Showing papers in "Rock and Soil Mechanics in 2007"

An anisotropic elastoplastic model for soft clays

Abstract: The modified Cam-Clay model is only applicable for isotropically consolidated soft clay;and it cannot describe the anisotropic stress-strain behavior of anisotropically consolidated clay.A new model,which can be adopted for anisotropically consolidated soft clay,was developed here based on the modified Cam-Clay model.An anisotropic tensor and a shape parameter were introduced in the yield surface of the model.Initial value of the anisotropic tensor was determined by the initial consolidation stress state,and its evolution was described by a rotational hardening law both depending on plastic volumetric and deviatoric strain.The shape parameter enhanced the flexibility of the yield surface.Effectiveness of the model was verified by the triaxial experimental data of kaolin clay,boston Blue clay and Otaniemi clay.

An adaptability study of Gaussian equation applied to predicting ground settlements induced by tunneling in China

Abstract: Gaussian equation suggested by professor Peck in 1969 is one of the most popular-used methods to predict the soft ground movement induced by shallow tunneling work. The application of this method in China should be carefully verified and evaluated since it was suggested based on case history studies. Because of the lack of the well-compiled measured data in China, there is no clear conclusion on the adaptability of the Gaussian equation to predict the ground movement induced by the metro tunneling until now. Based on the collection of the published case history analysis, including more than 30 sets of measured data from eight areas in China, the accuracy of the Gaussian equation applied to predict the ground settlement is discussed and evaluated in detail. some typical key parameters are also suggested to the designers.

Introduction of hypoplasticity

Abstract: A hypoplastic constitutive model and hypoplasticity are described briefly.A procedure for the determination of its parameters is described in detail.This model is based on nonlinear tensorial functions and is developed without recourse to concepts of elastoplasticity theory such as yield surface,plastic potential,flow and hardening rules and decomposition of deformation into elastic and plastic parts.The paper also presents the hypoplastic model that is capable of capturing the salient features of granular materials,in particular,for problems involving extremely low stress levels or extremely high stress.

Influence of matric suction on shear strength behavior of unsaturated soils

Abstract: It is evident that unsaturated soils are usually encountered in nature and engineering practiceHence,the understanding of unsaturated shear strength behavior is necessaryThe shear strength behavior of unsaturated soils is investigated on the basis of pressure plate test and direct shear testThe results show that the angle of internal friction decreases with increasing degree of saturation and the cohesion attains its peak value at degree of saturation 40 %-60 %According to these results,the unsaturated shear strength behaviors of non-cohesive and cohesive soils are demonstratedThe internal friction angle of non-cohesive soils increases with increasing matric suction;and the pseudo-cohesion attains its peak value in primary transition zone and then decreasesThe cohesion of unsaturated cohesive soils increases with increasing matric suction until threshold suction reachesThe threshold suction is the minimum suction at the residual zone of unsaturationThe cohesion of unsaturated cohesive soils can be predicted before the threshold suction,becauseφ b value is approximately constantMoreover,the state paths influence cohesion in excess of the threshold suction

Discuss on the correlations between landslides and rainfall and threshold for landslide early-warning and prediction

Abstract: Historical data of rainfall and landslides in Shenzhen City are researched in detail.And the partial correlation between landslides and rainfall,the correlation between landslides and rainfall intensity,and the statistic relation between landslide and rainfall time are analyzed.The results show that: First,the partial correlation coefficient is better between landslide and the two-days rainfall,and the sum of rainfall.It is clear that the regional landslides will appear when the rainfall reaches some numerical values.Second,the relation is very strong between rainstorm and landslides;the correlation coefficient is beyond 0.8,especially the great rainstorm and the unusual rainstorm.They can trigger landslides.Third,the occurrence time of landslide is parallelism to the rainfall season,the more is rainfall,the more is landslide.In addition,the occurrence time of landslide accords with or lags behind that of rainfall and rainstorm;but the lagged time is less than four days,and the landslide probably arises in the same day and the second day when the rainstorm takes place.The threshold values of critical rainfall and critical rainfall intensity are discussed.At the last,the indexes and grades of spatial early-warning are taken by the coupling between the model of landslide hazards and rainfall so that it predicts the time and space that the landslides occur.

Application of strength reduction finite element method to road tunnels

Abstract: Strength reduction finite element method is used in road tunnel. Based on latent slip surface and safety factor, the rationality of the designed parameters of support and excavations can be judged. The result shows that the distribution of plastic zone is effected greatly by Poisson’s ratios; however, safety factor varies little with it. The higher quality of rock, the more distribution of plastic zone under limit state is got. But the failure zone is lower. Additionally, the more thickness of upper rock layer, the more distribution of plastic zone under limit state is found. Meanwhile, the safety factor decreases. The example also shows that the plastic zone of good rock quality is larger than that of poor rock quality at failure condition. So it is deserved to study about judging safety of tunnel only by its plastic zone.

Investigation on shear rheological mechanical properties of shale in Longtan Hydropower Project

Abstract: Shear rheology experiments were carried out for the shale in Longtan Hydropower Project by using a servo-controlled shear rheology testing machine.The variation law of shear displacement of shale with the time is analyzed.The variation tendency of shear rheological rates under different stress states is discussed.At the same time,the variation of shear strength of rock with the time is discussed.When the proposed nonlinear rheological component(NRC model) by the author is parallel connected with Kormanura model;so that a new nonlinear rheological model can be gained,which can describe the accelerating rheological properties.Using the shear rheological curves of shale,the proposed nonlinear viscoelastoplastic shear rheological model of rock is identified,which gets the viscoelastoplastic rheological parameters of the shale.The comparison between the rhelogical model and experimental result shows that the proposed rheological model behaves reasonably well.

A developed dynamic viscoelastic constitutive relations of soil and implemented by ABAQUS software

Abstract: Based on the one dimension dynamic stress-strain relation Davidenkov skeleton curve,which is corrected by using sectional functions;and the upper limit of failure shear strain amplitude is used as the sectional point.The dynamic shear stress-strain circle curves are constituted by referencing Mashing rules and implemented by sub-program based on ABAQUS finite-element software.A representative soft site in Nanjing City is selected;and its dynamic nonlinear response are calculated when a cosine time-history curve and seismic acceleration waves with different peak values are inputted from bedrock interface.The results prove that the developed dynamic elastoplastic constitutive relations of soil are feasible and accurate;and it can be used to investigate the nonlinear earthquake response analysis of general site.

Experimental study on coefficient of thermal conductivity and specific volume heat of loess

Abstract: Experimentation for coefficient of thermal conductivity and specific volume heat of loess is explored first;and the experimental installation is installed.Then taking loess samples with different densities and moisture contents,the coefficient of thermal conductivity and specific volume heat of loess are tested.The test data show that for loess with constant water content,if the density is increased,the coefficient of thermal conductivity and specific volume heat both are increased.And for loess with constant density,if the water content is increased,the coefficient of thermal conductivity and specific volume heat both are increased too.Based on research on the test data,two formulas are obtained to determine the coefficient of thermal conductivity and specific volume heat of loess.After that it is pointed out that compared with soil density changes,water content change can result in more variations of coefficient of thermal conductivity and specific volume heat of loess.

Test study on permeability properties of coal specimen in complete stress-strain process

Abstract: The permeability test of coal in the complete stress-strain process is carried on the MTS815 Servo-controlled Rock Mechanical Test System.Test results indicate that the permeability rate curve and the complete stress-strain curve of coal almost have the same variation trend.But the permeability rate-strain curve is relatively “hysteretic” which indicates that the close relativity between the evolvement of damage and the development of permeability rate and the permeability developing needs a time.The micro-structure and micro-composition of coal are more complicated and different than other sedimentary rocks;so the permeability of different kinds of coal is much different.Moreover,the micro-porosity and micro-fracture of coal are more developed,so the influence of available confining pressure on penetrability of coal is more obviously.

Experimental study on thrust force during tunnel excavation with earth pressure balance shield machine

Abstract: Thrust force is an important construction parameter during tunnel excavation with shield machine.It not only determines the advance rate but also influences other working parameters,such as cutter torque and rotation speed,etc.But so far,the research on thrust force,especially on frictional force between soil and shield tube,still does not go into enough depth.Therefore,a special model test scheme on tunnel excavation with earth pressure balance(EPB) shield machine is designed by the authors;while the silt sandy ground in Shanghai area is taken as prototype,then different parameters of shield machine and ground properties can be combined for testing.Based on the test results,studies are carried out to discover the variation law of thrust force,the mechanism of friction between soil and shield tube and its influence factors.Furthermore,a calculation formula of thrust force is deduced.

Evaluation of slope stability based on genetic algorithm and fuzzy neural network

Abstract: Slope engineering is a dynamic,fuzzy,open and complicated nonlinear system,which makes it difficult to evaluate complicated slopes conforming to reality by the traditional analysis method.The factors which control and affect slope stability are random and fuzzy variables.As artificial neural network can consider both quantitative and qualitative factors,it is suitable to solve the uncertain problems,such as the estimation of slope stability.In consideration of influencing factors of the slope stability,a fuzzy neural network model based on genetic algorithm is established to predict slope stability,and many engineering data are collected to train and examine the model.Compared with the BP neural network,the presented model has higher predicting accuracy.

Study on attenuation parameters of blasting vibration by nonlinear regression analysis

Abstract: Determination of the blasting attenuation characteristics is of most importance for blasting design.The linear and nonlinear regression analysis methods to evaluate the parameters in the S formula and revised S formula are investigated.Based on the investigation,the concept for the residual sum of squares and nonlinear residual sum of squares(standard residual sum of squares) are proposed.The algorithm of nonlinear residual sum of squares is also introduced in detail.In accordance with the engineering case,the standard residual sum of squares is used to evaluate the deviation of vibration velocity acquired by the site monitoring and the regression analysis results by S and revised formula.It is shown that the accuracy of the nonlinear regression analysis method is significantly better than the linear regression analysis method.It is also reported the residual sum of squares and the standard residual sum of squares obtain by S formulas and revised S formula are almost similar;for this case,the two blasting attenuation formulas are available.

Forecasting nonlinear time series of surrounding rock deformations of underground cavern based on PSO-SVM

Abstract: The in-situ monitoring data of the surrounding rock displacements reflect the changing of mechanical situation of surrounding rock-supporting systemTo overcome the excessive learning problem of ANN,a new method of PSO LS_SVM to forecast the nonlinear displacements of surrounding rock in underground engineering is presented based on monitoring dataParticle swarm optimization is used to choose the parameters of support vector machine,which can avoid the man-made blindness and enhance the efficiency and capability of forecastingThe method can rolling forecast the surrounding rock deformations based on monitoring data,in order to adjust the supporting schemes dynamically and ensure the stability of the cavernIt is used to forecast the surrounding rock convergent deformations of Qingjiang Shuibuya Underground Powerhouse;and it is shown that the method is feasible and precise

A new earth reinforcement method by soilbags

Abstract: The reinforcement principle and some properties of soilbags are introducedSome practical application cases of soilbags in Japan,such as the reinforcement of building and road foundations,the construction of retaining wall and debris-diversion embankment,are presentedThe reinforcement of soilbags is contributed by a tensile force,which thereafter induced an apparent cohesionThe tensile force is produced due to the extension of the perimeter of soilbag under the application of external force

Test research on three-dimensional swelling pressure of remolded expansive clay

Abstract: A series of tests were conducted for the unsaturated expansive soil using three-dimensional swell-shrinking deviceThe three-dimensional swell-shrinking characteristics of remolded expansive soils was studiedAccording to the test results,three-dimensional swelling pressures are unequalThe lateral swelling pressure is smaller than vertical swelling pressureThe swelling pressure of remolded expansive soil is decreasing with wet-dry cyclesAnd a small strain could bring obvious decreasing of swelling pressureThe logarithmic relation between lateral pressure and strain is proposedA formula of vertical swelling pressure with initial water content and dry density is given

Stability assessment of rockmass engineering based on failure approach index

Abstract: On the basis of the YAI(yield approach index) concept and its phase complementary parameter ω called danger coefficient,which were proposed in the previous paper by the authors,the new concept,FAI,is introduced.It is the sum of danger coefficient ω and FD(failure degree) which describes the damage degree of rockmass in the plastic zone.Two cases are presented to analyze the validity and utility of FAI.The results proved that this parameter can line out the position and extent of failure zones,damage zones and disturbed zones,and characterize the stability rules of rockmass.In the stability assessment of slope engineering,the sliding surface denoted by this parameter perfectly agrees with that measured in situ.So,FAI is of very importance in the stability assessment,the election of excavating and supporting schemes.

Study of characters and formation mechanism of hard crust on tidal flat of Yellow River estuary

Abstract: The typical area was chosen on the tidal flat of the Yellow River estuary where hard crust develop,simulating wave load test with a light compactor was carried out to study the repeat liquefaction and change law of crust layer within one meter through the monitoring of pore water pressure and in-situ soil strength measure.The results revealed that the dynamic response varies a lot along depth and the strength of silt increases after repeat liquefaction;the upper crust layer increases more obviously than the down.The undisturbed samples were taken before and after vibrating;and quantitative analyzing of SEM structure images is carried out on the samples of different depths.The results revealed that some indexes of microstructure change evidently;such as asymmetry coefficient descended,granularity direction strengthed,granulometric fractal dimensions improved and figure coefficient improved and ratio of pore will descended etc.The transformation varied a lot along depth with the range mainly concentrated above the upper site.The relation between the dynamic response character and their microstructure changes has been discussed;and the character and formation mechanism of hard crust under the control law of microstructure are grasped primarily on this base.

Three-dimensional effects in low strain integrity test of PCC pile

Abstract: The objective of this paper is to present a theoretical evaluation of the capabilities of low strain integrity tests in estimating the pile length and integrity of PCC pile,a type of large diameter thin-wall cast-in-place concrete pile.One-dimensional and three-dimensional finite element modelling are performed to gain an understanding of the dynamic response of intact and defective PCC pile subjected to impulse loads.The three-dimensional effects in low strain integrity test of PCC pile are studied in detail.The result shows that the impact response of a PCC pile is affected by bending vibration,longitudinal vibration and waves reflected between the side boundaries at the surface of pile top.Based on the numerical results,suggested guidelines for selection of the locations of the hammer impact point and the recording point and test results interpretation of this new type of pile are given.

Study on mechanism of thermal damage fracture for limestone

Abstract: Under uniaxial compressive and tensile loading,limestone’s meso-structure has been investigated by real-time under high temperature with advanced experiment instruments such as High Temperature Fatigue Testing System with Scanning Electron Microscope in Tsinghua University.Thermal damage evolution process,thermal crack growth and fracture characteristic of rock are discussed.The damage model of rock thermal crack growth is established.Combining test results with the concept of dislocation and diffusion creep,thermal damage deformation fracture mechanism of rock are researched.The results show that the thermal crack is intercrystalline crack mainly within the scope of 500 ℃.

The electrical resistivity characteristics of unsaturated clayey Soil

Abstract: Electric current flows through clayey soils by the conduction of void water,electric double layer and clay minerals.The electrical characteristics of clay are discussed at first.Based on the hypothesis that there are three conductance pathways in unsaturated clay,and aiming at the deficiency of the existing models of unsaturated clayey soil,the electrical resistivity model of unsaturated clayey soil is studied.The main factors which affect the electrical resistivity of soils are summarized and analyzed.Through the laboratory tests,the characteristics of the influences of water content,the resistivity of pore fluid,temperature and porosity on the electrical resistivity of Hefei expansive soil are studied.

Experimental study on degradation of stiffness of saturated soft clay under undrained cyclic loading

Abstract: Degradation of stiffness and strength will occur in soft clay under cyclic loading because of the generation of pore pressure.Unfortunately,previous work rarely incorporated the effect of initial deviator stress on cyclic degradation behavior.In this paper,the degradation of stiffness characteristics of normally consolidated soft clay subjected to undrained cyclic triaxial loading are investigated at different numbers of cycles,different cyclic stress levels and different initial deviator stresses.The influence of these three factors on the shear stiffness is investigated by analyzing the secant shear modulus in each unload-reload loop.It’s observed that the stiffness reduced with progressive number of cycles.On the other hand,higher cyclic stress level and initial deviator stress accelerate the degradation of stiffness.A linear relationship is obtained if secant shear modulus is plotted against numbers of cycles on a semi-log scale.This relationship has been synthesized in the form of an empirical equation.The failure stiffness ration for Xiaoshan soft clay is obtained from the equation.

Stability analysis of slope reinforced with piles by using strength reduction FEM

Abstract: By using elasto-plastic finite element method(FEM) based on the technique of shear strength reduction(SSR),stability of slope reinforced with piles against sliding and the behavior of stabilizing piles in slope are numerically analyzed. The criterion of iteration non-convergence conventionally used for assessing the instability state of slopes is combined with that of the uncontrolled displacement at a certain characteristic node on the slope surface to evaluate the limit-equilibrium state of pile-soil-slope system,and its validity is demonstrated through comparative studies for typical example. Furthermore,a typical case slope reinforced with piles is investigated through numerical analyses. The results show that the potential slip surface determined by SSR-FEM is shallower than that by limit equilibrium method and limit analysis method for a slope reinforced with piles.

Experimental study on ultrasonic properties of sandstone before and after high temperature

Abstract: Ultrasonic velocities and dimensions of sandstone specimens from Jiaozuo that experience different temperature(100 ℃-1 200 ℃) are measured;and their uniaxial strength are tested.P-wave attenuation coefficients of rock specimens are calculated.Crack densities are calculated using relations between ultrasonic velocity and elastic moduli.The volumes,ultrasonic velocities and attenuation coefficients of sandstone specimens before and after high temperature are compared.Results show that the cracks of sandstone specimens develop so that the compactness and ultrasonic velocities of specimens decline.For sandstone specimens that experience different temperature the changes of ultrasonic velocity and uniaxial strength are obviously discrepant.But changes of attenuation coefficients are similar to uniaxial strength.It can be taken as reference to evaluate uniaxial strength at a certain extent.

Study on slope failure criterion based on strength reduction and gravity increase method

Abstract: According to deformation observation data analysis of typical landsides,such as Jiming Temple landslide etc.,the deformation failure mechanism is summarized;and then the mechanism is applied to slope stability analysis as the failure criterion by adopting strength reduction method and gravity increase method.By classical instances,the failure criterion based on deformation trend is suggested;various factors effecting computation accuracy are also analyzed;and at the same time,the reliability and stability of previous methods are demonstrated.The result indicates that the failure criterion has a clear concept,special physical meaning and distinct definition for slope stability analysis.By comparison,the precision and reliability on basis of strength reduction method is better,and can be applied to most slope stability analysis.

Analysis of soft soil with viscoelastic fractional derivative Kelvin model

Abstract: Soft soil is a material with the characteristics of viscosity,elasticity and plasticity.Till now,we have not yet seen researches on the creep laws of soft soil by using the models of fractional calculus.A number of one-dimensional consolidation tests for the Pearl River Delta typical soft soil have been carried out;and the creep laws of the soft soil are simulated by using fractional Kelvin model.The creep compliance and relaxation modulus of fractional Kelvin model and its application are discussed.It is shown by comparison with the traditional Kelvin model that fractional Kelvin model can simulate the creep compliance curves accurately.It has a simple and uniform form,only a few of the parameters should be adjusted in the calculation processes.It is predictable that fractional Kelvin body will have wide applications to the field of rheological constitution of soft soil and its calculation.

Experimental results and numerical predictions of ground vibration induced by high-speed train running on Qin-Shen Railway

Abstract: Qin-Shen Railway is the first special passenger traffic line in China.In 2002,the Ministry of Railways organized the second homologation test.The authors performed the field vibration measurements at various distances from the track.Firstly,the field measured vertical accelerations are analyzed.The attention is paid on the effect of train organization and train speed on the ground vibration.Secondly,the track is modeled as a beam on an elastic foundation;the vibration model of moving train-track-ground is constructed.Based on the theory of thin layered method(TLM),the quadratic shape function TLM is derived.The Green function of a layered half-space is obtained.At last,the ground vibration from the track is analyzed and the result is validated by means of in-situ vibration measurements.The results show that the acceleration spectrum has a maximum at the fundamental axle passage frequency.Vibration amplification is not appeared because the trains speed far less than the Ralyeigh wave velocity of the ground.The vibration acceleration is proportion to the wheel load.

Numerical simulation of abutment pressure distribution laws of working faces

Abstract: The dynamic movement of the working face abutment pressure during the coal mining is simulated with the FLAC-3D simulating software.And the curve about the relation between the abutment pressure concentrated coefficient and working face advanced distance is fitted according the simulating results.Contrasting simulation results,the relationships between abutment pressure concentrated coefficient and the distance which distant S meter from the rip with factors such as working face advanced distance,face length,coal seam thickness and buried depth are obtained.Then gained the attribution laws of abutment pressure.Those laws can offer gist for roadway maintenance,coal and gas outburst and caving evaluation of top coal.

Laboratory study of permeability of lightweight soil with EPS under different consolidation pressures

Abstract: The influences of age,cement-mixed ratio and EPS content on the permeability of LWS-EPS are studied mainly based on the permeability test.For simulating the change of coefficient of permeability of LWS-EPS in real situation,the permeability test is carried out under different consolidation pressures by routine triaxial apparatus.The results indicate that the coefficient of permeability decreases with the increasing of cement mixed ratio,age and consolidation pressure and the decreasing of EPS content,which have a power function relationship approximately.But the degree of decreasing to the coefficient of permeability varies with mixture ratio and age.

Development and implementation of Duncan-Chang constitutive model in FLAC~(3D)

Abstract: According to the further developing platform,the Duncan-Chang constitutive model is developed in FLAC3D with VC++ environment.A series of numerical simulations of triaxial tests are performed to verify the correctness of the compiled model.The provided basic procedures and programming essentials of the constitutive model redefining in FLAC3D offer a reference for other developers.Because of the characteristic of the development environment,a user-defined model has the same execution efficiency with the in-built model in FLAC3D.