The M25 London Orbital Motorway is one of the busiest highways in Europe. To reduce serious congestion, the UK Government put in place a construction programme to increase the capacity from 3 to 4 lanes in each direction.

To accelerate delivery, time-consuming statutory processes involving Compulsory Purchase Orders were avoided by the decision to build within the existing land-take. The ‘Initial Upgrade Sections’, together with refurbishment of the A1(M) Hatfield Tunnel, were let as part of the M25 DBFO contract for the maintenance and upgrading of the M25 and associated trunk road links as a 30 year concession.

Following the contract award Connect Plus began the widening with Section 1 (J16 to J23) in the north-west quadrant, and Section 4 (J27 to J30) in the north-east. The combined length of these sections is 64km.

Project Challenges

• The paramount requirement was that the works had to be complete before the London 2012 Olympic Games. To meet this deadline the design and construction had to be twice as fast as previous comparable projects, expending around £1 million per day and widening the motorway at a rate of 1.6km/month.
• Upgrading the road to Controlled Motorway standards brought the requirement for comprehensive gantry-mounted signs and signalling for driver information and traffic control.
• Extensive lengths of retained cut and fill were needed to ensure the works could be built within the existing fence lines.
• Surface water runoff from the increased paved area could not exceed previous discharge rates. Three traffic lanes in each direction had to be maintained throughout the working day.

Collaborative working around a 3D Model

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, which were narrowed not only by the new traffic lane, but also at “blister” sites where provision had to be made for the gantry pile caps, communications equipment, drainage attenuation or treatment facilities.

Both Skanska Balfour Beatty & 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. There was not enough width in the cross section simply to allocate spaces to each discipline; the linear services had to weave around the many obstacles: piles, foundations and drainage chambers for example.

Atkins built upon a decade of experience in developing accurate interactive 3D models of road projects, it was agreed that all, above and below ground infrastructure should be modelled to an appropriate level of detail. In developing optimal solutions for the proposed works the model would go through several iterations, thus to minimise abortive work, the modelling methods had to be as streamlined as possible – in essence 3D reports of the design.

There are as yet no international or national BIM standards for geometry and data adequate for civil infrastructure. The project therefore defined its own standards for each discipline. Atkins created automated software tools, methods and objects to achieve this.

The automation could work only if the CAD drawings (which remained the contractual deliverable) were correctly structured to the required standard and their content showed the locations and orientations exactly. As far as possible the drawings were read by bespoke software tools, similar in approach but tailored to each discipline, which generated the corresponding 3D representations. Other bespoke tools simplified the task of location by precise chainage and offset from features. As an encouragement to adopt this innovative method of working, the tools also generated consistent schedules and other structured data. The aim was that, overall, the new approach should be quicker than the old.

The 3D models from the designs of all those disciplines involved were brought together in Autodesk Navisworks. Its automatic clash detection function was used to detect and track any conflicts between disciplines. These were both physical conflicts and infringements of clearance zones. A report accompanied each issue of the model, expanding clash details with highway-specific information: chainage, offset, and carriageway, and assigning an owner responsible for each conflict.

At an agreed stage iterations of the model would be shared with SBBJV for design review and to assist in planning and phasing of the works. The final BIM models for each section were issued to site for the construction phase; over three quarters of the site staff (approx 120 personnel) received training and had constant access to the models for a range of purposes. They proved invaluable for daily briefings, for understanding the space available for the current activity and assessing the implications of design changes. As the only medium where all teams could view their part in the context of the scheme, they viewed potential issues in Navisworks Freedom and liaised on their resolution. Viewing the project in 3D also highlighted aspects that could be improved – even if they do not give rise to a clash.

Atkins’ site team in collaboration with SBBJV Engineering Department maintained the models to show any design changes that were made. Greater realism was added to make the models more intuitively comprehensible. For example, to check that the design is within the fence lines, a boundary extending 5m above and below the ground is suitable; but making it a recognisable post-and-rail fence is a better option during construction phase. SBBJV would go on to include accurate models of Temporary Works, Traffic Management and Models from Laser Scanning and would facilitate access to drawings through the Model environment. The model was used to brief Network Rail prior to a possession of the East Coast Main Line and virtual road safety audits were concluded by the site team prior to a shovel hitting the ground.

Constraints

The highway design involved meticulous attention to lane widths and hard shoulder provision, given the constantly varying constraints of under-bridges, overbridges, fence lines and infrastructure. As far as possible the central reserve works were minimised to reduce traffic management changes. Design surfaces were subdivided into separate appearances appropriate to their functions: running lanes, hard shoulders, verges etc. before transfer to the clash detection model.

Nearly all the existing bridges were retained, many with strengthened piers. Provision had to be made for extra ducts 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, where an extra lane had to be stitched 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 asbuilt drawings.

Geotechnical design of retaining and re-grade solutions was driven by the need for programme certainty in construction. The undulating terrain of Section 1 needed some fairly large retained heights. SBBJV’s innovative King Sheet Pile offered the fastest means of opening up working space for other construction activities. However, the extents of this sheet piling utilised much of the appropriate plant in the UK, meaning that 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%, moreover a provision for a 20% increase in rainfall intensity in line with Environment Agency requirements, both without increasing discharge rates to watercourses. The water quality requirements were also more stringent than as-built 30 years ago. The design therefore 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 completely new chambers and ducts to replace those under the old verge. More cabinets and supply interfaces were needed to house the upgraded equipment, each with access requirements as well as the space they occupy physically. Modelling the lighting design was a similar process.

The Vehicle Restraint System design adopted concrete step barrier (CSB) in the central reservation, for its lower maintenance and whole-life cost. In the verges the provision varied considerably, depending on the working width space available. At gantries this was often very 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, to flag up potential clashes that could then be assessed individually. Existing utilities always carry risks that cannot be fully 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 for these were modelled from first principles. Buried services were initially 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 to show their 3D extents. On more than one occasion this necessitated redesign, but the only clashes were virtual ones.

Key benefits and success factors

The BIM approach saved millions of pounds in programme efficiencies and helped to deliver the project ahead of time and within budget.

Feedback and continuous improvement over the duration of the project have added further aspects of BIM methodology to the process. Initially the aim was purely to achieve a buildable design without clashes. Later models have far more data structure, enabling property data (stored within the model or in external databases) to be attached to recognisably named objects.

Atkins and Skanska Balfour Beatty Joint Venture were jointly awarded an Autodesk BIM Experience Award for use of BIM through Design and Construction in April 2011. SBBJV & Atkins aim to adopt and enhance its BIM methodology on the recently awarded M25 Later Upgraded Sections 2 and 5.

Contract data

Contract value: £850M
Duration: 39 months
Completion date: June 2012