Strength enhancement of clay by encapsulating geogrids in thin layers of sand

Large size direct shear tests (i.e.300 × 300 × 200 mm) were conducted to investigate the possibility of strength enhancement of clays reinforced with geogrids embedded in thin layers of sand. In this paper test results for the clay, sand, clay–sand, clay–geogrid, sand–geogrid and clay–sand–geogrid samples are presented and discussed. Thin sand layers with thicknesses of 4, 6, 8, 10, 12 and 14 mm were used to quantify their effect on the interaction between the clay and the geogrids. In this regard effects of sand layer thickness, normal pressure (i.e. confinement) and transversal members of geogrids were investigated. All the tests were conducted using saturated clay with no drainage allowed. Test results indicate that provision of thin layers of sand for encapsulating the geogrids is very effective in improving the strength and deformation characteristics of saturated clay. Maximum strength enhancement was derived at an optimum sand layer thickness of 10 mm which proved to be independent of the magnitude of the normal pressure used. For a particular sand layer thickness, increasing the normal pressure resulted in enhanced strength improvement. Results also showed that removal of the geogrid transversal members resulted in reducing the strength of the reinforced samples by 10% compared to geogrids with transversal members. Encapsulating geogrids in thin layers of sand not only will improve the performance of clays if used as backfill it would also provide drainage paths preventing pore water pressure generation on saturation of the backfill.