Undrained dynamical behavior of Nanjing flake-shaped fine sand under cyclic loading

A series of dynamic behavior tests on Nanjing flake-shaped fine sand were performed by using the WFI cyclic triaxial apparatus made in England. The dynamic behaviors of Nanjing flake-shaped fine sand under different static deviator stress levels and cyclic stress ratios were studied. Through comparing the effective stress path under cyclic loading with static loading, the processes of liquefaction of saturated Nanjing flake-shaped fine sand with development of dynamic pore-water pressure, including the initial compact state, compression state and dilative state, were investigated. The variation of the shear stiffness with the number of cycles and cyclic strain was investigated by analyzing the secant shear modulus in each unload-reload loop of dynamic stress-strain relationship. And by means of the exponential function, the empirical equations of the relationship between secant shear modulus Gsec, shear modulus ratio Gsec/Gmax and cyclic strain ε were established based on series of test results. The results show that according to different combinations of static deviator stress and cyclic stress, two kinds of failure patterns with deviator stress reversal or no deviator stress reversal are observed in the samples tested in this series, including cyclic mobility and the failure of accumulation residual strain. In addition, the degradation of dynamic shear modulus is due to the development of vibration pore-water pressure and it is observed that the shear modulus reduces with the progressive number of cycles.     

Scattering wave field around a cavity with circular cross-section embedded in saturated soil using boundary element method

Based on Biot＇s theory and considering the properties of a cavity, the boundary integral equations for the numerical simulation of wave scattering around a cavity with a circular cross-section embedded in saturated soil are obtained using integral transform methods. The Cauchy type singularity of the boundary integral equation is discussed. The effectiveness of the properties of soil mass and incident field on the dynamic stress concentration and pore pressure concentration around a cavity is analyzed. Our results are in good agreement with the existing solution. The numerical results of this work show that the dynamic stress concentration and pore pressure concentration are influenced by the degree of fluid-solid coupling as well as the pore compressibility and water permeability of saturated soil. With increased degree of fluid-solid coupling, the dynamic stress concentration improves from 1.87 to 3.42 and the scattering becomes more significant. With decreased index of soil mass compressibility, the dynamic stress concentration increases and its maximum reaches 3.67. The dynamic stress concentration increases from 1.64 to 3.49 and pore pressure concentration improves from 0.18 to 0.46 with decreased water permeability of saturated soil.     

Characterization of EBPVD micro-layer composites and simulation of its internal stress state

Based on the basic operating principal and the technology characteristic of electron beam physical vapor deposition(EBPVD) technique, EBPVD was used to prepare the micro-layer composites. The effect on the substrate preheating temperature was taken into accounts and the finite element analysis package ANSYS was used to simulate the internal stress field and the potential displacement changing tendency. The results show that one of the most important quality factors on the judgment of micro-layer composites is the adhesion between the substrate and the deposition layers as well as among the different deposition layers. Besides the existance of temperature gradient through the thickness of layers, the main reason for the internal stress in micro-layer composites is the mismatch of various properties of the layer and the substrate of different thermal expansions and crystal lattice types. With the increase of substrate preheating temperature, the inter-laminar shear stress also takes on a tendency of increase but the axial residual stress decrease.     

Experimental study on stress-strain-temperature models for structural steel

For the research on steel structure in fire,it is very important to determine the properties of structural steel at elevated temperature.Up to now,the high-temperature properties of material is believed to be related to only temperature state,which is not precise enough to simulate the behavior of steel structures under different combinations of heating,cooling,loading,and unloading.To analyze the influence of the temperature-load history on the steel properties,a series of tests were carried out under different temperature-load paths about steel Q235,which is widely used in steel structures in China.In this paper,the method to set the temperature-load paths was introduced;the variety regulation of steel properties changing with temperature was analyzed under different paths;according to experimental results,the formulas of elastic modulus and yield strength at elevated temperature were fitted,and the stress-strain-temperature 3D relationships of structural steel under different paths were presented.     

Experiments and analysis for rheological properties of MRF

The work principle of flat-plate structure under shearing mode is expounded based on a vertical type rheometer for MRF which combined data acquisition with treatment and result display.The formula to calculate shearing stress is deduced.Based on different recipe for MRF,experiments under different working gap length were done by altering the intensity of magnetic field.The rheological model for MRF was established and the relationships between shearing stress,viscosity and magnetic field intensity were deduced.Experiments indicate that MRF has the flowing characters:with an increase of the magnetic induction and the nominal shear rate,the shear stress of MRF increases.However,as the working gap decreases,the shear stress increases.MRF has shear thinning property under magnetic field.     

Effect of pore pressure on deformation and unstable snap-back for shear band and elastic rock system

Fast Lagrangian analysis of continua（FLAC） was used to study the influence of pore pressure on the mechanical behavior of rock specimen in plane strain direct shear, the distribution of yielded elements, the distribution of displacement and velocity across shear band as well as the snap-back （elastic rebound） instability. The effective stress law was used to represent the weakening of rock containing pore fluid under pressure. Numerical results show that rock specimen becomes soft （lower strength and hardening modulus） as pore pressure increases, leading to higher displacement skip across shear band. Higher pore pressure results in larger area of plastic zone, higher concentration of shear strain, more apparent precursor to snap-back （unstable failure） and slower snap-back. For higher pore pressure, the formation of shear band-elastic body system and the snap-back are earlier; the distance of snap-back decreases; the capacity of snap-back decreases, leading to lower elastic strain energy liberated beyond the instability and lower earthquake or rockburst magnitude. In the process of snap-back, the velocity skip across shear band is lower for rock specimen at higher pore pressure, showing the slower velocity of snap-back.     

A DEM investigation on simple shear behavior of dense granular assemblies

A micromechanical investigation on simple shear behavior of dense granular assemblies was carried out by discrete element method.Three series of numerical tests were performed to examine the effects of initial porosity,vertical stress and particle shape on simple shear behavior of the samples,respectively.It was found that during simple shear the directions of principal stress and principal strain increment rotate differently with shear strain level.The non-coaxiality between the two directions decreases with strain level and may greatly affect the shear behavior of the assemblies,especially their peak friction angles.The numerical modelling also reveals that the rotation of the principal direction of fabric anisotropy lags behind that of the major principal stress direction during simple shear,which is described as fabric hyteresis effect.The degrees of fabric and interparticle contact force anisotropies increase as particle angularity increases,whereas the orientations of these anisotropies have not been significantly influenced by particle shape.An extended stress–dilatancy relationship based on ROWE-DAVIS framework was proposed to consider the non-coaxiality effect under principal stress rotation.The model was validated by present numerical results as well as some published physical test and numerical modelled data.     

PHYSICAL SIMULATION AND ANALYSIS OF METHANE TRANSPORT IN COAL SEAM

This paper studies the effect of ground stress, pore gas pressure and adsorbed methane on methane transport in coal seam, and researches into the applicability of Darcy‘s law to methane transport. The additional expansion stress of coal induced by adsorbed methane is measured. The paper establishes the constitutive equation of methane transport, taking ground stress, pore gas pressure and Klinkcnburg‘s effects into considcration, The features of methane transport under the condition of given stress or strain have been analyzed.     

Vibration pore water pressure characteristics of saturated fine sand under partially drained condition

Vibration pore water pressure characteristics of saturated fine sand under partially drained condition were investigated through stress-controlled cyclic triaxial tests employed varied fine content of samples and loading frequency. In order to simulate the partially drained condition, one-way drainage for sample was implemented when cyclic loading was applied. The results show that the vibration pore water pressure’s response leads the axial stress and axial strain responses, and is lagged behind or simultaneous with axial strain-rate’s response for all samples in this research. In addition, the satisfactory linear relationship between vibration pore water pressure amplitude and axial strain-rate amplitude is also obtained. It means that the direct cause of vibration pore water pressure generation under partially drained conditions is not the axial stress or axial strain but the axial strain-rate. The lag-phase between pore water pressure and axial strain-rate increases with the increase of the fine content or the loading frequency.     

Study of the Hole Wall Soil’s Capacity by the Action of the Bearing Device of Rodless Drilling Rig

In order to ensure the bearing device of rodless drilling rig to press stably against the hole wall, it has to analyze the contact between the soil pore wall and the bearing device to study whether the soil pore will shear failure. This paper uses the method to calculate the additional stress of any point in soil mechanics to get the three-dimensional stress state of any point of the soil pore under the support plate, and use the numerical analysis method to calculate the shear stress and its relative intensity. Under the circumstances of maximum torque and maximum pressure, ABAQUS is used to make a finite element analysis of the capacity of the soil pore. The results of numerical analysis and FEA indicate that in the condition of the support plate will not deform; the contact area between the soil pore and the support plate is rectangular; in the force process, the soil under the ends of the support plate have the trend of yield, while it meets the condition of Mohr-Coulomb not to yield generally.     

Rainfall-induced landslide stability analysis in response to transient pore pressure——A case study of natural terrain landslide in Hong Kong

Transient pore pressure in response to short intense rainfall process plays animportant role in shallow landslide occurrence. Using GIS technology, we carry out the rainfall-induced landslide stability analysis in response to transient pore pressure bymeans of transient and unsaturated rainfall infiltration modeling. A case study isperformed on the shallow landslide stability analysis in Hong Kong. Detailed analysis and discussion reached some useful conclusions on the tempo-spatial behavior andcharacteristics of slope stability response and pore pressure response to typical rainfall process. Comparison analysis is performed on some important issues including landslidestability response in different types of slopes with different hydraulic properties,antecedent rainfall and landslide stability, and the nature of pore pressure response time.These studies might give us an important insight into landslide tringgering mechanismand the hydrological process in response to rainfall, and provide systematic informationand evidences for effective risk assessment and warning system establishment.     

Experimental Study on Electrorheological Performance of a Composite Material

By using SrTiO3/PANI as a dispersed phase and silicone oil as a dispersion medium,the rheological characteristic of the dispersion medium under an electric field and the ERF performances of the mixture of Sr-TiO3/PANI composite particles and silicone oil under different dispersed phase consistencies and diffetent medium viscosities have been investigated.The measurements of the rheological characteristic,that is the shear stress of ERF,under shearing flow were performed with a Couette-type theometer manufactured by HAAKE.It is proved that the rheological characteristic of the dispersion medium follows Newtonian fluid viscosity law and the ERF rheological characteristic of the mixture of the dispersion medium and dispersed phase obeys Bingham fluid model under electric field.The experimental result shows that the effects of different phase consistencies and different dispersion medium viscosities on ERF perfomances are great.The testing expressions of shear stress vs.electric field under different conditions have been obtained by oplynomial fit,which is the theory foundation of application in engineering.     

Effects of stress conditions on rheological properties of granular soil in large triaxial rheology laboratory tests

In order to study the rheological properties of red stone granular soil,a series of rheological experiments were executed on large tri-axial rheological apparatus.Under 100,200 and 300 kPa confining stress conditions,the rheological tests were carried out.These experiment results showed that the stress conditions,especially the stress level were the critical influencing factors of the rheological deformation properties.Under the low stress level(S=0.1),the granular soil showed the elastic properties,and there was no obvious rheological deformation.Under the middle stress level(0.20.8) creep curves showed the non-linear viscous plastic rheological properties.Especially,under the stress level of S=1.0,the accelerated rheological phase of creep curves occurred at early time with a trend of failure.The stress level had obvious effects on the final rheological deformation of the soil sample,and the final rheological deformation increments nonlinearly increased with stress level.The final rheological deformation increment and step was little under low stress level,while it became large under high stress level,which showed the nonlinearly rheological properties of the granular soil.The confining pressure also had direct effects on final rheological deformation,and the final rheological deformation linearly increased with confining pressure increments.     

Experimental study on formation mechanism of compaction bands in weathered rocks with high porosity

Since Mollema and Antonellini observed compaction bands in the field in 1996,different patterns of compaction bands have been found in laboratory experiments.There are some discrepancies between the laboratory experiments and the field observations:compared to the field observation,the stress levels required to induce compaction bands in laboratory experiments are usually higher than the inferred in the field,and the grain crushing are more intense in the laboratory experiments.In this paper,compaction bands were observed at the maximal principal stresses below 8 MPa,which is lower than the stress level inferred in the field,and there was no severe comminution inside the compaction bands.Experimental results indicate that the porosity and confining pressure have great impacts on the types of localization bands.Lower porosity and confining pressure can promote the growth of shear bands and high-angle shear bands.Higher porosity and confining pressure can promote the growth of discrete compaction bands.Intermediate porosity and confining pressure are favorable for the growth of hybrid modes involving two of the three,i.e.,discrete compaction band,diffuse compaction band and high-angle shear band.The formation of discrete compaction bands is more unstable compared to diffuse compaction bands.The two types of compaction bands can appear in the same type rocks,and diffuse compaction bands are formed under lower confining pressure compared to discrete compaction bands.The reduction of permeability was within 2 orders of magnitude in this study,and it is 2 3 orders of magnitude lower than those obtained by other researchers.     

Hydro-mechanical coupling mechanism on joint of clay core-wall and concrete cut-off wall

The joint of clay core-wall and concrete cut-off wall is one of the weakest parts in high earth and rockftll dams.A kind of highly plastic clay is always fixed on the joint to fit the large shear deformation between clay core-wall and concrete cut-offwall,so the hydro-mechanical coupling mechanisms on the joint under high stress,high hydraulic gradient,and large shear deformation are of great importance for the evaluation of dam safety.The hydro-mechanical coupling characteristics of the joint of the highly plastic clay and the concrete cut-off wall in a high earth and rockfill dam in China were studied by using a newly designed soil-structure contact erosion apparatus.The experimental results indicate that：1） Shear failure on the joint is due to the hydro-mechanical coupling effect of stress and seepage failure.The seepage failure will induce the final shear failure when the ratio of deviatoric stress to confining pressure is within 1.0-1.2; 2） A negative exponential permeability empirical model for the joint denoted by a newly defined principal stress function,which considers the coupling effect of confining pressure and axial pressure on the permeability,is established based on hydro-mechanical coupling experiments.3） The variation of the settlement before and after seepage failure is very different.The settlement before seepage failure changes very slowly,while it increases significantly after the seepage failure.4） The stress-strain relationship is of a strain softening type.5） Flow along the joint still follows Darcian flow rule.The results will provide an important theoretical basis for the further evaluation on the safety of the high earth and rockfill dam.     

Application of BP Neural Network in Prediction of Cu-Pb Composite Plates Properties

Abstract： The artificial neural networks（ANN） , which have broad application, are proposed to develop Cu-Pb composite plates materials. Based on the back propagation（BP） algorithm of the forward muhilayer perceptron, the model to predict the shear stress under different ingredient of the third element and the hot dipping temperature for Cu-Pb composite plates are established. Then the relational model among the third element, hot dipping temperature and shear stress by using the limited data are studied, and the forecast average error is 4%. This model can satisfy the requirements of the precision of forecast in the project experiment process. The results show that the corresponding shear stress is greater when the third element in the element contains more Sn; the most appropriate temperature of hot-dip plating about is 340℃, ＇after predicted with lead/the third element/the best performance of copper composite material element of the third group is the one-element Sn, hot dip plating temperature is 335 ℃ ; two-element is 90% Sn 10% Bi, and hot dip plating temperature is 345 ℃. The prediction results can be used for a reference in instructing the further experimental design.     

Strength and elastic properties of sandstone under different testing conditions

A laboratory experimental program performed on Wuhan sandstones was presented under monotonic loading, partial cyclic loading during loading path and sine wave cyclic loading with different strain rates to compare uniaxial compression strength and elastic properties （elastic modulus and Poisson ratio） under different conditions and influence of pore fluid on them. When the loading strain rates are 10^-5, 10^-4 and 10^-3/s, uniaxial compression strengths of dry sandstones are 82.3, 126.6 and 141.6 MPa, respectively, and that of water saturated sandstones are 70.5, 108.3 and 124.1 MPa, respectively. The above results show that the uniaxial compression strength increases with the increase of strain rate, however, variation of softening coefficient is insignificant. Under monotonic loading condition, tangent modulus increases with an increment of stress （strain） to a maximum value at a certain stress level, beyond which it starts to decline. Under the partial cyclic loading during loading path condition, unloading or reloading modulus is larger than loading modulus, and unloading and reloading moduli are almost constants with respect to stress level, especially unloading modulus. Under the sine wave cyclic loading condition, tangent modulus and Poisson ratio display asymmetric ‘X＇ shape with various strain, and the average unloading modulus is larger than the average loading modulus.     

Elasto-plastic analysis of the surrounding rock mass in circular tunnel based on the generalized nonlinear unified strength theory

The present paper aims to establish a versatile strength theory suitable for elasto-plastic analysis of underground tunnel surrounding rock. In order to analyze the effects of intermediate principal stress and the rock properties on its deformation and failure of rock mass, the generalized nonlinear unified strength theory and elasto-plastic mechanics are used to deduce analytic solution of the radius and stress of tunnel plastic zone and the periphery displacement of tunnel under uniform ground stress field. The results show that: intermediate principal stress coefficient b has significant effect on the plastic range,the magnitude of stress and surrounding rock pressure. Then, the results are compared with the unified strength criterion solution and Mohr–Coulomb criterion solution, and concluded that the generalized nonlinear unified strength criterion is more applicable to elasto-plastic analysis of underground tunnel surrounding rock.     

Calculation and analysis of stress in strata under gob pillars

Aiming at the difficulty in stress analysis for strata under pillars with actual bearing conditions, an approach was proposed to apply multi-sectional linear approximation to the characteristic curves of pillar loads, and stress of strata was calculated under pillars with linear load by calculation method for uniform load. This approach leads to a rapid analyzing method for strata stress under pillars with any form of loads. Through theoretical analysis, strata stress expressions for pillars under linear bearing conditions are obtained. In addition, two concepts, stress increase factor and stress factor, are proposed for the approximate analysis of strata stress by uniform load approximation method. It is also found that the stress increase factor of strata is related to the strata stress factor and the ratio of the minimum load on the pillar’ two ends to the maximum one; and the distribution features of stress factors and the sizes of their influencing areas in strata influenced by overlying pillars are obtained. Combining with the gob pillar conditions of Jurassic coal seam in Tongxin Coal Mine, it is demonstrated that the results obtained by stress distribution analysis of the strata stress in non-influencing areas of pillars with linear bearing through uniform load approximation are in basic accordance with the results obtained for pillars under linear bearing condition. Therefore, it is feasible and accurate to calculate stress in non-influencing area in strata under pillars with linear bearing condition by uniform load calculation method.     

Elastic-plastic solution and experimental study on critical water pressure inducing hydraulic fracturing in soil

It is widely believed that hydraulic fracturing will occur in the clay core of an earth-rockfill dam if the water pressure in the core increases to levels that are high enough to allow a fracture to form. An elastic-plastic solution to critical water pressure inducing hydraulic fracturing(fracture initiation pressure) in soil is derived based on Mohr-Coulomb shear failure criterion and the theory of cavity expansion. In order to verify the applicability of the criteria presented and study the relations among fracture initiation pressure, tensile strength and stress state of soil, laboratory tests are performed on compacted cuboid specimens by true triaxial apparatus. According to the test results, the cracks of hydraulic fracturing existed perpendicular to the minor principal stress plane. The hydraulic fracturing pressure pf increases with the increase of dry density of specimen, pf shows good linear relationship with σ2 and σ3. The prediction from presented equation is compared with test results and other three predictions, of which two are tensile failure(TS) criterion, and the other is Mohr-Coulomb(M-C) criterion. The presented solution is verified, and the other three approaches for pf are evaluated. The comparison indicates that the predicted values from the presented equations agree well with the test values for specimens of low dry density, and the error of the prediction is larger for those of high dry density, especially in lower minor stress states. The predicted average relative error of absolute value Ra from TS1 criterion is 13.3% for all specimens of different dry densities, and each prediction is lower than the test data. On the contrary, most of the predicted values from M-C criterion are greater than the test data, but the average relative error from the presented equation is the minimum. Considering the safety of soil works, an equation from TS1 criterion is suggested to evaluate the occurrence of hydraulic fracturing in earth-rockfill dam designing.