Investigation Into the Use of Geosynthetics for Subgrade Stabilisation

Abstract

The aim of this project is to investigate the stabilisation effects of geosynthetics on weak subgrade common in Queensland. Subgrade stabilisation is a necessary part of pavement design to counteract the negative effects of weak subgrade. There are many methods of stabilising subgrade, but there are high carbon and financial costs associated with many of these methods. Geosynthetics are one method of stabilisation that is gaining popularity in Australia as engineers look for alternative subgrade stabilisation techniques. It was identified that the guidelines from Queensland’s Department of Transport & Main Roads and Austroads on subgrade stabilisation are based on research studies overseas and are not tailored to Australian soils. In this study, research on the effect of geosynthetics as subgrade stabilisation was performed by comparing the strength of a weak subgrade with and without geosynthetic reinforcement. The subgrade examined in this study was sourced from Mackay, Queensland. The soil was classified under the Universal Soil Classification System as Sandy Silt. Soaked CBR and UCS tests were performed, both with five different arrangements of geosynthetic reinforcement. The groups were a control group with just the soil, a group with combigrid, a group with combigrid and geotextile, a group with geogrid, and a group with geogrid and geotextile. The soil was determined to be unsuitable to perform UCS tests without a chemical binder. In the CBR tests, geogrid was shown to out-perform combigrid in the Sandy Silt, as was in line with expectations from the literature that was examined. Combigrid was shown to increase the CBR of the soil by +2% and geogrid was shown to increase the CBR of the soil by +5%. Both samples with the addition of geotextile performed better than their counterparts without, with the geotextile adding an average +2% CBR in comparison to only one piece of geosynthetic. It is recommended that Engineers use geogrid to stabilise Sandy Silt subgrades in Queensland. It is recommended that more tests are performed for different types of soil found in Queensland and that the geotextile has its stabilisation capability individually tested by the soaked CBR method, as the strength that it added was not insignificant.