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This case study looks at how the use of modular and offsite manufacturing solutions helped save 6 months construction time and reduce site labour by 30% on the A453 Road Widening project.
Following the Secretary of State’s decision in 2012 to proceed with the A453 widening, the project team was challenged to deliver 20% savings from the budget. To preserve the scheme objectives but achieve the savings, the team concluded that the project must be delivered more quickly by accelerating construction of the programme critical bridges.
Utilising an innovative Design for Manufacture and Assembly (DfMA) approach, structural components were manufactured off site and assembled on site by small teams. The strategy reduced the construction period by six months and on site labour by 30% and indirectly delivered significant safety and environmental benefits.
Originally built in the 1960’s, the single carriageway A453 was congested, had a poor accident record and placed a constraint on access to Nottingham from the south which was detrimental to road users and businesses.
In 2006, the Highways Agency awarded an Early Contractor Involvement (ECI) contract to Laing O’Rourke and their Designer, WYG to develop proposals to widen the road from M1 Junction 24 to the A52 at Nottingham. The Public Inquiry in to the proposals concluded in 2009 but due to the financial climate and a government spending review, the scheme was deferred. In 2012, the Secretary of State announced that the scheme was to proceed but the Project Team was challenged to find 20% savings in the scheme budget.
Limited opportunity existed for scope or specification reductions and so to preserve the scheme objectives, the Team concluded that the project would need to be delivered in a shorter period. Attention was focussed on the 10 major structures and in particular the 8 underbridges which dictated the programme critical path. At this stage, Tony Gee and Partners joined the project to bring additional structures expertise to the design team.
A radical approach was required to the design and construction of the bridges and so the benefits of Design for Manufacture and Assembly (DfMA) and Building Information Modelling (BIM) were explored. The concept of DfMA seeks to move the balance of construction activities from the site to a controlled factory environment with the ultimate objective of reducing site activities by 70%, reducing labour on site by 60% and shortening construction periods by 30%. BIM is an essential parallel activity using 3-D modelling to achieve the tighter tolerances demanded by DfMA.
Laing O’Rourke has championed the DfMA concept in building projects and sought to extend the concept to civil engineering environments. In particular, significant benefits of off site manufacture have been realised in the water industry and light rail schemes and the project team embarked on a programme of re-evaluation of the bridge structural forms to identify opportunities for modular construction. The Highways Agency was very supportive of this approach because of the synergy with the Agency’s drive for efficiency through innovation.
Precast concrete beams have long been feature of bridge construction but on the A453 project, abutments, cill beams, piers, crossheads parapets and wing walls have all been manufactured off site.
Once the revised concept designs were developed, studies were carried out to compare the construction periods, resource requirements and cost with the traditional insitu form of construction. The results of months, a 30% reduction in these studies indicated an overall programme saving of six the number of operatives on site and reduced cost which assisted in providing savings against the ministerially approved budget.
The following series of images illustrates the innovative forms of construction adopted for the A453 Bridges.
Reinforced concrete abutment and pier bases were constructed by traditional insitu construction on bored pile foundations with starter bars projecting for the next stage.
Modular abutment shell units, manufactured offsite, were craned in to position at the back of the abutment and propped. The units had projecting reinforcement and act as participating form work to the abutment structure. The sequence continued along the back of the abutment.
Reinforcement was fixed to the centre of the abutment. Modular shell units were then craned in to position at the front of the abutment, propped and tied to the rear units with macalloy bars. End closure units completed the shell.
The final activity was to fill the void with structural concrete.
Bridge piers were cast as single units and transported to the site. They were craned into pockets in the base slab, propped in position and then concreted in.
The crossheads were also manufactured offsite. These units were craned into position with the starter bars from the piers projecting through voids in the crossheads. Reinforcement was completed and the voids concreted leaving the crossheads ready to receive precast bridge beams complete with integral parapet units.
Figure 14 shows the River Soar Floodspans structure substantially complete. All visible superstructure elements were manufactured offsite and assembled on site by a small team.
This departure from traditional insitu construction required commitment from the Designers, WYG and Tony Gee and our Client, Highways Agency and their Supervisor URS to adopt an entirely different approach to facilitate this modular system. An important consideration however was to ensure that the design complied with existing codes and standards to avoid the necessity to seek and justify Departures from Standards.
Close collaboration was required between the permanent works designers, the temporary works designers and the precast module detailers to ensure that the element is adequate throughout all stages of its lifecycle from casting bed to transportation, lifting and placing and incorporation in the permanent works.
Modular construction tends to reduce flexibility on site to accommodate construction tolerances and so more rigorous control of design detailing and component manufacture is necessary. BIM assisted the process and all structures were detailed as 3-D object models to ensure accurate and efficient transfer of information between the design and fabrication teams.
Reinforcement was modelled in 3-D using the Tekla software to ensure accurate detailing and eliminate clashes.
The adoption of this modular form of construction allows the individual elements to be constructed in advance of their incorporation in the works however the decision to compress and overlap the construction periods for eight bridges required a large number of major components to be delivered in a relatively short period. It was therefore essential that the supply chain had the capability and capacity to manufacture and deliver the units to the specified quality and to the required schedule.
The units were made at Explore Industrial Park which is a state of the art precast concrete manufacturing facility just 45 minutes from the A453 site. The factory is dedicated to manufacturing products for the Laing O’Rourke Group and employs automated processes which can interrogate the design models and transfer the information directly in to the manufacturing process. The facility has the capability to run 24 hours a day if demand dictates.
Transportation and craneage of the units was managed by Select, Laing O’Rourke's plant and logistics division to ensure “just in time” deliveries in the required erection sequence which were coordinated with night time planned road closures.
Off site manufacture reduces the number of activities which take place on site and so with fewer material stockpiles, fewer operatives and less plant competing for space, the potential for conflict from concurrent activities is reduced creating a safer working environment. The reduction of steelfixing on site and virtual elimination of formwork minimises the amount of work at height both in total and the number of operatives exposed.
Manufacture of the components in factory conditions ensures a high quality product with minimal wastage of materials. Operatives are not exposed to adverse weather conditions and manufacture can continue uninterrupted.
Components are delivered to the site when and in the order they are needed reducing the need for additional land for storage of either the units themselves or the constituent materials for traditional insitu construction. Overall, the number of material deliveries to site is reduced minimising construction site traffic. The reduction in raw materials on site minimises waste and the potential for environmental incidents from hazardous materials.
A further benefit is that the compressed construction period reduces the disruption to the travelling public.
The Early Contractor Involvement (ECI) form of contract facilitates the establishment of the entire project team at an early stage when all parties can bring their expertise to influence the development of the proposals before design concepts are finalised. Partnering Workshops were held throughout the design development process enabling trust to be built in the team leading to the commitment to develop new and radical solutions. Workshops extended beyond the immediate project team to consider impacts on other parties and ensure adequate provision for future operation and maintenance.
To ensure that information was always available to all parties, a web based collaboration tool was used. Designs were circulated and the system ensured that all input was recorded and traceable and highlighted where allocated actions had not been completed. A document control protocol was established and the status of designs was tracked from preliminary status to construction issue, ensuring that all comments by approving parties were incorporated in the final version.
The approach adopted by A453 project team has demonstrated that a modular approach to the design and construction of highway structures can deliver significant programme savings and add safety and environmental benefits. A wealth of learning has been generated and shared with visitors from many parts of the industry and the experience gained by the project team will be transferred to other projects to build on the success of the A453.
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