Smarter Cambridge Transport

Repairable defects or design flaws in the Cambridgeshire Guided Busway?

Update June 2020: Cambridgeshire County Council are proceeding with a legal action against BAM Nuttall: “The claim is for the cost to repair defects on the Guided Busway. This follows a thorough review and advice from external legal and technical experts, who have assessed the work and the contract.” The court case is likely to start late 2021 or 2022.

The Cambridgeshire Guided Busway is deteriorating fast. It is six years old, with another thirty-four years of design life left. Already £8m has been spent on repairs and professional fees. A report published in November 2016 concluded that at least a further £36.5m of repairs are required. The urgency of those repairs will entail closing sections of the Busway and diverting buses while the A14 works continue (until 2021).

The County Council will attempt to recoup repair costs from the constructors, BAM Nuttall. There is no certainty that it will succeed.

Smarter Cambridge Transport has prepared a detailed review of reports commissioned by the County Council. It identifies more than twenty areas of concern with the busway not identified in those reports. The council leader, Councillor Count, has indicated that two companies, Capita and Atkins, are continuing investigations and that our findings have have already been investigated. We cannot verify this until a new report or statement of claim is published.

Perhaps the most important question we pose is whether the problems are due in part not to construction defects, but to inherent design flaws that repairs alone cannot fix.

But it’s not only cost that’s a concern; safety too needs to be scrutinised. The three most recent serious accidents on the Guided Busway have been blamed on driver error. In the aviation industry that would not suffice as an explanation. People make mistakes: always have, always will. The answer is to design out possibilities for error and to minimise the damage that an error may cause.

Use of GPS-based speed limiters on guided buses would help. But there are many scenarios in which a guide wheel could fail or disengage, leading to a sudden and severe loss of control. A derailment involving a double-decker bus could be very much more serious than any of the incidents witnessed to date.

Smarter Cambridge Transport is asking the County Council to conduct an open re-evaluation of the busway, independently of any action it takes against BAM Nuttall. Once the true cost of the busway is added up (over £200m), is it any longer accurate to say it was cheaper than light rail? And is it as safe as light rail?

Download the full review

An extract of our review, not including the detailed technical report, follows.

View of the middle of a Guided Busway beam
Cracks are apparent in nearly all of the Guided Busway beams

Executive Summary

Smarter Cambridge Transport has reviewed the Capita reports on the Cambridgeshire Guided Busway, which catalogue and analyse various defects. The remedial action recommended in the November 2016 report involves rebuilding almost 50% of the shallow foundations. This will entail closing sections of the busway for extended periods, during which buses will have to be routed onto other roads. The lowest cost estimate, based on work being carried out proactively, is £36.5m. The cost rises to over £160m if repairs are carried out reactively.

However, the report states that this will not resolve all of the problems. In light of this we have analysed the design, particularly regarding the foundations, the cracks in the concrete beams, and the poor ride quality evident along many sections of the track. We now believe these may be symptoms of fundamental flaws in the design and construction, which appear not to have been adequately examined to date.

Our report sets out some of these symptoms, and outlines possible theories to explain them. These have led us to believe there may be a high risk of future structural failure of the busway, possibly catastrophic. This would create additional, currently unquantified, liabilities for the County Council and further prolonged periods of closure of the Busway.


Failure by the council to obtain a complete understanding of the Busway defects and their causes exposes it to a risk of considerable further expense and reputational damage in the future, possibly within five years. In particular, the risk of a catastrophic failure of the busway, which could lead to deaths or serious injuries, must be quantified and agreed by councillors.

Capita is explicit in its reports that its investigations to date are not comprehensive. For instance, Capita has relied on just one theory to explain movement in the foundations (disturbance by tree roots), and has not proposed a theory to explain the extensive cracking found in most of the beams.

The reliability of cost estimates is unclear. We would expect all recommended courses of action to be presented with a range of costs and confidence levels to reflect uncertainties. What, for example is the probability that the proactive repair programme will cost £45­-50m rather than £36.5m? Uncertainties are inescapable: for instance, it cannot be known yet how many components, including guideway beams, will need to be re-manufactured; or how long the recommended repairs will last; or whether foundations deemed OK now will fail in the future.

The economic and social impacts of temporary closure of the busway and cycleway (‘maintenance track’) also need to be examined.


The council needs a complete picture of the defects and design flaws in the busway; a comprehensive diagnosis of the causes, with all uncertainties spelt out in detail; and for, each of the proposed remedies, a range of costs along with an economic and social impact assessment. These are needed for four reasons:

  1. To give the council the strongest negotiating hand with the original contractors in seeking a settlement for the repair bill.
  2. To minimise taxpayers’ current and future liabilities, and indirect social and economic costs of carrying out repairs.
  3. To ensure that the repairs carried out are sufficient to ensure trouble-free operation until 2051, the end of the Guided Busway’s design life.
  4. To ensure that any design flaws in the existing busway are not carried over to a future guided busway, such as is contemplated by the Greater Cambridge City Deal.

We further recommend that the council consider and commission cost-benefit analyses of alternatives to repairing the busway, ranging from refining the design to converting the busway to a restricted-access road or a light rail line.

Observations and Initial Questions

We have summarised here some of the key issues that we believe need additional research beyond that already carried out by Capita. A theme running through these questions is the need to give greater consideration to dynamic effects, i.e. the interactions between moving buses and components of the busway.

  1. The busway section around Histon has significant undulations. Capita has recommended only limited remedial works here.

What assessment has been made of ride quality? Has this been correlated with proposed remedial works to ensure that all vertical and horizontal deviations have been accounted for?

  1. Capita considered hedges and trees rather than geology to be the major contributory factors in the need to replace shallow foundations.

What evidence is there that heave and subsidence are caused by tree roots alone?

  1. NHBC (National House Building Council) construction standards for foundations typically apply to static buildings, not subject to high-impact dynamic loading.

Are these proven and accepted standards for construction of a busway?

  1. Some fifty per cent of the foundations Capita deems necessary to replace are between Swavesey and Longstanton. North of Swavesey, most of the foundations are piled, in accordance with the original design specification.

Are the geological conditions that required piled foundations found elsewhere along the busway?

  1. The foundations consist of compacted sand and gravel, contained by the surrounding soil, which partly consists of clay. Clay swells and contracts as it hydrates and dehydrates. This means that the sides of the soil ‘box’ containing the shallow foundation move in and out slightly over time. This could be what is allowing the foundations to move.

To what extent has geology been examined as a possible explanation for subsidence and heave? Has the interaction between shallow foundations, the surrounding soils and changing water table been examined? What will be the impact of Northstowe, which is expected to lower the water table?

  1. Many beams are cracking over much of their length, not just at the central V-notch. This is most notable between Histon New Road and Histon Station Road. Capita has recommended replacing only some of the foundations supporting affected beams.

What is causing the extensive cracking of beams?

  1. Water ingress at cracks is creating opportunities for corrosion and frost damage. In particular, when the busway is gritted during the winter, if salt solution is reaching the reinforcing bars, it will be corroding them. These effects will be shortening the safe operating life of the beams.

What analysis has been carried out on the integrity of beams that have cracks, in particular whether there is any sign of internal corrosion?

  1. Rebuilding 821 foundations will entail lifting and relocating over 3,284 beams and connected spacers. Drilling the anchor pin holes will entail lifting all of the remaining beams.

What proportion of beams and spacers does Capita expect to be re-usable after lifting? What is the uncertainty, and hence cost risk associated with the estimate being wrong?

  1. Capita are recommending sealing cracks in the beams with bitumen.

What research has been done into the effectiveness of bitumen filling of cracks in preserving the beams? How frequently will beams need to be inspected and re-treated? Has this been costed?

  1. Bearings and shims have moved sideways which might suggest that beams are rolling or twisting slightly as buses run over them.

How much analysis has there been of the movements, stresses and vibrations induced in the beams, bearings, shims, foundations and underlying geology by dynamic loading of the beams? Of particular concern are the effects of bus and guide wheels transferring loads from one beam to the next, especially at bends.

  1. There have been two derailments of single-decker buses on the Cambridgeshire Busway, and one of a double-decker on the busway in Leigh (Greater Manchester).

What analysis has been carried out to quantify the risk of derailment (e.g. owing to a beam up-stand breaking away under load, or a guide wheel riding up a damaged beam end)? Has the effect of derailment been modelled for a double-decker bus, in particular on a section where the centre of the guideway is open or in-filled with loose rubber, as shown in Fig 1 below?

  1. The stresses placed on the up-stand of the outer guide rail on bends can be exceptionally high, especially at the joins between beams.

Has this been analysed in detail?

  1. In light of plans currently in development to build further stretches of guided busway, it would be prudent to re-evaluate the business case for the existing busway in light of cost overruns, identified defects and design flaws.

What is now the safe operating life for the busway? What is the revised benefit-to-cost ratio for the Guided Busway, based on actual cost and ridership? (Ridership should not include journeys that do not reach the guided section, such as Peterborough to St Ives, but which are currently counted in reports on busway ridership.)

Where next?

In view of the doubts raised by Capita over the design integrity of the Busway, the concerns we have raised here, and the huge cost and disruption that is now inevitable, should the council not consider more options than just repairing the busway?

We note the suggestion that Construction Trials be considered [document reference 2 – p56, para 175] to test the practicality of construction methods. This seems entirely appropriate. In addition, we would suggest that some of the beams which have suffered degradation should be removed from service and subject to accelerated life tests. With an appropriate test rig subjecting the beams to an impact loading at, say 30 second intervals, the beam would be tested at approximately 35 times its working rate. Thus within a year there would be confirmation that the Busway would indeed have a total working life of forty years.

If these tests determine that the construction design needs to be modified to extend the working life, then that should be considered as another option. That would also provide an opportunity to make any necessary modifications to the design to accommodate other vehicles the council may want to permit onto the busway in future: maybe triple-axle buses (which are likely to accelerate wear in the current busway implementation), articulated (‘bendy’) buses, cargo vehicles, and driverless vehicles of a range of sizes.

Whilst the wheel-on-kerb guidance may appear reassuring, there are reasons to doubt this, for example:

  1. Several accidents have occurred on busways in Cambridgeshire and elsewhere, most of which have been attributed to driver error.
  2. Vehicles have to slow down even at light-controlled intersections in order to re-engage safely with the guideway.
  3. Buses cannot overtake an obstruction or reverse along the guideway.
  4. Vehicle breakdowns cause major delays and recovery is slow.

Guidance technology is making considerable advances: optical guidance combines precision with the flexibility of manual override; and collision avoidance technology can already monitor and respond to traffic conditions one hundred metres ahead on a crowded motorway.

We should be asking: What can we learn from the experience of building and running the existing busway? What design and technology is most appropriate for the twenty-first century?

These questions are especially pertinent in light of the fact that the County Council and City Deal are considering building busways to link Cambourne and Waterbeach to Cambridge.

We believe that a number of options should be considered and subjected to cost-benefit analysis. At a minimum these should include:

  1. Repair the busway to meet the original construction specifications.
  2. Modify design elements and rebuild in order to extend the busway life and to accommodate vehicles that we may want to run on it in future.
  3. Replace the busway with a kerbed, tarmac road (as in Runcorn).
  4. Replace the busway with a light rail line.

The Detailed Report in the full review sets out in more detail the engineering issues that lie behind these observations, and poses further, more detailed questions.


The report published by Capita in November 2016 indicates that the design of Cambridgeshire Guided Busway is deficient in several respects. The investigations upon which the report is based were conducted within five years of the Busway going into service in August 2011.

We believe there are significant grounds for concern about the integrity of the busway in its present form. There is a lack of data in the public domain to reassure us.

Although some of the issues of the last five years may be put down to initial settlement, there is little to indicate that movement has stopped. The geology is varied along the route of the Busway, and will continue to change as development continues close by. Establishing secure foundations has proved difficult in the past, and Capita confirm that even rebuilding with revised foundation design carries no guarantees.

It appears that no account has been taken of the root cause of degradation of the beams. Lifting them to replace the foundations is likely to further degrade the superstructure. Problems have developed in the beams in the past five years. We are not convinced that patching them with resin and bitumen paint is likely to provide the 40 year lifetime which is the basis for the original costing of the busway.

In addition, there is no indication that the effects of on-going building work, for example at Northstowe, have been considered.

The work on this occasion should be a definite fix, rather than something which is repeated every few years. The monetary cost of £36.5m plus legal fees plus potential compensation to the operators may be recoverable on this occasion; if it is, it will likely be ‘a full and final settlement’. Local taxpayers will have to foot any future bill and suffer the disruption and loss of amenity while further repairs are carried out. Therefore, any remedial action must resolve all design issues now.

We submit that questions posed in this document should be fully addressed and that taxpayers deserve comprehensive answers, supported by robust data. There is only one opportunity to fully resolve these issues whilst maintaining any likelihood of recovering losses from the original contractor.

It may be that pursuing litigation in order to repair and rebuild the busway to the current design specification is not the best option for the County Council and the residents it represents. Other options should be examined and compared.

The whole concept of a kerb-guided column-and-beam busway should be reviewed, particularly in the light of advancing guidance technology.

Smarter Cambridge Transport


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  • I did some investigations in the earlier years into causes of jolty, bucking riding one experiences, particularly on double deckers. Using a laser pointer, I determined that each beam sags by about 7mm between support bases (two sags per beam). This was enough to set up a 2Hz oscillation in the ride – which tallied with a bus going at 56mph = 30m/s with a beam length of 15m, and the fact that the poor ride quality was practically the same, independent of location. Subsequently, subsidence in the rails has meant that this regular up-and-down movt is now overshadowed by a more random movt. It underlined my contention that the busway could either be “cheap” or ” smooth” but not both.

  • I took a close interest in the Busway at the time of construction, making regular on site visits and can confirm that although the cracks may have worsened and became more apparent they were in fact there on the vast majority, if not all beams at the time of construction. I seem to remember a comment being made at the time, that these were to be expected and were a design feature. I’m not an expert but I found this it quite amazing .