Long-term deformation test and model study of one-dimensional consolidation of South China Sea coral mud under graded loading and unloading

South China Sea coral mud is a calcareous ooze of marine rock and soil formed by the accumulation of bones and debris after the death of coral groups, which has special engineering properties. It is significant to investigate its time dependent long-term nonlinear deformation under load for the construction and long-term stability analysis of dredger fill islands and reefs. Three groups of one-dimensional consolidation compression tests of coral mud with different loading and unloading schemes were carried out, and the influence of stress history on its long-term deformation properties was explored by changing the loading time and loading-unloading loops. According to the three-stage law of coral mud’s instantaneous deformation, delayed attenuation deformation and delayed stable deformation under loading and unloading test conditions, the improved Burgers model was used to fit ε-t curves under different vertical stresses, which has high fitting accuracy. Meanwhile, after analyzing the model parameters, it was found that the instantaneous strain increment and its growth rate decrease with the increase of graded loading time, the duration of delayed attenuation under the final load decreases, and the delayed stable strain rate and the total strain increment under the final load decrease. In the same loading-unloading loops, the instantaneous strain increment in the unloading stage is smaller than that in the loading stage under the same vertical stresses. With the increase of loading-unloading loops, the instantaneous strain increment in each stage of loading and unloading decreases and is close to each other, the duration of delayed attenuation deformation, the delayed stable strain rate and total strain increment under the final load decrease. The effects of graded loading time and loading and unloading cycles on the long-term deformation properties of coral mud were studied to provide theoretical basis for surcharge preloading scheme in island reef construction.