In order to study the characteristics of pile–rock action of steel pipe driven pile in coral reef limestone stratum, coral reef limestone at the China–Maldives Friendship Bridge site was selected to carry out indoor physical and model tests with red sandstone as the control group. The test outcomes indicate the following: (1) when substantial deformation is permitted, the coral reef limestone has a considerable strength dispersion, a low post-peak stress decrease rate, and a high residual strength, roughly 30% of the peak strength; (2) when the steel pipe pile penetrates the coral reef limestone, the pile top load shows an obvious sawtooth shape, and with the increase in penetration depth, the pile end load of the high-porosity rock sample gradually decreases, and the pile end load of the low-porosity rock sample gradually increases; (3) when the steel pipe pile is penetrated, the strain value of the red sandstone is about twice that of the coral reef limestone at the same position from the steel pipe pile. These findings indicate that the high porosity and heterogeneity cementation characteristics of the coral reef limestone make the extrusion effect during piling significantly less than that of the red sandstone. In addition, the steel pipe pile penetration process is numerically simulated using a four-dimensional discrete spring model method based on the multi-body damage criterion. The numerical simulation results further demonstrate that the pile-side rock fragmentation during steel pipe pile penetration is the primary reason for the lower bearing capacity of steel pipe piles in coral reef limestone stratums. This method provides a novel approach for studying the mechanical properties of reef limestone. The findings can serve as a guide for the design and construction of steel pipe piles in the reef limestone stratum.
In this work, a surface image-based 3D grain-based model (GBM) reconstruction method based on the digital image processing (DIP) technique, periodic random packing and the simulated annealing algorithm is proposed. Taking a surface digital image of Beishan granite as an example, a K-means clustering algorithm is employed to extract the mineral compositions, the distribution of which is quantified by the two-point probability function (TPPF). Given the 3D volume fraction of each mineral composition, their corresponding 3D two-point probability functions can be evaluated from their 2D forms by the linear interpolation method. The 3D GBM reconstruction is generated by a simulated annealing algorithm, with the Monte Carlo algorithm extending the calculation of the two-point probability function as the target function to the random particle model. To improve the computational efficiency, the periodic boundary condition is applied to both the random particle generator and the evaluation of the two-point probability function, allowing large-scale GBM to be generated for the Distinct Lattice Spring Model (DLSM). The elastic response and advantages of the random particle model for the DLSM are investigated. Finally, the DLSM is further enriched with a tension-cutoff Mohr-Coulomb failure criterion, and the GBM reconstruction method is successfully tested for reproducing the mechanical behaviours of granite and interpreting the failure mechanism at the mesoscale under different loading conditions.
Three-dimensional (3D) geometric reconstruction is a fundamental requirement to realistically predict the mechanical behavior of cemented granular materials (CGMs). In this work, a four-phase geometric reconstruction method for CGMs, involving aggregate, cement, in-between interfacial transition zone (ITZ), and void phases, is developed by using digital image processing techniques and an optimization algorithm. The reconstruction method includes four steps. (1) The planar outlines of aggregates are extracted from a two-dimensional (2D) aggregate image (e.g., gravels, pebbles, and crushed stones) based on the image segmentation algorithm. (2) Through spatial geometric transformations on the acquired planar outlines, the 3D aggregates are generated, whose reliability is verified by comparison with those obtained by a 3D scanner. (3) According to the volume ratio of each phase, the initial four-phase model is reconstructed by randomly placing aggregates into the user-defined domain with considering the size distribution of aggregates, followed by the generation of the ITZ represented by their surrounding minimum envelope surface, and the insertion of voids in the rest cement phase. (4) The final realistic CGM is reconstructed by optimizing the spatial distribution of the void and cement phases based on a simulated annealing algorithm. Afterwards, the rationality of the reconstructed CGM is verified by comparing two-point probability functions between the physical model and the reconstructed model. With the aid of a four-dimensional lattice spring model (4D-LSM), the feasibility of the reconstructed CGM to reproduce its mechanical behavior is demonstrated by numerical examples and a comparison with existing experimental or other numerical solutions.
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