Seymour Street
Joseph Slattery
Technical Director
Client
Galliford Try and The Portman Estate
Structural Engineer
Furness Partnership
Services
- Ground Movement Report
- Full Temporary Works Design
- Party Wall Design Report
- Highways AIP
- Royal Mail Movement Modelling
- Advice on Resequencing
- Monitoring Strategy and Contingency Planning
- Full Contractor Support During Construction
- CAT II Temporary Works Design Checks
Temporary works design
The former Marylebone Police Station has been demolished to be developed into a mixed use scheme in Central London. The new development will have seven storeys with a double storey basement across the entire footprint. The existing liner and party walls along the two boundaries of the site will be retained along with the raft within the footprint of the existing double storey basement that occupied around half of the site.
CGL was engaged by Galliford Try (GT) to provide contractor support, pre-tender outline designs, support regulatory approval for the project, then design the perimeter support systems and temporary works during excavation and basement construction. The whole area is prone to movement and the neighbouring buildings are very sensitive as they are fragile, Victorian and Grade II listed. The terraced buildings were bookended using the existing concrete lining walls and supported by four large steel towers. These towers were erected during demolition on mini piles that potentially clashed with the position of the new RC transfer structure in the permanent works design.
Value engineering
CGL divided the site perimeter into 8 salient design sections, each of which required a different design solution for piles or foundations depending on site conditions. Each perimeter design has 7-10 stages of basement construction thus providing a sequence and control for excavation to limit movements. CGL has provided considerable value engineering by rethinking original proposals, for example by changing from an underpinned design into a pile design which saved months on the construction programme and reduced risks. In another example 5000m3 of construction waste was originally planned to be retained onsite to provide ballast against heave induced uplift. CGL’s Ground Movement Analysis concluded this was not necessary to achieve sufficient stability so it was removed, significantly increasing the usable volume of the basement.
The constrained site presented an additional challenge as the towers erected during demolition to support neighbouring party walls were on a mini pile concrete support grillage that clashed with the position of a transfer structure required by the final construction design. Furthermore, 3m deep attenuation tanks were required to be located under the 10m deep basement and less than a metre from the tower bases. CGL developed a strategy to re-support the demolition towers, with a very fine tolerance for movement, so that the RC grillage and mini piles could be removed and the main construction could take place. CGL also provided the primary temporary works designs for walers and props to support the perimeter retaining wall during construction including calculations of load forces and support geometries that allowed minimal obstruction to piling whilst maximising working space on site.
CGL used geotechnical and geostructural design software including:
- WALLAP to analyse the retaining walls and underpins in the lateral condition.
- PLAXIS 3D to model the forces in the perimeter support props and reaction caps, the support piles for the tower cranes and impact on the adjacent Royal Mail Tunnel.
- CADS A3D frame analysis to inform the RC capping beam and steel walling design.
CGL’s Services
- A review of earlier site investigation data to establish an optimised ground model and design parameters.
- Detailed Ground Movement Analysis for the basement, perimeter walls and whole development.
- A conceptual and detailed temporary works scheme for the basement including designs of critical sections, predicted wall deflections and waler/prop loading.
- Outline design and detailing of temporary propping framing and reaction piles, including structural checks on retaining systems (piled wall and mass concrete underpins).
- Recommendations for underpin construction including an assessment of groundwater infiltration rate within the gravels during underpin construction.
- Producing a recommended groundwater control strategy and movement monitoring and contingency plan for each boundary location based upon observational methods.
- Advising on construction sequencing for the ramp removal.
- Ground Movement Analysis on the impact of the development on a Royal Mail Tunnel close to the site. It identified movement would be <2mm: within acceptable limits and avoided the need for costly tunnel movement monitoring.
- Tower crane base design, developing the detailed geostructural design on detailed sequencing drawings how to construct the base.
- CAT II checks on the subcontractor’s pile and basement propping temporary work designs.
