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Flexible Pavement is usually composed of several asphalt concrete layers, a granular base course and a soil subgrade. For mechanistic design of pavement systems based on elastic theory a modulus of elasticity must be designated for each design layer including the soil subgrade. The resilient modulus is used to characterize the soil in pavement design. The resilient modulus is defined as [equation] Where [symbol] is the deviator stress or the difference between the axial and confining stress, and [symbol] is the recoverable axial strain. The standard procedure for obtaining [symbol] is a repeated load tri-axial test at a constant confining pressure. There is not a singular resilient modulus value for a particular soil but rather the modulus is a function of the stress state. The standard test produces a range of resilient modulus values in a series of stress conditions. The resilient modulus test is inherently complicated, time consuming, and expensive. For these reasons, most commercial and design laboratories will not conduct these tests but instead rely on empirical relationships. Therefore, it has been recommended that alternative tests be developed to approximate resilient modulus. The Alternative Test Method was developed to be a simple and effective way of determining resilient modulus. The ATM design was based on a single degree of freedom, lumped mass spring system in which a hammer of known mass falls onto a volume of soil. Originally, there appeared to be good correlation between the Alternative Test Method and the standard test method for obtaining resilient modulus. However, subsequent testing failed to produce consistent results or confirm the correlation. Improvements have been made to the ATM to improve the overall consistency of results and correlation with the standard resilient modulus test results. The improvements to the ATM device include a new, more consistent drop mechanism, better data acquisition software, and a new calculation method. ATM tests were conducted on 4 different soils from TDOT research sites. Standard tri-axial resilient modulus tests were conducted for comparison with ATM resilient modulus tests. The improved ATM appears to measure a material response that correlates reasonably well with the standard triaxial resilient modulus test results for those soils tested. Furthermore, the Improved ATM produces much more consistent results than the Original ATM. It is also believed that the limitations of the device are outweighed by its simplicity and commonality with other lab tests. Therefore, the improved Alternative Test Method for resilient modulus of fine grained soils is believed to be a viable alternative to the standard test method for obtaining resilient modulus values.
The mechanistic design of flexible pavement systems requires the specification of mechanical properties for asphaltic concrete, base course, and soil subgrade. The stiffness of the soil subgrade is represented by the resilient modulus, Mr, which is determined in the laboratory from cyclic triaxial tests and is usually a function of the level of applied stress. For a typical pavement project where numerous soils are encountered, cyclic triaxial testing to determine Mr for each subgrade soil may be too complex and time consuming to be applicable on a production basis. Therefore, alternative methods for the determination or estimation of Mr are appropriate and are in fact suggested in the 1993 AASHTO Guide for Design of Pavement Structures (AASHTO Guide 1993).
The objectives of this research are to establish a resilient modulus test results database and to develop correlations for estimating the resilient modulus of Wisconsin fine-grained soils from basic soil properties. A laboratory testing program was conducted on representative Wisconsin fine-grained soils to evaluate their physical and compaction properties. The resilient modulus of the investigated soils was determined from the repeated load triaxial (RLT) test following the AASHTO T307 procedure. The laboratory testing program produced a high-quality and consistent test results database.
"Resilient modulus indicates the stiffness of a soil under controlled confinement conditions and repeated loading. The test is intended to simulate the stress conditions that occur in the base and subgrade of a pavement system. Resilient modulus has been adopted by the U.S. federal highway administration as the primary performance parameter for pavement design. We thank those who prepared these papers, the reviewers who provided anonymous peer reviews, and those who participated in the symposium. We hope this STP encourages more work to improve the testing standard and the value of the Resilient Modulus test."
The objective of this study was to investigate the applicability of the cone penetration test to determine the resilient modulus of coarse-grained materials. Field and laboratory investigations were conducted at the sites of two pavement projects in Louisiana. Field tests consisted of continuous intrusion miniature cone penetration tests and soil sampling. Laboratory tests included the repeated load triaxial test and other tests for materials characterization. The test results were used to develop a correlation for predicting the resilient modulus of coarse-grained materials using the cone penetration test parameters and basic soil properties. Another laboratory investigation was conducted to investigate the effect of moisture content and unit weight on the cone penetration test parameters and resilient modulus. Test results were used to validate the model developed for predicting the resilient modulus. The resilient modulus values predicted were consistent with those obtained using the repeated load triaxial test.