Numerical Analysis of Drained undrained Triaxial Tests for Overconsolidated Clays

In the framework of critical state soil mechanics, a simplified hardening law is proposed to describe the change between subloading surface and the normal consolidation surface with plastic deformation based on subloading surface concept proposed by Hashiguchi. The characteristics of overconsolidated clays, such as stress strain relationship, strain hardening and softening and stress dilatancy are simulated, and the results are in accordance with the data from triaxial drained compression test. Numerical model is also used to predict the stress strain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests.     

Test on engineering properties of unsaturated undisturbed loess Q2

A series of triaxial shear tests with controlled constant confining pressure are conducted to study the failure modes, effects of deformation, strength and yield properties of unsaturated undisturbed loess Q2, with improved SJ-1A triaxial shear equipment. The test results show that the failure mode of unsaturated undisturbed loess Q2 is shear failure with the rupture angle between 55 and 61 degree; Shear dilatancy is shown under low confining pressure, and shear shrinkage is shown under high confining pressure; The stress-strain curves of undisturbed loess Q2 will change from perfectly plastic to weak strain softening as the confining pressure increases; A effective method is suggested to identify the field stress of undisturbed loess Q2 under triaxial shear condition with the curves of - q/p ; when the confining pressure increases, the deviatoric yield stress qy has a logarithmic relationship with net mean yield stress py .     

红层泥岩桩岩接触面本构模型试验及数值模拟

桩-岩（土）接触面力学特性的研究是桩基承载机理研究的基础。通过红层泥岩桩岩接触面大型直剪试验,研究了红层泥岩桩岩接触面的力学特性,结果表明：接触面剪应力先随剪切位移增大而增大,在达到峰值后,剪应力随着剪切位移增大而降低,并最终趋于稳定值,应力应变曲线呈现出应变软化的特征。根据剪切试验结果,推导出桩岩接触面应变软化本构方程。利用fish语言对FLAC^3D中自带的理想弹塑性接触单元进行二次开发,并应用开发的模型对桩岩接触面直剪试验进行了数值模拟,分析剪应力与剪切位移之间的关系,证明了该本构能够较好地模拟接触面间的应变软化特性。     

Degradation of Geogrids under Cyclic Tensile Load

The degradation characteristics of geogrids have important influence on the stability of reinforced embankments and retaining walls. A series of unidirectional cyclic load-extension tests under stress control were carried out on plastic geogrids. The influence of cyclic tension, pretension and load frequencies on strain and degradation characteristics of geogrids was studied. It is observed that, with the increase of cyclic tension and pretension and the decrease of load frequencies, the accumulative plastic strain and degradation index of geogrids increase, and the stiffness of geogrids decreases. Based on experimental data, the empirical equation for softening of geogrids was proposed. A modified Iwan model was put forward to describe the stress-strain relationship of the geogrids subjected to cyclic tensile load. The proposed model was proved to be reasonable by comparing the experimental data with the numerical results.     

Elasto-plastic analysis on surrounding rock of roadways based on seepage, strain softening and dilatancy

According to seepage body force, strain softening and rupture expansion of rock mass, elastic-plastic mechanics theory, distribution laws of stress and displacement of surrounding rock in seepage field are derived, and relationships between different zonation range of the surrounding rocks, pore water pressure, strain softening and rupture expansion are established. The results show that the pore water pressure and rupture expansion rock mass have a more obvious impact on the surrounding rock rupture area than the plastic zone. Both the plastic zone and the rupture area are larger when the seepage field is considered. The impact of strain softening on both the plastic zone and the rupture zone of surrounding rock is very significant, and both seepage and strain softening rupture expansion affect deformation of surrounding rock of roadways obviously. It provides certain reference values for the roadway support engineering in seepage field.     

Study on mechanical behaviors of interface with cemented soil slurry between gravel and concrete by simple shear tests

The mechanical behaviors of the interface between coarse-grained soil and concrete are investigated by simple shear tests under conditions of cemented soil slurry (clay mixed with cement grout). The results show that the relation curve between shear stress and shear strain appears stress-strain softening and shear dilatation is significant. The point of peak strength and the position when the shear dilatation occurs are related to normal stress. In addition, shear dilatation occurs before the shear stress reaches peak value. In shear failure state, with the same height, the shear displacement increases as the normal stress increases. While with the same normal stress and at the same height, the shear displacement increases as the concrete content increases. A particle flow model of simple shear test between interface between coarse-grained soil and concrete is constructed by PFC (particle flow code). The disturbed height of the sample and the main influence factors are determined by analyzing the laws of particle motion at different heights inside the sample. The PFC results show disturbed height of the sample is related to maximum particle diameter of the soil, normal stress and roughness of the interface (with or without slurry) etc. In terms of the coarse-grained soil, the shear displacement is significant in the area which is close to the interface and about 3-4 times of the maximum particle diameter, and informed the obvious shear band. Further, the thickness of the interface can be regarded as the value.     

Elasto- plastic Analysis of Pressure Circular Tunnel with Liner

According to the strain nonlinear softening constitutive model having inflexion point after peak stress of practical rock and considering intermediate principal stress, the plastic zone of circular cavern with liner under the condition of uniform pressure is analyzed. The elasto - plastic boundaries both for the lining and surrounding rock are discussed. The stress distribution law is different due to the different position of elastic - plastic boundary. Based on this, different stress formulas are gained. The critical pressure caused by excavation of cavern leading to the yield zone and that caused by inner pressure leading to the yield of the liner and surrounding rock is introduced.     

Influences on Engineering Properties of AS Firming Agent Modified Soil by Freeze-thaw Cycles

To study the freeze-thaw durability of Aught-Set(AS) firming agent modified soil, a series of triaxial tests were conducted. The mutual relationships between stress-strain relationship, static strength, dynamic modulus and critical dynamic stress of AS firming agent modified soil and curing time, freeze-thaw cycles, cooling temperature were deeply analyzed. The results show that the unconfined compressive strength of 7 d is 75% than that of 28 d. The stress-strain curves of AS firming-agent modified soil are strain-softening and in brittle failure shape. The static strength, critical dynamic stress and resilient modulus decrease in exponential form with the increase of freeze-thaw cycles, and after 6 cycles of freezing and thawing, the trend is stable. Under the same dynamic stress, the cumulative plastic strain of samples develop from stable state to failure because of freezing and thawing. After many times of freeze-thaw cycles, the lower the negative temperature, the little influence on the mechanic properties.     

Stress Distribution in Fixed Anchor Length of Compression Type Anchor in Soft Rock Mass

To study stress distribution in fixed anchor length of compression type rock anchor cable, field test was carried out. Strain sensors were placed in the grout paste in fixed anchor length with certain interval. Axial and radial strains of the grout were noted by test. Shear stress distribution curve along the fixed anchor length was obtained by stress-strain relationship and static equilibrium analysis. Analysis results prove that maximal shear stress on the interface can achieve upper level and move forward when the load increases; the transmission length is short; shear stress transfers forward; and three dimensional state is obvious around capacity plates in soft rock mass.     

Slip Line Field of Plane Strain Problem Obeying the GeneralizedHoek Brown Failure Criterion

According to the characteristics of plane strain problem of elastic and plastic mechanics, the bi parametric expressions were derived for stress components satisfying the generalized Hoek Brown failure criterion. Being substituted into the static equilibrium partial differential equations, a group of first order hyperbolic pseudo linear differential equation partial differential equations was obtained. Utilizing the determinant method and proper variable transformation, the characteristic direction and the differential relation equation for the stress partial differential equations were acquired. The characteristic direction indicted that in plastic zone obliquely intersecting conjugate shear slip surfaces formed two families of non orthogonal slip lines, in which conjugate angle varied with the limit stress state and the material physical properties of Hoek Brown rock mass. Since the principal direction of the maximum principal stress was circumferential in perfectly elasto plastic surrounding rock of the plastic zone around a circular opening suffering the symmetric initial stress field, the angle between the tangential direction of slip line with the principal direction of the maximum principal stress was the function of the minimum principal stress(i.e. the radial stress). Combined with the analytical solution, the polar coordinate differential equation was derived and furthermore, the polar coordinate curve equation for the slip line was obtained.     

Constitutive Model for Negative Simple-sheared Soil-concrete Interface

Negative shear property of soil-concrete interface is one critical issue in civil engineering. According to negative simple shear tests of soil-concrete interfaces with various water content, quantitative analysis on changes of shear strength, friction angle and cohesion force of negative sheared interface against positive shear experience were conducted. Several empirical formulae were proposed for above changes, and critical positive shear ratio was put forward. Based on improved stress-strain equation, mathematical model for tangent modulus of negative sheared soil-concrete interface was established. This new model consists ten parameters which can be determined by simple shear test. Process for determining these ten parameters was presented, and empirical values of them were obtained. Simulations for stress-strain curves of soil-concrete interface were carried out, and good ability of the model was found.     

Experimental Study on Influence of Stress Path on the Constitutive Relations for Reshaped Clay

In order to study the influence of stress path on the constitutive relations for reshaped clay,the triaxial compression tests under stress paths with drainage in cases of increased,constant and decreased mean normal stress and undrained conventional triaxial compression test have been carried out,the stress-strain relations under the four stress paths are obtained.Through visualization the three-dimensional surfaces of shear and volume strain in the whole stress field under the four stress paths are given respectively by numerical modeling.In addition,the four families of shear and volume yield loci are plotted respectively.It is found through comparing the deformation results under the four stress paths that there are obvious differences in the stress ranges,the strain peaks,the shapes of strain surfaces and the trends of variation of volume yield loci,however,for the four families the shear yield loci are similar.These results demonstrate that the influence of stress path on the constitutive relations of clay is considerably large and could not be neglected,and it relates to the effective mean normal stress closely.     

Validity Judgment Computation of a Causal Relation Model of Strain gauge Arrays

The balance principle of the strain gauge arrays observation is analyzed in detail according to the first invariant of the stress. A partly damaged strain gauge arrays' strain conversion is discussed. The stress computation through the strain gauge arrays under different condition is confirmed. A method for determining the validity of the strain gauge arrays is studied and summarized. A collection procedure containing the strain gauge arrays' validity judgment, strain conversion, and stress computation is established based on the causal relation model.     

Analysis of the Lateral Deformation of Structural Soft Dredger Fill with True Triaxial Test

In order to study the mechanical properties and structural properties of soft dredger fill under lateral deformation. Based on true triaxial and WF stress path apparatus, lateral unloading undrained tests were carried out. The results derived from true triaxial test, WF stress path test and routine triaxial shear test were analyzed. The contrastive conclusions are as follows: The stress-strain curve of true triaxial unloading test shows a softening behavior, which is different from the hardening behavior for that of conventional triaxial shear test. With the increase of initial confined pressure, the dilatancy of soil transfers from shrinkage to expansion. The structural yield stress of the unloading soil with true triaxial test method is dramatically larger than that with the latter two tests due to the effect of intermediate principal, which exhibits a nonlinear growth as the coefficient of intermediate principal bd stress jumps. The shear strength indexes of unloading soil with true triaxial test is larger than that with WF stress path test. Compared with the results of routine triaxial test, the internal friction angle of true triaxial test is bigger while the cohesion is smaller.     

Design for Object-Oriented FEM Analysis Program of Three-dimension Solid

In the nonlinear analysis, stress state and stress history are in respect to stress-strain law. It is needed to store its stress history in calculation and analysis. Based on stress state of material, Object-Oriented Programming method used has the advantage in storing stress history intelligently. It overcomes the shortcoming of Process-Oriented Programming method, saves space and reduces difficulty of programming. Based on the three-dimension solid element, how to design for Object-Oriented nonlinear FEM Program.     

Analysis of ultimate load and mechanismsof failure in large cast steel joints

Based on a project of pipe truss roof framing in Chengdu, the ultimate limit state analysis is performed on cast steel joints by using FEM, in which Linear hardening elastic-plastic constitutive model is considered. The results show that, the joint has a great accumulation of plastic strain in ultimate loads, and the point of maximum stress is different from that in elastic with the development of the yielding zone. The paper propoes to use rate-type tensor for the large strain geometric nonlinear problem, shows the criterion of failure formation , and indicates that its essential to apply limit state analysis to the determination of the weakest location on cast steel joint design, which provides basis for understanding the failure modes and failure mechanism of cast steel joint and the cast steel joint design.     

约束再生混凝土足尺试件受压应力应变全曲线试验研究

为研究箍筋约束再生混凝土的单轴受压应力应变全曲线,对9个直径为500 mm、高度为1 500 mm的再生混凝土圆形柱进行试验,采用20 000 kN伺服液压试验机进行位移控制加载。试验参数主要为纵筋率、箍筋间距与直径、加载应变速率。试验结果表明,箍筋间距、配箍率对试件延性影响较大。当加载应变速率由0.000 003/s增大到0.003 3/s时,试件的峰值应力增大1.14倍。分析表明,再生混凝土应力应变全曲线与普通混凝土类似,但下降段较普通混凝土陡峭,脆性更为明显。     

a rational method for defining damage variables in one dimension

To address the issue of whether it is rational to define damage variables through the degradation of elastic modules, we essentially clarified which material module should be used to describe the damage variables in unidimensional cases, and pointed out that the reference undamaged working state of material was needed firstly in order to measure the degree of damage if the strain equivalence hypothesis was used. We maintained it was baseless to consider the linear stress strain working state as an undamaged reference state. We deduced a rational definition of elastic and elasto plastic damage variables. We stated that the degradation of an unloading module can be used to define the damage variables. To illustrate the definition of damage variables, we presented an example of establishing a damage constitutive equation of concrete in compression, validate the elastic strain equivalence hypothesis, and provide an appropriate approach for establishing a damage constitutive model.