THE PROJECT
The construction of a 500m2 new basement and underpinning of the existing gable end of the joining hotel.
The completed building is to comprise of seventeen flats, with all the drainage running into the basement pumping station which is installed to a depth of four meters below the basement slab. The project is located in Thames Ditton in the suburbs of London, close to Surbiton and Hampton Court Palace.
The original design of the basement construction involved the perimeter of the basement to be piled using a secant piled wall method with a concrete frame extending from the basement. This method of the piled wall would allow the mass excavation of the basement. However, prior to the start of these works we completed the underpinning.
The original design of the basement when priced made the project financially unviable and put the build of the project in jeopardy.
The work was in partnership with MCD Construction (http://www.mcdesigners.co.uk) as the main contractors.
Construction Structural Services Management Limited (www.csmanltd.com) were appointed as the principle groundworks contractor to construct the new basement
THE SOLUTION
We proposed a complete redesign of the project, which the client and the main contractor accepted.
The following design changes were accepted, which we were done by our in house engineering team led by https://www.inertiastructures.co.uk/
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- Design change from a concrete frame building to a steel frame building
- Design change of the secant piled wall to a contiguous piled wall
- Removal of the internal concrete walls to block work
- Removal of the attenuation tank to below ground
- The concrete floors replaced with hollow rib decked floors
THE RESULT
We started the works in the first stage doing the underpinning to the gable end of the neighbouring hotel and this bought us time whilst the structural design of the building was being completed.
The works started before the public utilities started their work and this enabled us to work with them ensuring each of us having equitable access to the site.
The design change has enabled the project to become financially viable and the project completion date has been revised to March 2021.
THE CHALLENGE
The first challenge we came across was that the original design of the building placed it exactly on the boundary between the footpath and the site.
This caused problems since it would mean that the piling works would encroach public land and would result in the closure of the footpaths. This was not acceptable to the local authority since the footpaths are heavily used and we had to maintain pedestrian access all the time as well as for the traffic.
This resulted in the redesign of the building with the building being setback to enable the piling works away from the boundary and the capping beam constructed, where it stretched to the edge of the site boundary.
At the time of starting the works the full structural redesign of the works was not completed since the works had to be started without delay to ensure our access to the site not being impeded as a result of external works commencing by the public utilities outside the gates of the site.
Furthermore, the COVID-19 pandemic resulted in the cessation of the works for a period of six weeks. However, upon return to work we did not have any problems with delivery of materials as many others were experiencing since we had anticipated and stocked the site with all the equipment and materials we needed upon our return.