PolyU IR Collection: CEE Theses
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Studies of slope stability problems by LEM, SRM and DEM
http://hdl.handle.net/10397/6496
Title: Studies of slope stability problems by LEM, SRM and DEM<br/><br/>Authors: Sun, Yingjie<br/><br/>Abstract: Slope stability problem is a major problem in geotechnical engineering with influence on structure and human life, and slope stability problem has drawn the attentions of many researchers and engineers for the past several decades. This study is aimed to investigate slope stability problem with a better understanding of the failure mechanism and some fundamental principles in slope stability analysis by several methods so that the complete stability and failure processes are investigated. From the present study, some outstanding fundamental questions in slope stability problem have been settled, and the works are beneficial to both academic and practical aspects. This study first begins with the typical upper bound limit equilibrium analysis where different modern heuristic optimization algorithms are modified and improved to locate the critical slip surface efficiently and precisely. This problem has been studied by many researchers in the past, but there is a major difficulty in this problem in that the objective function is non-smooth and non-convex and the solution might be trapped into local minimum easily. Towards this complicated problems, two modified optimization algorithms: improved harmony search method MHS and coupled algorithm of HS/PSO are developed. These two algorithms are demonstrated to be more efficient than the original methods, and are particularly suitable for highly complicated problems where there are several strong local minima in the solution domain. The knowledge and works gained in this part of work are useful for practical engineering and also become part of the tools for the later sections. Secondly, the extremum principle and the concept of variable factor of safety based on Pan's postulate and equivalent variational principle are developed in this study. Using the new concept which can be viewed as an equivalent lower bound method, the long outstanding question on the interslice force function is finally settled using the mathematical tool developed in the first part of the present study. Slope stability problem can now become a statically determinate problem, and the interslice force function is actually taken as a variable instead of a prescribed function. This function is now determined by an equivalent lower bound principle which is missing in all the previous limit equilibrium formulation, which is major breakthrough in the basic formulation of the limit equilibrium method. Besides the new extremum LEM formulation, the author has also employed SRM to study the interslice force function. In general, it is found that the interslice force function is close to a bell shape and is also in agreement with the results from LEM, and such results clearly demonstrates that this function cannot be arbitrarily specified as what has been done for more than 40 years. The location of the thrust line also agrees well with the Janbu's Rigorous method which is at 1/3 of slice height from the base for normal cases. As a further extension of the works, the extremum formulations are further extended to the concept of variable factor of safety formulation which can satisfy all the global and local equilibrium. Using this new concept, the stress re-distribution and residual strength concept can be cast into the LEM framework under a rigorous lower bound formulation. Progressive failure can now be cast into the framework of limit equilibrium method which is not possible in the past.; The limitation of both LEM and SRM is the requirement on continuity which is not possible after the initiation of failure. The failure and post-failure mechanism are investigated by the use of Distinct Element Method (DEM) due to the demand in the consideration of large scale post-failure deformation. The use of DEM to investigate the slip surface is seldom considered in the past but has been achieved in the present study. The effect of water seepage on slope stability using a DEM approach is also an outstanding work which is worth to be investigated. In this study, it is found that the geometry of slope changes continuously, and tensile failure at the crest and shear failure in the middle of the slope are found. The failure mode for soil nailed slope and slope with by water flow are also studied by the DEM with interesting results obtained. For three-dimensional problems, there are several interesting problems to be considered. Three-dimensional effect of curvature with different nailing modes is considered by SRM, and the intercolumn force function is investigated (which is an outstanding item up to present). It is found that the intercolumn force function within the principal section containing the sliding direction is dominating over other sections. Concave geometry also gives higher global stability which is important for many highway slopes. For nailing pattern, the radial nailing mode gives lower factor of safety for convex slope but higher factor of safety for concave slope as compared with the parallel nailing mode. These results are both useful to the engineers as well as to the basic understanding of three-dimensional slope stability problem. Based on the above research involving different methods, many fundamental principles and outstanding problems in slope stability analysis have been settled in the present research. For example, the search for critical failure surface can now be carried out with very high level of confidence even for very complicated problem. Many engineers arbitrarily assign interslice force function (f(x)) equal to 1.0 (or sine function) without any thought, as all textbooks and research papers give the view that this function is "fundamentally indeterminate" and is not critical for normal condition. The author has however pointed out the mistake of this common belief accepted by engineers/researchers for more than 40 years, and has also demonstrated that there are also cases where f(x) is important and has proposed a systematic way to determine this function for arbitrary problem based on the equivalent lower bound principle. This work is then extended to three-dimensional condition where no one has ever proposed any interslice force function, and this three-dimensional function can now be treated as determinate function. The knowledge about the initiation and post failure movement of slope by DEM has also provided clearer picture about the movement and internal stress distribution within a slope at different stages which is useful to assess the post-failure behaviour and the precautions that are required.<br/><br/>Description: xi, 203 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P CEE 2013 SunFailure mechanism of slope under several conditions by two-dimensional and three-dimensional distinct element analysis
http://hdl.handle.net/10397/6484
Title: Failure mechanism of slope under several conditions by two-dimensional and three-dimensional distinct element analysis<br/><br/>Authors: Li, Na<br/><br/>Abstract: Landslide is a major disaster resulting in considerable loss of human lives and property damages in hilly terrain in Hong Kong, China and many other countries. The factor of safety and the critical slip surface for slope stabilization have been the main considerations for slope stability analysis in the past, while the detailed post-failure conditions of the slopes have not been considered in sufficient details. There are however increasing interest on the consequences after the failure which includes the amount of failed mass and the runoff and the affected region. To assess the development of failure of slope in more details and to consider the potential danger of slopes, the slope stability problem is analyzed by the distinct element method (DEM) in the present research. There are very few studies about slope stability using the DEM in the past due to various difficulties in DEM modeling. To investigate the progressive failure of slope, the author has managed to model the complete failure processes under several important conditions by particle flow analysis, and important new results have been obtained in the present study. In this study, Particle Flow Code (PFC) is used for the detailed investigation of the failure mechanism of slopes. Large displacement simulation of slope failure by distinct element analysis using PFC has been carried out for both two-and three-dimensional analysis to reveal the failure patterns of simple slope under the effect of self-weight, reinforced action of soil nails, buoyant force and seepage of water flow, local surcharge and also curvature effect on curvilinear slope. In two-dimensional DEM analysis, firstly, it is found that for a slope with cohesionless soil, failure firstly occurs at the crest of the slope, and the failure gradually extends to the base of the slope until finally the slope angle is equal to the friction angle of the soil and slope is completely collapsed. The analysis successfully simulates the whole flow process of simple slope failure, and the modeling includes the flow of sand from crest to toe of slope. The large scale soil movement and major change of slope geometry have revealed some important stress change and soil movement due to soil failure and flow of soil not considered in the past which is one useful new contribution in this study. Secondly, soil nails can effectively reinforce the slope stability, especially when nail head is used, and the overall stability is greatly enhanced which is demonstrated by the numerical analysis using distinct element method in this study. It is demonstrated that the soil nail can greatly stabilize the stress change along the potential failure surface even when large scale soil movement is mobilized which is another new contribution. Moreover, four influencing factors are considered in the study of the failure mode by rainfall induced water flow as followings: buoyant effect of ground water, seepage effect, strength reduction during slope failure and persistent rainfall effect. The simulation results show that the slope crest is scoured smoothly under continuous effect of rainfall induced water flow. More obvious settlements occur at the top of the slope, and extended failure zone and noticeable upheaval at the toe are developed compared with normal failure. Thus rainfall induced water flow can accelerate slope failure with an obvious decrease in the stability of the slope leading to collapse at the end. The detailed stress change and large soil movement under these cases are important and useful innovative works with no previous study.; Three-dimensional DEM analysis possesses many major difficulties and is not commonly considered. Practically, most of such studies are devoted to qualitative study with very few quantitative studies at present. In the present three-dimensional DEM analysis, the author has carried out a tedious and very time-consuming analysis together with laboratory test aiming at quantitative slope failure analysis which is an innovative and new work. The comparison of the physical modeling and numerical simulation indicates that the relations between the force against displacement curves are basically similar between the physical model test and the numerical model, and the final failure loadings from the physical model test and the numerical model are also very close which is a success seldom accomplished in DEM studies. For the three-dimensional curvature influence on slope stability with or without localized loading, the study shows that the curvature defined by R0/H0 (where R0: radius of curvature, H0: height of slope) has a beneficial effect on the global stability of concave slopes, especially when the resistance of soil is relatively high and also arching effect is illustrated clearly for concave slope. As a result, only partial/localized failure is initiated in concave slope under local loading. These studies which are seldom considered in microscopic scale in the past are both difficult to be considered as well as useful in giving clearer insights about the development of failure, microcosmic failure mechanism and the post-failure mechanism of slope. The various findings from the present work are pioneer works in slope stability analysis and they are very useful and important to the better understanding of the failure and post-failure condition of a slope.<br/><br/>Description: xvii, 130 leaves : ill. (some col.) ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577M CEE 2013 LiComputational models for FRP-confined concrete and FRP-confined RC columns
http://hdl.handle.net/10397/6432
Title: Computational models for FRP-confined concrete and FRP-confined RC columns<br/><br/>Authors: Xiao, Qiongguan<br/><br/>Abstract: The use of FRP jackets to strengthen RC columns has become popular in recent years due to the well-known phenomenon that lateral confinement can significantly enhance the strength and the deformation capacity of concrete. However, the related confinement mechanism of concrete, particularly when under non-uniform confinement, is still inadequately understood. This thesis is thus concerned with the development of a deeper understanding of the confinement mechanism of concrete in FRP-confined RC columns. This thesis first presents a series of axial compression tests on FRP-confined high strength concrete cylinders. These tests are an important supplement of the existing test data. Based on these tests, a stress-strain model applicable to both normal strength concrete and high strength concrete under active confinement is proposed. Moreover, an existing analysis-oriented stress-strain model for FRP-confined concrete is shown to be applicable to concretes of different strength grades. This analysis-oriented stress-strain model served as a basis of the subsequent studies on the numerical modelling of FRP-confined concrete and FRP-confined RC columns presented in the thesis. Attention is then shifted to the performance of plasticity models and plastic-damage models in predicting the stress-strain behaviour of confined concrete. In the plasticity models or the plasticity part of the plastic-damage models, two techniques have been utilized to define the plastic deformation process: the scaling technique in which the hardening law is defined as a function of the confining pressure and the plastic volume strain technique in which the plastic volume strain serves as the hardening variable. While both techniques are shown to lead to accurate predictions for actively-confined concrete, they are shown to be incapable of providing accurate predictions for FRP-confined concrete. This is because both approaches cannot accurately simulate the lateral deformation process of FRP-confined concrete. In addition, the thesis also presents a study of the use of Bazant's micro-plane model in predicting the behaviour of confined concrete; an improved version of the M4 model, referred to as the M4+ model, is presented for the numerical modelling of FRP-confined concrete. Several important parameters of the M4+ model were set to be confinement-dependent. The improved model provides accurate predictions for FRP-confined concrete.; The next part of the thesis is on the development and application of advanced finite element models for FRP-confined non-circular columns. Two constitutive models, that is, Yu et al.'s plastic-damage model and the M4+ model, were employed in the finite element models to predict the behaviour of FRP-confined square and elliptical columns. Numerical results from the finite element model show favourable agreement with the experimental results. The final part of the thesis presents a three-dimensional finite element model for FRP-confined RC columns based on Yu et al.'s plastic-damage model. For this finite element model, a local analysis-oriented stress-strain model is proposed for adoption to avoid the double counting of end restraint effects. This finite element model is shown to produce accurate predictions of the stress-strain behaviour of transverse steel-confined concrete columns and FRP-confined RC columns.<br/><br/>Description: xvii, 384 p. : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P CEE 2013 XiaoFire resistance of FRP-strengthened RC beams : numerical simulation and performance-based design
http://hdl.handle.net/10397/6421
Title: Fire resistance of FRP-strengthened RC beams : numerical simulation and performance-based design<br/><br/>Authors: Gao, Wanyang<br/><br/>Abstract: The wide use of externally bonded fiber reinforced polymer (FRP) laminates (including wet layup FRP sheets and pultrued FRP plates) in the strengthening of existing reinforced concrete (RC) structures has been a significant development in structural engineering over the past three decades. However, the technology also suffers from one serious limitation when employed for indoor applications in buildings: FRP composites have a poor resistance to fire as organic polymers (normally epoxies) used both as the matrix material and the bonding adhesive soften quickly around the glass transition temperature. Furthermore, when exposed to a high heat flux, the polymer matrix may ignite, resulting in flame spread and smoke generation. To facilitate a safe and economic use of the FRP strengthening technique in building applications, an in-depth understanding of the fire performance of FRP-strengthened RC members is deemed necessary. Against this background, this dissertation aims to develop a comprehensive approach to simulate the fire performance of un-protected and insulated FRP-strengthened RC beams. The dissertation is composed of three main parts: (a) theoretical analyses on the bond-slip behavior of the FRP-to-concrete interface at elevated temperatures; (b) advanced finite element (FE) modeling of the structural behavior of un-protected and insulated FRP-strengthened RC beams exposed to fire; and (c) design-oriented solutions for predicting the fire resistance of un-protected and insulated FRP-strengthened RC beams. In the first part of the dissertation, a set of closed-form theoretical solutions was developed for tracing the Mode II debonding process of FRP-to-concrete bonded joints subjected to combined thermal and mechanical loadings. In order to represent the behavior of a wide range of bonded joints, five different bond-slip models were considered in deriving the closed-form solutions, including the elastic-brittle, bi-linear, elastic-plastic-brittle, rigid-softening, and exponential models. For each bond-slip model, explicit expressions for the debonding load, effective bond length, interfacial shear stress, interfacial slip as well as the load-displacement response were derived. It was found that the debonding of FRP-to-concrete interfaces may be delayed or accelerated by temperature variations (i.e., thermal loading) during service. The theoretical solutions indicate that, provided the bond length is sufficiently long, the debonding load of the FRP-to-concrete interface depends only on the interfacial fracture energy and the temperature variation. A temperature increase leads to an increase in both the debonding load and the effective bond length, and the rate of increase of the latter depends on the interfacial bond-slip model. Based on the theoretical solutions, a nonlinear local bond-slip model was also developed for FRP externally bonded to concrete at elevated temperatures. Two key parameters of the proposed bond-slip model, the interfacial fracture energy Gf and the interfacial brittleness index B were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperatures. The proposed bond-slip model provides the first-ever constitutive law for describing the local performance of the FRP-to-concrete interface at elevated temperatures.; In the second part of this dissertation, advanced FE models were developed to trace the thermal and structural responses of RC beams (i.e., equivalent to un-protected FRP-strengthened RC beams) and insulated FRP-strengthened RC beams exposed to fire. In the models, the temperature-dependent thermal and mechanical properties of concrete, steel, FRP and interfaces are all appropriately considered. The thermal and structural responses predicted by the FE models were compared with existing fire test data to examine their validity. For RC beams, the comparison showed that the inclusion of the steel-to-concrete interfacial behavior leads to more accurate predictions of the deflection. Besides, the proposed FE model allows the complex distribution and evolution of stresses in the reinforcing steel and concrete to be examined in detail, leading to a better understanding of the local responses of RC beams exposed to fire. For insulated FRP-strengthened RC beams, FE predictions showed that the assumption of perfect bonding between FRP and concrete as adopted by previous numerical models leads to an underestimation of deflections and thus an unsafe prediction of fire resistance. Unless a very thick insulation layer is provided (usually an impractical solution), it was revealed that the main role of the insulation layer is to minimize the degradation of the original RC beam rather than to protect the FRP strengthening system. Therefore, the fire resistance evaluation of an insulated FRP-strengthened RC beam can be conservatively but closely approximated by that of an insulated un-strengthened RC beam. In the third part of this dissertation, simple design-oriented solutions for predicting the fire resistance periods of un-protected and insulated FRP-strengthened RC beams are presented. For the un-protected FRP-strengthened RC beams (i.e., equivalent to bare RC beams), the validated FE model for RC beams was used for extensive parametric studies to investigate the effects of various influencing parameters on the fire resistance periods and to generate sufficient data for regressing explicit design formulae. For insulated FRP-strengthened RC beams under fire, a two-phase design-orientated approach was proposed to predict their fire resistance. The first phase is the development of explicit solutions for the temperature field analyses of insulated FRP-strengthened RC beams exposed to the standard fire. The second phase is the structural response analysis, for which the degradation of the load-carrying capacity of the beams are assessed using the "500 °C isotherm method" in combination with the temperature predictions of insulated beam sections. The fire resistance periods of insulated FRP-strengthened RC beams are defined to be reached when the fire load action exceeds the loading carrying-capacity of the beams during fire. In order to validate the simple design-oriented method, parametric studies using the advanced FE model were carried out to generate the fire-resistance data for comparisons. The comparisons showed that the simple design-oriented method provides a reliable prediction of the fire resistance periods for insulated FRP-strengthened RC beams.<br/><br/>Description: xxviii, 330 p. : ill. ; 30 cm.; PolyU Library Call No.: [THS] LG51 .H577P CEE 2013 Gao