Permanent Deformation Behaviour Of Two Victorian Subgrade Soils Under Repeated Loading

Mechanistic design methods of pavement structures require the knowledge of permanent deformation behaviour of subgrade soils under anticipated repeated loading. Limiting subgrade deformation up to an acceptable level is one of the main design philosophies of mechanistic pavement design approaches. In the Austroads pavement design guide, a general subgrade failure criterion is given, which was originally derived from CBR pavement design chart. However, for a proper design, one should know the permanent deformation potential of the subgrade that is being dealt with. In this respect, development of predictive equations through dynamic testing gains importance. This paper presents some of the findings of a continuing research project on permanent deformation and resilient characteristics of subgrade soils. Repeated load triaxial tests were performed for two Victorian fine grained soils, in which the level of loading was defined as percentages of the soil static strength. The permanent deformation test results based on various stress- strength ratios were then analysed by using power model. Correlations between model constants and other parameters such as stresses and soil physical properties were investigated. Stress-strength ratio was found to be highly correlated to the logarithm of permanent strain occurring at the first load application. This was because the inclusion of strength parameter into the model which may also indirectly represent the effect of soil physical properties. Model constants of predictive equations were established by means of regression analysis. Predicted permanent strains were compared with measured strains and Austroads subgrade failure criterion.