Rochdale's gyratory system is being replaced by a traffic light controlled crossroads and widened approaches.
Traffic light controlled pedestrian crossings will also replace old graffiti covered subways beneath the roads, says Rochdale Borough Council project team leader John Ashley.
Rochdale, together with consultant Mouchel makes up the Impact Partnership, which implements local traffic schemes. Bringing pedestrian crossings up to the surface then left a small problem of what to do with the uninviting tunnels afterwards.
Bricking up the empty subways up would have left potentially dangerous spaces, and filling them in with concrete would have been time consuming and disruptive in the city centre, not least because of the number of readymix trucks which would have had to visit the site. Using granular fill would have been awkward too.
Instead, the Impact Partnership has gone for a relatively new foam infill product supplied by specialist firm Uretek.
Originally a Dutch product called Benefil, it is an amino-plastic resin which expands to 12 times its normal volume when mixed. It then hardens rapidly before solidifying over about two hours.
The material arrives on site in liquid form and is pumped into the subways through tubes, filling more or less any shaped space required. Uretek has kitted out a container lorry with tanks and a compressed air unit for pushing it through the pipes. The lorry carries enough to fill around 100m3 of void says Steve Woolley, one of the subcontractor's technical design advisors.
"We deliver two liquid components, a hardener and the base material, along with compressed air, to a mixing gun by flexible umbilical tubes," says Woolley.
The mixing gun produces a foam whose density can be changed by varying the compressed air content using valves incorporated in the gun.
In the case of the Rochdale subways, the answer has been to brick up the entrances, leaving a contained space that the material can easily flow into and fill. Two were successfully filled last September and a third, beneath a two lane dual carriageway, this March.
"A major advantage of the foam is that it can be placed from a distance rather than backing up concrete trucks," says Woolley. At Rochdale the subcontractor was able to park its container in a nearby carpark. The tubes are about 50m long at full stretch.
Another 50m or so of delivery tube from the gun can be used and Uretek says it is possible to reach areas up to 150m from the lorry. In Rochdale, material was pushed through sacrificial placing pipes. Uretek uses 63mm medium density polyethylene pipes through which the foam is delivered to the back of the void, filling it in about three hours.
A faint smell of formaldehyde is apparent, given off by the foam's reaction but Uretek says the chemical is very low concentration and disperses fairly quickly.
Compared to the need to have someone working inside the space as concrete is poured or granular fill moved , the foam filler is much safer.
The foam is very light at 30kg/m3 in its least dense grade, and has enough strength to provide structural support. Depending on the exact mix strength, that is around 0.14N/mm2 to 0.33N/mm2, reaching 95% of that in two days and full strength in 28 days.
Uretek was in and out of the Rochdale job visited by NCE in March within a long morning. It says most jobs can be done as quickly. For Rochdale it used a "double gun" producing around 30m3 of foam each hour but the company also has triple guns, used on jobs like the infilling of basements.