and consultation meetings with the local community and stakeholders Cantillon developed a neighbourly strategy to manage very sensitive stakeholders.

Cantillon became active leaders in the community liaison working group and formed a key part of the Client communications team in developing relations with important stakeholders, setting up residents’ meetings individually and as groups, sending communication about events and activities on-site and generally providing a conduit for dialogue.

Enabling Works Demolition and Retention

The outline scope included the demolition and clearance of a majority of existing buildings site-wide, down to and including the lowest slabs and foundations.

There are several buildings identified to be retained, notably the 18th-century warehouses and original 19th-century facades bounding Folgate and Blossom Street. Our in-house engineering team led on all temporary works concepts, design, and detailing to ensure that the support systems were sympathetic to the fabric of the retained elements and future permanent works interfaces.

In addition to the retained buildings, a large volume of salvaged materials including, bricks, structural timber, slates, and setts were carefully removed and set aside for storage and for reuse in the future development.

Archaeological Support

Due to the aforementioned presence of the Scheduled Ancient Monument and in line with the Written Scheme of Investigation, there was a significant archaeological excavation to be completed prior to the commencement of intrusive excavations. This was undertaken site-wide and required significant coordination and support of over 30 archaeologists overlapped through a series of sequenced digs released as each phase of demolition progressed to minimise the impact of over 20 weeks of archaeology.

Piled Foundations and Value Engineering

During the design team development from stage 3 to stage 4, Cantillon’s team were actively involved in identifying alternative Value Engineering solutions to reduce package cost and to reduce critical path reliance on the activities related to Client risk items involving substation disconnection strategy and Network Rail.

This involved working with the consultant engineer to develop the largely LDA piling strategy to become a hybrid piling solution, using CFA technology to ratify the diameters and increase installation quality plus a change from LDA to mini piling to reduce working space requirements in strategic locations, which brought a significant benefit to the Clients plan.

Cantillon undertook the design and installation of the piled foundations engaging our supply chain partner Bachy Soletanche to undertake the piling package to be carried out under Cantillon’s scope.

The works included the installation of a 750mm diameter secant wall to be installed to the main site 1 perimeter boundary site perimeter, as well as the bearing piles to support the main structures, notably developing the design to avoid the requirement to use fluid support.

Excavation and Reduced Level Formation

Following the demolition, archaeology and piling phases Cantillon shall carry out and complete the 22,000 m3 bulk excavation down to the final formation-level below the adjacent ground level highways and shall install the permanent works RC capping beams as necessary prior to the installation of the basement temporary works propping.

Cantillon shall install underpinning and sheet piling as the excavation progresses, to the required formation level, prior to installing an imported granular subbase and blinding to form the required levels in preparation for the substructure construction.

Substructure Works

Cantillon shall undertake to complete the waterproofing and gas membrane design detailing which will be coordinated into the stage 4 / 5 permanent works design, including and incorporating design detailing for both new RC structures and existing slabs/walls.

The substructure construction shall be carried out by Cantillon using the supply chain partner, Morrisroe to undertake the construction, benefitting from the relationship between the two to maximise the ability to overlap and work seamlessly on the site.

The proposed substructure varies between the 5 sectional handovers on-site, ranging from traditional piled foundations to raft structures and incorporating existing slabs and structures into the process. The substructure scope includes the construction of basement slabs and vertical elements including lift shafts, columns, and walls up to and including the ground floor slab.

Overall, the project duration is planned to extend to complete in November 2020, incorporating further basement construction in addition to each of the aforementioned numerous packages and technical challenges, maximising the overlap potential between each phase and providing a seamless project delivered with our partners for our Client British Land.

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