1,720,971 research outputs found
Instability criteria for quasi-saturated viscous soils
No full textThis paper presents a theoretical framework to interpret the inception of unstable undrained creep in quasi-saturated soils. For this purpose, the effect of gas bubbles occluded in the fluid phase is embedded into an augmented compressibility of the fluid mixture, while the mechanical characteristics of the solid skeleton have been simulated through a viscoplastic strain-hardening model. This constitutive framework has been been used to formulate a theoretical platform able to detect runaway failures resulting from extended stages of undrained creep. It is shown that the conditions identifying the onset of spontaneous accelerations are governed by the same stability index associated with the initiation of static liquefaction. At variance with soils saturated by incompressible fluids, the conditions for undrained instability are altered by the appearance of the Skempton coefficient B, thus reflecting the beneficial effect of the fluid compressibility and its ability to decrease the liquefaction potential. The capabilities of the theory are verified through a sequence of undrained creep simulations showing the transition from stable to unstable behavior resulting from an increase of the degree of saturation. The proposed findings provide a conceptual framework to interpret the effects of gas bubbles in loose soils, as well as to assess effectiveness and longevity of liquefaction mitigation strategies based on desaturation technologies
Parameter calibration for high-porosity sandstones deformed in the compaction banding regime
No full textThis paper discusses the parameter calibration procedure for an elastoplastic constitutive model for high-porosity rocks. The model selected for the study is formulated in the frame of the critical state theory, which is here used in a form able to accommodate non-associated plastic flow and softening effects due to volumetric and deviatoric plastic strains. The goal of this study is to generate a set of model constants able to capture both the stress-strain response and the compaction localization characteristics (e.g., stress and inclination at the onset of the deformation bands). For this purpose, data about the compaction localization properties of four extensively characterized sandstones have been considered. In particular, the strain localization theory has been used as a calibration tool, using explicitly information about the pressure-dependence of the localization mechanisms observed in experiments. The model constants have been defined by matching the constitutive response upon hydrostatic compression, as well as the stresses at the transition from high-angle shear bands to pure compaction bands, and from compaction bands to homogeneous cataclastic flow. It is shown that such procedure generates a set of model constants able to capture satisfactorily both the rheological response upon triaxial compression and the salient features of the compaction localization process
A Generalized Backward Euler algorithm for the numerical integration of a viscous breakage model
No full textThis paper discusses the formulation and the numerical performance of a fully implicit algorithm used to integrate a rate-dependent model defined within a breakage mechanics framework. For this purpose, a Generalized Backward Euler (GBE) algorithm has been implemented according to two different linearization strategies: The former is derived by a direct linearization of the constitutive equations, while the latter introduces rate effects through a consistency parameter. The accuracy and efficiency of the GBE algorithm have been investigated by (1) performing material point analyses and (2) solving initial boundary value problems. In both cases, the overall performance of the underlying algorithm is inspected for a range of loading rates, thus simulating comminution from slow to fast dynamic problems. As the viscous response of the breakage model can be recast through a viscous nucleus function, the presented algorithm can be considered as a general framework to integrate constitutive equations relying on the overstress approach typical of Perzyna-like viscoplastic models
Compaction localization in granular rocks: Modeling grain-size effects
No full textThis paper focuses on the simulation of localized compaction in granular rocks. For this purpose, a continuum framework referred to as Breakage Mechanics is used to capture the role of microscopic crushing on the mechanical response of grain assemblies. In particular, grain size dependencies are introduced by connecting the physics of grain-scale fracture to the energetics of collective crushing. It is shown that this approach enables the simultaneous consideration of changes in grain sorting and average grain size, where the role of the latter is modeled via central splitting and contact fracture laws. Using this constitutive framework, the localization potential of Bentheim sandstone has been studied with the purpose to emphasize the role of grain scale characteristics in the inception of compaction banding. The analyses show that the model captures correctly the increase of the localization potential resulting from a coarser gradation or a narrow grain size distribution
Modeling Delayed Flow Liquefaction Initiation after Cyclic Loading
No full textGround shaking during earthquakes can be a prominent cause of strength loss in loose saturated soils. In some cases, uncontrolled pore pressure build-up and liquefaction may occur with a time lag with respect to strong ground motion, especially when the time-dependent properties of the soil interact with the constraints brought by undrained and/or partially drained conditions. Here a rate-dependent law based on the critical state theory has been used to replicate the cyclic response of sand by taking into account density effects. The model equations have been inspected to define indices of delayed undrained instability. Then, the model has been calibrated by considering laboratory tests for both monotonic and cyclic undrained loading on loose Hostun sand. Numerical simulations of undrained creep stages following cyclic loading have been performed to illustrate the transition from stable to unstable creep as a function of the number of cycles, thus providing a conceptual framework to evaluate the risk of delayed flow liquefaction in loose sandy deposits
Mathematical interpretation of delayed instability in viscous unsaturated soil
No full textFluid-infiltrated soils are vulnerable to wetting and often exhibit delayed deformation characterised by acceleration stages. In this paper, mathematical tools are developed to diagnose delayed failure in unsaturated materials subjected to saturation. For this purpose, stability criteria for saturated viscous soils are extended to account for hydraulic state variables (e.g. suction and water content), which may provoke an unexpected increase of the creep rate. A simple one-dimensional constitutive law is used to test the proposed theory and assess its capability to distinguish stable and unstable creep. Numerical simulations revealed that, although the mathematical conditions associated with a loss of stability resemble those of rate-independent models, high viscosity delays strain acceleration and accentuates suction dependence (i.e. drier states are less susceptible to tertiary creep than wetter states). Most importantly, the analyses indicate that a violation of the stability criteria is a precursor of sharp suction loss and consequent fluidisation
Viscoplastic Interpretation of Localized Compaction Creep in Porous Rock
No full textRecent laboratory evidence shows that compaction creep in porous rocks may develop through stages of acceleration, especially if the material is susceptible to strain localization. This paper provides a mechanical interpretation of compaction creep based on viscoplasticity and nonlinear dynamics. For this purpose, a constitutive operator describing the evolution of compaction creep is defined to evaluate the spontaneous accumulation of pore collapse within an active compaction band. This strategy enables the determination of eigenvalues associated with the stability of the response which is able to differentiate decelerating from accelerating strain. This mathematical formalism was linked to a constitutive law able to simulate compaction localization. Material point simulations were then used to identify the region of the stress space where unstable compaction creep is expected, showing that accelerating strains correspond to pulses of inelastic strain rate. Such pulses were also found in full-field numerical analyses of delayed compaction, revealing that they correspond to stages of inception and propagation of new bands across the volume of the simulated sample. These results illustrate the intimate relation between the spatial patterns of compaction and their temporal dynamics, showing that while homogeneous compaction develops with decaying rates of accumulation, localized compaction occurs through stages of accelerating deformation caused by the loss of strength taking place during the formation of a band. In addition, they provide a predictive modeling framework to simulate and explain the spatio-temporal dynamics of compaction in porous sedimentary formations
Influence of Clay Anisotropy on Model Simulations of Wetting Collapse
No full textRecent interpretations of wetting-induced compaction revealed that water sensitivity can cause a loss of controllability in samples subjected to fluid injection. This paper elaborates these findings by focusing on fabric anisotropy, i.e., a feature of unsaturated clays not encompassed by previous studies. For this purpose, a hydromechanical elastoplastic model with rotational hardening is developed to capture fabric effects. The model performance has been validated under both saturated and unsaturated conditions with reference to laboratory tests on Lower Cromer Till (LCT). To inspect the role of the material properties, parametric analyses have been conducted, thus identifying the parameters which govern the transition from stable to unstable conditions upon wetting. The results show that fabric anisotropy affects only the deformations prior to wetting-collapse, without changing the value of suction at the onset of volumetric instability. By contrast, it is found that the model parameter governing the intensity of suction-hardening is able to alter the value of critical suction at the loss of control regardless of the intensity of fabric evolution. These results corroborate previous findings obtained through isotropic constitutive laws and emphasize the crucial role of hydromechanical constitutive couplings on the inelasticity of unsaturated porous media
A Rotational Hardening Model Capturing Undrained Failure in Anisotropic Soft Clays
No full textInduced anisotropy is known to play a major role in the undrained strength of clays, especially in case of weakly consolidated deposits. This paper discusses undrained failure in soft clays in light of material stability principles. For this purpose, a strain-hardening elastoplastic model widely used to study undrained instability in isotropic soils is enhanced to account for stress-induced anisotropy. A versatile yield function able to flexibly control the shape of the elastic domain is augmented through a hardening variable related to the evolving fabric, while rotational hardening is used to replicate the reorientation of the surface as a function of the loading history. Parametric analyses are used to illustrate the model capabilities, and instability indices for undrained failure are derived in analytical form. Finally, the model performance is tested against experimental evidence available for two widely tested soils: soft Chicago clay and Boston blue clay. The analyses illustrate how the proposed model allows an accurate representation of soil responses under extension and compression paths. In addition, it enables the identification of undrained failure resulting from the monotonic growth of shear stresses, as well as from a post-peak strength decay
Constitutive modeling approaches for cross-anisotropic porous rocks
No full textSimulating the anisotropy of rock properties is a major challenge for geomechanical modeling. Despite numerous techniques that have been proposed to account for the orientation of the material reference system with respect to the loading directions, such methods often involve a major increase in the number of model parameters, especially if the anisotropy influences both elastic and plastic properties. In this paper, two approaches to model the mechanical behavior of cross-anisotropic porous rocks are examined. The first approach relies on a tensorial projector used in conjunction with standard strain-hardening plasticity. It is illustrated how such projector is able to map the stress conditions into a modified stress space, thus distorting the yield surface and inducing the dependence of yielding on the direction of sample coring. Afterwards, a novel energy-based approach to replicate such mapping effects is discussed. The methodology relies on the Breakage Mechanics theory, i.e. a constitutive framework expressing the yielding of granular rocks in terms of a strain energy threshold associated with the release of the elastic energy stored in the brittle grains constituting its skeleton. It is shown that energy-based yielding produces the same benefits of a tensorial projection without the need of introducing additional model parameters, but solely relying on the directional properties of the elastic stiffness tensor. The capabilities and performance of both approaches with respect to data available for a porous rock are outlined, discussing their relative merits and providing guidelines for the formulation of constitutive laws able to reduce the number of model parameters by relying on more detailed insights on the microscopic causes of inelasticity
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