Hypoplastic Constitutive Model Research Articles

A comparative study of the different responses of circular and horseshoe-shaped tunnels to an advancing tunnel underneath

Previous studies on tunnel interaction mainly focused on parameters such as cover depth, pillar depth and construction sequence with little attention given to the influence of tunnel shape. With discontinuous hoop stress distribution along the tunnel lining, non-circular tunnels are expected to respond to the construction of new tunnels nearby differently from circular tunnels. This paper investigates the effect of tunnel shape on multi-tunnel interaction by means of physical and numerical modelling. Two, three-dimensional (3D) centrifuge model tests were carried out simulating the existing circular and horseshoe-shaped tunnels. The test results were back analysed using PLAXIS 3D. An advanced hypoplastic constitutive model with small-strain stiffness was adopted for the numerical back analysis. Centrifuge test results show that the existing horseshoe-shaped tunnel experienced ∼ 20% more vertical elongation in the mid-plane than the existing circular tunnel. The measured and computed bending strains at the invert of the horseshoe-shaped tunnel were three times larger than those at the invert of the existing circular tunnel in the transverse direction. However, hoop stress along the tunnel lining in the existing circular tunnel was larger than that in the horseshoe-shaped tunnel.

Seismic vulnerability assessment of reinforced concrete silo considering granular material‐structure interaction

SummaryAs a special shell structure, silos are used in storing a wide range of multitudinous granular materials. However, seldom have researchers assessed seismic vulnerability of the reinforced concrete (RC) silo. This paper aims at studying the seismic vulnerability assessment of a silo, which is located in Zhoushan, China. Prior to assessing the seismic vulnerability of the selected RC silo, the validation of the hypoplastic constitutive model, which is used to simulate granular materials, is studied. After discussing the validation of the hypoplastic theory, the numerical model considering granular material‐structure interaction is developed by means of the ABAQUS software. And the numerical simulation results are compared with the experimental data obtained from a shaking table test discussed in order to confirm the validation of the numerical model, which is used to study the seismic vulnerability of the RC silo. Then the seismic fragility assessment of the selected RC silo is performed using the incremental dynamic analysis. The analytical results showed that the hypoplastic theory can be used to simulate the stored materials in the silo considering the collapse property before entering plastic state of granular materials. For the design basic acceleration, the fragility curves of the selected RC silo showed that the probability of exceeding the no or light damage state was about 2.12%. For the maximum considered earthquake, the exceeding probability of no or slight and the moderate damage states was 17.63% and 1.31%, respectively. With respect of the severe and total damage state, the exceeding probabilities were almost zero. Therefore, the selected RC silo structure has enough safety stock to withstand strong earthquakes in the future. Finally, a general design procedure considering seismic fragility assessment was presented in order to provide references for other structure design. Copyright © 2016 John Wiley & Sons, Ltd.

Buckling analyses of metal cylindrical silos containing bulk solids during filling

ABSTRACTThe paper presents 3D results on stability of thin-walled cylindrical metal silos made of isotropic rolled and corrugated plates containing bulk solids. The behavior of bulk solids was described using a hypoplastic constitutive model. Nonlinear finite element (FE) analyses with both geometric and material nonlinearity were performed with a perfect and an imperfect silo shell wherein initial geometric imperfections were taken into account in the form of the first eigenmode. The influence of the presence of the bulk solid (sand and wheat) stored during filling on the buckling strength of silos was calculated and compared with that on the strength of the empty silo. Our numerical results indicate a clear strengthening effect of the stored solid on the silo buckling strength.

Modeling the viscous behavior of frozen soil with hypoplasticity

SummaryThis paper presents a hypoplastic constitutive model for the viscous behavior of frozen soil. The model is composed of a ‘solid’ part and a ‘fluid’ part. The solid part is based on the extended hypoplastic model, and the fluid part is dependent on the second time derivative of strain. The performance of the model is demonstrated by simulating some uniaxial compression tests at different strain rates. Moreover, the model can describe in a unified way the three stages of typical creep tests, that is, primary, secondary, and tertiary stage. Copyright © 2016 John Wiley & Sons, Ltd.

Numerical investigation of pile installation effects in sand using material point method

The installation of displacement piles in sand leads to severe changes in the stress state, density and soil properties around the pile tip and shaft, and therefore has a significant influence on the pile bearing capacity. Most current numerical methods predicting pile capacity do not take installation effects into account, as large deformations can lead to mesh distortion and non-converging solutions. In this study, the material point method (MPM) is applied to simulate the pile installation process and subsequent static pile loading tests. MPM is an extension of the finite element method (FEM), which is capable of modelling large deformations and soil-structure interactions. This study utilizes the moving mesh algorithm where a redefined computational mesh is applied in the convective phase. This allows a fine mesh to be maintained around the pile tip during the installation process and improves the accuracy of the numerical scheme, especially for contact formulation. For the analyses a hypoplastic constitutive model for sand is used, which takes into account density and stress dependent behaviour. The model performs well in situations with significant stress level changes because it accounts for very high stresses at the pile tip. Numerical results agree with centrifuge experiments at a gravitational level of 40g. This analysis confirms the importance of pile installation effects in numerical simulations, as well as the proposed numerical approach’s ability to simulate installation and static load tests of jacked displacement piles.

Effect of bulk solid on strength of cylindrical corrugated silos during filling

The paper presents 3D numerical analysis results on the effect of bulk solid on strength and stability of metal cylindrical silos with corrugated walls (without stiffeners) during filling. The behaviour of two bulk solids (dry sand and wheat) was described with a hypoplastic constitutive model. Non-linear FE analyses with both geometric and material non-linearity were performed. The numerical results were compared with the Eurocode formulae. The strengthening effect of the stored solid, wall thickness, solid granular hardness, initial void ratio of solid and wall friction angle on buckling strength was investigated. The major contribution of the paper is the quantitative estimation of the effect of the bulk solid stiffness on the silo stability and wall stresses.

Effects of construction sequence and cover depth on crossing-tunnel interaction

An appropriate construction sequence for crossing tunnels can help minimize the adverse impact on the tunnel that is constructed first (considered as the existing tunnel). However, the influence of construction sequence on crossing-tunnel interaction is complex. Two pairs of three-dimensional centrifuge tests were carried out to investigate the effects of construction sequence on crossing-tunnel interaction. In the first pair of tests, the new tunnel was excavated beneath the existing tunnel in a reference test, while in the other test the new tunnel advanced above the existing tunnel. To study the effects of cover depth on the construction sequence, the depths of the existing and new tunnels were increased in the second pair of tests. An advanced hypoplasticity constitutive model with small-strain stiffness was adopted to back-analyze the tests. The existing tunnel was found to be vertically compressed when the new tunnel was excavated underneath, but vertically elongated when the new tunnel advanced above. This is because the reduction of stress acting on the existing tunnel in the horizontal direction was larger than in the vertical direction when the new tunnel was constructed beneath. On the other hand, the decrease in vertical stress on the existing tunnel was larger than the horizontal stress reduction when the new tunnel was excavated above. This behavior was observed in both pairs of tests, irrespective of the cover depths of the tunnels. As the cover depths of the existing and new tunnels increased, settlement of the existing tunnel due to the new tunnel construction beneath decreased. This is because with the larger cover depths of the tunnels, the increase in mobilized shear stiffness of the soil dominated the increase in stress relief caused by the tunnel excavation.

A SPH approach for large deformation analysis with hypoplastic constitutive model

A hypoplastic constitutive model is implemented in a smoothed particle hydrodynamics code for the first time. An improved wall boundary treatment is presented for better performance. The proposed approach is first validated by comparing the numerical results with the analytical solutions for oedometer test. Two more problems involving large deformation, i.e., the collapse of sand between two parallel plates and the failure of a homogeneous slope, are analyzed to demonstrate the performance of the proposed method.

Simulation of buckling process of cylindrical metal silos with flat sheets containing bulk solids

The paper presents 3D results on stability of thin-walled cylindrical metal silos made from isotropic rolled plates containing bulk solids. The behaviour of bulk solids was described with a hypoplastic constitutive model. Non-linear FE analyses with both geometric and material non-linearity were performed with a perfect and an imperfect silo shell wherein 3 different initial geometric imperfections were taken into account. The influence of a stored bulk solid (dry medium-dense cohesionless sand) during filling on the buckling strength of silos was compared with the strength of an empty silo and with the experimental results available in the literature. Our numerical results indicate a clear strengthening effect of the stored solid on the silo buckling strength as in the experiments, depending upon the wall thickness, wall loading way and imperfection type and amplitude.

Simplified numerical modelling of pile penetration - the press-replace technique

This paper presents a simplified finite element analysis technique, the ‘Press-Replace’ technique, to model pile penetration problems in geotechnical engineering, particularly, pile jacking. The method is employed in standard finite element analysis software. The method involves a straining and a consequent geometry update phase. First, a cone penetration test in (undrained) clay is modelled and compared with the results of analytical, semi-analytical and more advanced finite element techniques. The model sensitivity for the step size and mesh is investigated using a hypoplastic constitutive model. An optimum way of modelling based on the numerical performance is shown. Copyright © 2015 John Wiley & Sons, Ltd.

Effects of Pillar Depth and Shielding on the Interaction of Crossing Multitunnels

AbstractAny new tunnel excavation may damage adjacent existing tunnels in congested cities. To evaluate the impact of new tunnel construction on nearby existing tunnels, a series of three-dimensional centrifuge model tests were conducted with numerical backanalyses using an advanced hypoplasticity constitutive model. The influences of the pillar depth-to-diameter ratio (P/D) on two-tunnel interaction and the effects of shielding on three-tunnel interaction were investigated. The maximum measured settlement of an existing tunnel caused by a new tunnel excavation at P/D of 0.5 underneath was approximately 50% larger than that when P/D was 2.0. This is attributed to a smaller mobilized shear modulus, resulting from a larger reduction in confining stress of soil acting on the invert of the existing tunnel in the former than in the latter. When the new tunnel was excavated underneath two perpendicularly crossing tunnels, the lower existing tunnel “shielded” the upper one from the influence of tunnel excavation...

Amelioration of Wolffersdorff Hypoplastic Constitutive Model Considering Intergranular Strain Tensor

Under cyclic load, the major shortcoming–ratcheting is produced in Wolffersdorff hypoplastic constitutive model. For eliminating ratcheting, Wolffersdorff hypoplastic model is ameliorated based on intergranular strain tensor. The added parameters in ameliorated model are determined by optimization method. Under cyclic load of triaxial consolidation undrained condition, the mechanics features of sand are described by the ameliorated Wolffersdorff hypoplastic constitutive model. Preliminary result shows that with increasing times of cyclic load excess pore water pressure is increased gradually and effective stress is reduced gradually. When effective is reduced to zero, the liquefaction happens. So in many projects, excess pore water pressure must dissipate by means of some measures. The sand liquefaction under the dynamic load is avoided.

Numerical analysis of a geosynthetic-reinforced piled load transfer platform – Validation on centrifuge test

Soft soil improvement techniques using a network of rigid inclusions and geosynthetic reinforcement are investigated to improve our understanding of load transfer mechanisms towards piles. The physical modelling of the system consists in simulating fictional soft soil settlement through downward displacement of a perforated tray above a network of rigid piles placed in the centrifuge swinging basket. Tests are used to validate the results of the numerical study.Elasto-plastic and hypoplastic constitutive models have been used to predict the behaviour of the granular mattress, which simulates a Load Platform Transfer (LPT). A two-dimensional, axisymmetrical model has been adopted, which fulfils the validation on the experimental test and the time needed for calculation.The results of the parametric studies show that load transfer increases with mattress thickness and closer pile spacing. Geosynthetic deflection is reduced when load transfer is high.

Size Effect on the Deformation Characteristic and Shear Band of Granular Material Based on Hypoplastity

The deformation characteristic of granular material is important. It is important factor to analysis the bearing capacity and the thickness of granular material footings. There are many factors influence the deformation of granular material, such as material characteristic, content of water, degree of compaction, test method, etc. For analyzing size effect on the deformation characteristic of granular material, hypoplastic constitutive model considering void ratio is introduced. The influence of an initial void ratio, pressure and a mean grain diameter on the deformation is investigated by polar quantities: rotations, curvatures, couple stresses and a characteristic length. Thus, the influence of density and pressure on the initiation of shear bands can be investigated with a single set of constitutive constants.

Prediction of permanent soil deformations due to cyclic shearing with a hypoplastic constitutive model

AbstractThe prediction of permanent soil displacements due to alternating loading is important for assessing the stability and serviceability of transport network, foundations of machines and wind turbines as well as structures in earthquake regions. The accumulation of permanent displacements due to alternating loading depends on many factors which engineers attempt to capture in a corresponding stress‐strain relationship. Typical hysteretic soil behaviour during cyclic shearing and its description with a constitutive model are shown in this article.The capability of the hypoplastic constitutive model with intergranular strains to reproduce the non‐linear and anelastic behaviour is discussed. The determination of the material parameters for describing shear stiffness degradation with increasing shear strain amplitude is considered and, simultaneously, an increase in stiffness after a change in shearing direction. A modification of the hypoplastic constitutive model, including one additional soil parameter, is proposed in order to describe accumulation effects more realistically. The additional parameter can be determined in cyclic shear or triaxial tests. How to reproduce realistically the accumulation of volumetric strains under drained conditions and excess pore pressures under undrained conditions during cyclic shearing is demonstrated.Prognose bleibender Verformungen infolge zyklischer Scherung mit einem hypoplastischen StoffgesetzDie Prognosen von bleibenden Bodenverformungen infolge Wechselbelastung spielen zum Beispiel für die Beurteilung der Gebrauchstauglichkeit und Stabilität von Verkehrswegen sowie von dynamisch belasteten Gründungen, wie z. B. Maschinenfundamenten oder Windenergieanlagen, aber auch für Bauten in Erdbebengebieten eine wichtige Rolle. Die Akkumulation der Verformungen infolge Wechselbelastung hängt dabei von vielen Faktoren ab, die in einer entsprechenden Spannungs‐Dehnungs‐Beziehung erfasst werden.Im Artikel wird das typische Bodenverhalten infolge dynamischer Beanspruchung beschrieben und gezeigt, wie man dieses nichtlineare und anelastische Verhalten mit dem hypoplastischen Stoffgesetz mit intergranularen Dehnungen wirklichkeitsnah erfassen kann. Dabei wird auf die geeignete Wahl der Materialparameter zur Beschreibung der Abnahme der Steifigkeit mit zunehmender Scherdehnungsamplitude sowie die Zunahme der Steifigkeit nach einem Richtungswechsel der Belastung eingegangen. Für die realistische Erfassung des Bodenverhaltens infolge zyklischer Belastung werden eine Modifizierung des hypoplastischen Stoffgesetzes sowie die Einführung eines zusätzlichen Stoffgesetzparameters vorgenommen. Es wird gezeigt, wie dieser Parameter in zyklischen Scher‐ oder Triaxialversuchen bestimmt werden kann und wie damit die Akkumulation von volumetrischen Verformungen bei drainierten Verhältnissen bzw. von Porenwasserüberdrücken bei undrainierten Verhältnissen zuverlässig prognostiziert werden kann.

Numerical simulation of a tunnel surrounded by sand under earthquake using a hypoplastic model

A numerical model of a centrifuge experiment on tunnel located in sand is being presented. The experiment was carried out under seismic loading using a dynamic actuator. The responses of the tunnel and of the sand were measured. The numerical model is based on a hypoplastic constitutive model with intergranular strains implemented in the FE-code TOCHNOG. The calculated accelerations in the sand match the measured results, while the surface settlement and the bending moments in the tunnel lining are only qualitatively reproduced by the numerical model.

Effect of particle friction and polydispersity on the macroscopic stress–strain relations of granular materials

The macroscopic mechanical behavior of granular materials inherently depends on the properties of particles that compose them. Using the discrete element method, the effect of particle contact friction and polydispersity on the macroscopic stress response of 3D sphere packings is studied. The analytical expressions for the pressure, coordination number and fraction of rattlers proposed for isotropically deformed frictionless systems also hold when the interparticle coefficient of friction is finite; however, the numerical values of the parameters such as the jamming volume fraction change with varying microscopic contact and particle properties. The macroscopic response under deviatoric loading is studied with triaxial test simulations. Concerning the shear strength, our results agree with previous studies showing that the deviatoric stress ratio increases with particle coefficient of friction μ starting from a nonzero value for μ = 0 and saturating for large μ. On the other hand, the volumetric strain does not have a monotonic dependence on the particle contact friction. Most notably, maximum compaction is reached at an intermediate value of the coefficient of friction μ ≈ 0.3. The effect of polydispersity on the macroscopic stress–strain relationship cannot be studied independent of initial packing conditions. The shear strength increases with polydispersity when the initial volume fraction is fixed, but the effect of polydispersity is much less pronounced when the initial pressure of the packings is fixed. Finally, a simple hypoplastic constitutive model is calibrated with numerical test results following an established procedure to ascertain the relation between particle properties and material coefficients of the macroscopic model. The calibrated model is in good qualitative agreement with simulation results.

Discrete modelling results of a direct shear test for granular materials versus FE results

The intention of this paper is to present a comparison of the results of discrete element and finite element simulations of a simple shear test for medium dense cohesionless sand. Such a comparison may provide useful information on the limitations and possible advantages of micro-polar continuum models for granular media as compared with discrete element models. To simulate the discrete nature of sand at the micro-level during shearing, the 3D discrete open-source model YADE developed at Grenoble University was used. Contact moments at spheres were assumed to capture the influence of force eccentricities due to grain roughness. Attention was paid to some micro-structural events (such as vortices, force chains, vortex structures, local void ratio fluctuations) appearing in a shear zone and kinetic, elastic and dissipated energies in granular specimen. The results of the discrete element simulations were compared with the corresponding finite element (FE) solutions based on a micro-polar hypoplastic constitutive model for granular material. A satisfactory agreement between discrete and FE results was achieved. Advantages and disadvantages of both approaches are outlined.

II cycle compressibility from satellite measurements

Knowledge of ground compressibility in unloading/reloading conditions is of paramount importance in several geomechanical processes, but its estimate is often affected by large uncertainties, especially for deep rocks. Satellite measurements of the land motion can help improve this information. The present study investigates the ground response due to underground gas storage activities as revealed by satellite interferometric measurements in three gas fields of the Po river basin, Italy. The ground proves to behave elastically, moving upwards and downwards in response to gas injection and extraction, respectively. The accurate calibration of a detailed geomechanical, transversely isotropic, finite-element model allows for an indirect estimate of the vertical ground compressibility in unloading/reloading conditions, which proves consistent with the results already available from the same basin. Assuming a hypo-plastic constitutive model for the compressibility in virgin loading conditions, the ratio between I cycle and II cycle compressibility at the load inversion turns out to be between 3·5 and 4·0 in the depth range 1100–1500 m, again proving to be in good agreement with other results obtained in the same basin.

Numerical study of the deformation of saturated soil in the vicinity of a vibrating pile

The paper presents a dynamic finite-element analysis of the deformation of saturated cohesionless soil in the vicinity of the toe of a vibrating cylindrical pile. The soil behaviour is described by a hypoplastic constitutive model with intergranular strain assuming locally undrained conditions for the pore fluid. Emphasis is placed on the detailed analysis of stresses and displacements in the soil. It is shown that, starting from a homogeneous stress state, the first several cycles of vibration lead to the formation of a permanent liquefaction zone with vanishing effective stresses at a certain distance from the pile. The displacement field reveals an accumulation of residual displacements in the soil in the form of rotation. The influence of the initial stress state, the pore fluid compressibility, the pile displacement amplitude and the soil density on the formation of a liquefaction zone, on the stress amplitude in the soil and on the intensity of the permanent rotation is investigated.