During the construction of the A500 Hough to Shavington bypass in Cheshire, UK, contractor Mowlem Major Projects encountered difficulty along the route of the new carriageway following the discovery of layers of soft formation soils. Tensar International provided a cost-effective and environmentally-friendly solution using a geogrid / geotextile composite to reinforce the roadway while avoiding the alternative, costly excavation solution.
In the scheme to improve highway links around Crewe in Cheshire, the majority of the construction was done on sound formation soils, but one area proved a significant exception. A detailed inspection showed that high water content had saturated the silts and clays, and disposal of the material would prove difficult and expensive for the contractor.
Initial recommendations were to remove 200mm of the soft soil and construct a minimum 600mm capping layer to support the pavement construction, which included a 150mm Type 1 sub-base layer. The Tensar solution used an SS geogrid / geotextile composite placed on existing ground at sub-formation level beneath a reduced capping layer of 400mm. The client, the Highways Agency and their consultants, based on the extensive experience of independently-monitored field trials and testing over many years with Tensar geogrids over poor formations, accepted this solution which was proposed by the contractor. The Tensar geocomposite was used to reinforce the reduced 400mm thickness of the 6F1 capping layer thereby reducing excavation and removal costs and minimising the amount of imported granular fill required.
Subsequent in-situ CBR tests on top of the reduced-thickness, reinforced capping layer achieved an average CBR value of 21% and comfortably satisfied the minimum target value of 15%.
A design CBR of 1% was adopted for the inspected sub-grade. The wet nature of the formation soils identified during the site visit made by Tensar, together with the grading of the capping material suggested the use of a Tensar SS30 geogrid / geotextile, placed grid side up, would be most appropriate. The geogrid component of the composite would mechanically interlock with the granular fill particles placed above. The geotextile component, a non-woven needle-punched fabric, bonded to the geogrid, would provide additional separation to prevent sub-grade fines from pumping up into the capping layer.