Finite-discrete Element Approach Research Articles

Assessing fracturing mechanisms and evolution of excavation damaged zone of tunnels in interlocked rock masses at high stresses using a finite-discrete element approach

Deep underground excavations within hard rocks can result in damage to the surrounding rock mass mostly due to redistribution of stresses. Especially within rock masses with non-persistent joints, the role of the pre-existing joints in the damage evolution around the underground opening is of critical importance as they govern the fracturing mechanisms and influence the brittle responses of these hard rock masses under highly anisotropic in situ stresses. In this study, the main focus is the impact of joint network geometry, joint strength and applied field stresses on the rock mass behaviours and the evolution of excavation induced damage due to the loss of confinement as a tunnel face advances. Analysis of such a phenomenon was conducted using the finite-discrete element method (FDEM). The numerical model is initially calibrated in order to match the behaviour of the fracture-free, massive Lac du Bonnet granite during the excavation of the Underground Research Laboratory (URL) Test Tunnel, Canada. The influence of the pre-existing joints on the rock mass response during excavation is investigated by integrating discrete fracture networks (DFNs) of various characteristics into the numerical models under varying in situ stresses. The numerical results obtained highlight the significance of the pre-existing joints on the reduction of in situ rock mass strength and its capacity for extension with both factors controlling the brittle response of the material. Furthermore, the impact of spatial distribution of natural joints on the stability of an underground excavation is discussed, as well as the potentially minor influence of joint strength on the stress induced damage within joint systems of a non-persistent nature under specific conditions. Additionally, the in situ stress-joint network interaction is examined, revealing the complex fracturing mechanisms that may lead to uncontrolled fracture propagation that compromises the overall stability of an underground excavation.

A finite-discrete element approach for modelling polyethylene pipes subjected to axial ground movement

ABSTRACT The response of medium-density polyethylene (MDPE) pipes subjected to ground movement is often investigated using soil-pipe interaction models that were originally developed for steel pipes. In this study, the behaviour of MDPE pipes buried in dense sand under pull-out force is investigated using a coupled finite-discrete element framework. The pipe is modelled using finite elements whereas the granular soil is modelled using discrete elements. The model is validated using experimental data and then used to investigate the response of the pipe and the surrounding soil. The response of the MDPE pipe-soil system to axial loading is found to differ significantly from that of steel pipes due to the elongation and distortion that develop in the MDPE pipes, which affect the mobilized friction forces along the pipe. This study demonstrates that caution must be exercised when using current methods in the analysis of MDPE pipes.

DYNAMIC STIFFNESS ANALYSIS FOR TORSIONAL VIBRATION OF CONTINUOUS BEAMS WITH THIN-WALLED CROSS-SECTION

An analytical method for determining natural frequencies and mode shapes of the torsional vibration of continuous beams with thin-walled cross-section is developed by using a general solution of the differential equation of motion based on Vlasov's beam theory. This method takes into account the effect of warping stiffness; it leads to an exact solution and is called the continuous mass method. Also, the approximate method based on the finite discrete element approach is presented. The mathematical relationship between the exact and the approximate methods is discussed, and the accuracy of the natural frequencies obtained by these analytical methods is investigated. Some typical continuous beams are analyzed to illustrate the applicability of the lumped, consistent, and continuous mass methods, and the computed results are given in tabular form.