Staffordshire County Council has brought a variety of innovative techniques together for a major A-road reconstruction project, as NCE reports.
When Staffordshire County Council’s highways department identified that a 3.5km section of the A515 needed resurfacing and haunch repairs, the main concern was how to do the work with minimum disruption to motorists.
The council’s first innovative decision was to opt for a full closure of the section, between Draycott in the Clay and Yoxall, rather than trying to manage the repairs under traffic management. “Although it’s a significant stretch of road, there are actually only five residential properties on it. This allowed a full road closure,” explains Staffordshire County Council site agent Steve Knott.
Minimising road closure
Having taken that decision, the council and its contractors looked for techniques that would minimise the closure period and get the road back open as quickly as possible.
As a result, the work was completed in just six weeks, compared with the 20-week estimate for more conventional methods.
“The biggest advantage is not that the guy on the roller can decide enough’s enough, but that it’s mapping where they haven’t compacted sufficiently”
Steve Knott, Staffs County Council
The £1M project involved resurfacing the entire 7.3m wide road over a length of 3.5km - a total area of 25,550m2 - and strengthening the haunches.
The road closure started on 4 July, and the first major task was to cold plane a depth of 80mm off the entire length and width using a 2m wide planing machine. This resulted in 2,050m3 of asphalt planings being removed and stockpiled at the site compound on a nearby airfield.
Second planing machine
A second planing machine - this time 1m wide - then went in to take out a 300mm depth of carriageway on the haunches on both sides of the road. Again, the 2,100m3 of asphalt, concrete and granular planings were stockpiled at the airfield.
Project A515 haunch and carriageway reconstruction, Draycott in the Clay to Yoxall, Staffordshire
Project value £1M
Client Staffordshire County Council
Term maintenance contractor Enterprise
Planing contractor Powerplane
All the planings were crushed and screened to a maximum particle size of 20mm, and then recycled for use in the FABM1 material used to reconstruct the haunches. FABM1 is a mixture of coarse and fine aggregates, used in combination with lime, water and pulverised fuel ash (PFA).
On the Staffordshire job the material was mixed using a Wirtgen KMA 220 mobile cold recycling mixing plant, located on the airfield. Initially, mixing the planings with PFA from nearby Rugeley power station in a 3:1 ratio created a pre-blend. This pre-blend was then mixed with more planings and quick lime to give a final mix of 85% planings, 12% PFA and 3% lime.
“Our highway laboratory performed pre-scheme tests to determine the optimum moisture content, and we had two technicians on site during the blending process to control the moisture content while also performing quality checks such as grading, dynamic plate testing and air void cores,” says Knott.
The haunches were reconstructed in three 100mm lifts of the FABM1 material, using a “Sidewinder” offset placing machine, and compacted using three Bomag tandem rollers.
The Sidewinder machine has a far greater output than a minipaver, and contributed significantly to the reduction in the overall programme. In fact, the machine allowed up to 800t of FABM1 to be laid and rolled per day, with the entire haunch reconstruction process taking just nine days in total.
The next step was a further planing operation, to ensure fine level control, with 20mm being removed from the road across its full width and length. “The Sidewinder enables an unprecedented output to be achieved, but one of the disadvantages is that you lose level control,” explains Knott. “We took the decision to optimise productivity for the haunch construction, and then plane off 20mm to achieve the required level and tolerances.”
One advantage of this decision was that it also ensured the surface of the base material was clean and dry before the next stage - application of a 2kg/m2 Seal O Flex sprayed binder. A GlasGrid CG50 geosynthetic reinforcing mesh was laid on top of the binder, followed by a 60mm asphalt binder course laid with a Bomag BF600P wheeled paver.
Standard asphalt was used on the majority of the job, but the local authority also took the opportunity to run a trial with 40t of low energy aspha
lt (LEA). LEA asphalt is designed to reduce environmental impacts by lowering the temperature at which it is produced from in excess of 150ºC to below 90ºC.
The final surface course for the road consists of a 40mm layer of 10mm stone mastic asphalt (SMA).
During the surfacing construction Staffordshire Highways specified a 7.1t pivot-steered Bomag BW 154 AP-4 Asphalt Manager roller for compacting the binder layer and the surface course. The machine automatically records the density of the material below the compacting surface, so the operator knows if they have achieved the required compaction (see box). The Staffordshire contract was the first time the equipment had been used in the UK.
“We were very pleased to get it for this scheme,” says Knott. “The advantage for the person on the roller is that it maps where they’re rolled and what dynamic stiffness module is being achieved.
“For us the biggest advantage is not that the guy on the roller can decide enough’s enough, but that it’s mapping where they haven’t compacted the material sufficiently, enabling them to concentrate on the areas where it doesn’t yet meet the requirements and make sure it’s a consistent mat.”
The final stage of the project, completed minutes before the road re-opened, involved applying the white lines, including distance marks for the speed cameras, and fixing 450 new cat’s eyes.
While Staffordshire County Council had used some of the techniques and materials before - including GlasGrid and FABM1 - others, such as the Seal O Flex binder, the Sidewinder and Asphalt Manager roller, were new on this job. Their combination has proved highly successful, resulting in significant time and cost savings.
Allu mobile mixer targets renewables
Allu has launched a mobile mixer that is specifically designed to produce hydraulically bound material (HBM) material for trench reinstatement. The Suremix SM50 machine has the capacity to recycle arisings from trench excavation and produce HBM at a rate of 65t per hour.
The company, which specialises in soil stabilisation and recycling equipment, developed the machine with input from consultant URS Scott Wilson and waste reduction organisation WRAP. It has been designed to produce high quality HBM that meets all current and anticipated UK highways specifications.
One feature of the Suremix SM50 is a very accurate weighing system, so contractors can produce laboratory specification material without adding higher levels of binder than are required. Proprietary binders can be expensive, and Allu was keen to offer contractors an accurate system that ensures only the exact amount is added to the mix.
The launch earlier this month follows a successful trial of a prototype Suremix machine in North Wales last year, in which arisings from 30km of cable trench went through the mixer to produce HBM for the trench reinstatement. In all, 48,000t of material was excavated from the highway, of which 63% was recycled.
The client estimated that £20,000 was saved in reduced lorry movements to landfill, as well as 47,000kg of CO2.
The new machine has been designed to be very light, compact and mobile, and Allu says it can ready to work within 30 minutes of delivery to site, allowing utilities contractors to set up in a local depot or in a field or yard alongside a run of trench excavation.
The firm anticipates considerable interest from contractors working in urban areas, where transporting arisings to landfill is expensive and time consuming. It also believes the machine will help local authorities meet their targets of reducing the volume of waste that goes to landfill.