Discrete element analysis of settlement of pile groups

Two discrete element models are described for the analysis of settlement of vertical pile groups. In both models, the piles are represented by discrete elements with an axial model of deformation, and the soil behaviour for the individual piles is represented by load-transfer curves. The essential difference between the models is the manner in which pile-soil-pile interaction is represented. The first model treats the soil as independent horizontal layers which permit interaction between piles to take place within the same layer only. Thus ignoring the continuity of the soil medium. The second utilizes Mindlin's solution which determines interaction effects in a homogeneous, isotropic elastic half-space, but an approximate procedure is used to account for soil inhomogeneity. Comparisons of both approaches with a rigorous integral equation method for pile groups in a homogeneous, isotropic elastic soil indicate that the second method generally gives good agreement while the first method tends to underestimate interaction, especially for shorter piles. For pile groups in nonhomogeneous soils, where rigorous theoretical solutions are generally not available, parametric studies comparing the two discrete element models solutions are presented. The studies performed using both methods on the nonlinear behaviour of an instrumented pile group compare favourably with the field measurements.     

Dynamics of Flexible Multibody Systems Using Virtual Work and Linear Graph Theory

By combining linear graph theory with the principle of virtualwork, a dynamic formulation is obtained that extends graph-theoreticmodelling methods to the analysis of flexible multibody systems. Thesystem is represented by a linear graph, in which nodes representreference frames on rigid and flexible bodies, and edges representcomponents that connect these frames. By selecting a spanning tree forthe graph, the analyst can choose the set of coordinates appearing inthe final system of equations. This set can include absolute, joint, orelastic coordinates, or some combination thereof. If desired, allnon-working constraint forces and torques can be automaticallyeliminated from the dynamic equations by exploiting the properties ofvirtual work. The formulation has been implemented in a computerprogram, DynaFlex, that generates the equations of motion in symbolicform. Three examples are presented to demonstrate the application of theformulation, and to validate the symbolic computer implementation.     

Rigid body concept for geometric nonlinear analysis of 3D frames, plates and shells based on the updated Lagrangian formulation

In the nonlinear analysis of elastic structures, the displacement increments generated at each incremental step can be decomposed into two components as the rigid displacements and natural deformations. Based on the updated Lagrangian (UL) formulation, the geometric stiffness matrix [k_g] is derived for a 3D rigid beam element from the virtual work equation using a rigid displacement field. Further, by treating the three-node triangular plate element (TPE) as the composition of three rigid beams lying along the three sides, the [k_g] matrix for the TPE can be assembled from those of the rigid beams. The idea for the UL-type incremental-iterative nonlinear analysis is that if the rigid rotation effects are fully taken into account at each stage of analysis, then the remaining effects of natural deformations can be treated using the small-deformation linearized theory. The present approach is featured by the fact that the formulation is simple, the expressions are explicit, and all kinds of actions are considered in the stiffness matrices. The robustness of the procedure is demonstrated in the solution of several benchmark problems involving the postbuckling response.     

Autonomous detection and loading of ore piles with load–haul–dump machines in Room & Pillar mines

Automation of machines in underground mines is a topic with increasing interest, both for research and industrial applications. Autonomous load–haul–dump (LHD) machines need to load material successfully before dumping it into a crusher or an ore pass. The autonomous loading method must be robust to enable reliable operation of the LHD during long periods of time. In this work, a method to perform autonomous loading in Room & Pillar mines is presented. It is based on detecting all ore piles in real-time, and then computing attack poses in each pile. Then, a positioning process is performed to get the machine in front of the selected ore pile, and an excavation algorithm is executed for loading until the bucket is filled. The proposed method is able to detect multiple ore piles, with different slopes and sizes, and to consider different possible trajectories for attacking and loading the most feasible pile. The method was tested in the Werra Potash Mine, under real operational conditions. The results show that the method can load about 80% of the material that an experienced operator can load. Also, the success rate of the autonomous loading process is very high, being able to load enough material in all trials, and performing successfully the full procedure in 88% of the cases. Thus, the proposed autonomous loading method is a suitable alternative to be used in Room & Pillar mines.     

The simulation of 3D elastic scattering produced by thin rigid inclusions using the traction boundary element method

A mixed formulation that uses both the traction boundary element method (TBEM) and the boundary element method (BEM) is proposed to compute the three-dimensional (3D) propagation of elastic waves scattered by two-dimensional (2D) thin rigid inclusions. Although the conventional direct BEM has limitations when dealing with thin-body problems, this model overcomes that difficulty. It is formulated in the frequency domain and, taking into account the 2-1/2D configuration of the problem, can be expressed in terms of waves with varying wavenumbers in the zdirection, k_z. The elastic medium is homogeneous and unbounded and it should be noted that no restrictions are imposed on the geometry and orientation of the internal crack.     

A special BEM for elastostatic analysis of building floor slabs on columns

This work presents a boundary element formulation for the analysis of building floor slabs, without beams, in which columns are coupled with the plate. An alternative formulation of boundary element method is presented, which considers three nodal displacements values (w, ∂w/∂n and ∂w/∂s) for the nodes at the boundary of the plate. In this formulation three boundary equations are written for all nodes at the boundary and in the domain of the plate. As the nodes of the column-plate connections are also represented by three nodal values, all these structural elements can be easily coupled. It is supposed that the cross-sections of the columns remain flat after the deflection and consequently the assumption of linear variation of the stress in the plate-column contact surface is also valid.     

3-D time domain BEM analysis with animated graphics

The intense computational requirements of a time domain solution of 3-D elastodynamic problems based on the boundary element method (BEM) have limited its implementation to main frame computers. In this work a simplified 3-D direct time domain BEM is described along with a 3-D interactive animated graphics code custom made to accompany the above BEM analysis. The simplifications introduced make its use efficient and accurate even in a microcomputer environment. Representative numerical results from the dynamic interaction of a rigid square machine foundation with an elastic half space are presented.     

A new BEM approach for reissner's plate bending

A new boundary element formulation for Reissner's plate bending is presented. This form of BEM has an advantage in that the bending stresses on the boundary can be calculated directly from the numerical solution, avoiding the use of tangential derivatives of displacement for finding plate bending stresses on the boundary. The effectiveness of the approach is also discussed through some test examples. In the present BEM formulation, the singular orders of the two kernels are the same as those in the standard BEM formulation of a Reissner's type plate—one of which is logarithmic singular and the other is 1/r singular.     

Fast multipole method based solution of electrostatic and magnetostatic field problems

Abstact Applications of boundary element methods (BEM) to the solution of static field problems in electrical engineering are considered in this paper. The choice of a suitable BEM formulation for electrostatics, steady current flow fields or magnetostatics is discussed from user's point of view. The dense BEM matrix is compressed with an enhanced fast multipole method (FMM) which combines well-known BEM techniques with the FMM approach. An adaptive grouping scheme for problem oriented meshes is presented along with a discussion on the influence of the mesh to the efficiency of the FMM. The computational costs of the FMM algorithm are analyzed for typical problems in practice. Finally, some electrostatic and magnetostatic numerical examples demonstrate the simple usability and the efficiency of the FMM. Communicated by: U. Langer     

Accurate modelling of rigid and soft inclusions in 2D elastic solids by the boundary element method

In this work, a particular sub-region technique that does not require traction approximations along interfaces is used to model rigid and soft inclusions. The stability and the accuracy of the formulation have been studied for several applications, including the degenerated cases where one dimension is strongly reduced to represent an embedded bar. The rigid inclusions have been modelled by increasing their elastic modulus. On the other hand thin inclusions with elastic modulus reduced to nearly zero can accurately model crack problems. In all analysed cases, the obtained results have shown that this simple strategy is reliable and can be applied to complex stress analysis problems.     

地基波动影响下非线性粘弹性桩的混沌运动分析

研究了在地基波动影响下非线性粘弹性桩中的混沌运动．假定桩体材料满足Leaderrnan非线性粘弹性本构关系,得到在轴向载荷作用下满足Winkler条件的地基土波动方程、桩与地基土耦合振动方程;利用Galerkin方法将非线性积分一微分方程简化,并进行了数值计算,揭示了非线性粘弹性桩包括混沌运动在内的动力学行为．     

基于Ansys的带侧向支撑桩的稳定性

为分析既有建筑物增设地下空间托换桩的稳定性,利用Ansys建立增设侧向支撑的钢筋混凝土桩的三维有限元模型,通过改变桩周土水平抗力系数的比例系数、桩周土开挖深度、桩身截面边长、桩身配筋率和载荷偏心距等参数,分析桩的受压极限承载力和相应支撑轴力的变化规律.结果 表明:桩受压极限承载力和相应支撑轴力随载荷偏心距和桩周土开挖深...     

A filter model for feature subset selection based on genetic algorithm

This paper describes a novel feature subset selection algorithm, which utilizes a genetic algorithm (GA) to optimize the output nodes of trained artificial neural network (ANN). The new algorithm does not depend on the ANN training algorithms or modify the training results. The two groups of weights between input-hidden and hidden-output layers are extracted after training the ANN on a given database. The general formula for each output node (class) of ANN is then generated. This formula depends only on input features because the two groups of weights are constant. This dependency is represented by a non-linear exponential function. The GA is involved to find the optimal relevant features, which maximize the output function for each class. The dominant features in all classes are the features subset to be selected from the input feature group.     

A boundary element formulation for the substation grounding design

A Boundary Element approach for the numerical computation of substation grounding systems is presented. In this general formulation, several widespread intuitive methods (such as Average Potential Method (APM)) can be identified as the result of specific choices for the test and trial functions and suitable assumptions introduced in the Boundary Element Method (BEM) formulation to reduce computational cost. While linear and parabolic leakage current elements allow to increase accuracy, computing time is drastically reduced by means of new completely analytical integration techniques and semi-iterative methods for solving linear equations systems. This BEM formulation has been implemented in a specific Computer Aided Design system for grounding analysis developed in the last years. The feasibility of this new approach is demonstrated with its application to a real problem.     

Dynamic response of 3-D embedded foundations by the boundary element method

The dynamic response of three-dimensional rigid embedded foundations of arbitrary shape, resting on a linear elastic, homogeneous, and isotropic half-space is numerically obtained. The foundations are subjected either to externally applied forces or to obliquely incident seismic body or surface waves of arbitrary time variation. The time domain boundary element method (BEM) is utilized to simulate the soil medium with the aid of Stokes' fundamental solutions. The dynamic response of the foundation-soil system is obtained in a step-by-step time-marching solution. Use of this time domain BEM requires a minimum amount of surface discretization only and provides the basis for an extension to nonlinear soil-structure interaction (SSI) problems.     

Soil-structure interaction and effect of axial force on the dynamics of offshore structures

The offshore structures are flexible systems subjected to various types of loadings. The heavy gravitational loads on the top decks, wind and water wave pressures acting on the platforms are transferred to the soil through the piles or mat foundations. Under the vibration, the variation in the pore pressures induces additional effects on the embedded part of the piles. The effect of the soil-structure interaction on the dynamics of the structure is taken into account as the deformations of the soil caused by the motion of the structure which in turn modify the response of the structure. The effect of the axial forces, within the individual members, on the vibration of the structure is included in the formulation. The dynamic stiffness matrix of the members are developed by considering the actual mass distribution and the effect of the axial force of the members. For the members embedded into soil, the soil reactions and the skin frictions are also considered as continuously varying over the members. Therefore, the equations of motion are satisfied along any infinitesimal element of the members. The new formulation is introduced in the general purpose computer code STDYNL, then the sensitivity of the overall dynamic response of the deep water platforms to the variation of the soil characteristics and to the effect of the axial forces of the members are investigated.     

基于双目视觉的不规则堆料体积测量研究

针对不规则堆料的体积难以直接测量的问题,提出一种基于双目视觉的不规则堆料体积测量方法。首先,使用双目相机采集堆料的俯视图像,对图像进行预处理;接着通过立体匹配求出堆料图像的视差图,并采用中值滤波和最邻近插值算法对于视差图中的空洞进行填充,同时计算出视差图对应的三维点云坐标;再采用改进的OTSU阈值分割算法将视差图中的堆料区域提取出来,计算出堆料区域的平均高度和面积,最后计算出堆料的体积。实验结果表明,体积测量误差小于5%,可以快速有效地测量出堆料的体积,能够满足工业检测领域的需求,同时为准确快速地测量堆料体积提供了一种新的思路。     

A hybrid learning network for shift, orientation, and scaling invariant pattern recognition.

A three-layer neural network is presented as a generic approach for visual pattern recognition invariant with respect to the geometric appearance such as translation, orientation and scale of the patterns. The invariant recognition is achieved by representing the geometric variations internally in the network by nodes in the input and middle layers, which are laterally connected and trained by a hybrid algorithm combining both competitive and Hebbian learning. As the result of the hybrid learning, each pattern will be represented by a particular subset of middle-layer nodes all specialized to respond to the same pattern but with different geometric appearances. The nodes in the output layer are then trained by competitive learning to recognize the different pattern internally represented by the middle-layer nodes, independent of their location, orientation and size. The proposed algorithm is generic and robust and can be applied to various practical recognition problems. Moreover, the network is relatively simple and biologically plausible and can serve as a computational model to account for the invariant object recognition in the biological visual system.     

Estimation of current-induced scour depth around pile groups using neural network and adaptive neuro-fuzzy inference system

The process of local scour around bridge piers is fundamentally complex due to the three-dimensional flow patterns interacting with bed materials. For geotechnical and economical reasons, multiple pile bridge piers have become more and more popular in bridge design. Although many studies have been carried out to develop relationships for the maximum scour depth at pile groups under clear-water scour condition, existing methods do not always produce reasonable results for scour predictions. It is partly due to the complexity of the phenomenon involved and partly because of limitations of the traditional analytical tool of statistical regression. This paper addresses the latter part and presents an alternative to the regression in the form of artificial neural networks, ANNs, and adaptive neuro-fuzzy inference system, ANFIS. Two ANNs model, feed forward back propagation, FFBP, and radial basis function, RBF, were utilized to predict the depth of the scour hole. Two combinations of input data were used for network training; the first input combination contains six-dimensional variables, which are flow depth, mean velocity, critical flow velocity, grain mean diameter, pile diameter, distance between the piles (gap), besides the number of piles normal to the flow and the number of piles in-line with flow, while the second combination contains seven non-dimensional parameters which is a composition of dimensional parameters. The training and testing experimental data on local scour at pile groups are selected from several precious references. Networks’ results have been compared with the results of empirical methods that are already considered in this study. Numerical tests indicate that FFBP-NN model provides a better prediction than the other models. Also a sensitivity analysis showed that the pile diameter in dimensional variables and ratio of pile spacing to pile diameter in non-dimensional parameters are the most significant parameters on scour depth.     

A linear Uzawa-type FEM–BEM solver for nonlinear transmission problems

We propose a fully discrete Uzawa-type iteration for the Johnson–Nédélec formulation of a Laplace-type transmission problem with possible (strongly monotone) nonlinearity in the interior domain. In each step, we sequentially solve one BEM for the weakly-singular integral equation associated with the Laplace-operator and one FEM for the linear Yukawa equation. In particular, the nonlinearity is only evaluated to build the right-hand side of the Yukawa equation. The algorithm includes the inexact solution of the BEM/FEM part by a preconditioned CG method. We prove that the proposed method leads to linear convergence with respect to the number of Uzawa iterations. Moreover, while the current analysis of a direct FEM–BEM discretization of the Johnson–Nédélec formulation requires some restrictions on the ellipticity (resp. strong monotonicity constant) in the interior domain, our Uzawa-type solver avoids such assumptions.