Low Effective Stress Levels Research Articles

Assessing swelling-induced damage in shale samples during triaxial testing

In shale testing, understanding the impact of effective stress and saturation conditions is crucial for accurate material behaviour assessment and parameter determination. In some cases, saturation in triaxial testing starts at low effective stress before ramping up for shearing. However, when in contact with water (or saline water), shales are prone to swelling, particularly at low effective stress levels, which can induce fissures and alter material properties. This study investigates the influence of fluid saturation strategies and stress/pressure variations on the mechanical behaviour of shales, particularly under low effective confinement. Building upon the comprehensive testing campaign (>140 tests) in Crisci et al. (2024), additional tests were conducted on Opalinus Clay shale, focusing on sample saturation methods and loading histories before shearing. The conditions under which tested specimens experience damage were detected through diagnostic indicators such as differences in stress path and lower strength and stiffness compared to intact specimens with identical basic properties. Micro CT scanning confirms that damage is related to the development of fissures. The volumetric changes in specimens were quantified throughout the testing phases and thresholds for tolerable strains and effective stresses, specific to this material, were established. Comparative analysis with Opalinus Clay from shallower depths and other shales globally revealed consistent findings. Notably, it is shown that, for all shale types analyzed, a linear failure envelope emerges in the low to intermediate effective stress regime when filtering out "damaged" specimens. This suggests that non-linear failure envelopes observed in some cases may stem from exposing specimens to low effective stress before shearing.

A hypoplastic model for pre- and post-liquefaction analysis of sands

This article proposes an extended constitutive model for liquefaction analysis of sands. The proposed model is formulated based on the hypoplastic model for granular soils by von Wolffersdorf (1996), enhanced with Intergranular Strain Anisotropy by Fuentes et al. (2019), which is usually referred to as ISA-hypoplasticity. The proposed extended model includes some modifications that are indispensable for the correct prediction of liquefaction-related analysis: (i) a new Lode-angle-dependent function for post-liquefaction shear strain accumulation, (ii) a modification for fabric change effects, (iii) the theory of the so-called semifluidized state. The first modification allows the model to predict shear strain accumulation in extension and compression during cyclic mobility. The second and third modifications, were initially developed by Liao et al. (2022) for hypoplasticity with conventional intergranular strain based on the pioneer work by Barrero et al. (2020) for Sanisand, and enable the model to reproduce fabric change effects upon loading reversal, and stiffness and dilatancy degradation at low effective stress levels, respectively. With the consideration of the new Lode-angle function, the proposed model realistically predict the increasing shear strain accumulation in both extension and compression without adopting a circular critical state surface, as adopted in Liao et al. (2022). The proposed model was carefully calibrated and validated based on a series of monotonic and cyclic triaxial tests on Zbraslav and Karlsruhe fine sands, considering various testing conditions. The comparison between experimental measurements and numerical predictions of undrained cyclic tests suggests that the proposed model accurately describes the pre- and post-liquefaction stages, as well as the stress attractors.

Shallow penetrometer tests: theoretical and experimental modelling of the rotation stage

Shallow penetrometers are a new type of device that measures the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In fine-grained soils such as silts and clays, these testing stages yield properties relevant to subsea pipeline and shallow foundation design; namely, undrained strength, consolidation, and interface friction. This paper describes the fundamentals of the rotation stage, including models required for data interpretation, encompassing both a total and an effective stress framework. Additionally, new relationships to evaluate the pore pressure scaling factor, which is a key interpretation parameter required to convert discrete measurements of pore pressure on the penetrometers to an average pore pressure over the contact area, are developed based on large-deformation finite element simulations. Results from an experimental campaign using kaolin clay samples are presented, illustrating the potential of the devices to rapidly and repeatably measure interface friction properties of fine-grained sediments offshore. The results compare well with comparative measures obtained from shear box tests conducted at similarly low effective stress levels. Recommendations regarding future in situ applications are given at end of the paper.

State-dependent constitutive model and numerical solution of self-weight consolidation

A new constitutive model is developed for the compressibility behaviour of soft clay sediments at low effective stress level. Central to this model is the realisation that at low effective stress levels the void ratio–effective stress paths of soil elements are non-unique, and dependent very much on their state. Beyond a threshold stress level, these relationships become unique. These observations have been used to develop a state-dependent constitutive model for the compressibility behaviour of soft sediments. Methods to obtain the input parameters of the model are discussed. The proposed model is used to solve finite-strain one-dimensional consolidation with pertinent initial and boundary conditions. The analyses performed using a finite difference computer code predict better the observed phenomena associated with self-weight consolidation, especially during its early stages. The comparison of the predictions shows that the present model can capture the decrease in void ratio patterns, as observed in experiments at low effective stresses, and which cannot be modelled using existing constitutive relationships.

Self Weight Consolidation: Compressibility Behavior of Segregated and Homogeneous Finegrained Sediments

Soil sediments formed during sedimentation will be either segregated or homogeneous in nature, depending upon the prevailing depositional environment. These sediments initially undergo self weight consolidation. The article reports information about the compressibility characteristics of soft sediments at very low effective stress levels due to self weight as a function of clay mineralogy.

Simplified Seepage Consolidation Test for Soft Sediments

Abstract The consistency of very soft sediments prevents the conventional oedometer test from being applied to study their compressibility and permeability characteristics. The hydraulic consolidation test in existence requires sophisticated instrumentation and testing procedures. The present paper proposes a seepage-force-induced consolidation testing procedure for studying the compressibility and permeability behavior of soft sediments at low effective stress levels. The good agreement that has been observed between the results obtained from the proposed method and the conventional oedometer test at overlapping effective stress levels indicates that the proposed method can be used to study the compressibility and permeability characteristics of soft sediments at low effective stress levels satisfactorily.

Back analysis of coalbed strength properties from field measurements of wellbore cavitation and methane production

Abstract Direct measurement of field representative strength and permeability characteristics of coalbed methane is difficult due to the fact that coal has a cleated structure. These cleats or discontinuities vary in orientation and size throughout the coal mass. The cleats' network largely controls the strength of the coal and also provides the passage for fluid flow. Permeability of the cleats varies with the confining stress and shear stress level. Laboratory test results are greatly influenced by the size and orientation of the cleats, and the magnitude and direction of applied stresses. To obtain the large-scale hydraulic and strength properties of coal, afield problem involving wellbore cavitation and a concomitant increase in gas production was numerically analysed. Calibration of the numerical model against the field observations which included a 10 fold enlargement of the wellbore radius and a four fold increase in gas production indicated that the apparent cohesion of coal is approximately 100 kPa and not 2–5 MPa, as was previously thought. The validity of this finding was later confirmed by performing a set of laboratory direct shear tests at low effective stress levels. The findings from this study have significant implications in the design of wellbores for production of coalbed methane.

Back analysis of coalbed strength properties from field measurements of wellbore cavitation and methane production

Direct measurement of field representative strength and permeability characteristics of coalbed methane is difficult due to the fact that coal has a cleated structure. These cleats or discontinuities vary in orientation and size throughout the coal mass. The cleats' network largely controls the strength of the coal and also provides the passage for fluid flow. Permeability of the cleats varies with the confining stress and shear stress level. Laboratory test results are greatly influenced by the size and orientation of the cleats, and the magnitude and direction of applied stresses. To obtain the large-scale hydraulic and strength properties of coal, afield problem involving wellbore cavitation and a concomitant increase in gas production was numerically analysed. Calibration of the numerical model against the field observations which included a 10 fold enlargement of the wellbore radius and a four fold increase in gas production indicated that the apparent cohesion of coal is approximately 100 kPa and not 2–5 MPa, as was previously thought. The validity of this finding was later confirmed by performing a set of laboratory direct shear tests at low effective stress levels. The findings from this study have significant implications in the design of wellbores for production of coalbed methane.

Response Prediction of Granular Materials at Low Effective Stresses

Analysis of laboratory experiments and model structures on cohesionless granular materials conducted at very low effective stress levels have shown that their response to loading differs significantly from that observed at higher stress levels. These differences may be attributed both to intrinsic material properties and external factors such as boundary conditions. The gravity induced self‐weight stresses that occur in typical laboratory specimens are often of the same order of magnitude as the externally applied tractions or stresses that result from displacement controlled modes of loading at very low confinement levels. A three stress invariant elastoplastic constitutive model recently proposed may be used to simulate the nonlinear stress‐strain‐strength behavior of granular soils at low stress levels. The constitutive equations have been integrated via a fully implicit technique. The model parameters are calibrated via an optimization scheme. In this study the model is implemented in a finite‐element...

Development of KO in soft soils

This Paper presents observations of the development of A',,, the coefficient of lateral earth pressure at rest, in a soft kaolin undergoing one-dimensional consolidation up to 150 kPa. A consolidometer with flush diaphragm transducers was linked to a system controlled by microprocessor for controlled gradient consolidation. Remoulded and artificially sedimented samples of kaolin in distilled and sea water chemistry were used in the experimental programme. Experimental observations indicate that A"0 is not a constant but varies linearly with void ratio. It is suggested that the traditional empirical correlations between K0 and friction angle may be applicable at low stress levels, given that ty may be higher at low effective stress levels than at higher values. L'article présente des études de la détermination de A,, coefficient du poids latéral des terres au repos, pour un kaolin mou soumis à une consolidation unidimensionnelle pouvant atteindre 150 kPa. Un consolidomètre, à capteurs à diaphragmes affleurants, a été relié à un système piloté par un microprocesseur permettant de controller le gradient de consolidation. Des échantillons de kaolin remariés et artificiellement sedimentes dans de l'eau distillée et dans de l'eau de mer ont été utilisés dans le programme expérimental. Les résultats expérimentaux montrent que A~0 n'est pas constant et qu'il varie linéairement avec l'indice des vides. Il semble que les corrélations empiriques traditionnelles entre K0 et l'angle de frottement 4>' peuvent exister pour de faibles niveau de contrainte si l'on suppose que 4>' est plus grand pour les faibles contraintes effectives que pour les contraintes plus élevées.

Analysis and calibration of a three-invariant plasticity model for granular materials

In this paper we briefly review issues related to the characterization of properties of granular materials subjected to micro-gravity and one-gravity conditions at very low effective stress levels. We describe the development of a three-invariant plasticity model that resembles the model devised by Lade. An inverseidentification scheme where the analysis tools are used to extract constitutive model parameters from experiments is also discussed.