July 20, 2020
In the busy Old Street roundabout area, Cantillon carried out soft strip, asbestos removal, structural demolition, pile probing, UXO magnetometer survey and installation of piling platform within the existing basement box of the two 1960’s office buildings along Featherstone Street, City of London, EC1 on behalf of Derwent London. Two substantial discoveries were to be overcome with minimal impact to programme.
As with most inner-city jobs in busy pedestrian and traffic areas there were times when street side scaffolding and pavement gantries had to be erected/dismantled out of normal hours with the agreement of Islington Council and prior notification to local residencies. To manage logistical & spatial constraints a 50m pit lane was also constructed along Featherstone Street while maintaining a two-way cycle lane in the road.
The original R&D survey excluded the window frames and so, once site set up was completed, some window frames were removed under controlled conditions to close out this exclusion. Numerous packers were exposed in each window reveal: This was the first discovery. Cantillon Project team re-sequenced programme with the scaffold access amendments and demolition re-phasing in tandem with Derwent and their cost consultant, Exigere, to minimise EoT duration to remove asbestos packers from 736 windows and our in-house asbestos team worked diligently to implement this plan.
One of the party wall properties, Matisse Court, enclosed upon structure to be demolished and relied upon it for stability. Cantillon’s engineering team assessed both structures and, utilising the retained end block infilled RC frame, designed a lightweight yet robust temporary works scheme of PFCs. These were tied into both retained RC frame and Matisse Court to provide structural stability without hindering later piling rig access. All openings were closed up and entire party wall weather proofed with felt & batten.
To reduce transmission of structure borne vibration between demolition area and Matisse Court, Cantillon employed isolation cuts beside the retained end RC frame.
Demolition started and ended with traditional top down techniques using internal material drop zones to limit the effect of noise, dust and vibration to the residential, business and open park outside. Half way through demolition Cantillon initiated the agreed demolition re-phasing to high reach 25% of the job.
Another Matisse Court party wall interface was that there were four shared foundations which had to be cut back 1m using non-percussive techniques so that the contiguous pile line was clear. A combination of horizontal, vertical diamond core holes and diamond wire cutting achieved this goal with +0 / – 25mm tolerance. Party wall movement monitoring was well established and continued up to Cantillon completion.
A gravity block had to be constructed adjacent to Matisse Court & Lexington Apartments to remove lateral loading generated by the 2.3m deep pile mat and piling works. At tender stage a 4N/mm2 concrete mix was agreed but after contract award the follow-on piling contractor stipulated a weaker 2N/mm2 mix. Following talks with client team on this risk ownership Cantillon successfully developed the alternative concrete mix alongside concrete suppliers and piling contractor with cube test results verifying strength gain for the 400m3 placed.
Pile probing to a depth of 3m below underside of B1 slab was carried out along the entire 194 linear meter perimeter contiguous pile line and for 123 bearing pile locations. During this process Cantillon discovered 76 existing piles of which 24 affected design pile layout. Again, Cantillon worked closely with Derwent’s structural engineer HTS to survey, plot and find alternative locations for affected piles whilst minimalizing programme impact to follow-on piling contractor. A total of 179 UXO survey positions were cleared during this time as the pile mat was laid and load tested.
98% of all materials removed from the site were diverted away from landfill.
Cantillon completed works on time and handed the site back to the client for their piling & construction contractor.