Bristol has long struggled with traffic clogging its road network. Now a new rapid transport system is being introduced to help try and combat that congestion.
Project: Ashton Vale-Temple Meads Bristol Metrobus
Client: Bristol City Council
Contract value: £29m
Contract type: NEC Option C – Target cost
Main contractor: Balfour Beatty
Piling subcontractor: BBGE
Materials supplier: Cemex
Start date: January 2015
Completion date: July 2017
As one of the country’s largest cities and most important financial centres, Bristol has fared better than many over the last few years.
Acting as the gateway to the West Country, it enjoys enviable road and rail connections with the rest of the UK, which have helped the city bear the worst of the recession well.
Unfortunately for the citizens of Bristol and those that commute into it for work or pleasure, the transport network within the city itself leaves a lot to be desired.
Despite regularly coming top of newspaper ‘best places to live’ lists, it also manages to vie for the ‘worst traffic congestion’ title, often coming top of the pile with average traffic speeds throughout the city centre lower even than those of central London.
In a move that aims to help boost transport efficiency within the city, local authorities Bristol City Council, North Somerset Council and South Gloucestershire Council have given the all-clear to a rapid transport system that will enable guided buses to pass through and around the city centre, slashing travel time for commuters.
Balfour Beatty Bristol busway BMB5
Contentious since the first planning stages, the initial 50 km Bristol Metrobus network is being constructed in a number of sections with Balfour Beatty looking after the southern-most element, the crucial Ashton Vale – Temple Meads segment.
This section will link the Long Ashton park and ride on the south western outskirts of Bristol with the city’s main rail terminus at Temple Meads. Part of the route through this section will be constructed as a fully guided busway, part as a dedicated busway with the driver in control, and part will merge onto the existing road network.
“Our section starts on newly constructed busway before tying into the existing road network near the old Bonded Warehouses at Cumberland Road on the north side of the River Avon,” says Balfour Beatty project manager Gary McHugh.
“It then carries on using existing bus lanes until finishing at Temple Meads.”
Difficult ground conditions
With most of the heavy civil engineering work concentrated within the first 3 km of the Balfour Beatty section, the team is working to bring the busway through the land east of the Long Ashton park and ride, a difficult area that is liable to flooding and also part of an infilled waste dump.
“The ground is very poor along here, not necessarily because of the waste site – we have very little interaction with that, but the alluvial deposits that sit on top of the mudstone make it very difficult,” Mr McHugh says.
Throughout most of this area the new busway sits on a new embankment, keeping the route clear of possible floodwater.
“There are only a certain number of variables that you can play with to get the right amount of settlement”
Gary McHugh, Balfour Beatty
But the construction of the embankment has meant the team have had to carry out some thorough ground consolidation measures, with band drains installed in a 1 m triangular grid at depths of 8-12 m and a surcharge layer averaging around 1 m thick.
Balfour Beatty Bristol busway BMB4
This granular material surcharge was removed at Easter 2016 after being placed in one of the earliest operations that the team carried out when it started on site in January 2015.
“There are only a certain number of variables that you can play with to get the right amount of settlement. You either add more band drains, place more surcharge or increase the amount of time the surcharge is in place,” Mr McHugh says.
“We couldn’t really fit more band drains in and we wanted to be working within a fixed time frame, so we increased the surcharge.”
There is one pinch point along the route where the embankment has been routed across a section of the old waste tip. Here, the use of band drains was dismissed over fears that they could allow contaminated ground water to migrate along their length. Instead the team bulked up the surcharge, placing three times as much as across the rest of the scheme.
At this point the busway crosses the Bristol – Portbury freight rail line and the A3029 via a 400 m-long skew bridge, before running along the line of another former freight railway and meeting the Ashton Avenue Swing Bridge that is being revamped under a separate contract by contractor VolkerLaser.
Balfour Beatty Bristol busway BMB11
The section between the bridge and tie-in with Cumberland Road sits at existing ground level before rising on a piled ramp to join the highway.
Of the almost 3 km of busway that Balfour Beatty is constructing, only around 700 m will be truly guided because of the steep gradients and curves that affect the route.
In the guided section the busway will be of single carriageway in each direction, constructed using in-situ reinforced concrete techniques, with the troughed 2.6 m cross-section of each carriageway dipping into a central drain featuring a gulley every 50 m.
UK concrete first
In the non-guided sections, however, the team is planning to use roller compacted concrete, the first time this material has been used on a public highway in the UK (see box).
Metrobus eyes concrete first
In a bid to address some of the issues that have blighted other guided busway projects across the world, the Balfour Beatty and Metrobus team have been looking hard at processes and applications that might help.
One process involves the use of roller compacted concrete (RCC) as a running surface throughout the non-guided sections of the busway, a technique that is being championed by materials supplier Cemex.
In a UK public road network first, current proposals will see 200 mm-thick layers of RCC placed above a cement bound material sub-base.
Essentially a zero slump concrete, RCC can be placed using standard asphalt pavers and compacted with rollers in the same way hot mix asphalt is placed.
According to Cemex it offers all the ease of installation advantages of flexible pavement construction without its associated long-term durability and maintenance problems.
“We had dismissed using precast concrete over fears of ride quality and maintenance, and jointed reinforced concrete was rejected because of similar concerns,” Mr McHugh says. “The client didn’t want to use a standard flexible pavement because of fears of rutting and associated maintenance costs. Cemex’s roller compacted concrete option is perfect. It is has a long design life and no joints,” he says.
Piling work at the Cumberland Road apron has been completed with hundreds of 600 mm diameter CFA piles at 12-20 m depths supporting the concrete slab as it ramps up to meet the existing highway, helping maintain the integrity of an existing historic wall.
At the skew bridge, subcontractor Balfour Beatty Ground Engineering is finishing off the installation of a forest of piles of similar size and depth for the north and south ramp and the four bridge piers that help carry the spans across the road and railway.
The team will then move on to the construction of the bridge proper, with the completion of this phase of the Bristol Metrobus project scheduled for July 2017.
For traffic-besieged Bristolians, any transport solution that offers some respite from the clogged streets must come as welcome relief.
Bus byway BIM
The AV-TM section of the Metrobus is the first project in the south west where Balfour Beatty has fully embraced the BIM revolution.
Across the site all its staff are using BIM 360 Field management software for the scheme, helping improve efficiency on the project. It’s fair to say that Mr McHugh is quite a fan.
“It has been fantastic. The feedback we have had from our staff and the supply chain has been brilliant,” he says.
The ability for all members of the project team to get instant, immediate access to current information is vital for the project and makes the management of the scheme much easier, Mr McHugh argues.
“The supply chain gets access to the database, generally through their desktops. The information we are able to input directly through the tablets we use means that everything is constantly up-to-date. We know exactly where we are at any one time with all aspects of the project,” he says.
“It has opened up the information feed, not just from the offices-out but from site-back too. It has been ideal for our client to keep abreast of progress on the scheme.”