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Cambridge: Park Street - Secant Basement




Waterman and CGL



DAWSON-WAM (DW) were contracted by Gilbert-Ash (GA) after an extensive period of early specialist involvement to design and construct 3 storey secant basement right in the heart of Cambridge. A multi-storey car park was demolished as part of the works to make way for an aparthotel of 229 rooms, above a modern basement car park with 225 public car parking spaces.

The location of the development is in the heart of Cambridge bounded by busy roads on two sides and by party walls on the other two sides. The basement walls were to be installed tight to the limitations of the ownership footprint to maximise the size of the new basement. This meant that new basement walls were to be installed extremely close to party walls. Most of the 27nr party walls were formed of occupied buildings dating back to the 1860s, with structures which had seen significant movement in the past. DW and GA worked closely together from an early stage to assess the risk and mitigate the impact of the works as much as possible whilst delivering value to the client.

The initial proposals for the design of the retaining wall to install a 900mm diameter Hard/Firm secant wall installed some 1.3m from the party walls, however with pressure to maximise the footprint of the basement and DW proposed a close proximity 750mm diameter Hard/Hard secant wall. Through complex Plaxis 3D models and ground movement assessments, the team were able to satisfy the numerous party walls, and in turn offering as solution which maximised the basement footprint. A robust vibration and movement monitoring plan was devised by the DW team to allow the party walls to be constantly monitored throughout the basement construction process. Various trigger limited were set with well thought out action points, with warnings sent immediately and directly to the construction team to implement actions if required.

Key Numbers

•12.5m retained height

•226 linear metres of 750mm secant piles

•27.5 linear metres of buttress walls

•30nr 525mm diameter tension piles to 27m

•Installation of 2nr dewatering wells.

•30nr existing piles of 500mm diameter and 14m long removed.

•27nr party walls

The first challenging was to remove some 30nr 14m long,  500mm diameter existing piles from the secant wall line. To do this DW used roatary bored techniques and a Delmag RH34 to bore out the piles and replace with piling friendly material. The position of the secant wall was fixed by a secant guidewall with piles to be installed within 100m off existing party walls. The secant wall itself was formed of 750mm piles installed at 640mm centres. Primary piles were unreinforced but formed with C30/37 concrete. Secondary piles were installed up to 17m in length and were reinforced over thier full length to allow a 12.5m retained height.