Slipformed concrete has been chosen over the precast concrete alternative for the £27M, 4.5km section of kerb guided busway on the 21km Leigh-Salford-Manchester Bus Priority project, NCE can report.
It is a change in construction approach after precast concrete was used to create the tracks on both the Cambridge and Luton Guided Busway projects.
Balfour Beatty is the main contractor for the Manchester scheme in partnership with slipform specialist Extrudakerb. Client is Transport for Greater Manchester.
Slipforming is due to begin in August and Balfour Beatty project manager Paul Robinson said there were practical and technical reasons for selecting slipform. “From the contractor’s point of view, the lower start up costs and much higher outputs are very attractive. “It’s a significantly more cost-effective technique,” he said.
Technical benefits include the ability to form any desired curve or superelevation, rather than being restricted to the limited range of radii normally associated with precast construction, he added.
“And we expect much improved ride quality, thanks to the use of continuously reinforced concrete pavement (CRCP) construction.”
CRCP road pavements have no movement joints. Instead, a multitude of very fine cracks form themselves as the concrete hardens.
These are kept to acceptable proportions by the reinforcement. Extrudakerb will form the busway as two 3m wide monoliths complete with upstand kerbs and a central drainage channel (see diagram).
Typical Busway design
A typical width for two lanes of kerb guided busway is 6.7m, made up of two, 2.6m wide bus lanes, a 800mm central reservation and a 700mm evacuation strip.
This contrasts favourably with the 9.3m width of a rural road built to 2009 standards, not including verges. Disused railway lines which once ran right into city centres are prime opportunities for guided busways. Under and overbridges, cuttings and embankments are generally too narrow for the former lines to be economically converted into conventional roadways, but guided busways will fit in with only minor modifications.
Compared to trams or light rail, guided busways’ other great advantage is that at either end of the guided sections, the buses can transit to and from existing roads and access city centres without infrastructure modifications. The buses themselves are basically standard vehicles with minor adaptations for the guided phase.
However, this ease of transition can make the busway vulnerable to trespass by other road vehicles. Despite various forms of “car trap” and a profusion of warning signs, there have been a number of incidents of cars and even HGVs finding their way onto the Luton and Cambridge tracks.
There have even been claims that some satellite navigation systems have directed drivers to take the busway route. Rising bollards are an obvious if expensive option at locations where trespass is an ongoing problem.
Where busways run in the middle of or alongside existing roads the raised kerbs can be an obstacle to pedestrians wishing to cross. Less obtrusive guidance options exist and are in use elsewhere in the world, mostly on guided trolleybus projects.
The French designed Translohr guided bus systems utilise central steel guide rails set in slots in the road surface, thus presenting less of an obstacle to pedestrians, but there have been problems
with running noise and concerns about safety during the transition phase. Even less obtrusive are magnetic and optical guidance systems.
Either permanent magnets beneath the track surface or electromagnetic fields generated by buried cables are used for magnetic systems, while optical systems as used in Rouen, France rely on road markings.
Guidance comes from an on-board camera and computer, which controls a guidance motor attached to the steering column of an otherwise standard bus.
The route is pre-programmed, and the system is claimed to enable precision docking at bus stations and stops, thus facilitating disabled access.
The crucial distance between the inner faces of the kerbs will deliberately be cast 10mm too narrow, then ground to a tolerance of plus or minus 1mm.
A very different approach was adopted for the troubled Cambridge busway.
Pairs of high strength 15m long precast elements were connected by three cross pieces to form massive “ladder” units which were placed in position by a specialised launching gantry.
Each ladder unit spans between three sets of supports, either precast pile caps or precast block foundations, with elastomteric bearings between the foundations and the ladders.
Design speed is 88km/h, slightly higher than Manchester’s 80km/h.
Delays and cost overruns dogged the Cambridge project from the start, and it opened two years late.
A bitter dispute between main contractor Bam Nuttall and client Cambridgeshire County Council dragged on for several years before a £33M out of court settlement was reached in August last year (NCE 5 September 2013). Since then, major problems with the foundations and the precast units have emerged that some estimate will cost up to £20M to rectify.
According to a recent council report based on an investigation by Atkins, many of the elastometric bearings have become displaced.
Cracking at the midpoint of the units is much worse than expected. Up to six cracks either side of the central induced crack can be observed on many if not most of the units, with signs of deflections under load well in excess of design predictions.
The slippage of the bearings is said to be down to the effects of dynamic loading and traffic-induced vibration at the joints between the units.
Meeting the very demanding tolerances essential for a smooth ride is a major challenge for both the precaster and the site placing team (see box).
Extreme accuracy in vertical alignment is also essential, and levelling shims under the bearings were used to achieve this.
Regular deformation of the bearings under traffic loading exacerbated by the “rocking” of the units across the central support would, over time, allow the shims to slip out.
Recent observations revealed joints between the ladder units varying between 3mm and 25mm in width, in an ambient temperature of 20˚C.
Local speculation is that vibration from the high speed impact of the buses’ tyres on the joints is the main cause of the bearing displacement problems.
There have also been reports of foundation settlement in some areas. On the Luton kerb guided bus there have been so many complaints of noise and vibration that the local authority are trialling a flexible joint filler in an attempt to mitigate the problem.
At Cambridge it is likely that the shims and bearings at the joints are most affected by the vibration. Any loss of support there will cause the unit to rock more around the centre bearing as the wheel loads move onto it and then “hog” more than predicted, leading to increased bending stresses at the midpoint.
Greater deflection at the ends of the units will also create a “step” between them as the bus moves along, thus making the vibration worse.
Bam Nuttall and Cambridgeshire County Council would not comment on the current situation, and it is understood the matter is in the hands of solicitors.
Challenges for guided bus trackways
All concrete shrinks as it sets and hardens. In high strength, low water/cement ratio mixes, initial, or autogenous shrinkage can be significant.
The mix water is rapidly drawn into the hydration process, creating capillaries in the fresh concrete.
Surface tension within the capillaries causes volume reductions, which can lead to early age cracking.
Shrinkage and cracking can be minimised by careful curing, although only keeping the surface of the concrete continually wet for at least seven days can achieve significant reductions in this early shrinkage.
Exactly how much a large high strength precast unit will shrink before it is placed in position is hard to predict, influenced as it is by such factors as ambient temperatures and humidities during the casting and curing process.
BS8110-1-1997 advises that the specified dimensional tolerances on a 15m long unit should be of the order of plus or minus 20mm.
Where there are very high standards of mould construction and production supervision, these can be halved, to plus or minus 10mm.
The temperature at which these dimensions are measured has to be specified as well, as a 15m long unit will vary in length by as much as 15mm over the normal ambient temperature range in this country.
However, tolerances of plus or minus 2mm were claimed to have been achieved on precast track units for Adelaide’s O-Bahn busway project, in Australia which opened in 1986.