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Stress-induced anisotropy in calcareous sand can cause an uneven displacement in island reef engineering. In this study, stiffness, as a quantitative indicator, is explored to reveal the stress-induced anisotropy in calcareous sand. Based on the discrete element method, the stiffness anisotropic characteristics of calcareous sand during shearing, as well as the impact of particle breakage, are investigated by numerical simulations. Both the macro and micro responses, i.e., the maximum shear modulus, contact normal, strong and weak contact normal force, and the direction of particle breakage, are explored for calcareous sand with different particle breakage ratios. The results show that calcareous sand exhibits notable anisotropy during shearing, with the maximum shear modulus in the vertical direction (deviatoric stress direction) being significantly greater than that in the horizontal direction. Moreover, the higher the particle breakage rate, the lower the stiffness and its anisotropy. The micro-mechanism results indicate that the primary particle breakage during the shearing process occurs in the vertical direction. That is, the particle breakage weakens the strong contact force in the vertical direction, leading to a redistribution of the strong contact forces from the vertical direction to other directions. This redistribution mainly manifests in a decrease in the anisotropy of contact normal and contact vector within the sample, as well as a decrease in the proportion of strong contact forces in the overall contacts. This, in turn, reduces the shear strength and stiffness of calcareous sand, particularly in the vertical direction, and results in a decrease in the maximum shear modulus and its anisotropy. The maximum reduction can be up to 50% of the original value. These insights can provide a certain theoretical support for the uneven displacement and long-term stability of calcareous sand for islands and reefs. [ABSTRACT FROM AUTHOR]

The mechanical response and deformation characteristics in calcareous sand foundations during pile driving and setup were studied using model tests combined with the technical methods of tactile pressure sensors and close-range photogrammetry. Different types of piles were considered, including a pipe pile, square pile and semi-closed steel pipe pile. The test results show that during pile driving, the pile tip resistance of different piles increases with an increase in the pile insertion depth, and an obvious fluctuation is also obtained due to the particle breakage of the calcareous sand and energy dissipation. Different degrees of particle breakage generated by different type piles make the internal stress variations different, as with the pile tip resistance. The pile tip resistance of model pile A, which simulates a pipe pile, is the highest, followed by model pile B, the simulated square pile. Model pile C, which simulates a semi-closed steel pipe pile, has the smallest pile tip resistance because its particle breakage is the most obvious and the pile tip energy cannot be continuously accumulated. The induced deformation such as sag or uplift on the surface and the associated influence range for the calcareous sand foundation are the smallest for model pile C, followed by model pile B and then model pile A. Model pile A has the most obvious pile driving effect. During the pile setup process after piling, the increase in the total internal stress of model pile B is the largest, and the improvement of the potential bearing capacity is the most obvious, followed by model pile A and model pile C. During the pile setup, the induced uplift deformation in pile driving is recovered and the potential bearing capacity increases due the redistribution and uniformity of the vertical and radial stress distributions in the calcareous sand foundation. Considering the potential bearing capacity of different model piles, the influence range of pile driving, foundation deformation and the pile setup effect, it is suggested to use a pointed square pile corresponding to model pile B in pile engineering in calcareous sand foundations. [ABSTRACT FROM AUTHOR]

• A deep multimodal fusion (DMF) model is developed for sand gravel moisture content prediction. • The DMF model effectively extracts and fuses information from image, spectral, and dielectric data. • The DMF model shows superior performance on sand gravel multimodal datasets. • The DMF model maintains good robustness when the dataset was disturbed by general noise. A fast and accurate moisture content (MC) measurement of sand gravel is essential for hydraulic engineering project sites. Most existing measurement methods are unimodal, facing non-robust against external interference. To address this issue, a deep multimodal fusion (DMF) model for measuring the MC of sand gravel using images, near-infrared (NIR) spectra, and dielectric data, is proposed. A modified bottleneck transformer network (BoTNet) added with an extremely efficient spatial pyramid (EESP) block is first proposed to extract image features from different receptive fields. The improved convolutional neural network with attention blocks added (A-CNN) and gated recurrent unit with attention blocks added (A-GRU) networks are then adopted to extract local and sequential features from NIR spectra, respectively. The square root of dielectric data and above multimodal features are effectively fused according to their contribution to the target indicator in the Fusion module. Among other comparative models, the DMF model yielded the best performance (R2 = 0.962, RMSE = 0.645, RPD = 5.124) on the original sand gravel dataset, and still maintained the best accuracy (the average R2 and RPD mostly exceeded 0.85 and 2.5, respectively) when against general external noise. [ABSTRACT FROM AUTHOR]

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 particles. • A modified Burgers model captured creep structure effect and failure is proposed. [ABSTRACT FROM AUTHOR]

In the study, the creep characteristics and meso-mechanism of calcareous sand under different stresses are explored through triaxial creep tests. The influence of particle size on calcareous sand creep is also considered. Furthermore, the particularity of calcareous sand creep is revealed by comparing with the creep behavior of silica sand. The results show that during creep, the calcareous sand is more stable than silica sand due to the interlocking of irregular particles. When creep deviatoric stress is small, the creep of calcareous sand is manifested as nonlinear attenuation creep (primary creep), and the creep deformation is smooth without creep failure. With the increase of creep deviatoric stress, the creep deformation is concentrated, resulting in a strain concentration zone similar to the shear band, and the associated creep rate first decreases and then suddenly increases, finally, creep failure occurs. With increasing particle size of calcareous sand, the particle crushing increases, which induces a higher creep deformation, however, the overall creep failure is not generated. During the graded creep of calcareous sand, an obvious creep structural effect is observed, which is temporary and results in a significantly reduction of creep rate. Only after the previous creep structure effect is destroyed by the continuously creep process, the creep response can restore to its original behavior. The meso-behavior is represented by particle morphology, i.e., the fractal dimension and aspect ratio. It is found that both confining pressure and particle size can influence the evolution of particle morphology during creep. With the increase of confining pressure, the fractal dimension of calcareous sand decreases which means that particle grinding is always existent, while the aspect ratio of small-sized calcareous sand after creep decreases first and then stabilizes (slightly increases), suggesting that the overall particle breakage is the main cause of deformation. However, the aspect ratio of large-sized calcareous sand continually decreases, indicating that the main contribution for its deformation is angular cracking. The meso scopic behavior of silica sand is basically unaffected by creep, and its creep deformation is mainly caused by particle rearrangement. Fig. 1 Creep rate and related evolution of particle morphology for calcareous sand under different creep stresses: (a) Creep rate-time relationship; (b) Photos of samples before and after creep; (c) Variation of fractal dimension D p ; and (d) Variation of fractal dimension of mean aspect ratio. [Display omitted] • Performing triaxial creep tests to explore the creep behavior of calcareous sand. • The creep behavior is influenced by confining pressure, creep deviatoric stress, particle size and particle crushing. • The particle shape before and after creep is characterized by fractal dimension and aspect ratio. • Only sand that can form strain concentration band by creep process can lead to creep failure. [ABSTRACT FROM AUTHOR]