Micro-mechanical analysis of caisson foundation in sand using DEM: Particle shape effect

Abstract

The caisson foundation has been extensively studied with experimental, analytical and numerical methods. However, the effect of particle shape on the caisson foundation is somehow ignored. In this paper, the caisson foundation is analyzed with DEM and the effect of particle shape is discussed in detail by introducing spherical and tetrahedral particles without rolling resistance, and spherical particle with rolling resistance. Firstly, a series of biaxial tests are simulated on samples with different particle shapes to obtain the soil properties, and typical shear behaviors are observed in all simulations. Then the effect of particle shape on the macro- and microscopic behaviors of caisson foundation is then investigated with different loading conditions. The results show that when subjected to external loading, compared with spherical particles, caisson foundation with angular particles or spherical particles with rolling resistance (1) has increased horizontal and rotational bearing capacities; (2) has much smaller particle rotation due to the strong interlocking capacity between particles; (3) exhibits higher stress concentration and magnitude of force chains; and (4) experiences significant increase on the magnitude of the anisotropy.

Introduction

Caisson foundation has been extensively used for offshore structures such as bridge piers and wind turbines. Caisson foundation is an upside down steel or concrete tube embedded in the seabed. During the installation process, a caisson is usually floated to the job site and sunk into place. And the caisson is push to the final position by the suction force induced by pumping the water out of the caisson. Caisson foundation is often subjected to the loading condition which can be characterized by large overturning moment and small vertical and horizontal loads. Compared with other types of foundations used for offshore structures, it is more cost-effective and environment-friendly.

Caisson foundation has been extensively studied with experimental, analytical and numerical methods. For the experimental aspect, the moment loading capacity, vertical loading response, penetration velocity, seismic behavior, and soil-structure interaction of caisson foundation subjected to monotonic or cyclic loadings were investigated in detail using small-scale model test, centrifuge test or full-scale test (Barari and Ibsen, 2012; Byrne and Houlsby, 2002; Chang et al., 2014; Cox et al., 2014; Jia et al., 2018; Kelly et al., 2006; Kou et al., 2019; Zhu et al., 2011; Zhu et al., 2019). In addition, analytical methods, including strain-hardening plasticity model (Cassidy et al., 2006; Villalobos et al., 2009), macro-element method (Jin et al., 2019c; Skau et al., 2018), upper bond plasticity method (Yun and Bransby, 2007) and hyperplasticity theory (Nguyen-Sy and Houlsby, 2005) were used to study the bearing capacity and failure mechanism of caisson foundation. In terms of the numerical analysis, finite element method (FEM) and finite difference method (FDM) were used to investigate the behaviors of caisson foundation at various working conditions, to validate the experimental results and to reproduce the failure mechanisms provided by analytical methods (Achmus et al., 2013; Bagheri et al., 2017; Jin et al., 2019a; Jin et al., 2019b; Mehravar et al., 2016; Mehravar et al., 2017). Recently, discrete element method (DEM) was used to study the behavior of a caisson foundation during its installation, operation and failure, in which the micro-mechanical analysis of the behaviors of caisson foundation was conducted (Wang and Yin, 2020). The advantage of using DEM to analyze the caisson foundation lies in the fact that it can physically capture the large deformation of soil particles, and thus provide us not only the bearing capacity but also failure mechanisms of caisson foundation. Therefore, in this follow-up study, the effect of particle shape, which is one of the most important characteristics of soil particles, on the micro-mechanical behavior of caisson foundation is investigated.

The seabed soils are usually non-homogeneous and subjected to natural spatial variation in terms of soil behavior and particle micro-structure (Peng et al., 2017; Sui et al., 2019). Therefore, geotechnical survey and soil classification are often conducted before the construction of offshore structures (Thusyanthan, 2012). Previous research showed that the bearing capacity of a caisson foundation is closely related to the soil condition and mechanical properties (Bagheri et al., 2017; Kelly et al., 2006; Yun and Bransby, 2007). In addition, as proved by many studies, particle shape has a strong influence on soil properties such as strength and frictional angle. For example, the critical state friction angle of sand was reported to decrease markedly with an increase of rounded fine contents in sand (Yang and Wei, 2012). Similar results were also found in (Podczeck and Miah, 1996; Yang and Luo, 2015), which was attributable to the increased interlocking effect among particles (Shinohara et al., 2000). Particle shape also influenced the distribution of stress among particles by propagating force chains more efficiently in rough particle than frictionless systems (Guises et al., 2009). In addition, soil compressibility and dilation are also closely related to particle shape (Afzali-Nejad et al., 2017; Cavarretta et al., 2010; Wang et al., 2019a). The effect of particle shape on soil behavior has also been extensively investigated and validated by using the DEM, in which the angularity can be approached by bonding a number of spheres, applying polygon-shaped or egg-shaped particles, and imposing rolling resistance torque to particles (Cui et al., 2020; Gong et al., 2019; Zhou et al., 2018a). In the first two methods, particles have shapes that are physically similar to real soil particles, while the rolling resistance method provides a numerical way to simulate angular particles. In the case of caisson foundation, the increase of soil particle angularity is expected to increase its bearing capacity due to the enhanced interlocking between particles and the improved engineering properties of soils. In addition, the particle shape also has an influence on the size of failure zone during the failure of a caisson foundation. Thus, it is important to account for the effect of particle shape in the analysis of caisson foundation, which, however, has been unfortunately ignored in previous studies.

In this paper, caisson foundation is analyzed with DEM, in which the effect of particle shape is discussed in detail with different load combinations. The particle angularity is achieved by introducing two simple but common particle shapes, i.e. spherical and tetrahedral, which allows the evaluation of the effect of particle shape at the engineering scale with relatively low computation cost. In addition to physically changing the particle shape, the rolling resistance method, which indirectly reproduces the behavior of irregular particles, is also applied. The property of soils is first investigated with a series of biaxial tests using DEM. Then the micromechanical analysis focuses the effect of particle shape on the behavior of caisson foundation.