Particle gradation effects on creep characteristics and the underlying mechanism in calcareous sand

Calcareous sand, a foundation material of marine origin, is an indispensable component in marine construction and has found extensive applications in building foundations and airport runways. The complex nature of the marine environment and the unique characteristics of calcareous sand present significant challenges for ensuring the stability and durability of engineering structures. To address these challenges, multistage loading triaxial creep tests were conducted on calcareous sands with different particle gradations to analyze their creep characteristics and underlying mechanisms. The result shows that the amount of ultimate axial creep strain is positively correlated with deviatoric stress but negatively correlated with the coefficient of uniformity (Cu). The creep process demonstrates a significant creep structure effect, characterized by creep rate lag and creep failure lag. The volumetric creep strain-time relationship at different creep stress ratios can be classified into three types. When the creep stress ratio is less than the stress ratio at the dilative point, the sample is first compacted followed by dilatancy; when the creep stress ratio is equal to the stress ratio at the dilative point, only compaction is obtained; as the creep stress ratio becomes larger than the stress ratio at the dilative point, pure dilatancy is found. The particle crushing modes and creep mechanisms of calcareous sand are also examined through meso-observations. It is found that the particle crushing modes can be categorized into three distinct types: grinding, angular breakage, and whole particle breakage. The particle gradation not only impacts the primary particle crushing mode but also governs the movement of particles and fragments, thereby influencing the occurrence of creep deformation. Specifically, a smaller Cu value corresponds to a higher fragmentation rate of the specimen, an increased number of overall crushing and angular fractures, a larger space for fragment migration after crushing, and eventually a greater creep deformation. Finally, a modified Burgers model is proposed based on the test results, which can effectively capture the creep deformation in the presence of creep structure effects and creep failure stages. •The ultimate creep strain is negatively correlated with coefficient of uniformity.•A significant creep structure effect is demonstrated by creep rate and failure lag.•The particle gradation impacts both crushing mode and movements of pa