A hybrid discrete–finite element model for numerical simulation of geomaterials

A hybrid discrete–finite element model is introduced for simulation of mechanical behavior of geomaterials. The soil or rock is modeled as a system of discrete balls that interact through normal and shear springs. The balls can be bonded at the contact points to withstand the applied deviatoric stresses. The important feature of this model is that the confining walls that can be imagined for example as the surrounding membrane or the mold in a physical test are modeled by deformable finite elements. This allows simulation of laboratory test features more realistically compared to the situations where the surrounding walls are rigid. The relationships between micro- and macro-properties are investigated in this paper as well. These relationships and the corresponding curves are helpful tools in calibration of the numerical model for the macroscopic elastic properties.     

Hybrid continuum–discrete simulation of granular impact dynamics

Acta Geotechnica - Granular impact—the dynamic intrusion of solid objects into granular media—is widespread across scientific and engineering applications including geotechnics....     

Combined spheropolyhedral discrete element (DE)–finite element (FE) computational modeling of vertical plate loading on cohesionless soil

This paper presents a combined spheropolyhedral discrete element (DE)–finite element (FE) computational approach to simulating vertical plate loading on cohesionless soils such as gravels. The gravel particles are modeled as discrete elements, and the plate is modeled as a deformable FE continuum. The simulations provide a meaningful step toward better understanding how deformable bodies transmit loads to granular materials. The DE–FE contact algorithm is verified through comparison with an analytical solution for impact between two symmetric bars. A parametric study is conducted to ensure boundary effects are not significantly influencing the simulations. Numerical simulations are compared to experimental test results of lightweight deflectometer loading on a gravel base course with satisfactory agreement. Future developments of the approach intend to simulate wheel loading of military aircraft on unsurfaced airfields.     

Single-particle crushing behaviour of carbonate sands studied by X-ray microtomography and a combined finite–discrete element method

This paper investigates the particle breakage behaviour of a carbonate sand based on single-particle compression experiments with in situ X-ray microtomography scanning (μCT) and a combined finite–discrete element method (FDEM). Specifically, X-ray μCT is applied to extract the information on grain morphology and intra-particle pores of carbonate sand particles to establish an FDEM model. The model is first calibrated by comparing the simulation results of two carbonate sand grains with the corresponding single-particle compression experiment results and then applied to model the stress evolution, cracking propagation and failure of other carbonate sand particles under single-particle compression. To study the influence of intra-particle pores, FDEM modelling of carbonate sands with completely filled intra-particle pores is also performed. The particle strength of carbonate sands both with and without pore filling is found to follow a Weibull distribution, with that of the sand with pore filling being considerably higher. This behaviour is associated with lower stress concentration, resulting in later crack development in the pore-filled sand than in the sand without pore filling. The cracks are found to usually pass through the intra-particle pores. Consequently, a larger proportion of particles fail in the fragmentation mode in the sand without pore filling.

     

An embedded bond-slip model for finite element modelling of soil–nail interaction

Soil nailing has been widely used as a reinforcing technique to retain excavations and stabilise slopes. Proper assessment of the interaction between the nails and the surrounding soil is central to safe and economical design of the composite reinforced soil structure. In this note, a new interface model, denoted as “embedded bond-slip model”, is proposed to model the soil–nail interaction numerically in a simplified manner. Combining the key features of the embedded element technique and the conventional interface element method, the proposed plane–strain interface model has the advantages that no special considerations have to be given to the arrangement of the finite element mesh for the soil nails, and that possible tangential slippage along the interface can be modelled. The formulation also allows pore water flow across the soil nails to be incorporated into the analysis. The proposed model has been implemented into a finite element code and verified by simple element tests under different uni-direction loading conditions. Using the proposed interface model, back analyses of a field test involving a soil-nailed cut slope subjected to a rise in groundwater table have been conducted. This note presents the details of the embedded bond-slip model and the numerical results which demonstrate that the proposed model is capable of simulating soil–nail interaction conveniently and realistically.     

GPU-accelerated iterative solutions for finite element analysis of soil–structure interaction problems

Soil–structure interaction problems are commonly encountered in engineering practice, and the resulting linear systems of equations are difficult to solve due to the significant material stiffness contrast. In this study, a novel partitioned block preconditioner in conjunction with the Krylov subspace iterative method symmetric quasiminimal residual is proposed to solve such linear equations. The performance of these investigated preconditioners is evaluated and compared on both the CPU architecture and the hybrid CPU–graphics processing units (GPU) computing environment. On the hybrid CPU–GPU computing platform, the capability of GPU in parallel implementation and high-intensity floating point operations is exploited to accelerate the iterative solutions, and particular attention is paid to the matrix–vector multiplications involved in the iterative process. Based on a pile-group foundation example and a tunneling example, numerical results show that the partitioned block preconditioners investigated are very efficient for the soil–structure interaction problems. However, their comparative performances may apparently depend on the computer architecture. When the CPU computer architecture is used, the novel partitioned block symmetric successive over-relaxation preconditioner appears to be the most efficient, but when the hybrid CPU–GPU computer architecture is adopted, it is shown that the inexact block diagonal preconditioners embedded with simple diagonal approximation to the soil block outperform the others.     

Effects of pre-existing discontinuities on the residual strength of rock mass – Insight from a discrete element method simulation

When rock failure is unavoidable, the designer of engineering structures must know and account for the residual strength of the rock mass. This is particularly relevant in underground coal mine openings. Pre-existing discontinuities play an important role in the mechanical behavior of rock masses and thus it is important to understand the effects of such pre-existing discontinuities on the residual strength. For this purpose, the present study demonstrates a numerical analysis using a discrete element method simulation. The numerical results indicate that fracture intensity has no significant influence on the residual strength of jointed rock masses, independent of confining conditions. As confining pressures increase, both peak and residual strengths increase, with residual strength increasing at a faster rate. The finding was further demonstrated by analyzing documented laboratory compressive test data from a variety of rocks along with field data from coal pillars. A comprehensive interpretation of the finding was conducted using a cohesion-weakening-friction-strengthening (CWFS) model. The effect of rock bolts on rock mass strength was also evaluated by using a discrete element method model which suggested that rock bolts can significantly increases residual strength but have limited effect on increasing the peak strength of rock masses.     

Mesoscopic pullout behavior of geosynthetics–sand–clay layered reinforced structures using discrete element method

Acta Geotechnica - The geosynthetics–sand–clay layered reinforced (GSCLR) structure has wide application prospects due to its massive adoption of the clay. To date, the coordination...     

A modification to dense sand dynamic simulation capability of Pastor–Zienkiewicz–Chan model

A modification to the nonlinear Pastor–Zienkiewicz–Chan (PZC) constitutive model without any change in the number of model parameters is introduced in order to simulate stiffness degradation of dense sands at dynamic loading. The PZC model is based on generalized plasticity and was verified by good prediction of liquefaction and undrained behavior of saturated sand. The PZC is a robust model that can predict drained dynamic behavior of sands, especially stiffness increase in loose sand at reloading of dynamic loading. Yet, this model does not show stiffness degradation of dense sand at reloading. The modification is made through modifying the stress memory factor, H _DM, which is multiplied by the plastic modulus, H _L. This modification does not influence reloading behavior of loose sand. The modified PZC model is verified via results of drained cyclic tests. Two cyclic triaxial tests on loose and dense specimens, along with two cyclic plane strain tests on dense sand are utilized for validation. The model simulation shows that the modified PZC model is able to predict the stiffness degradation of dense sand at reloading well.     

Coupled deformation–seepage analysis of dynamic capacity tests on open-ended piles in saturated sand

Open-ended piles such as tubular piles or I-beams are used as foundations for offshore and nearshore construction. After the pile installation a load test to estimate the bearing capacity of these open-ended piles is necessary. Due to the offshore conditions and the high bearing capacity of the installed piles a static load test is not normally feasible. Therefore, dynamic load tests are carried out where the wave propagation due to an dynamic impact at the pile head is measured. The methods to estimate the bearing capacity from the measured signal of the dynamic tests were derived for solid pile profiles. It is questionable whether these evaluation techniques are applicable for open-ended piles. Hence, the influence of various important system parameters as well as the differences between static and dynamic load tests on open-ended piles is investigated in this paper.     

A finite element–based approach for predictions of rigid pile group stiffness efficiency in clays

This paper presents a finite element parametric study of several variables that affect the stiffness efficiency of rigidly capped pile groups with a view to developing a solution for preliminary design purposes. Previous empirical solutions from linear elastic work had identified a significant dependence of stiffness efficiency on pile group size and group spacing, and in this study, the effect of the pile length-to-diameter ratio, the compressibility of a stiff bearing stratum beneath the pile group and the depth below ground level to the stiff bearing stratum are also considered. Pile groups in a soft clay/silt are modelled using PLAXIS 3D Foundation in conjunction with a soil model that captures the stress dependency of soil stiffness. The trends from the soft soil study have been formulated into a set of equations which can be used to predict the stiffness efficiency of pile groups. This new approach captures more variables than previous simpler empirical prediction methods and performs better when applied to a database of 29 published pile group case histories.     

Meso-mechanism of rolling dynamic compaction to reinforce loose landslide dam materialEI北大核心

为系统研究冲击碾压过程中松散堰塞坝料的细观密实机制,基于自行设计的可视化冲击碾压模型装置及粒子图像测速技术,研究了不同冲击碾压参数对堰塞坝料地基的表面变形、内部变形及颗粒位移规律的影响。试验结果表明,冲击碾压加固过程是冲击和碾压两者共同作用,由于水平冲击作用,冲击点下方地基的变形具有非对称性。“高速轻轮”的施工参数会强化冲击效果,弱化碾压效果,造成地基表面平整性差。堰塞坝料冲击轮加固过程中的最大位移发生在三边形冲击轮圆弧面较平滑处与土体接触时,随后由于模型冲击轮重心上升,地基出现部分弹性回弹。提高冲击轮的牵引速度能够促进冲击能量向深层传递,但水平影响宽度有限;提高冲击轮的质量则能促进能量向两侧水平方向传递,但影响深度有限。对于模型试验的易贡堰塞坝料地基,冲击碾压最佳牵引速度约为0.75 m/s。结果可为堰塞坝料地基的冲击碾压浅层加固提供理论依据。     

DEM–SPH simulation of rock blasting

A numerical model is proposed for the simulation of rock blasting. A bonded particle system is utilized to mimic the behavior of rock. The particles interact at the contact points through normal and shear springs to simulate rock elasticity. To withstand the deviatoric stresses, the particles are glued to each other. If the applied force exceeds the contact strength, local failure occurs and microcracks are developed in the synthetic rock. For simulation of gas flow, the smooth particle hydrodynamic method is implemented. The interaction of gas particles with the rock grains is assumed to follow a perfect plastic collision model in which the initial momentum of the colliding particles is preserved. A detailed examination of the interaction of gas with blast hole is investigated. It is shown that the proposed hybrid model is capable of simulating the induced shock waves in the gas together with wave propagation in the rock material. The model successfully mimics crack propagation in rock. In particular, the crushed zone around the borehole, radial cracks, and surface spalling are all captured successfully. The results of numerical analysis suggest that gas–rock interaction can, in fact, generate a few successive compressive waves in the rock specimen, causing further extension of radial cracks with time as the weaker secondary and tertiary waves interact with the crack tips.     

Finite element simulation of an embankment on soft clay – Case study

Numerical simulations and field measurements of an embankment constructed on a deposit of soft Ariake clay in Saga, Japan are compared and discussed. The simulations were made both before (Class-A) and after (Class-C) the field data became known. It is shown that the Class-A prediction resulted in poor simulations of the measured settlement–time curves, mainly due to over-estimation of the magnitude of the yield stresses of the subsoils (i.e., the sizes of the yielding loci) and under-estimation of the compressibility, hydraulic conductivity and the slope (M) of the critical state line. It is demonstrated that: (a) appropriate site investigation, soil testing and interpretation of the test results are essential for accurate prediction of the behaviour of an earth structure constructed on soft clayey deposits; (b) when using a soil model developed within the framework of Critical State Soil Mechanics to make such predictions, M value should be directly determined from tests with an appropriate effective stress path; and (c) yield stresses of soft soil layers can be calibrated by comparing the predicted undrained shear strengths (S_u) with measured values, provided the effect of strain rate and/or strain softening on the value of S_u is properly considered. The results of this analysis indicate that Bjerrum’s strain rate correction factor can be adopted as a first approximation of the correction factor applied to field or laboratory measured values of S_u.     

Numerical simulation of migration–accumulation of oil resources

Numerical simulation of oil migration and accumulation is to describe the history of oil migration and accumulation in basin evolution. It is of great value to the evaluation of oil resources and to the determination of the location and amount of oil deposits. This thesis discusses the characteristics of petroleum geology and permeation fluid mechanics. For the three-dimensional problems of Dongying hollow of Shengli Petroleum Oil Field, it puts forward a new model and a kind of modified method of upwind finite difference fractional steps implicit interactive scheme. For the famous hydraulic experiment of secondary migration–accumulation, the numerical simulation test has been done, and both the computational and experimental results are basically identical. For the actual problem of Dongying hollow, the numerical simulation test and the actual conditions are basically coincident. Thus, the well-known problem has been solved.     

Fourier amplitude sensitivity test applied to dynamic combined finite-discrete element methods–based simulations

Fracture propagation plays a key role for a number of applications of interest to the scientific community, from dynamic fracture processes like spallation and fragmentation in metals to failure of ceramics, airplane wings, etc. Simulations of material deformation and fracture propagation rely on accurate knowledge of material characteristics such as material strength and the amount of energy being dissipated during the fracture process. Within the combined finite-discrete element method (FDEM) framework material fracture behavior is typically described through a parametrized softening curve, which defines a stress-strain relationship unique to each material. We apply the Fourier amplitude sensitivity test to explore how each of these parameters influences the simulated damage processes and to determine the key input parameters that have the most impact on the model response. We present several sensitivity numerical experiments for the simulation of a split Hopkinson pressure bar (SHPB) test for weathered granite samples using different combinations of model parameters. We validate the obtained results against SHPB experimental data. The experiments show that the model is mostly sensitive to parameters related to tensile and shear strengths, even in the presence of other parameter perturbations. The results suggest that the specification of tensile and shear strengths at the interfaces dominate the stress-time history of the FDEM simulation of SHPB test.     

Deformation analysis of reinforced soil retaining walls—simplistic versus sophisticated finite element analyses

Over the last two decades, different kinds of modular block have been increasingly used in the geogrid-reinforced soil retaining walls. The simulation of such wall behavior, which involves interactions between different structural components and backfill soils, requires a rigorous numerical procedure. Finite element is usually a preferred method, but this procedure, especially the soil constitutive models, is of different degrees of sophistication. It is always an issue of how simple or sophisticated should an analysis be conducted in replicating the actual behavior. In this article, a full-scale test wall was used to validate simplistic and sophisticated finite element analyses. Different types of finite element and material models were used in the two kinds of analysis to differentiate the level of simplicity or sophistication. The results obtained from stress-deformation analyses are presented and compared. It is shown that for wall construction that involves static loading conditions, simplistic nonlinear elastic and sophisticated elastoplastic analyses produced close and acceptable results.     

EM2INV — A finite difference based algorithm for two-dimensional inversion of geoelectromagnetic data

The paper presents an efficient finite difference based 2D-inversion algorithm, EM2INV, for geoelectromagnetic data. The special features of the algorithm are

Effect of vehicle–pavement interaction on dynamic response of rigid pavements

In the present paper, dynamic response of rigid pavements subjected to moving vehicular loads is presented using an analytical procedure based on the finite element method. The vehicle–pavement interaction effects are taken into account while developing the solution algorithm. The concrete pavement is discretized by finite and infinite beam elements. Infinite elements are helpful in appropriate modeling of end conditions. The main purpose of the development of infinite elements is to model the unbounded domain. The underlying soil medium is modeled by Pasternak model which assumes the existence of shear interaction between the spring elements. The moving vehicle is represented by a mass supported by a spring-dashpot system. The vehicle -pavement interaction force is modeled with Dirac-delta function. Dynamic equilibrium equation is solved with Newmark-Beta integration scheme. It has been observed that the dynamic interaction between the moving load and the pavement has significant effect on pavement response. Parametric study is carried out to investigate the effect of vehicle–pavement interaction (VPI) and soil parameters on the response of pavement.