Experimental investigation into the shakedown state of unsaturated road base aggregate at various fine contents and matric suctions

Abstract

The road base aggregate usually reaches the shakedown state under large number of cycles of traffic loads. But this shakedown state is expected to depend on the inclusion of fines and the matric suction. In this study, monotonic and cyclic triaxial tests were performed under controlled matric suctions on the road base aggregate with different fine contents (FCs) and matric suctions. Results show that the plastic creep limit of unsaturated road base aggregate is affected by the two-side effects of fines: reduction of friction angle and enhancement of matric suction. For the coarse-grain controlled soil (FC range from 0% to 4%) and fine-grain controlled soil (FC range from 8% to 16%), the plastic creep limit decreases with increasing FC due to the friction angle reduction caused by the inclusion of fines. For the transition soil (FC range from 4% to 8%) under unsaturated state, the plastic creep limit increases with the increase of FC as the dominant effect of fines shift to the enhancement of matric suction. By adopting an allowable cyclic stress ratio (CSR), a novel method is proposed, allowing prediction of the plastic creep limit based on the results from standard monotonic triaxial test for coarse-grain controlled soil and fine-grain controlled soil, and the results from post-cycle monotonic triaxial test for the transition soil. This method can be applied to road base aggregate with different FCs and matric suctions.