Taking the plunge
It is not everyday that a leading ground engineering firm has the opportunity to work in the renowned surroundings of London’s Westminster. However, Bachy Soletanche Limited (BSL) has been doing just that at the site of Park Plaza Westminster Bridge Hotel. The geotechnical specialist completed the five million pound piling package at the development in January, which included the installation of plunge column bearing piles and a secant piled wall.
The Park Plaza Westminster Bridge Hotel will have, without doubt, one of the best scenic positions in Westminster located on the south side of Westminster Bridge. In the shadow of landmark buildings such as the Houses of Parliament and Big Ben, the aparthotel will provide unrestricted and spectacular views to its guests.
The joint venture between Frogmore Property Co, Galliard Homes and Park Plaza has established an innovative enterprise where investors can buy one of the hotel rooms on the project’s completion. Consequently, the project has a fast-track programme that has dictated the construction process and the techniques used.
Jonathan Spyvee, Project Engineer for Bachy Soletanche Limited explained: “To enable this project to run on a tight programme, the project manager, GCPM, has adopted the top-down method which requires the early installation of the ground floor slab. With this in place, work above and below the slab can continue, simultaneously knocking two months off the schedule. However, before the ground floor slab can be put into place, a range of ground works had to be conducted and BSL was awarded these advance works together with the piling works contract.”
The majority of the bearing piles incorporate plunge columns which were specified by the client to support the ground floor slab, this being possible as the steel column extends from the cut-off level of the concrete pile at lower basement level to ground slab level.
The plunge column installation technique at first follows the general piling process by boring out the core, placing the reinforcement inside and then pouring the concrete into the pile. With this complete, an adjustable steel frame is immediately lowered into the pile shaft and suspended from the top by the temporary steel casing. The steel column is then fed through the frame and held firmly into position whilst position and verticality are checked. Finally, the column is embedded into the top of the pile concrete leaving up to two metres of the column exposed above current ground level.
Jonathan Spyvee continued: “It is extremely important to install the steel column accurately down the centre of the pile. For this particular project, we had to meet tolerances in the region of 10mm to ensure compatibility with the follow-on works.
“The construction of these plunge columns is relatively straight forward, however the design included the installation of four jumbo plunge columns, and to place each one of these correctly required specially prepared plunging frames, meticulous planning and each required a couple of days to construct.
“We left the installation of the jumbo plunge columns until last as the logistics for getting them onto site and putting them into place required as much space as possible. The columns were delivered, on two separate occasions, via closed roads and using police outriders, which demonstrates their enormity.
“To install plunge columns of this size is something quite unusual, but we put a lot of careful preparation into the design and installation method in conjunction with the project manager.”
BSL has also constructed a secant piled wall which runs around the perimeter of the site and forms the retaining wall for the excavation creating the deep basement for the hotel. The wall was split into two sections so different techniques could be used.