An Improved Alternative Test Method for Resilient Modulus of Fine Grained Soils

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

Where s_d is the deviator stress or the difference between the axial and confining stress, and e_R is the recoverable axial strain.

The standard procedure for obtaining M_R 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.