Effective Stress Parameter Research Articles

Shear characteristics of root–matrix composites under various interface friction and moisture content conditions

Analysing the composite's adhesion mechanism is necessary to gain a deep understanding of the pipeline blockage phenomenon encountered during negative pressure matrix removal. This study considers the interface suction and liquid bridge changes between the roots and the matrix. In particular, the liquid bridge volume and the interface's effective stress parameters were combined to establish a theoretical model of the shear strength of the root–matrix composite. The main factors affecting root–matrix composite adhesion stability were the root friction coefficient and the composite moisture content. Regarding the influence of the root friction coefficient, the surface structures of the primary and lateral roots were observed using a laser confocal microscope. Using the principal component analysis method, the main factors influencing the friction properties of the root system were identified as the taproot's maximum valley depth (Rv) and the average roughness (Ra) of the profile, as well as the lateral root's maximum peak-to-valley height (Rz) and the average roughness (Rq) of the lateral root profile. The influence of both the main and lateral roots cannot be overlooked when removing inferior seedlings from the substrate. Furthermore, the bond efficiency of the root–matrix composite was calculated, revealing values of 16.03%, 49.87%, and 58.31% under low, medium, and high moisture content conditions, respectively. Finally, direct shear tests were conducted on the complex under dry-wet-dry conditions, yielding an internal friction angle of 14.1656° and a cohesion force of 1.738 ± 0.3 kPa at a water content of 20% (dry-wet). In conclusion, it is recommended to perform the negative pressure removal of inferior seedling substrate blocks during the seedling stage with low water content and underdeveloped root systems. Related research lays the foundation for preventing blockages in negative pressure pipelines.

A fully coupled micro-hydromechanical (micro-HM) model for partially saturated soils based on DEM

A fully coupled micro-hydromechanical (micro-HM) model is developed for partially saturated soils in this study by integrating two-dimensional pore morphology (PM) approach and discrete element method (DEM). In the proposed model, the PM approach is employed to predict the tentative water distribution. The porous media marching cubes (PMMC) algorithm is adopted to evaluate the interphase interfaces and to further calculate the capillary forces. The combined effects of interparticle contact forces and the capillary forces on the motion of particles are handled by DEM. The developed model was then employed to conduct a series of numerical biaxial shear tests on a partially saturated soil with real particle shapes. The typical macroscopic responses such as stress–strain relationship, volume change, and saturation change can be well simulated by the micro-HM model. Based on the micro-HM model, a novel equation is proposed to directly evaluate the effective stress from the pore water distribution. The effective stress parameter and the suction contribution to effective stress calculated by the new equation well matches the experimental data, thus confirming the validity of the micro-HM model and the new equation of effective stress. The microscopic responses are then revealed and discussed through the proposed model.

Water retention behavior and shear strength of artificially cemented granite residual soil subjected to free drying

Artificially cemented soils have been widely used as filling materials in highway and railway construction. The shear strength evolution of filling materials upon moist variation can determine the stability of subgrade and embankments. This study conducted water retention tests, MIP tests, and multi-stage triaxial shear tests on cement-treated granite residual soil (GRS) to determine its water retention curve (WRC) upon free drying, pore structure, and peak shear strength qf, respectively. The water retention behavior and shear strength evolution upon free drying were modeled based on the dual-porosity structure of cement-treated GRS and the effective stress principle, respectively. Results show that the drying-WRC is bimodal and higher cement dosage yields a more severe decrease in the water retention capacity within a specific suction range. For a given confining pressure, the peak shear strength qf increased with increasing cement dosage or suction value s. The peak shear strength qf also solely depends on the suction value in the peak stress state. In addition, the cement-treated GRS has a bimodal pore size distribution curve, and its macro- and micro-void ratios remain almost unchanged after free drying. The bimodal drying-WRC of the cement-treated GRS can be modeled by differentiating the water retention mechanisms in macro- and micro-pores. Moreover, using the macro-pore degree of saturation as the effective stress parameter χ = SrM, the qf–pf′ relationship (where pf′ is the effective mean pressure at failure) under various suction and stress conditions can be unified, and the qf–s relationships at various net confining pressures σ3,net can be well reproduced. These findings can help design subgrade and embankments constructed by artificially cemented GRS and assess their safe operation upon climate change.

Effect of volumetric deformation on saturation of unsaturated soils under constant suction

The soil–water retention curve (SWRC) represents a fundamental relationship in the study of unsaturated soils, where volumetric deformation plays a crucial role in shaping the water retention characteristics by impacting the pore size distribution. This study conducted a thermodynamic analysis to derive a relationship between the effective stress parameter and the water retention properties of unsaturated soils. Consequently, a streamlined formula was introduced to forecast the impact of volumetric deformation on saturation at constant suction. This formula, incorporating a singular parameter from the effective stress parameters equations, can be seamlessly integrated with any prevailing SWRC model to characterize the influence of volume deformation on the SWRC. The effectiveness of the proposed model was corroborated through seven sets of experimental data spanning various soils, affirming its capability to precisely describe the water retention behavior of deformable soils under deformation.

Evaluation of Design Parameters (α and β) for Analysis and Design of Piles on Soft Clays

This paper presents the analyses of 12 prestressed concrete instrumented test piles (TPs) that were driven in different bridge construction projects of Louisiana as part of a load testing program. The 12 TPs were driven mainly in cohesive soils. Detailed soil characterizations including both laboratory and in situ tests were conducted to determine the different soil properties. The TPs were instrumented with vibrating wire strain gauges to be able to calculate the distribution of skin friction and end-bearing capacities separately. Static load tests and dynamic load tests were conducted on each TP after end of driving to calculate the ultimate load capacity. Fifty six soil layers exhibiting clayey soil behavior and sandy soil behavior dominated the remaining 15 soil layers. The total stress parameter (α) and the effective stress parameter (β) were back-calculated for each soil layer. The α values ranged from 0.22 to 1.80, and the β values ranged from 0.11 to 0.83. Regression analyses were performed on 42 soil layers (75% data) to develop an empirical model to predict α as a function of undrained shear strength. The remaining 25% of the data were used to verify the developed model, and good agreement was observed between the measured and predicted values.

Effective stress and shear strength parameters of unsaturated soils as affected by compaction and subsequent shearing

AbstractWheeling induced compression and shearing forces are main stresses accounting for soil deformation and changes of hydraulic, gaseous and thermal properties. There are reports about the combined effects of compaction and subsequent shearing on soil hydraulic properties, but their consequences on soil strength properties (i.e., effective stress and shear strength) need to be further analysed. This study investigated the dynamics of soil mechanical properties as affected by pore water pressure () during compaction and shearing. Soil samples from an A‐horizon of Gleysols derived from glacial sediment and a Stagnic Luvisol from loess were analysed. The repacked and structured samples were compressed under static and cyclic loading and then sheared at two speeds (0.3 and 2.0 mm min−1) with three loading levels (50, 100, and 200 kPa). During each stress application, the , chi factor (χ) and effective stress () were measured and calculated. The shear strength (), angle of internal friction () and cohesion (), were determined and fitted by the Mohr–Coulomb failure criterion. The results showed that compaction and shearing increased and χ in all homogenized soils while on structured soils this phenomenon only occurred when the applied loading stress exceeded the soil precompression stress. The increased resulted in soil hydraulic and mechanical stresses, which ultimately reduced the , especially at −6 kPa initial matric potential. Soils with finer texture, higher loading stresses and faster shear speed normally exhibited more reduced values. The structured soil had higher values with higher and compared to the homogenized soils. The changes of at high loading stress (i.e., 200 kPa) may overlap the normal pattern of the Mohr–Coulomb failure line that results from the theoretical Mohr envelope. Thus, to minimize the destruction of soil structure and stability induced by wheeling, it is important to consider field water content, traffic loading and wheeling speed.

Evaluating friction angles for clays: piezocone tests compared with Atterberg limits

A limit plasticity solution for evaluating the effective stress friction angle in clays from piezocone tests was assessed and compared with the popular notion of using laboratory Atterberg limits on remoulded specimens. The results showed that the in situ piezocone-based method provided a far more accurate and robust interpretation of the friction angle values of 155 separate clays than empirical correlations with plasticity index when compared with laboratory benchmark values obtained from triaxial compression tests. For illustration, the piezocone-based method was shown successfully to capture the effective stress parameter of four clays with friction angles of 20–37°: normally consolidated kaolin clay in chamber tests; natural soft clay deposit located at Bothkennar, UK; Troll offshore clay in the North Sea; and soft plastic lacustrine deposits in Bogota, Colombia.

Elastic core concept for modified Bree problems considering in-phase and out-of-phase loading conditions

The elastic core concept developed from the Bree problem can be used for structural integrity evaluation of high temperature components, such as creep ratcheting and creep fatigue. The simplified inelastic analysis method of the ASME 2013 III-1 NH Code adopts the idea of core stress, which protects against creep rupture and collapse of the structure by limiting the accumulated inelastic strain. Currently, the formulas of core stress and evaluation diagram of effective creep stress parameter for Tests B-1 and B-3 adopted by the APPENDIX NH-T are developed from the classical loading Bree problem, however, the related concepts and evaluation diagrams of modified Bree problems considering generalized loading conditions have not been reported. This paper first reviews the development history of the elastic core concept and its corresponding analysis methods. Based on the modified Bree problems considering in-phase and out-of-phase loading developed by Bradford, the corresponding formulas of the core stress in the non-plastic ratcheting zone are completely proposed, and the corresponding evaluation diagrams of effective creep stress parameters are constructed for the first time. By comparing the core stress formulas and effective creep stress parameter diagrams under three loading cases, the conservatism and applicability of the classical core stress method are discussed. In addition, the differences and relations between two creep ratcheting assessment methods applicable to transient and sustained thermal load conditions respectively are clarified. The new results and conclusions presented in this paper can deepen the understanding of structural response under complex loading conditions, and provide guidance for shakedown analysis and creep ratcheting assessment of high-temperature components.

A fully coupled hydro-mechanical model for the analysis of partially saturated multiphase geo-structures using the bounding surface plasticity

This paper presents a fully coupled plasticity model for the rigorous analysis of fluid flow and solid skeleton deformation in variably saturated porous media. The flow deformation behaviour of unsaturated soils is described through the simultaneous solution of governing partial differential equations, including the equilibrium equation and the continuity equations of pore fluids. The coupling between solid and fluid phases is established based on the effective stress concept with the effective stress parameter as a function of suction. The elasto-plastic behaviour of geomaterials is formulated using the bounding surface theory. The fully coupled flow deformation framework along with the UNSW bounding surface constitutivelaw are implemented into COMSOL Multiphysics commercial software to simulate the results of several laboratory tests conducted on soils with different degrees of saturation under various drainage conditions and loading paths. The close agreement between the numerical solutions and experimental data from the literature indicates the good performance of this new elastic-plastic unsaturated model and emphasises its capability to capture the different characteristic features of response of soils with a variety of saturation degrees and over-consolidation ratios.

Gaining insight into the effective stress parameter using pore-scale numerical modelling with the multiphase lattice Boltzmann method

We investigate the factors influencing the effective stress parameter, χ, in unsaturated granular soils using pore-scale numerical modelling. The Shan-Chen multiphase lattice Boltzmann method (LBM) is used for this purpose. A procedure is outlined for measuring χ using LBM data. A full hydraulic cycle is simulated for a granular soil column and individual contributions of suction and surface tension forces to χ are measured as a function of the degree of saturation, Sr. It is observed that while χ = 0 at Sr = 0 and χ = 1 at Sr = 1, χ > Sr for 0 < Sr < 1. The trend of the χ-Sr curve is explained based on the trend in the individual components. Schematic plots are utilized to facilitate the understanding of the trends in the data. It is also observed that the χ-Sr relationship is hysteretic; particularly, χ is larger during imbibition compared to drainage, for a wide range of Sr, due to the more pronounced contribution of surface tension forces.

Pore Pressure Estimation for Shaly Sand Formations Using Rock Physics Models For Compressional and Shear Velocities: A 1D Geomechanical Approach

Pore pressure estimation in sedimentary basins has been made exclusively through the compressional velocity data since the 1960s, using the normal compaction trend and lithostatic pressure profile derived from wireline logs. Considering that seismic velocity is highly dependent on petrophysical parameters such as porosity and clay volume, pore pressure estimation is commonly associated with a high degree of uncertainty due to simplistic assumptions that neglect those dependencies. To improve that, we propose two empirical velocity models based on compressional and shear rock physics relations for pore pressure prediction in shaly sand formations. These formulations extend Bowers and Doyen formulae, linking compressional and shear velocities with effective stress and petrophysical parameters. Finally, we used a non-linear multidimensional inversion approach to calibrate the proposed models and apply them in the context of a 1D geomechanics and pore pressure prediction study of an upper cretaceous overpressured shaly sand oil reservoir. The results show good agreement with pore pressure data and pressure predictions from the traditional Eaton method. The advantage of the proposed approach is its consistency throughout the entire well-log petrophysical interpretation workflow, especially concerning porosity, clay volume and saturation.

Contribution of capillary pressure to effective stress for unsaturated soils: Role of wet area fraction and water retention curve

Estimating the contribution of capillary pressure to the effective stress is critical for studying the mechanical and hydraulic behavior of unsaturated soils. Based on experimental evidence, we clarify that the degree of saturation (Sr) rather than matric suction (s) is more suitable as a variable for modeling Bishop's effective stress parameter (χ). The wet area fraction, defined as the ratio of the wet external area of soil particles to the total surface area, is then introduced as an alternative to the parameter χ. Because the contact area between soil particles and pore water is affected by both hydraulic (matric suction) and mechanical (volumetric deformation) states, the soil water retention curve (SWRC) is embedded in the proposed model to simulate the influences of pore size distribution and hydraulic hysteresis on shear strength. The proposed model requires only the material parameters of soils in the reference state and then can be used to predict the strength characteristics in other mechanical or hydraulic states. The shear strength data under different test conditions and the corresponding SWRC data are adopted to validate the new model. The results reveal that the wet area fraction could well predict the contribution of capillary pressure to the shear strength of unsaturated soils.

A Review of Techniques for Measuring the Biot Coefficient and Other Effective Stress Parameters for Fluid-Saturated Rocks

Abstract Predicting the behavior of a saturated rock with variations in pore fluid pressure during geo-energy production and storage, deep geological disposal of nuclear wastes, etc. with skeletal mechanical behavior in the linear elastic range is carried out using the isothermal theory of poroelasticity that incorporates Biot's effective stress principle. For conditions that are not within linear elasticity, other effective stress coefficients are used. Several experimental methods for determining Biot's and other effective stress coefficients have been documented in the literature. The objective of this study is to review the fundamentals of these techniques, their advantages and disadvantages, and to include several case studies. Current techniques for Biot's coefficient are based on different premises: jacketed and unjacketed bulk moduli or compressibility values; volume changes of the bulk and pore fluid from a drained triaxial test on a saturated sample; isotropic-isochoric compression tests on a saturated sample; matching volumetric strains for dry and saturated samples; estimation of the Biot coefficient from other poroelastic parameters; and approximation of the jacketed bulk modulus from ultrasonic wave velocities and/or unjacketed bulk modulus from the mineralogical compositions. Other effective stress coefficients are based on matching failure envelopes for dry and saturated samples and variations of rock properties (such as volumetric strain, permeability, and ultrasonic wave velocities) with respect to confining stress and pore pressure. This article discusses variations in Biot's and other effective stress coefficients produced using the different techniques and how factors such as pore geometry, test conditions, stress path, and test temperature affect the coefficients.

Determination of shear strength parameters of compacted high plasticity clay soils based on different laboratory tests

In civil engineering projects, soils are compacted to improve their engineering behavior and properties. Compacted soils are widely used in dams, embankments and road infrastructure. Compacted fine-grained soils, especially clay-containing soils, are frequently used as barriers to water and pollutant movement in landfills. Shear strength parameters of compacted high plasticity clay soils depend on many variables such as consistency limits, dry density, and degree of saturation. In this study, 20 high plasticity clay soil samples were used and geotechnical identification tests were performed on each of them. Direct shear box and unconfined compression tests were carried out to determine the shear strength parameters of the samples prepared by compression in their compaction characteristics. As a result of this study, the relationships between the geotechnical properties of soil samples and both the effective shear strength parameters and total shear stress parameters were evaluated.

Effect of Water Content on Strength of Alluvial Silt in The Lower Yellow River

In order to study the strength characteristics of alluvial silt in the lower Yellow River Channel at Luokou, Jinan, by varying the moisture content five times, direct shear tests were conducted and soil–water characteristics curves were obtained to explore the relationships between moisture content and matric suction as well as the bishop coefficient and shear strength. The soil–water characteristics curve test shows that the water retention curve of silt samples in the lower Yellow River can be fitted by the VG (Van Genuchten) model with the appropriate fitting coefficients. The direct shear test reveals that the relationship between shear stress and shear displacement alters from the shear softening type to the hardening type with the increase in moisture content and normal stress. The cohesion has a nonlinear inverse relationship with moisture content while a small variation is reported in the internal friction angle. Finally, a simple shear strength equation for silt in the lower Yellow River is proposed in relation to moisture content, to define the relationship between the effective stress parameter and the matric suction for future engineering purposes.

Mechanical response of a cemented soil over a wide range of porosities and cement contents

The adjusted porosity/cement index η/(Civ)a has proved its effectiveness in modelling the strength and stiffness of a variety of artificially cemented soils. In this regard, by establishing a specific η/(Civ)a value, either by increasing the cement content and decreasing the density (or the opposite), a similar mechanical response is expected. Nonetheless, the role of this sort of dosage changing has not yet been verified for triaxial testing and, particularly, the effect of different dosages assembled at the same η/(Civ)a value has not yet been evaluated considering the effective peak stress parameters. As a reason, present research fulfills this gap by conducting a series of CID triaxial tests on a cemented clayey sand and, as well, small strain stiffness tests and the unconfined compressive strength tests. The last two being part of a preliminary assessment which comprehended the study of, at least, three dosages assembled at five different η/(Civ)a values. The test data have revealed that both the stiffness and the strength outcomes could be properly correlated to the η/(Civ)a, regardless of the dosage type. Yet, the effective peak stress parameters (ϕ′peak and c′peak) could not be related to the η/(Civ)a index owing to the dosage-dependence of the strength mobilization mechanisms.

Computational intelligence models for predicting the effective stress of unsaturated soil

In the design of underground spaces, the value of the effective stress (χ) parameter is important while dealing with unsaturated soils. The estimating procedure might be complex due to the non-linearity of the connection between χ parameter and dependant parameters. As a result, two novel hybrid optimization models are proposed in this paper: relevance vector regression (RVR) optimized using artificial bee colony (ABC) and harmony search (HS) algorithms (RVR-ABC and RVR-HS models). The RVR-HS and RVR-ABC models' prediction abilities were demonstrated using 120 datasets from open-source literatures. In this dataset, characteristics of unsaturated soils such as bubbling pressure, soil–water characteristic curve fitting parameter, saturated volumetric water content, net conning pressure, residual water content, and suction were used as the inputs, while χ parameter was used as the output. The proposed models performances were assessed by comparing them regarding some statistical metrics, e.g., correlation coefficient (R2). For the RVR-ABC and RVR-HS models, the R2 of 0.86 and 0.89 were attained, respectively. The findings showed that the RVR-ABC and RVR-HS models were effective in predicting the χ parameter. However, the RVR-HS model, outperformed the RVR-ABC model as this issue shows, the HS algorithm contributed significantly to the optimization of the suggested model.

Generalized Terzaghi’s Effective Stress Equation for Unsaturated Soil: An Independent Phase Balance Approach That Considers a Pore Water Content Gradient

The effective stress equation for unsaturated soil is the most important equation in unsaturated soil mechanics. It has been derived by many scholars using different methods. However, none of them considered the gradient of the pore water content, which results in unreasonable force balance equations for different constituent phases in unsaturated soil. To introduce the gradient, we propose an extended three-phase physical model that includes capillary water, air, and generalized soil skeletons. Based on this model, three balance equations for these three constituent phases are separately formulated by considering the gradient of the pore water content. Comparing the result of the superposition of these three balance equations with the total balance equation, we derive a generalized Terzaghi’s effective stress equation. This equation states that the effective stress is equal to the total stress minus the neutral stress. In comparison with the classical Bishop’s equation, the generalized Terzaghi’s equation ensures a smooth and continuous transition from unsaturated to saturated conditions not only in mathematical expression but also in physical meaning. Furthermore, the different pressure effects of capillary water and adsorbed water, their volumetric (or areal) effects, and the transformation between them can be considered by adopting the effective saturation of the capillary water as the effective stress parameter. Therefore, the generalized Terzaghi’s equation can provide a better choice for estimating the effective stress in unsaturated soils.

Stress dependence of the gas permeability of montmorillonite sediments in the presence of methane hydrate

The evolution of porosity and permeability of silty clay sediments in the South China Sea has significant effect on gas hydrate production due to the stress effect of submarine overburden rocks. The gas permeability of silty clay specimens under different effective stresses was measured in a cylindrical stainless steel reaction vessel. The experimental results in this paper show that application of effective stress destroys gas permeability of methane hydrate specimens. The power function relation between permeability and effective stress and exponential function relation between porosity and effective stress are given. The experimental data show that the formation of methane hydrate improves the compressible channel of montmorillonite deposits. The permeability damage rate increases with increase of effective stress, and stress sensitivity coefficient increases first and then decreases with increase of effective stress. Finally, an effective permeability equation of sediment containing hydrate saturation and effective stress parameters is established. The results are of guiding significance for the study of the stress dependence of porosity and permeability of gas hydrate silty clay reservoirs in the South China Sea. • Power law is superior to exponential law for showing permeability stress dependency. • Hydrate formation improves the compressible space of montmorillonite sediments. • Permeability damage rate increase with increasing effective stress in sediments. • Stress sensitivity coefficient show a trend of decreasing first and then increasing. • Relational expression between effective stress and permeability is proposed.

Capturing dynamic behavior of propped and unpropped fractures during flowback and early-time production of shale gas wells using a novel flow-geomechanics coupled model

Hydraulic fracturing has been widely used to exploit unconventional oil and gas resources for decades. The fracture closure during flowback and early production may be characterized by fluid dehydration and pressure drop in oil and gas production. In this paper, a fully coupled flow and geomechanics model is proposed to capture the dynamic behavior of key fracture parameters for flowback and early-time production. In this coupled model, the controlled volume method is used to numerically simulate the fracture flow, which can consider the geometry and conductivity distribution of the propped and unpropped fractures. For the fracture geomechanics, the joint-closure relationship is introduced to describe the fracture aperture of unpropped fracture. The empirical formula of effective normal stress and proppant parameters is applied to characterize fracture conductivity. The fracture aperture can be calculated coupled with the discontinuous displacement method (DDM) and the matrix transient linear flow. The coupled geomechanically model can consider the flow of the propped and unpropped fracture system, which is easy model-setup and convenient for practical application due to its excellent computational performance. Detailed flow behavior analysis shows that the aperture of the unpropped fracture is relatively slow during the fracture closure process. Compared with the fracture section opening under higher normal stress condition, the fracture section opening under lower normal stress condition has larger initial aperture and faster attenuation of aperture. The crucial fracture parameters, including fracture permeability and fracture length, can be well interpreted by matching flowback and production data. Furthermore, the relationship between fracture conductivity and effective stress for both propped and unpropped fractures can be explored. These new findings can also be applied to interpret reservoir/fracture properties from ChangNing shale fractured wells in China during flowback and early production.