Geotechnical Temporary Works Design

CGL’s pragmatic tender conceptual geo-temporary works design with value-engineered improvements was incorporated into the Client’s, Volker Fitzpatrick’s (VF), bid for this scheme, making the project more buildable overall and aiding in VF being awarded the contract for this £30m Tribeca development in Camden consisting of the design and construction of an office building with a ground floor plus six storeys and a 10m deep basement with retail shell units. 

The 42×36m site with challenging split-level topography has highly complex and constrained with muti-asset protection challenges that included onerous party-wall limits/impacts to consider, including commercial properties, the Regency Canal, St Pancras Way (highway) and Thames Water Sewers. The post-demolition presence of the 1970s building foundations with many obstructions to piling, including existing piles and the presence of the Fleet mid-level sewer only 0.5m below the basement slab formation level added complexity. 

Key constraints from the Victorian sewer necessitated smart hit-and-miss sequencing to mitigate a lower Confinement Pressure Ratio during excavation. CGL’s recommendation of using Magnadense concrete over the sewer was controlled with a fully illustrated ‘Tetrislike’ counterweighting. Challenging 6m wide no-pile gaps across the sewer exclusion zone were swiftly dealt with by a top-down skin wall. The magna-dense slab was connected to piles on both sides of the sewer, forming a rigid capping structure designed to resist extreme event sewer uplift pressure both in the raft reinforcement capacity and pile skin friction.

CGL implemented innovative ground movement monitoring techniques using innovative Bluetooth sensors that were placed in the Magnadense slab sections over the sewer to provide real-time readings of strength, which allowed the Client to load with
kentledge weight as soon as the bay reached the required strength. This technique was recognised as innovative by the Considerate Constructors scheme and one of the first to be used in the UK. Live back analysis of data enabled slab casting and prop removal sequence with confidence observationally.

CGL’s PLAXIS- 3D temporary works design enabled the canal side permanent exterior column piles to be utilised as a sustainable temporary and permanent solution offering a clutter-free excavation. CGL also designed the Plot A to B boundary contiguous pile wall depths for ‘future-proofing’ the deeper Plot B basement. VF/CGL’s contribution to BREEAM Excellent in sustainable development included using the below-ground demolition arisings for working platforms and re-using existing piles in temporary condition for bolstering berm stability/earth/retention/pile guides and beneficial re-use of historic canal side masonry walls and highway masonry for temporary works. The Apex project combined sustainable alternatives with traditional construction methods by using bricks made from excavated clay from the site to construct the basement perimeter external walls.

The project delivery was a collaborative team effort across VF and the design team, with the relationships established in place for continuing collaboration on the next stages of the development.