Three Dimensional Discrete Element Modelling of Open-Ended Tubular Pile Penetration in Weak Rocks

Publication Type:
Thesis
Issue Date:
2020
Full metadata record
Open-ended tubular piles, usually made of steel, are commonly used in offshore structures and bridge projects due to the high capacity and less required installation effort. Predicting the load – displacement response of the open-ended piles is of interest to many practicing engineers and researchers. Although the behaviour of the piles embedded in sands and clays has been studied extensively, clear and adequate recommendation is not currently available for predicting the load – displacement response of open-ended piles embedded in weak rocks. However, the increasing number of projects in Australia requires adopting the open-ended piles embedded in weak rock layers to sustain the axial and lateral structural loads. Current recommendations for the design of tubular piles in weak rocks are mainly based on the methods originally proposed for gravels, sands and clays, which cannot predict the shaft and base resistances of tubular piles in weak rocks accurately. Furthermore, the behaviour of weak rock masses is complex due to their characteristics, and the plugging mechanism of tubular piles influenced by the characteristics of the weak rock is not well understood. Therefore, there is a need to enhance the understanding of the load bearing mechanism of open-ended tubular piles embedded in weak rocks to optimise the construction cost and improve safety. In this thesis, initially the effects of joint properties on the behaviour of weak rock masses are discussed. The discrete element method (DEM) is adopted which can simulate interacting rock grains. The interaction between grains is controlled by the adopted contact models. The flat-joint model that follows the elasto-plastic force-displacement constitutive law is employed in the analysis. Meanwhile, the discrete fracture network (DFN) along with the smooth-joint model are adopted to replicate the sliding of the joints. Initially, the effects of joint dip, joint density, joint aperture, and joint length on the mechanical behaviour of the rock mass are investigated through simulating unconfined compressive strength and triaxial tests. Moreover, this research provides an insight into the impacts of joint dip and joint density on the internal shaft friction of open-ended tubular piles through DEM analysis. The flat-joint and smooth-joint contact models are adopted to replicate the rock mass and the sliding of the joints, respectively. The push-up load tests are performed on the intact rock plug and jointed rock plug to analyse the effects of joints on the internal shaft frictions of tubular piles. It is noticed that the joints could reduce the capacity of the rock plug, and the joints parallel to the loading direction have the largest impact compared to other joint dips, where the joints parallel to the axial loading direction in the triaxial test do not result in the lowest strength. This indicates that the internal shaft friction of open-ended tubular piles is not solely a function of the rock strength, and the effects of joint properties need to be taken into consideration. Furthermore, the axial load bearing mechanism of open-ended tubular piles penetrating into weak rock is discussed. The numerical modelling using the discrete element method is adopted since tubular pile driving involves extremely large displacements that DEM can accommodate. Effects of an inner driving shoe attached to the open-ended tubular pile on the load bearing mechanism is also investigated. The incremental filling ratio (IFR) and plug length ratio (PLR) are used to assess the plugging of the piles. As expected, the base resistance can mobilise at the early stage of the pile penetration, while the internal and external shaft frictions increase continuously as the pile penetrates deeper. The unit internal shaft friction is mainly mobilised at the bottom portion of the rock plug due to the arching effect. Moreover, partial plugging is observed for both piles with and without driving shoe, and the correlations between IFR and PLR are proposed.
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