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Constructing a guided busway on a disused railway line in Cambridge presented many unique challenges that would rule out the use of standard construction equipment. Existing railway bridges needed to be negotiated and local farmland had to be considered. The solution needed to be light enough to travel along sections of already laid track, as well as being able to operate in narrow and sometimes difficult ground conditions along the route.
Bennett Associates was contracted by Edmund Nuttall Limited for Cambridgeshire County Council to design a bespoke track laying solution that could overcome these difficulties. The design would need to have minimal impact on the environment as well as being compact and flexible enough to be able to work within the constraints that the project presented.
The route of the guideway will run along the disused railway track of the former Huntington to Cambridge railway line, which was closed in 1998. The existing narrow railway bridges on the track would make it particularly difficult to lay the new track using standard crane machinery. Bennett Associates had to find a custom built solution that could operate effectively within very tight confines and make optimum use of the space available. The bridges allowed for a maximum height of 4.8m and a width of only 8.4m, enough to allow the passage of a single decked bus in either direction.
Had a traditional crane been used to lay the guideway, significant work would have been needed to prepare the ground and surrounding area to allow for movement of the crane. This would have increased both the width of site required as well as having a significant environmental impact.
Innovation, flexibility and efficiency were brought to bear by Bennett Associates to create a practical and cost effective solution. A lightweight, compact system in the form of a gantry crane was developed that was productive, yet versatile enough to deal with the constraints of the limited track width and mixed ground conditions.
Cambridge County Council wanted a smooth track, upon which buses could travel at a speed of up to 60 mph and that would also allow passengers to work on a laptop or take a cup of coffee on board. To achieve a quality smooth surface, pre-fabricated concrete segments were decided upon at a very early stage of the project. These are transported by trailer to the rear of the crane, along already laid sections of the track. The segments are lifted by a system of 4 forks, which act as a 'grabber' and are able to fit within the limited space available. The segment is then lifted by the crane to the front of the gantry and manoeuvred precisely into position onto pre-laid foundations. Much of the Guided Busway track is 2 lanes and the structure is able to deliver the concrete segments either side of the crane. Once the segment has been laid, the hydraulic legs lower the gantry’s wheels back onto the track and then the whole structure moves forward, ready to lay the next section of track.
Bennett Associates designed the unique gantry crane to operate by means of a hoist system comprising two bogeys running along the length of the structure and across its width. These work in unison so as to be able to lift and manoeuvre the concrete segments precisely in all directions. The gantry crane is able to lay 10 segments (5 pairs) of concrete track per day, covering a distance of 150 metres.
The compact design of the crane made optimum use of the limited width available when negotiating existing railway bridges. The crane was able to lay track underneath the railway bridges on the route with a clearance of only 30mm either side.
A key challenge for Bennett Associates was to design a gantry crane that was able to drive along the track without breaking the concrete segments already in place. The track, which was designed to support a 13 tonne single-decker bus, would clearly not be strong enough to support the 70 tonne crane whilst lifting a 32 tonne concrete segment. Eight hydraulic legs were positioned on the crane, through which the load is distributed when laying the track. The legs also needed to be able to operate in, and adapt to, poor ground conditions that are present for much of the route. These include the existing, often steep, embankments and a wide variety of materials that they would be placed upon.
The feet of the hydraulic legs can extend over a range of 1.2m and have feet mounted ball and socket joints. The flexible design enables the feet to adapt to the local ground conditions where they are placed. The pad that lands on the ground was designed to limit the ground pressure whilst also being small enough to allow the guideway beams to be laid.
Power and welfare facilities for a crew of 7-8 were included in the design comprising a cabin that houses an office, kitchen and toilets, as well as a diesel generator with fuel tank. The cabin, which is mounted on top of the crane, was designed so that it could be detached to enable the gantry to fit under the bridges situated on the route of the guideway.
"The design of the crane is completely unique, there is nothing else quite like this. We had to develop a bespoke solution to be able to overcome the difficult conditions presented by the project," said James Hill, Mechanical Design Engineer at Bennett Associates. "The biggest challenge was designing a crane that was lightweight enough to drive along the guideway without breaking it, yet strong enough to lift the 32 tonne concrete track segments," he added.
Bennett Associates assisted in the procurement for the project and looked to Hadee Engineering in Sheffield to fabricate their final design. In September 2007, the crane was assembled on site in Longstanton and began the first stage of construction towards Cambridge Science Park. It was important that the crane did not require any specialist transport requirements as it will need to be stripped down, transported by road and then rebuilt at various stages of the project.
When completed, the Cambridgeshire Guided Busway will be the longest guided busway in the world. It is estimated that 11,500 journeys a day will be made on the guideway. Running between St Ives and Cambridge, this innovative transport system will help to ease congestion on the busy A14, which currently links Huntingdon and Cambridge, and provide an alternative and convenient mode of transport for car users in the area. The tramway will pass through the proposed new town of Northstowe, a significant area of urban expansion that will accommodate nearly 10,000 new houses.
The bespoke crane has proved a practical and cost effective solution to the various challenges presented by this project. The innovative and adaptable design is able to overcome difficulties along the route and yet still deliver a high level of productivity. The guided busway is due to be completed in the summer of 2009.
March 2008
For further information, please contact Atkins Rotherham on 01709-373782
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