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The M25 London Orbital Motorway is one of the busiest highways in Europe. To reduce congestion, the UK Government put in place a construction programme to increase capacity from three to four lanes in each direction.
The project avoided compulsory purchase orders because of the decision to build within the existing land-take.
The 'Initial Upgrade Sections', together with refurbishment of the A1(M) Hatfield Tunnel, were included in the DBFO (design, build, finance and operate) contract. This involved a 30-year remit to maintain and upgrade the M25 and associated trunk road links.
Connect Plus - a consortium including Balfour Beatty, Skanska, Atkins and Egis Road Operation UK - was awarded the contract. It began work on Section 1 (J16 to J23) in the north-west quadrant, and Section 4 (J27 to J30) in the north-east - combined length of 64km of highway.
The project presented a number of challenges.
The physical limit of the existing fence lines and the deadline of the Olympics dictated the approach to the project. Design and construction methods had to be fast and right first time. The focus was on the verges, narrowed not only by the new traffic lane, but also at 'blister' sites for the gantry pile caps, communications equipment, drainage attenuation or treatment facilities.
Both Skanska Balfour Beatty Joint Venture (SBBJA) and Atkins recognised that the greatest design risk came from this need to fit far more infrastructure, from many disciplines, into these highly constrained verges between the new lanes, and retaining walls. Teams from different disciplines could not work in isolation as the final design had to be free from inter-disciplinary conflicts. Plus, there was not enough width in the cross section to allocate spaces to each discipline. The linear services had to weave around the many obstacles - piles, foundations and drainage chambers for example.
Atkins used a decade of experience in developing accurate interactive 3D models of road projects. The project team agreed that all infrastructure (above and below ground) should be modelled to an appropriate level of detail. To find the most effective solutions, the model would be improved through several versions. To minimise abortive work, the modelling methods had to be as streamlined as possible – in essence, 3D reports of the design.
At the start of the project, there were no BIM standards for geometry and data good enough for civil infrastructure. For that reason, the project had to define its own standards for each discipline. Atkins created automated software tools, methods and objects to achieve this.
The automation could work only if the computer-aided design (CAD) drawings met the required standard. Their content must show the locations and orientations exactly. As far as possible, bespoke software tools read the drawings, similar in approach but tailored to each discipline. These tools then generated the corresponding 3D representations. Other bespoke tools simplified the task of location by precise chainage and offset from features. To encourage the use of this innovative method, the tools also generated consistent schedules and other structured data. The aim was to make the new approach quicker than the old.
The 3D models from the designs of all disciplines were compiled in Autodesk Navisworks. Its automatic clash detection function could detect and track any conflicts between disciplines. These were both physical conflicts and infringements of clearance zones. A report was produced with each version of the model, expanding on clash details with highway-specific information such as chainage, offset, and carriageway, and assigning an 'owner' to each conflict.
At an agreed stage, versions of the model were reviewed by SBBJV to help plan and phase the works. The final BIM models for each section were sent to the site for the construction phase. Over three-quarters of the site staff (about 120 personnel) received training and had constant access to the models for a range of purposes. They were invaluable for daily briefings, understanding the space available and assessing the implications of design changes.
This gave all teams the opportunity to see their area in the context of the scheme and, by viewing potential issues in Navisworks Freedom, they could work together to resolve them. Viewing the project in 3D also highlighted aspects that could be improved – even if they do not highlight clashes.
Atkins' site team, in collaboration with the SBBJV engineering department, maintained the models to show any design changes made. By adding greater realism, the models became more intuitive and easy to understand. For example, to check that the design is within the fence lines, a boundary extending 5m above and below the ground is suitable. But to make it recognisable, a post-and-rail fence is a better option during the construction phase.
SBBJV went on to include accurate models of temporary works, traffic management and models from laser scanning, and provide access to drawings from the model. The model was used to brief Network Rail before a possession of the East Coast Main Line, and virtual road safety audits were concluded by the site team before a shovel hit the ground.
The highway design involved meticulous attention to detail regarding lane widths and hard shoulder provision. This was so it could take into account under-bridges, over-bridges, fence lines and infrastructure. To reduce traffic disruption, the project aimed to minimise central reserve works. Design surfaces were subdivided into separate appearances appropriate to their functions - such as running lanes, hard shoulders, verges etc. - before transferring it to the clash detection model.
Most of the existing bridges remained, many with strengthened piers. Extra ducts were needed for communications and lighting. One footbridge with inadequate headroom over the new hard shoulder was demolished and replaced.
The biggest structural challenge was widening Berry Lane Viaduct, near Chorleywood. This required stitching an extra lane onto either side of the existing structure. The piers, abutments and foundations were particularly important for both new and existing structures. Most had to be modelled from the original 'as-built' drawings.
The need for programme certainty in construction drove the geotechnical design of retaining and re-grade solutions. The undulating terrain of Section 1 required large retained heights. SBBJV's innovative King Sheet Pile was the fastest way of opening up working space for other construction activities. The extents of this sheet piling utilised much of the appropriate plant in the UK. This meant alternative solutions – such as slipformed concrete walls and gabions – were preferred on the flatter ground in Section 4.
The drainage design had to accommodate an increased paved area of approximately 25%, and allow for a 20% increase in rainfall intensity in line with Environment Agency requirements. The design had to deliver both, without increasing discharge rates to watercourses.
The water quality requirements were also more stringent than when the M25 was built 30 before. The design had to include a wide range of attenuation tanks, ponds, ditches, soak-aways, separators, flow control devices, penstocks and emergency impoundment tanks. The 3D model had to include correctly dimensioned objects for these as well as the standard chambers, carrier drains, filter drains and slot drains.
The gantries and communications system required new chambers and ducts to replace those under the old verge. More cabinets and supply interfaces had to house the upgraded equipment, each with access requirements as well as the space they physically occupy. Modelling the lighting design was a similar process.
The vehicle restraint system used a concrete step barrier in the central reservation, as it's easier to maintain and has a lower whole-life cost. The verges varied, depending on the working width space available. For gantries this was often little, requiring a proprietary systems with a ground beam. The posts for all types of steel barriers were modelled as continuous walls for clash detection purposes.
This helped flag up potential clashes that could then be assessed one by one. Existing utilities always carry risks that cannot be completely assessed until physical surveys have found and measured the equipment and checked for clashes against its safe clearance. Overhead power lines can impact the design of lighting columns, CCTV masts and gantry-mounted signs.
The safety clearance zones were modelled from first principles. Buried services were first modelled as walls from the utility companies' 2D record drawings. As more information became available from trial pits and site surveys, the services were remodelled in 3D. On more than one occasion this necessitated redesign, but the only clashes were virtual ones.
The BIM approach saved millions of pounds and helped to deliver the project ahead of time and within budget.
Feedback and continuous improvement throughout the project also added to the BIM methodology for the process. To begin with, the aim was to achieve a buildable design without clashes. But later models have far more data structure, which meant property data (stored within the model or in external databases) could be attached to named objects.
Atkins and Skanska Balfour Beatty Joint Venture were both awarded an Autodesk BIM Experience Award in April 2011, for their use of BIM in design and construction. SBBJV and Atkins aim to adopt and enhance their BIM methodology on the M25 'Later Upgraded Sections' 2 and 5.
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