The redevelopment of this one-block site in the City of London, transforming it from legal offices into the shopping and restaurant destination One New Change, came with many challenges. Bounded by public roads and in close proximity to St Paul’s Cathedral, many physical and legal restrictions were imposed on the project from the start. Arup came in to support Ateilier Jean Nouvel in the redevelopment from the beginning.

Finding solutions to the restrictions imposed by St Paul’s historic status was an important part of Arup’s work – the Cathedral’s surroundings must be dealt with delicately. We designed a conceptual model of the groundwater regime surrounding the Cathedral, and modelled the proposed One New Change designs with this system, demonstrating that the Cathedral would not be impacted.

One New Change has revitalisted the City of London, turning it from business-hours-only into a seven-days shopping and restaurant destination. The project has also been successful on a design level, winning a number of awards including The Fleming Award for excellence in geotechnical design and construction (2009), the RIBA Award (2011), and the Best Built Project at the London Planning Awards (2011/12).

Ground engineering to protect St Pauls

St Paul’s Cathedral is built on shallow foundations in the Quaternary brickearth. These deposits are underlain by terrace gravel which forms the upper aquifer. Over the years, damage has occurred to the Cathedral as a result of ground movements arising from consolidation settlements, 19th century deep utility excavations and changes in upper aquifer groundwater levels.

The One New Change development required consideration and design to take into account the groundwater and geotechnical effects on St Paul’s Cathedral. In 1935 an Act of Parliament was passed to control works within a prescribed area, known as St Paul’s Depths, to protect the fabric of St Paul’s Cathedral from further damage.

Arup developed a conceptual model of the groundwater regime surrounding the Cathedral, taking into account the complex interaction between natural and anthropogenic induced groundwater flows. Numerical modelling using MODFLOW was undertaken to model the current groundwater regime. The proposed basement was integrated into the model to simulate future effects on the groundwater levels beneath the Cathedral. Scenarios were modelled which demonstrated that the proposed development would not significantly alter the groundwater levels in the upper aquifer.

During construction, monitoring of the groundwater levels around the site and structural monitoring of the cathedral was carried out. The results demonstrated that the groundwater levels in the shallow aquifer at the Cathedral were not affected  by the construction of the new basement. The structural monitoring demonstrated that negligible movement occurred to the Cathedral during construction of the works.