Investigation into the influence of caisson installation process on its capacities in clay
Introduction
Caisson is a thin-walled bucket structure with a closed top and an open bottom, as illustrated in Fig. 1. After self-weight penetration, caisson installation can be carried out by using suction or jacking. Ballast system or synchronous vibrating system composed of large hydraulic vibration hammers (Xiao et al., 2020) can be used to install the caisson during jacking installation. Caisson foundations have been widely used in offshore engineering to support their superstructures, including the recent applications as foundations for offshore wind turbines (Liu et al., 2015) and breakwaters (Xiao et al., 2012). Understanding the bearing capacity of caisson under combined vertical loads (V), horizontal loads (H) and moments (M) generated from the superstructure deadweight and the environmental loading (such as wind, wave and current transferred to the foundation) is of great importance for the confident application of caisson foundations. It is especially important to evaluate the short-term undrained foundation capacity of caissons after installation in clayey soil, because the short-term capacity is a critical scenario as the foundation capacity increases with time due to consolidation. The consolidation coefficient cv of clayey soils is in the range of 1–5 m2/year (Mahmoodzadeh and Randolph, 2014). It can take several years even decades for clay to finish consolidation after foundation installation (Gourvenec and Randolph, 2010). The short-term bearing capacity here means the caisson foundation undrained capacity prior to soil consolidation.
Many studies focusing on the bearing capacity of caisson in clay have been conducted, including field tests (Houlsby et al., 2005; Le et al., 2018), model tests (Guo et al., 2018; Zhu et al., 2018), limit analysis (Aubeny and Murff, 2005; Tang et al., 2016), finite element analysis (Gourvenec and Barnett, 2011; Fu et al., 2018), finite element limit analysis (Ukritchon and Keawsawasvong, 2016) and artificial intelligence methods (Masoumi Shahr-Babak et al., 2016; Kim et al., 2017; Derakhshani, 2017 and 2018). However, in most of the available numerical studies, caissons were assumed ‘wished-in-place’, where the process of installation was neglected (Aubeny and Murff, 2005; Monajemi and Razak, 2009; Gourvenec and Barnett, 2011; Hung and Kim, 2012; Masoumi Shahr-Babak et al., 2016; Mehravar et al., 2016; Tang et al., 2016; Ukritchon and Keawsawasvong, 2016; Kim et al., 2017; Derakhshani, 2017 and 2018; Fu et al., 2018).
For the post-installation bearing capacity of caisson foundation, the installation process of caissons causes two effects, i.e. the seabed soil heaving and the surrounding soil remoulding. Soil heaving is that the soil inside the caisson becomes higher than the original soil surface, which can lead to a premature failure of installation without reaching the designed installation depth (Guo et al., 2016). According to Andersen et al. (2005), the height of soil heave inside a caisson can exceed 10% of its penetration depth in normally consolidated clay. On the other hand, the installation process of caisson remoulds the soil along the penetration path, which can decrease the soil strength due to strain softening effect. In the process of caisson installation, both soil strain softening and rate effects should be considered. However, the strain rate effect disappears once the caisson foundation has been installed while the recovery of the soil strength degradation takes a long time. Therefore, only the soil remoulding caused by soil strain softening is necessary to be considered to predict the post-installation bearing capacity of caisson foundation. The strain rate effects after full installation of the caisson has not been studied as it has only a minor influence on the response, as discussed later in the paper. Studies on offshore foundations (i.e. anchors by Kim et al., 2015 and Liu et al., 2016 and spudcans by Zhang et al., 2014), showed that the installation process had a significant influence on the short-term bearing capacity of foundations.
This paper models the installation process of caisson using large deformation modelling in order to examine the two effects of soil heaving and remoulding. The short-term bearing capacity of caisson foundations influenced by the two installation effects is then evaluated, which is expressed with combined loading envelopes, as detailed in the following sections.
Section snippets
Methodology of this study
The caisson installation process requires large deformation modelling technique as traditional small deformation Lagrangian method is inadequate due to the excessive mesh distortion. There are three large deformation numerical methods commonly used in offshore geotechnical engineering including the Arbitrary Lagrangian-Eulerian (ALE) method, the remeshing and interpolation technique by small strain (RITSS) and the coupled Eulerian-Lagrangian (CEL) method. The ALE method, in a more generic
Coupled Eulerian–Lagrangian
In a traditional Lagrangian finite element analysis, nodes are fixed with the material, and elements deform with the material. In contrast, nodes are fixed in space in an Eulerian analysis, and elements do not deform while the materials flow through elements. Different from Lagrangian elements which are always fully filled with material, an Eulerian element may be completely or partially filled with material or even completely void. The fullness of an element can be described by the Eulerian
Tracking the free surface of soil and mapping the updated shear strength
Small strain Lagrangian finite element method is extensively adopted to calculate the undrained bearing capacity of skirted foundations (Hung and Kim, 2014; Vulpe, 2015; Fu et al., 2017; Xiao et al., 2017), due to its advantages of higher computational efficiency and accuracy. In order to calculate the capacities of caisson post installation, it is essential to map the free surface of soil and the updated soil strength profile influenced by the installation process from the Eulerian model in
Conclusions
The effects of caisson installation on the short-term capacity of caisson have been investigated in this paper, accounting for the soil heave inside the caisson and the surrounded soil degradation. Firstly, continuous penetration of the caisson from the seabed surface was simulated using the large deformation CEL finite element method. The soil remoulding was accounted for using the model of Einav and Randolph (2005), which was implemented in CEL method using the user subroutine VUSDFLD in
CRediT authorship contribution statement
Zhong Xiao: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Yumin Lu: Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Yuanzhan Wang: Resources, Supervision, Project administration. Yinghui Tian:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgements
This study is supported by the National Natural Science Foundation of China (Grant No. 51879187, 51539008, 51890915, 51479133, 51879183, 51890913), Natural Science Foundation of Tianjin (18JCYBJC40600).
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