The Effect of Cyclic Load Characteristics on Unbound Granular Materials

Abstract

Vertical load characteristics used in laboratory repeated load triaxial tests (RLTT) have a significant impact on evaluating unbound granular materials (UGMs) for the flexible road pavements. Many studies and standard testing protocols suggest a diverse range of these characteristics (i.e. stress magnitude, pulse shape type, loading period and rest period). Several studies have been conducted to identify the factors that affect the permanent deformation (PD) and resilient modulus (Mr) of UGMs. However, the effect of the pulse shape types has not yet been investigated. The aim of this study is to experimentally investigate the effect of vertical stress pulse shape type on the PD and Mr behaviour of the UGMs using RLTT. For further assessment, a parametric analysis was also conducted using eight existing PD and Mr constitutive models. Three typical vertical stress pulse shape types were investigated, namely trapezoidal, haversine and triangular, as suggested by several international testing protocols. The results show that tested UGMs, using trapezoidal stress pulse, produced higher PD and lower Mr than haversine and triangular pulses under controlled experimental conditions. As the loading span of the pulse increased, the amount of PD also increased, and Mr decreased. Some of the regression parameters of the investigated constitutive models for both PD and Mr showed correlations with the type of applied stress pulses. Moreover, it was found that the PD and Mr models were material dependent, as a better statistical fit was achieved for the granite than the basalt samples. It is recommended to take extra precautions while adopting a particular type of vertical stress pulse shape as the results may vary widely.