Particle breakage and uneven settlement characteristics of calcareous sand foundation

The distinct particle breakage characteristics of calcareous sand can induce extra settlement in calcareous sand foundations, posing a significant challenge to the safety of island and reef engineering. To explore the particle breakage, settlement characteristics and internal stress variations of calcareous sand foundations, the laboratory loading tests for calcareous sand foundations with different particle gradations were conducted. Particle Image Velocimetry (PIV) technology and tactile pressure sensor systems were also utilized. The study reveals that the tested calcareous sand foundations have differential settlements subjected to external loading, which has a strong relationship with the particle breakage. It is found that the nonuniform internal stresses between the sand particles can induce different degrees of the particle breakage, which in turn changes internal stresses and redistribution of particle positions in calcareous sands, and further causes the uneven settlement of the foundation. The degree of uneven settlement in calcareous sand foundations increases with an increase of external load and decreases with an increase of the coefficient of uniformity C u for calcareous sands. During creep, the vertical and lateral stresses on the inter-particle contacts within the calcareous sand foundation exhibit an overall trend of decrease in weak forces and increase in strong forces. This continuous increase in strong forces results in a growth of creep deformation in calcareous sand foundation, while the degree of differential settlement in the foundation decreases with the progression of creep. The distinct particle breakage characteristics of calcareous sand can induce extra settlement in calcareous sand foundations, posing a significant challenge to the safety of island and reef engineering. To explore the particle breakage, settlement characteristics and internal stress variations of calcareous sand foundations, the laboratory loading tests for calcareous sand foundations with different particle gradations were conducted. Particle Image Velocimetry (PIV) technology and tactile pressure sensor systems were also utilized. The study reveals that the tested calcareous sand foundations have differential settlements subjected to external loading as shown in Fig. a for the calcareous sand foundation with Cu = 2 as a representative. The uneven settlement coefficient δ is defined to quantify the degree of differential settlement. As shown in Fig. b, δ exhibits a strong correlation with both the vertical load and the coefficient of uniformity ( Cu ) of the specimen, that is, δ increases with increasing load and decreases with increasing Cu , and the closer the location is to the load center, the greater δ becomes. The uneven settlement also has a strong relationship with the particle breakage. It is found that the nonuniform internal stresses between the sand particles can induce different degrees of the particle breakage. As observed from Fig. 1b, there is a discrepancy in the particle breakage rate between the left and right sides of the calcareous sands at the same distance from the load center. Overall, the particle breakage rate on the left side is greater than that on the right side, indicating that particle breakage in the left region of the calcareous sand foundation is more pronounced compared to that in the right region. This increased particle breakage leads to an aggravation of the deformation, resulting in a greater settlement on the left side of the calcareous sand foundation compared to the right side (Fig. a), which can enhance the uneven settlement. With increasing time, a decreasing trend in δ was observed which may due to the rearrangement of particle positions and redistribution of internal stresses, that is, the differential settlement can relieve to a certain extent. As the vertical external loading increases, the internal vertical stress in the calcareous sand foundation consistently follows a Weibull distribution (Fig. c), whereas the lateral stress distribution gradually transitions from a Weibull distribution to a normal distribution (Fig. d). The coefficient of variation (COV) is utilized to quantitatively characterize the variation of internal stresses, which is defined as the ratio of the standard deviation to the mean value. The COV ranges from 0 to 1, and a smaller COV indicates a lower dispersion of stress data. It is evident that as the external vertical stress increases, the internal stress distribution undergoes change, resulting in an increase in both the mean stress value and the COV. During creep, the internal lateral and vertical stress distributions as changes with time. With increasing creep time, both vertical and the lateral stresses within the calcareous sand foundation generally show a trend of decreasing weak forces and increasing strong forces. This increase in strong forces leads to greater creep deformation of the foundation.Figure The uneven settlement of calcareous sand foundation and the related characteristics: (a) displacement nephogram and corresponding displacement vector diagram of calcareous sand foundation with Cu = 2; (b) relationship between settlement difference and vertical load, time and particle breakage; (c) histograms of internal vertical stress distributions in calcareous sand foundation with Cu = 2; and (d) histograms of internal lateral stress distributions in calcareous sand foundation with Cu = 2. • The calcareous sand foundations have differential settlements subjected to external loading. • The degree of uneven settlement is positively correlated with particle breakage and negatively correlated with coefficient of uniformity. • The nonuniform internal stresses induce different degrees of the particle breakage, and further causes the uneven settlement of the foundation. • The creep can decrease the uneven settlement of calcareous sand foundation.