Basement impact assessment, ground modelling, specification and design

No. 79 Fitzjohn’s Avenue is a £28 million development of prestigious retirement apartments in Hampstead, London Borough of Camden. Pegasus Life is building a five and seven storey structure on the site with a lower ground basement almost 12m below ground level to be used as an underground car park using a car stacker system. The site is very sloped and was previously occupied by a 1970’s hotel.

CGL worked for the client, Pegasus Life and the project manager, Gleeds. CGL provided geotechnical advice before the planning and within all phases of the substructure construction.

CGL’s initial brief was to provide a Basement Impact Assessment (BIA) for planning, which has since been updated to become a post-planning Ground Modelling Report (GMR). CGL has also produced enabling works specifications and designs that were used for third party stakeholder approvals including party walls, highways and protected trees around the development boundaries.

Phase 1 – design of construction sequence

CGL was appointed as the primary temporary works designer (TW) and Certifying Engineer for the Basement Construction Plan (BCP). The TW design report includes construction sequencing and a fully coordinated TW design.

Camden Council is particularly concerned about flooding in certain streets, so monitoring ground water on this site is important. CGL put in place a ground water monitoring scheme by allowing one of the boreholes to be salvaged at any point in the future to detect if this development has had any impact on rising ground water.

Within the proposed site were trees with TPO’s. CGL advised, plotted and witnessed the air spade survey which detected no tree roots would be damaged during construction. The site also has a contamination history, so CGL reviewed the existing ground investigation report, and advised on its adequacy.

CGL devised the construction sequencing for the sensitive boundary perimeter protecting the assets around the sites. CGL identified nine design sections each requiring a different design depending on the constraints at each boundary. A range of pragmatic and cost effective solutions were introduced throughout the site, for example CGL made use of the existing reinforced concrete retaining wall by introducing king posts to reduce new piling. CGL also designed propping corbels to limit ground movement and reduce risk.

Phase 2 – excavation and installation of stage 1 props

After demolition there were remnants of the previous structure in the ground that required partial propping to facilitate the final excavation and removal of the earlier structure. CGL made sure the sensitive boundaries were fully supported and not undermined during this phase.

Phase 3 – preparation for piling

CGL advised on a practical solution to attain a clear site that would facilitate piling safely and effectively. CGL devised a detailed sequence involving sheet piling, kingposts, steel walings and propping to retain the boundaries and protect the sensitive trees. CGL advised on use of the demolition material as piling platform material as a sustainable solution.

Temporary works design

To design the temporary works CGL used WALLAP to predict the forces applied from each retaining wall boundary condition. The forces and the results were transferred into CADS A 3D frame software and CGL interfaced the model with Steel Work Member Design (SWMD) to select appropriate section sizes for each prop.

CGL used the permanent works as reaction piles to support the boundary walls during construction rather than design all new temporary piles which increased the sustainability of the construction.  Laterally loaded piles were designed using PLAXIS to calculate the forces from the propping scheme into the piles in the ground and the temporary works designs were used to gain approvals from London Borough of Camden, highways and party wall engineers.

Monitoring plan

CGL provided movement monitoring and contingency plans. Targets around the perimeter of the site party walls and highways were put in place and monitored to make sure the assets are kept safe.

CGL has also reviewed the contractor’s designs and advised how to address weaknesses to make the strategies more effective. High levels of collaboration between the primary contractor, piling contractor, enabling works contractor and structural engineers allowed for seamless construction sequences. CGL’s efficient designs and teamwork have led to the enabling works being completed ahead of programme.