Centrica (British Gas) is based across a large site in Staines Upon Thames. As part of their redevelopment strategy for the site, they required additional car parking space without expanding their footprint.
Fox (Owmby) Limited was awarded a Design and Build Contract to construct a single deck car park structure along with upgrading sitewide drainage, surfacing, lighting and access.
Prior to the works, we were required to work closely with the clients representatives to undertake early planning. This was due to the site bordering a railway meaning the project required a Network Rail BAPA, a requirement for a Section 278 temporary entrance with required works to several statutory undertaker apparatus and demolition of an existing building requiring a Section 80. All of these agreements were completion prior to commencement on site.
The site is adjacent to a lake which was a former gravel pit. The site was partially filled in an uncontrolled manner, and used for surface level car park for several years. We were approached to offer a value engineered foundation system as an alternative to the cost prohibitive options being considered such as piling. We proposed a solution with reinforced shallow pad foundations supported on CMC column improved ground. This was developed between our design team and specialist subcontractor, and assessed and accepted by the clients representatives.
Once on site, we began with demolition and site entrance works. The site entrance required us to coordinate access for 6 service providers to undertake lowering and diversion works without our 2 week road access. We then securely barriered the new entrance to segregate our vehicles and personnel from the Centrica offices area. The first construction operation was to demolish an existing building, cap off service connections and grub out foundations.
Following completion of this, we opted to work using the existing asphalt surfacing as the construction platform, with an additional piling platform where the controlled modulus columns (CMC) were to be installed. This temporary works design was subject to a CAT3 check, and approved by Network Rail prior to use. The CMC installation was undertaken locally at the pad foundations, with three columns per pad, installed to the base of the made ground, approximately 6m deep. On completion of installation, the piling platform was removed and foundation construction commenced. Our value engineered foundation scheme had a key benefit of raising foundation levels above the groundwater level. Therefore the pad foundations could be dug without dewatering in stable excavations. The structure required over 70 pad foundations of various sizes, and preformed bolt sets hung and cast into the bases. Groundworks also included service ducting, surface water drainage pipework and channel ACO drains. Two new oil interceptors were also constructed, along with outfalls into the adjacent lake.
Due to the tight programme, erection of the car park commenced concurrently with the groundworks. Detailed lift studies were completed, and we were able to design craneage to ensure that the crane collapse radius did not impact on the railway corridor. This made the approvals process with Network Rail much quicker on a tight programme. The structure comprised a steel framework with precast concrete deck units, stitch poured to provide composite action. The deck was waterproofed with an asphalt surfacing and clad with cedar. One side of the structure also supports climbing wires to form a green wall. Erection of the main structure was completed in a 16 week programme.
The structure also included a comprehensive M&E fitout, which included motion sensing low power LED lighting to the upper and lower decks. The site is in a residential area, and light pollution was a key issue during the planning process. Our lighting scheme utilised low level strip lighting to the internal perimeter of the top deck, supported on headlight screening panels. This prevented off site light dispersion. The central parts of the top deck used low level column lighting (2m height), with light assemblies angled to provide adequate lighting to meet minimum agreed standards. A fire alarm system was also installed with a wireless comms link back to the wider site system to provide seamless integration.
On completion of the superstructure works, the lower level was overlaid with 30mm depth of new surface course asphalt and line marked. The project was part of a whole site refurbishment by Centrica, and an extensive landscaping package was included in these works. This included construction of a new lakeside footpath, along with new shrub planting, boundary fencing and trees. The green wall to the car park included an automatic irrigation system, with specific tree planting and climbing plants which will provide an attractive façade. We were subsequently also asked to renew block paving on the footpath which links the new car park to the offices.
Our value engineered foundation solution provided sufficient savings to bring the project within the clients budget for the work, without sacrificing quality of finish. It also reduced vehicle movements through the site, which held thousands of Centrica workers. The new surface water drainage system collects runoff and diverts it to the lake through oil interceptors, which will improve surface water quality in the lake.