THE PROBLEM
The improvement work to the highway involved the remodelling of a side road junction.
The new layout required that the side road should pass over Melsonby Tip, the site of a former quarry that had been filled with domestic wastes. This meant that, in the alternative solution, not only did the waste material need capping but also there was a concern about differential settlement – particularly at the edge of the quarry wall where the road transitioned from a firm to a compressible subgrade.
THE SOLUTION
A reinforced granular mattress, comprising multi-layer geogrids and good quality aggregate was installed over the waste. Then a transition detail was devised for the quarry edge. The opportunity was taken to introduce a Tensar TriAxTM geogrid and take advantage of the extra benefits the geogrid brings to stabilisation applications.
PROJECT DESCRIPTION
In this Design & Build contract, the contractor and his designer conducted value-engineering workshops in order to meet the incentives and address the target costs that are part of the innovations that the Highways Agency has introduced into many of its contracts. One subject was how to engineer the requirement for a road to pass over the Melsonby waste disposal site. Traditionally, removing the waste and replacing with engineering fill would have sufficed. Environmental legislation and taxes now mean that this approach is no longer viable and the reinforced granular mattress solution introduced Tensar TriAx geogrids into one of the first projects for new form of geogrid.
The benefits that Tensar TriAx geogrids brought to this project, compared with traditional Tensar geogrids, are derived from its radial stiffness and improved interlock with the granular fill. This means that the performance of a TriAx geogrid is available in any direction of strain. For this project, it means:
- The potential for differential settlement emanating from the boundary of the quarry could be better controlled regardless of the direction of strain.
- The in–service traffic loading will be quite random around the curvature of the junction and the geogrid will be lined-up effectively to respond to the loading however it arrives into the lower layers of the pavement.
For one year after construction, the side road was monitored and the Engineer’s reports to the client indicate that the road is performing satisfactorily. Some three years after installation there is no visual evidence of differential settlement or distress in the asphalt surface.
Apart from the financial gain, there was also an environmental gain of a reduced carbon footprint. Compared with removing the waste and backfilling with an engineering fill, the financial gain is self-evident. This solution also reduces the carbon emissions and an estimate has been produced using the Tensar Carbon Calculator. This solution reduced carbon emissions by a emphatic 90% - which means a staggering 147kg of greenhouse-effect gases were saved, on site, for each square metre of this alternative installation.
