A 3-Dimensional RITSS Large Deformation Finite Element Method And Its Application On The Foundation Punch-through Failure And Plate Anchor Uplift Resistance Problems

In soil mechanics,many incremental plasticity models have been proposed in attempts to deal with the issue of material nonlinearity.But little result has been reported regarding the effect of geometrical nonlinearity,i.e.,large deformation/displacement.However,the large deformation behaviors are involved in many geotechnical problems,such as the penetration/pullout of piles,cone penetrometer,soil slope failure,and large strain soil consolidation,et al.In these cases the soil failures may be due to the development and accumulation of localized soil deformation.The large deformation upon the overall geometry of the structure has great effect on the structure bearing capacity and should not be ignored. Moreover,the deformation or failure processes also need to be investigated for some engineering practices.Therefore,large deformation numerical procedures considering the effect of geometrical nonlinearity are required for these problems.In recent years,TL,UL or ALE large strain finite element methods have been ultilized to study many geotechnical problems involving large deformation.Among these large strain FE methods,RITSS is a convenient and robust one.It falls essentially within the ALE category, in which,conventional small strain FE analysis is combined with fully automatic mesh genetration and plane linear stress interpolation techniques to deal with large deformation problems in soil.RITSS method has already been applied to simulate the continuous deforming process of various foundations in soil.And many valuable results have been obtained.However,most of the available publications focus on plane strain or axisymmetric 2-D problems,and the implementations of the 3-D large deformation analyses into the problems of practical geotechnical engineering are relatively rare.The reasons may include the difficulties in deriving the 3-D large deformation FE formulations,generating and controlling the 3-D mesh automatically,tracing the deformed boundary in 3-D space,and include the unacceptable time consuming in calculation.On the other hand,2-D numerical analyses would not be accurate enough for some practical problems,such as the penetration of square/rectangular footings,the uplift resistance of the inclined circular and square anchors, and the keying behavior of 3-D anchors,et al.3-D large deformation analyses are essential for these problems. In order to deal with the above 3-D problems,an efficient and robust 3-D large deformation numerical method,which can be implemented on personal computers,was proposed in the present thesis.Based on the 2D-RITSS method,a convenient and robust 3-D stress interpolation method and an automatically 3-D mesh generation approach were presented.Thus a procedure of 3-D large deformation FE analysis was proposed.Furthermore, the present 3D-RITSS method was applied to investigate the behaviors of punch:through failures of square footings on double layered clays.This method was also applied to simulate the keying processes of 3-D plate anchors.Some conclusions of these two problems were obtained,which may be valuable to the engineering practice.In addition,the emphasis of most of the published researches about the anchor uplift resistance problems was on vertical or horizontal plate anchors in uniform clay,the results related to inclined anchors in normally consolidated(NC)clay were relatively rare.So in the present thesis a large amount of 2-D and 3-D small strain FE analyses were implemented to calculate the uplift resistance of inclined palte anchors in NC clay.The research work carried out in the present thesis could be summarized as follows:1)The 2D-RITSS approach is developed to deal with 3-D geotechnical problems.Several types of elements are tested and the unstructured 20-node hexahedral element is found to work well both for generating mesh automatically.and for predicting collapse loads accurately for 3-D undrained geotechnical problems involving material incompressibility. A convenient method named UED is proposed to interpolate the stress variables of the new elements after each remeshing.A 3-D nodal joint element is developed to simulate the soil-structure interface.Aseries of B-spline lines are used to fit the deformed surface boundary.Thus the 3-D RITSS large deformation FE procedure can be run automatically. Finally the 3-D RITSS method is proved to be efficient and robust by several numerical examples.2)The present 3D-RITSS method is used to simulate the continuous penetration processes of square footings into double layered clays with a strong layer overlying a weak one. The effects of the relative thickness of the top layer and the shear,strength ratio of the bottom layer to the top layer soil on the punch-through critical be,aring capacity factors and critical depths are investigated.The load-displacement curves during penetration of various cases are presented in chart form and are also fitted by approximate equations, which may be convenient for practical design.The developments of soil flow mechanisms and plastic zone distributions during penetration,which may be useful for analytical and semi-analytical calculations,are also presented:3)2-D small strain FE analyses are implemented to investigate the uplift resistances of strip anchors in uniform and normally consolicated clay.The effects of the soil-structure interface(attached or vented),anchor inclination,the soil self-weight,and the soil nonhomogeneity on the anchor bearing capacity are analyzed in detail.For the vented plate anchors in NC clay considering soil weight,the bearing capacity factor N_c can also be calculated from the weightless result N_c0plus the dimensionless parameterγH/c,but remaining not larger than the limit uplift resistance factor N_c^*.The inclination angleβhas great effect on the N_c^* for shallow anchors.The larger is theβ,the lower is the N_c^*.The effects of the soil nonhomogeneity on the bearing capacity factor N_ckand limit bearing capacity factor N_ck^* of plate anchots in NC clay are both investigated.A systematic design procedure and approximate equations are proposed for the bearing capacity factors of plate anchors with any inclinations in uniform or NC clay.The 2-D RITSS approach is used to investigate the breakaway behavior of the soil beneath,the anchor.The results show that the dimensionless parameterγH/c is an important factor which determines whether the bottom soil would break away from the anchor or,not.4)3-D small strain FE analyses are implemented to investigate the uplift resistances of inclined circular and square plate anchors.And the effects of anchor inclination on the uplift resistance and on the soil flow mechanisms are analyzed in detail.An equation which is to calculate the N_c of an inclined anchor from the N_c of vertical and horizontal anchors,is verified to be suitable for 3-D circular and square plate anchors.A simplified equation is proposed and is proved to be accurate for shallow horizontal anchors.5)The 3D-RITSS method is also used to simulate the keying processes of 3-D strip and square anchors in NC clay.The effects of loading eccentricity,pullout angle and anchor aspect ratio(length/width)on the embedment loss during keying are investigated.The developments of the uplift resistance and the soil flow mechanisms are both presented. The numerical results show that the loading eccentricity e/B has a great effect on the value of embedment loss.The pullout angle has also a little effect on the embedment loss. The keying process and embedment loss of square anchors are almost the same as those of the strip anchors.The maximum uplift resistance factors of square anchors are about 1.10 to 1.19 times of those of strip anchors.The work in this thesis is surported by the National Natural Science Foundation of China (Grant Nos:50679093,50538080,50578029),the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry and the Program for Changjiang Schorlars and Innovative Research Team in-University (IRT0518).These supports are gratefully acknowledged.