Flexible pavement is a complex structure built with the main purpose of carrying the traffic loading. A typical flexible pavement structure consists of several layers – surface course, base course, sub-base course and subgrade soil. The behaviour of the flexible pavement is complicated and is commonly characterised by the resilient modulus (Mr) of subgrade soils and granular materials in the pavement structure. Technically speaking, Mr is defined as the ratio of the deviator stress and the consequent recoverable strain, and can be determined from the repeated load triaxial test in the laboratory. Inherently, it is a challenge to perform repeated load triaxial tests as a routine basic test due to the complicated, time-consuming and expensive procedure; hence, several empirical approaches to estimate Mr from other soil mechanical properties – California Bearing Ratio, unconfined compressive strength or physical properties – moisture content, plasticity index or dry density have been proposed and recommended by several pavement design guides. With the same philosophy, the current research evaluated the resilient response of some different fine-grained subgrade soils, granular, base and sub-base materials and developed several simple, less time-consuming, and inexpensive methods and models to evaluate and determine Mr for the design of flexible pavements. Based on the experimental results and the findings from numerical investigation, several important developments were achieved, as follows: 1. Development of dynamic lightweight cone penetrometer for the evaluation of Mr of soils in the field as well as in the laboratory. 2. Development of a prediction model for Mr based on the dynamic lightweight cone penetration index and stress levels for fine-grained soils. 3. Development of the simplified testing method for the repeated load triaxial test to determine Mr for fine-grained soils. 4. Development of a simplified testing method for the repeated load triaxial test to determine Mr for granular materials. 5. Development of a three-dimensional numerical model for the study and prediction of the resilient behaviour of granular materials using discrete element method. 6. Development of a prediction model for the resilient behaviour of fine-grained soils based on the California Bearing Ratio values and stress levels from laboratory tests results. From the results and prediction models developed in this study, fifteen papers have been prepared and published or submitted in international journals. In addition, some of the results have been presented in five international conferences.
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