Bachy Soletanche completes 5GBP piling contract

Bachy Soletanche completes five million pound piling contract at landmark Park Plaza Westminster Bridge Hotel.

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".

"We divided the contract into two main sections including the installation of 140 bearing piles and the construction of a 260 linear metre secant piled wall around the perimeter of the site".

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 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.

The column sizes range from 200 x 200mm up to 400 x 400mm with varying lengths up to 20metres.

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".

Specifically, the first day is taken to begin augering the 2.1metre diameter pile to its final depth of 35metres and the second day is used to place the massive 21metre, 37 tonne column, into the pile using a similar process to the smaller plunge columns previously described.

The four jumbo piles had to be specially commissioned due to their huge non-standard size - two are 300 x 1,100mm and the other two are 500 x 800mm.

With each weighing 37 tonne, these jumbo columns are some of the heaviest plunge columns ever installed by BSL.

Jonathan Spyvee added: "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 actually 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.

On the east side of the site, BSL used the classic Kelly/segmental casing method with a large diameter heavy duty rotary rig as this method has the strength to bore through the ground profile of made ground, gravels and finally clay.

For the secant piled retaining wall on the opposite site boundary, BSL used the cased CFA secant piling technique to construct the 750mm diameter piles.

Jonathan Spyvee concluded: "Overall this ground engineering project has proved to be a varied and interesting project and we are pleased with the final results".

"By working very closely with the project manager and by having enough flexibility and budget to make design changes when necessary, the piling works have run on course ready for the future works to continue apace.".

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