Model testing of geogrids in unpaved roads

Simple unpaved roads consist of a layer of coarse granular material placed directly onto the surface of weak or compressible ground. It is thought that the construction of such roads can be considerably improved by the incorporation of a geogrid at the base of the granular fill layer. Geogrids are a type of geotextile, distinguished by their relatively large aperture size. Laying out a geogrid on the surface of the ground before placing the fill layer may in many cases allow a reduced thickness of fill material to be used, and may also substantially increase the load required to cause a complete failure of the system. No generally accepted design method exists for the construction of reinforced unpaved roads, due to the complex mechanisms which govern deformations in the system. The primary aim of this dissertation was to investigate the performance, in such a construction, of a particular geogrid, namely Tensar, manufactured by Netlon Ltd. A detailed model study into failure mechanisms was undertaken using laboratory apparatus constructed to conduct work at 1/4 full scale. Simple plane-strain, monotonic footing tests were carried out on systems consisting of a fill layer compacted onto a consolidated clay subgrade, both with and without the incorporation of a model grid at their interface. The testing technique included a comprehensive study of photographs taken of marker movements in the clay through the transparent sides of the test-box during tests. The relevant failure mechanisms associated with reinforced and unreinforced systems were established. In addition the significance of shear stresses acting at the subgrade surface was recognised and a concept whereby the appropriate subgrade bearing capacity factor is related to these shear stresses was developed. The modelling techniques adopted in this work obviated the need for a centrifuge