Denison Street rail upgrade still going strong, two decades on
In 1996, the rail line which travels along Denison Street in Rockhampton, Queensland was completely reconstructed including the laying of new precast concrete sleepers to replace the old timber sleepers, laying of new rail tracks and repairing the remaining road pavement and resurfacing.
Although the basic concept was fairly simple, problems were anticipated.
• Major services cross Denison Street at most intersections – gas, stormwater, sewerage, water and telephone – at less than adequate depths.
• The underlying subgrade material consists of alluvial silts with a CBR of 1.5.
Queensland Rail’s 20 year design criteria for the required loading would require a capping layer depth of 800mm. At this depth major problems with services would be encountered. This meant the capping layer depth needed to be reduced by 300-400mm to alleviate costly relocation of services.
Geofabrics was approached for a solution and after consultation with Tensar® International, a design suggestion utilising Tensar® Geogrid was offered and subsequently accepted. The design adopted reduced the capping layer pavement by a sufficient depth so as not to incur the costly service relocation.
The design called for:
1. bidim® A34 geotextile (to separate the insitu subgrade).
2. Tensar® SS20/SS30 geogrid (to reinforce the capping layer).
3. Megaflo® 300mm (flat pipe drainage system).
The construction included the removal of the existing rail and timber sleepers, and excavation of old ballast and sub-ballast to the required depths and widths. bidim® A34 geotextile was placed on the floor of the excavation prior to placement of the Megaflo® 300mm flat pipe drainage system along each side of the excavation. All service and communication conduits were protected by a sand sleeve. Tensar® geogrid was then placed directly upon the geotextile prior to the placement of the CBR pavement capping layer which was compacted to a finished depth of 400mm. The ballast, new PSC sleepers and rail track were reinstalled before the adjoining road surface was repaired with a new asphalt surface.
A number of site inspections with the relevant rail designers over the years since construction has indicated that this section of rail line is still performing well to this day.