Influence of Loading History on the Creep Behavior of Rockfill under Confined Oedometric Conditions

Abstract

Creep, once considered an inherent characteristic of granular materials, is primarily governed by time and the current stress state. However, recent studies indicate that creep development is also influenced by the loading history. To better reveal the creep revolution law of the rockfill under the influence of loading history such as historical stress rates, creep tests were conducted under oedometric loading. Alternative loading–creep steps, different stress increment sizes, and various precreep stress rates were considered. Independent of other factors, the development of the creep rate was governed by the recent precreep stress rate (the prior stress rate defined in this study). When the prior stress rate was higher than a threshold value, the relationship between the creep rate and time was double logarithmic linear; thus the creep strain–time relationship tended to converge on a power law (referred to as the creep baseline herein). However, when the prior stress rate was lower than the threshold value, the initial creep rate was lower than that of the creep baseline and did not decrease until several minutes after the start of the creep. The development of the creep rate with time in the initial stage can be generalized as a straight horizontal line, suggesting that the rate remains almost unchanged for a certain time, until the straight horizontal line approached the creep baseline. The inheritance and hysteresis of different strain rates in the initial stage of subsequent creep resulted in differences in the creep magnitude and time development process of the creep rate. The above findings are constructive for predicting the deformation of deep layers of rockfill, such as embankments, with more accuracy, especially for that with some large-sized rigid-structure buildings on its surface. Creep of granular materials such as rockfills has a significant impact on the safety of upper and inner thin-walled structures, most notably the effect of rockfill deformation on the curvature and the stress–strain behavior of upper concrete slab in concrete-faced rockfill dams and the effect of fill deformation on the leveling of upper runways in high-fill airports. Conventional analytical models consider creep as an inherent property independent of the loading history. The paper answers the question of whether there is a relationship between creep and the prior loading process through a unique series of tests. It is clarified that there is a positive correlation between the prior loading rate and creep. A low prior loading rate may inhibit the initial rate of creep and reduce the amount of deformation accumulated in the early stage of creep. A mathematical model for creep analysis is proposed that can be used for deformation prediction in engineering to improve the prediction accuracy.