25 BEACON, BOSTON, USA

CCL was engaged to design a specialised solution to enable the construction of a parking basement beneath two luxury townhouses in a conservation area in Boston. 

As there was no existing basement, the construction of the parking garage involved excavating beneath the properties. There was a critical requirement to retain the facades so it was crucial that the excavations did not cause any adverse structural movement which could damage the above-ground structure or the structural walls shared with neighbouring properties.

The original design proposal was to use structural steel beams and would have required underpinning. This plan carried high levels of risk to the integrity of the structure and neighbouring properties, as well as significant logistical, cost, and length of programme issues. It also included a large number of steel columns to support the structure which would make it difficult for vehicles to manoeuvre and would have required deeper excavation in order to achieve the desired height clearance.

CCL proposed a solution for a post-tensioned slab tied into the structural walls to support the above-ground structure during excavation and post-construction. Mini piles were drilled around the internal perimeter of the external wall to allow post-excavations of 3 m of headroom and provide further anchoring into the ground. Small openings were created in the existing brick walls at regular intervals through which the post-tensioning tendons were fed.

This option accommodated the large span required for an open-plan basement, with just two support columns, overcame logistical issues, offered a faster construction schedule, and reduced the cost 0f materials. A top-down approach was the ideal method to protect the facade and adjoining buildings with excavation carried out once the slab was in place, enabling it to support the structure above ground and brace the retaining elements below ground.