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Abstract: Any failure criterion can be represented as a surface in principal stress space σ 1, σ 2, σ 3 (with no order implied), and the shape of the surface depends on the functional form of the criterion. For isotropic rock that exhibits a pressure dependence on strength, the simplest failure criterion is a linear function, and the failure surface is a hexagonal pyramid with a common vertex V o on the tension side of the hydrostatic axis, where V o =(theoretical) uniform triaxial tensile strength. An example of a pyramidal failure surface is the popular Mohr–Coulomb criterion, which is independent of the intermediate principal stress σ II (σ I ≥σ II ≥σ III ) and contains two material parameters, such as V o and the internal friction angle ϕ. The Paul–Mohr–Coulomb failure criterion Aσ I +Bσ II +Cσ III =1 is linear with three principal stresses, and it is formulated with three identifiable material constants, where A=(1−sinϕ c )/(2V o sinϕ c ), B=(sinϕ c −sinϕ e )/(2V o sinϕ c sinϕ e ), C=−(1+sinϕ e )/(2V o sinϕ e ) and ϕ c , ϕ e are internal friction angles for compression (σ II =σ III ) and extension (σ I =σ II ). The convex nature of the failure surface at constant mean stress can be approximated by additional planes with appropriate material parameters. To demonstrate the utility of the linear failure criterion, a series of conventional triaxial compression and extension experiments were performed on an isotropic rock. The results were processed using the developed data fitting techniques, and the material parameters for the six-sided pyramidal failure surface were determined. A multi-axial experiment was also performed to evaluate the convexity of the failure surface, and a twelve-sided pyramid was constructed and the appropriate equations were derived. [Copyright &y& Elsevier]

A study was conducted to evaluate the effects of varying the gradation of aggregate base course (ABC) on material performance. Five different gradations consistent with a range of common ABC were selected. Experimental trials were performed to classify the materials and assess the performance differences between the five gradations. The gradation was found to affect the behaviour of the aggregate in that coarser gradations gave better strength and resilience values under conditions most similar to those found in the field. From research and experience during the laboratory testing, it was concluded that the coarsest blends became too difficult to realistically work with and they also lacked the stability of the well-graded combinations. It was also observed that as the amount of fines in the specimens exceeded 8–12% by mass, the fines governed the behaviour of the material. Results are interpreted using previously published work on specific surface, ‘ideal’ gradations and micromechanical models for the response of granular materials. [ABSTRACT FROM PUBLISHER]

Extensive damage to residential wood-frame buildings caused by failures of roof-to-wall connections during extreme wind events underscores the need to improve their performance. Most of these connections use mechanical connectors, e.g., metal clips and straps (sometimes referred to as hurricane clips and hurricane straps). The allowable capacity of these connectors is based on results of unidirectional component tests that do not simulate multiaxial aerodynamic loading effects induced by high wind events. The objective of this research was to facilitate a better understanding of these loading effects on roof-to-wall connections of a typical low-rise gable roof residential structure subjected to combined impacts of wind and a potential breach of the building envelope. Large-scale experiments on a heavily instrumented building model generated multiaxial aerodynamic loading data on roof-to-wall connections for various wind angles of attack and internal pressure conditions. The results showed the severity of increased loading on connections in certain configurations of wall openings that could occur from the breach of the building envelope in windstorms. It was also shown that lateral components of the wind load acting simultaneously with the uplift may be significant, depending on wind angles of attack and internal pressure conditions. Aerodynamic test data were used to perform component-level triaxial-load testing on hurricane clips to determine their load capacities and compare results to those obtained using the traditional approach of uniaxial-load testing. Component level test results showed that current uniaxial testing methods tend to overestimate the actual load capacities of metal connectors. Neglecting triaxial loading effects in testing of connectors and in the design of connections could potentially cause the type of failures frequently documented in hurricane damage reconnaissance reports. [ABSTRACT FROM AUTHOR]

Abstract: Carbon dioxide (CO2) sequestration in deep, unmineable coal seams may provide an immediate and economically feasible solution for mitigation of anthropogenic CO2 emissions. Coal contains natural cleats, which largely control fluid movement in coal seams. This study uses experimental and numerical methods to investigate the variables (i.e. injection pressure, injection depth, and coal temperature) that will influence cleat performance in the CO2 sequestration process for black coal. The steady-state downstream pressure (and pressure differential from the injection value) that develops during undrained triaxial tests performed on fractured black coal, without deviatoric load, is taken as a measure of cleat performance. The results of tests show that increasing injection pressure has a significant detrimental effect on cleat performance, in accord with greater adsorption-induced swelling (and cleat closure) with higher injection pressure. Testing also shows that increasing injection depth has a moderate (negative) effect on cleat performance and that temperature has a minor (positive) effect on cleat performance. The experimental results were used to validate a numerical model produced using the COMET 3 numerical simulator, which was used to further explore coupling between the variables that influence cleat performance in coal-seam CO2 sequestration. The numerical results show that the influence of injection pressure on cleat performance is dependent on injection depth, whereby a given increase in injection pressure will have more detrimental effects on cleat performance at greater depths than it would at more shallow depths. The influence of injection depth on cleat performance is similarly dependent on the absolute depth range considered. Coupling was observed in the influence of temperature on cleat performance and both injection pressure and depth. Modeling shows that increasing temperature has a negative influence on cleat performance for low injection pressures (i.e. 6MPa), but a positive influence on cleat performance for higher injection pressures (i.e. 12MPa). Modeling shows that temperature will have an influence on the manner by which depth effects cleat performance if injection pressures are high (i.e. 12MPa), but no such influence if injection pressures are low (i.e. 6MPa). [Copyright &y& Elsevier]

Abstract: This paper studies the behavior of a number of gravelly modeled materials by conducting large-scale triaxial testing. Undrained monotonic compression tests were carried out on the high compacted gravelly soil specimens with different fine content and stress levels. It was observed that high compacted gravelly soil specimens show dilative behavior at failure in a wide range of fine content. The effective stress paths of the materials having less than 10% and more than 22% of material finer than 0.2 mm are completely different. Variations of the CU material parameters with respect to the confining pressure, fine content, shape, strength and particle size were investigated. The CU material results were compared with those in CD conditions. The value of internal effective friction angle in CD conditions is slightly higher than those of the CU conditions. Moreover, where the volume strain in CD conditions is minimum, the excess pore water pressure in CU conditions decreases to zero. [Copyright &y& Elsevier]

Abstract: A comprehensive laboratory investigation was performed on municipal solid waste (MSW) from a landfill located in northern California using a large-scale triaxial (TX) apparatus. An improved, standardized waste specimen preparation method was developed and used to prepare 27 large-scale TX specimens (d =300mm, h =600–630mm). The effects of waste composition, confining stress, unit weight, loading rate, and stress path on the drained stress–strain response of MSW were investigated. Waste composition has a significant effect on its stress–strain response. The commonly observed upward curvature of the stress–strain response of specimens composed of larger-sized waste materials results from the fibrous constituents (primarily paper, plastic and wood) reinforcing the waste matrix. This effect is greatest when the MSW specimen is sheared across the long axis of the fibrous particles. Due to this significant strain hardening effect and waste’s in situ stress state, a limiting strain failure criterion of 5% axial strain from the K o field consolidation state is judged to be most appropriate. Results from this test program and data from the literature indicate that the TX compression secant friction angle of MSW varies from 34° to 44°, with 39° as a best estimate, at a confining stress of one atmosphere (assuming c =0). The friction angle decreases as confining stress increases. The friction angles measured in this testing program are representative of failure surfaces that are oriented at an angle to the predominant orientation of the long axis of the fibrous waste particles. These friction angles are higher than those obtained in direct shear tests where shearing typically occurs parallel to the orientation of the fibrous waste particles. [Copyright &y& Elsevier]

This paper presents the results of a laboratory testing program involving consolidated undrained triaxial tests and direct shear tests on remoulded peat, remoulded peat fibre, and Shelby specimens of peat obtained from a field site located in the Edson subdivision of the Canadian National railway in Alberta, Canada. These results were analyzed within the frameworks of elastic behaviour of cross-anisotropic materials and shear strength of fibre-reinforced soil. Shelby specimens were found to be inherently cross-anisotropic, whereas the remoulded peat and peat fibre specimens showed a transition from isotropic to cross-anisotropic with increasing vertical strain and effective confining pressure. The horizontal stiffness of Shelby specimens was found to be 2.6 to 2.9 times their vertical stiffness. The shear strength of intact peat is made up of interparticle friction as well as tension in the peat fibres. A novel procedure for estimating the interparticle frictional strength of fibrous peat from CU triaxial test results is proposed. It involves extrapolating the linear strain-hardening portion of the stress-strain curve to obtain the deviatoric stress at zero axial strain and plotting the deviatoric stress values thus obtained against initial mean effective confining pressure to obtain the frictional strength. Using this procedure, a value of 31° was obtained for the interparticle friction, which compares favourably with a value of 31° obtained from direct shear tests. It is recommended that further studies be undertaken to assess if interparticle frictional strength is an appropriate strength parameter for evaluation of the stability of structures founded on fibrous peat. [ABSTRACT FROM AUTHOR]

This article presents a constitutive model for cohesive soils that exhibits the post-peak strain-softening behavior. The model combines the elements of plasticity with damage mechanics to simulate the stress–strain behavior. The total strain increment is composed of an elasto-damage strain increment and a plastic strain increment. The post-peak stress drop is captured by the elasto-damage formulation, while the plasticity is superimposed beyond the elastic range. The elasto-damage strain increment is found using the elasto-damage formulation, while the plastic strain increment is found using either the Drucker–Prager classical plasticity model or as a function of the damage strain. To calibrate this model, an experimental program was conducted on a cohesive soil at different densities and water contents. The various physical and mechanical properties of this soil were determined. Triaxial compression tests, unconfined compression test, and hydrostatic tests were performed, in order to obtain the model parameters at different conditions. These parameters were used to calibrate the model, which was coded in FORTRAN computer programs to simulate the stress–strain behavior of cohesive soils. The model was verified and found to be a good predictor of the response of cohesive soil for the selected stress path. [ABSTRACT FROM PUBLISHER]

Abstract: Synthesized gas hydrate in lab was mixed with kaoline to form core samples, so as to simulate the hydrate-bearing sedimentary layers in the seabed. Under conditions of different confining pressure, triaxial compressive tests were conducted on hydrate-bearing sediments with different volume content of kaoline. The results show that: (1) The failure strength of gas hydrate-bearing sediments increases with the confining pressure at a low-pressure stage. The strength tends to decline gently with further increases of confining pressure. (2) The modulus of elasticity E 0 keeps unchanged under various confining pressures. But the secant modulus E 50 presents a great dependency on the confining pressure. Secant modulus increases to the peak and then decreases with the increase of confining pressure; (3) The internal friction angles of gas hydrate-bearing sediments are not sensitive to the volume ratio of kaoline, but its cohesion depends on the volume ratio of kaoline. The sediment strength increases with the increase of kaoline content. [Copyright &y& Elsevier]

Abstract: This study investigates an application of genetic programming (GP) for modeling of coarse rotund sand–mica mixtures. An empirical model equation is developed by means of GP technique. The experimental database used for GP modeling is based on a laboratory study of the properties of saturated coarse rotund sand and mica mixtures with various mix ratios under a 100kPa effective stresses, because of its unusual behavior. In the tests, deviatoric stress, and pore pressure generation, and strain have been measured in a 100mm diameter conventional triaxial testing apparatus. The input variables in the developed GP models are the mica content, and strain, and the outputs are deviatoric stress, pore water pressure generation. The performance of accuracies of proposed GP based equations is observed to be quite satisfactory. [Copyright &y& Elsevier]

A triaxial system is designed with a temperature range from −20 °C to 25 °C and a pressure range from 0 MPa to 30 MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments. The mechanical properties of synthetic gas hydrate-bearing sediments (gas hydrate-kaolin clay mixture) were measured by using current experimental apparatus. The results indicate that: (1) the failure strength of gas hydrate-bearing sediments strongly depends on the temperature. The sediment's strength increases with the decreases of temperature. (2) The maximum deviator stress increases linearly with the confining pressure at a low-pressure stage. However, it fluctuates at a high-pressure stage. (3) Maximum deviator stress increases with increasing strain rate, whereas the strain-stress curve has no tremendous change until the axial strain reaches approximately 0.5%. (4) The internal friction angles of gas hydrate-bearing sediments are not sensitive to kaolin volume ratio. The cohesion shows a high kaolin volume ratio dependency. [ABSTRACT FROM AUTHOR]

Abstract: In the 1920s and 1930s, Terzaghi and coworkers realized that the failure of various porous geomaterials under internal pore pressure is given through evaluating the failure function for the same materials at zero pressure, with ‘total stress plus pore pressure’ instead of ‘total stress alone’ as argument. As to check, probably for the first time, the relevance of this (‘Terzaghi''s’) failure criterion for trabecular bone, a series of poromechanical and ultrasonic tests was conducted on bovine and human trabecular bone samples. Evaluation of respective experimental results within the theoretical framework of microporomechanics showed that (i) Terzaghi''s effective stress indeed governs trabecular bone failure, (ii) deviatoric stress states at the level of the solid bone matrix (also called tissue level) are primary candidates for initiating bone failure, and (iii) the high heterogeneity of these deviatoric tissue stresses, which increases with increasing intertrabecular porosity, governs the overall failure of trabecular bone. Result (i) lets us use the widely documented experimental results for strength values of bone samples without pore pressure, as to predict failure of the same bone samples under internal pore pressure. Result (ii) suggests a favorable mode for strength modeling of solid bone matrix. Finally, result (iii) underlines the suitability of microfinite element simulations for trabecular bone microstructures. [Copyright &y& Elsevier]

Abstract: Coal permeability is sensitive to the effective stress and is therefore coupled to the geomechanical behaviour of the seam during gas migration. As coal shrinks with gas desorption and swells with adsorption, understanding this coupling to geomechanical behaviour is central to interpreting coal permeability. Existing coal permeability models, such as those proposed by Shi and Durucan (2004) and Palmer and Mansoori (1996), simplify the geomechanical processes by assuming uni-axial strain and constant vertical stress. However it is difficult to replicate these conditions in laboratory tri-axial permeability testing and during laboratory core flooding tests for enhanced coal bed methane. Often laboratory tests involve a hydrostatic stress state where the pressure in the confining fluid within the tri-axial cell is uniformly applied to the sample exterior. In this experimental arrangement the sample is allowed to undergo tri-axial strain. This paper presents two new analytical permeability model representations, derived from the general linear poroelastic constitutive law, that include the effects of tri-axial strain and stress for coal undergoing gas adsorption induced swelling. A novel approach is presented to the representation of the effect of coal sorption strain on cleat porosity and thus permeability. This involves distinguishing between the sorption strain of the coal matrix, the pores (or cleats) and the bulk coal. The developed model representations are applied to the results from a series of laboratory tests and it is shown that the models can predict the laboratory permeability data. As part of this characterisation the various sorption strains are identified and it is shown that the pore strain is significantly larger than (approximately 50 times) the bulk sorption strain. The models also provide further insight into how coal permeability varies with coal shrinkage and swelling and how the permeability rebound pressure depends upon the effective stress applied. [Copyright &y& Elsevier]

Marine clays at Gangavaram port on east coast of India are known to contain seashells which affect the undrained shear strength of the soft clays. This paper reports the results of consolidated undrained triaxial tests (CIU) on undisturbed samples of marine clay collected from the site. The findings from the tests are then compared to the results of the common elasto-plastic soil model used in the back analysis. The results seem to suggest that the presence of seashell fragments influence the measured pore pressure response and the strength especially at low consolidation pressure when stiffness of the clay is less. [ABSTRACT FROM AUTHOR]

A series of isotropic consolidated drained triaxial tests were conducted on three carbonate sands with different physical properties and particle breakage potential. Tests were performed on loose and dense ungrouted and loosely grouted specimens. Two sands were obtained from north shores of Persian Gulf, south of Iran, named Kish and Hormoz sands. Another sand was taken from south of England named Rock sand. Some of specimens were cemented using injection of a chemically grout named sodium silicate. Effects of grains physical properties on the shear behavior and strength of grouted sands under constant conditions such as initial relative density (before consolidation), grout, confining pressure and loading are studied. Tests results showed different compressibility, shear behavior, shear strength parameters, deformability and particle breakage for the studied sands. Totally, bonding the particles to each other improves the shear strength parameters and reduces the compressibility of soil. However, the three grouted sands demonstrated different shear behavior and strength response including bond strength, initial relative density and confining pressure under the same conditions due to different physical properties of grains. [ABSTRACT FROM AUTHOR]

This paper investigates the results of laboratory experiments and numerical simulations conducted to examine the behavior of mixtures composed of coarse (i.e. Leighton Buzzard Sand fraction B) and fine (i.e. Leighton Buzzard Sand fraction E) sand particles. Emphasis was placed on assessing the role of fines content in mixture and strain level on the deviatoric stress and pore water pressure generation using experimental (i.e. Triaxial testing) and numerical approaches (i.e. genetic programming, GP). The experimental database used for GP modeling is based on a laboratory study of the properties of saturated coarse and fine sand mixtures with various mix ratios under a 100 kPa effective stresses in a 100 mm diameter conventional triaxial testing apparatus. Experimental results show that coarse–fine sand mixtures exhibit clay-like behavior due to particle–particle effects with the increase in fines content. It is shown that GP modeling of coarse–fine sand mixtures is observed to be quite satisfactory. The results have implications in the design of compressible particulate systems and in the development of prediction tools for the field performance coarse–fine sands. [ABSTRACT FROM AUTHOR]

The idea of mixing mine tailings and waste rock to form “paste rock” prior to disposal is now receiving significant attention from the point of view of sustainable mine waste management practices. This approach has been viewed as a favourable alternative to traditional methods of mine waste disposal because paste rock has the potential to overcome deficiencies (e.g., acid rock drainage and liquefaction-induced failures) associated with traditional methods. To advance the current limited knowledge, a laboratory research program was undertaken to study the mechanical response of paste rock. Testing was undertaken on paste rock specimens prepared so that the tailings would “just fill” the void spaces within the waste rock particle skeleton. The findings suggest that the material is unlikely to experience flow deformation under monotonic shear loading conditions, at least up to the tested initial effective confining stress conditions of 400 kPa. The monotonic shear response of paste rock was found to be similar to that of rock-only material; this finding is in agreement with previous observations from one-dimensional consolidation testing where it had been shown that the rock particle skeleton would carry almost 90% of the externally applied stress on a given paste rock mass. Le concept de « pâte résidus-stériles (PRS), c’est-à-dire le mélange de rejets de concentrateur et de roches stériles avant leur déposition, reçoit maintenant beaucoup d’attention d’un point de vue de gestion durable des résidus miniers. Cette approche est considérée comme une alternative favorable aux méthodes traditionnelles de gestion des résidus miniers puisque la pâte résidus-stériles peut potentiellement pallier à certaines préoccupations (ex. drainage minier acide et ruptures induites par liquéfaction) associées aux méthodes traditionnelles. Afin d’améliorer les connaissances actuelles, un programme de recherche en laboratoire a été élaboré pour étudier le comportement mécanique de la PRS. Les essais ont été effectués sur des échantillons de PRS préparés de façon àce que les rejets remplissent « tout juste les vides entre les particules des roches stériles. Les résultats suggèrent que le matériau a de faibles chances de subir des déformations sous des conditions de chargement en cisaillement monotonique, du moins jusqu’aux conditions testées de contraintes effectives de confinement initiales de 400 kPa. Le comportement en cisaillement monotonique de la PRS est similaire à celui de la roche stérile seule. Ce résultat concorde avec des observations précédentes obtenues lors d’essais en consolidation unidimensionnelle avec lesquels il avait été démontré que le squelette des particules rocheuses pouvait supporter près de 90% des contraintes externes appliquées sur une masse de PRS. [ABSTRACT FROM AUTHOR]

Mixing of waste rock and tailings as a homogeneous mixture (referred to as “paste rock”) has been suggested as a favourable approach to overcome deficiencies associated with traditional methods of mine waste disposal. In consideration of the current limited understanding of the fundamental mechanical response of paste rock, a laboratory research program was undertaken to investigate the monotonic and cyclic shear response of paste rock. This paper presents the findings from undrained cyclic triaxial shear testing conducted on saturated paste rock specimens reconstituted such that the tailings would “just fill” the void spaces between the coarse particles of the skeleton. During undrained cyclic loading, paste rock typically exhibited a cumulative decrease in effective stress along with a progressive degradation of shear stiffness. The material generally displayed a higher tendency for strain development under cyclic loading when compared with rock-only and tailings-only specimens subjected to similar consolidation and cyclic loading conditions. However, no strain-softening or loss of shear strength was observed, suggesting that the material is unlikely to experience flow failure under undrained cyclic loading. The cyclic shear resistance was relatively insensitive to the initial effective confining pressure ([IMG]) for the tested stress levels of [IMG] < 400 kPa. Le mélange des roches stériles et des rejets de concentrateur sous forme de pâte homogène (appelée pâte résidus-stérile ou PRS) a récemment été suggéré comme approche de gestion des résidus miniers qui permet de contourner certains problèmes associés aux approches traditionnelles. Puisque les connaissances sont limitées dans le domaine du comportement mécanique fondamental de la PRS, une étude en laboratoire a été initiée dans le but d’évaluer le comportement en cisaillement monotonique et cyclique de la PRS. Cet article présente les résultats obtenus lors des essais triaxiaux en cisaillement cyclique non drainés effectués sur des échantillons de PRS saturés reconstitués de telle façon à ce que les rejets remplissent tout juste les vides entre les particules de roches stériles. Pendant le chargement cyclique non drainé, la PRS présente typiquement une diminution cumulative de la contrainte effective ainsi qu’une dégradation progressive de la rigidité en cisaillement. Le matériau démontre généralement une plus grande tendance au développement de déformations en conditions de chargement cyclique, comparativement aux échantillons de stérile seul et de résidu seul soumis aux mêmes conditions de consolidation et de chargement cyclique. Cependant, aucun relâchement des déformations ou perte de résistance au cisaillement n’a été observé, ce qui suggère que le matériau n’est pas susceptible de subir une rupture en écoulement lors de chargement cyclique. La résistance au cisaillement cyclique était relativement insensible à la pression initiale de confinement ([IMG]) pour les contraintes testées de [IMG] < 400 kPa. [ABSTRACT FROM AUTHOR]

This paper presents results of shear strength testing of intact and recompacted samples of municipal solid waste (MSW). A method for in situ sampling of MSW from landfills using a push-in sampler was developed and used to obtain intact samples of MSW from a large municipal landfill. Shear strength testing of MSW was carried out using a large triaxial compression apparatus as well as a large direct shear apparatus. The results are presented in terms of cohesion intercept (c′) and angle of shearing resistance ([IMG]) and are compared with those available in published literature. Based on these results and their favourable comparison with this literature, it can be concluded that meaningful shear strength parameters for MSW can be obtained using consolidated undrained triaxial tests on large-diameter intact and recompacted samples. A fairly consistent picture of the shear behaviour of MSW obtained from effective stress paths in triaxial tests appears to suggest that shear behaviour of MSW can be explained using the effective stress principle. It is suggested that recompacted samples could be used for obtaining reasonable estimates of c′ and [IMG] for MSW; however, it may be necessary to use intact samples to establish the pre-failure deformation behaviour of MSW. [ABSTRACT FROM AUTHOR]

Abstract: The paper describes the development of a constitutive model for a poorly graded sand, which was used in geotechnical experiments on buried pipes (reported elsewhere). The sand was tested extensively in the laboratory to determine the state parameter constants. Triaxial tests on the sand included conventional drained triaxial compression tests, as well as more specialized shearing tests at constant mean effective stress and others at constant volume. Single element simulation of the triaxial tests was performed to validate the proposed constitutive model. The adopted model allowed non-linear elastic behaviour prior to yielding. After yielding of the sand, the state parameter-based model for the sand permitted non-associated plastic flow. Dilation and frictional strength were both dependent on the current value of the state parameter. The combination of laboratory testing and single element modelling resulted in the selection of a single set of material constants for the soil, which adequately described the full range of triaxial tests. Subsequently the model was applied to the problem of a plate loading test on the sand and the model predictions were compared with the test data. [Copyright &y& Elsevier]

A comprehensive research project was undertaken to evaluate the suitability of triaxial testing in determining formwork pressure developed by self-consolidating concrete (SCC) under both drained and undrained conditions. The cohesion (C) and angle of internal friction (φ) were determined on 10 concrete-equivalent mortar (CEM) mixtures made with various binder types and water-cementitious material ratios (w/cm). Correlations were established with respect to thixotropy and formwork pressure determined using SCC. Test results show that the cohesiveness of CEM mixtures is greatly affected by the drained and undrained conditions under which the tests are conducted. Mixtures made with ternary cement and/or lower w/cm exhibited higher C and lower φ values compared to those made with binary or Type I cement, as well as those prepared with increased water content. Lateral pressure of SCC developed right after casting can be predicted using either thixotropy or C values. For pressure drop evaluation over time, however, the CEM triaxial approach was not appropriate given the absence of coarse aggregate, which underestimates the development of internal friction between solid particles. [ABSTRACT FROM AUTHOR]

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• Monotonic and RLPD triaxial tests of CIR mixtures were conducted. • Shear parameters and Mohr–Coulomb envelopes were obtained. • Higher BC provided higher cohesion but was detrimental to internal friction. • Permanent strains under different SR were assessed, and two M−E prediction models were fitted. • Mixtures with 2.50% BC exhibited the best response to permanent deformation. The cold in-place recycling (CIR) technique is a road rehabilitation method suitable for a circular economy. Triaxial tests have proven to be suitable for studying the mechanical behaviour of cold mixtures. In this study, CIR specimens were prepared using a gyratory compactor with different proportions of bitumen emulsion. The Mohr–Coulomb diagrams and shear parameters were obtained by conducting monotonic triaxial tests at different confining pressures. The binder content provided adequate cohesion; however, an excess of binder reduced internal friction. Subsequently, repeated load permanent deformation triaxial tests were performed. Accordingly, critical stress ratios between 20% and 30% were obtained, and the mix with a 2.50% binder exhibited the best response. Lastly, two permanent deformation prediction models were fitted to the measured results. For high deformations, the Paute model underestimated the deformations, whereas the Huurman model demonstrated the best fit. [ABSTRACT FROM AUTHOR]

• The nonlinear elastic behaviour of CIR mixtures with 100% RAP was studied. • Triaxial tests were conducted assuming CIR behaviour similar to granular material. • The influence of different proportions of bitumen emulsion and water was assessed. • The evolution of the resilient moduli was observed for different curing times. • The resilient moduli obtained were adjusted using prediction numerical models. This study evaluates the short-term resilient behaviour of cold in-place recycled (CIR) asphalt mixtures with bitumen emulsion when they are more similar to non-cohesive granular materials than to conventional hot mixtures. Gyratory compacted CIR specimens were manufactured using different proportions of bituminous emulsion and water, and dynamic triaxial tests were conducted at different curing times. The resilient moduli were obtained experimentally and fitted using three numerical models. Characteristic nonlinear elastic behaviour and an increase in stiffness with curing time were observed. Water loss during curing and stiffness increase were found to be related. Mixtures with 2.50% residual bitumen and 2.75% added water showed the best short-term stiffness evolution. [ABSTRACT FROM AUTHOR]

Abstract: The experimental investigation of clayey rocks which are typical for cap rock formations above CO2 storage reservoirs is described and results are given. One series of claystone samples is gathered from the injection well of the Ketzin pilot test site in Germany. Another series comes from the same Keuper formation in a near-surface outcrop in the Southwest of Germany. The near-surface condition allowed drilling of samples with a standard size of 100 mm diameter and 200 mm height as well as large samples with a diameter of 550 mm and a height of 1200 mm. A special triaxial testing procedure is applied both on standard and large size samples which allows determination of the strength, stiffness and viscosity behavior of the rock in one path. A multi-stage technique (stepwise variation of the confining pressure) gives the strength behavior of each single sample. Stepwise varied deformation rates, over two orders of magnitude, lead to steps in the stress–strain-curve from which the viscosity index can be calculated. The viscosity index can be directly used in the Norton’s constitutive relations for a viscoplastic simulation. The comparison of results from standard and large samples shows that for clayey rocks a scale effect is negligible. Strength, stiffness and viscosity are not dependent from the sample size. The observed failure modes of the samples show that the transition from cataclastic to non-cataclastic behaviour occurs in the range of the applied levels of pressure and deformation rates even at room temperature. This transition limit is very important for the judgment of the sealing capacity of a cap rock. If numerical simulations result in deformation rates which, for the given pressure and temperature conditions in a cap rock formation during the CO2 injection, stay sub-critical for cataclastic behaviour, the formation of new cracks can be excluded. [Copyright &y& Elsevier]

Hydration occurring in cemented paste backfill (CPB) is shown to generate matric suction through self-desiccation. This complicates determination of the water-retention curve and mechanical properties during curing, which are important in stope design, and renders problematic the use of axis-translation testing procedure to control suctions in strength testing. An alternative is to monitor suction directly during testing. To this end, a miniature tensiometer is inserted into the base of a triaxial cell. A series of unconfined and confined tests are performed on early-age (less than two weeks old) CPB. The observed behaviour is similar to that observed in weakly cemented soils, showing a distinct tendency to dilate during shearing. On a montré que l’hydratation qui se produit dans les remblais de pâte de ciment « CPB génère une succion matricielle par auto-dessiccation. Ceci complique la détermination de la courbe de rétention d’eau et des propriétés mécaniques durant le mûrissage, qui sont importantes dans la conception de gradins, et rend problématique l’utilisation de la procédure d’essais par translation d’axe pour contrôler les succions dans les essais de résistance. Une alternative consiste à mesurer la succion directement durant l’essai. À cette fin, un tensiomètre miniature est inséré dans la base de la cellule triaxiale. Une série d’essais confinés et non confinés sont réalisés tôt dans l’histoire de la pâte « CPB (moins de deux semaines). Le comportement observé est similaire à celui observé dans les sols faiblement cimentés, montrant une tendance distincte à se dilater durant le cisaillement. [ABSTRACT FROM AUTHOR]

Abstract: The poromechanical behaviour of hardened cement paste under isotropic loading is studied on the basis of an experimental testing program of drained, undrained and unjacketed compression tests. The macroscopic behaviour of the material is described in the framework of the mechanics of porous media. The poroelastic parameters of the material are determined and the effect of stress and pore pressure on them is evaluated. Appropriate effective stress laws which control the evolution of total volume, pore volume, solid volume, porosity and drained bulk modulus are discussed. A phenomenon of degradation of elastic properties is observed in the test results. The microscopic observations showed that this degradation is caused by the microcracking of the material under isotropic loading. The good compatibility and the consistency of the obtained poromechanical parameters demonstrate that the behaviour of the hardened cement paste can be indeed described within the framework of the theory of porous media. [Copyright &y& Elsevier]

Under cyclic loadings, the plastic strain of soft soil will take place under very small shear strain. So the visco-plastic model is appropriate to be used to model the dynamic characteristics of soft soil. Based on the principles of geotechnical plastic mechanics, the incremental visco-plastic memorial nested yield surface model is developed by using the field theory of nonlinear isotropic materials and the theory of kinematical hardening modulus. At the end of anyone time increment, the inverted loading surface, the damaged surface and the initial loading surface which is tangent with the inside of inverted loading surface are memorized respectively. The kinematical behavior of yield surface is defined by using these three surfaces. The developed model in this paper is successfully implemented in ABAQUS using FORTRAN subroutine. The predicted stress-strain relationships of soft soil are compared with the test results given by dynamic triaxial tests. It is proved that the cyclic undrained stress-strain relation of soils can be fairly simulated by the model. At last, the nonlinear earthquake response of a representative soft site in Nanjing city is calculated with the dynamic behavior of soils modeled by the new developed model. The results are accordant to the earthquake response of soft site given by other scholars. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the evaluation of the critical-state parameters with respect to the matric suction for saturated and unsaturated undisturbed residual clayey soils from Turkey. In order to conduct the unsaturated triaxial compression testing procedures a conventional triaxial compression apparatus was redesigned. The data for critical-state conditions from these tests are presented with respect to matric suction, based on the critical-state parameters of M, q o, Γ, λ, which is commonly proposed by many authors. The critical state of the unsaturated samples is compared with that of the saturated samples. This experimental study has demonstrated that matric suction has no influence on parameters of M and λ. The parameters of M and λ are approximately 0.85 and 0.074 respectively for saturated and unsaturated conditions. The relationships between matric suction (u a − u w) and the intercepts q o and Γ have been observed as nonlinear, and thus they can be defined as a function of matric suction (u a − u w). Furthermore, a method is developed to predict the intercepts q 0 according to matric suction for unsaturated clayey soils. [Copyright &y& Elsevier]

Road rehabilitation technology is currently under the global spotlight. Considering that cold bituminous binders, e.g. emulsion and foamed bitumen, have become more commonly used in cold recycling operations, there is a need to understand the performance properties of these materials. Great strides have been made in the modelling of foamed bitumen treated materials in recent years. Performance functions that may be used in the design of pavements incorporating these materials become increasingly important. The challenge of modelling the behaviour of these mixtures is complicated by the variety of foamed mixtures that are produced and the range of mix variables that prevail. Binder content (BC), active filler content, parent rock type, aggregate gradation, plasticity, moisture content, etc. can vary significantly from mix to mix. A unified approach to designing with these materials that accounts for all of these variables is exceptionally challenging. This publication investigates the shear and dynamic properties of foamed bitumen stabilised mixes and their role in the performance modelling of these materials. Latest research findings are synthesized with the relevant aspects of the mix evaluation and classification procedures. Correlation of different testing protocols and mix compositions is made with a view to providing synergy to the research results and direction to their application in mix design and pavement design. [ABSTRACT FROM AUTHOR]

Abstract: The structure of constitutive laws for powder compaction is examined in a companion paper, where a suitable framework for incremental and deformation theory models is developed. In this paper the response of a commercial steel powder is investigated using triaxial compaction experiments. The tests were designed to critically evaluate the underlying assumptions of the two modelling approaches and to calibrate and validate the models. It is demonstrated that for loading histories that are characteristic of industrial die compaction processes, a deformation plasticity model is capable of describing the material response, while the incremental plasticity response is very sensitive to the detailed form of the model in the early stages of compaction, where the material has a very low yield stress and undergoes large deformations without being able to sustain a sizeable load. [Copyright &y& Elsevier]

This paper examines and compares the mechanical behaviour of two different unsaturated clay mixtures comprised of bentonite clay (Saskatchewan or Wyoming) and quartz sand. The two mixtures have been proposed as compacted barrier materials for reducing groundwater flow in the vicinity of waste disposal repositories. Triaxial specimens were compacted to consistent properties, and then specified suction conditions were applied to the specimens using the vapour equilibrium technique. Following equilibrium at the specified initial suction, specimens were subjected to isotropic and shear loading in a conventional triaxial cell to measure the mechanical response under selected stress paths. The results are interpreted in terms of the yield, strength, and stiffness behaviour at the various suction levels. Results suggest that the clay component of the mixture dominates the behaviour at suctions less than approximately 30 MPa, and the sand component dominates the behaviour above approximately 30 MPa. The transition from clay- to sand-dominated behaviour is attributed to volume strain during application of the initial suction bringing the sand particles into contact. The discussion highlights how the results can be used to modify constitutive models to incorporate the transitional behaviour in numerical modeling. [ABSTRACT FROM AUTHOR]

The PSU-EVP model's constitutive parameters for alumina powder are presented. The PSU-EVP model was also used to back-predict the triaxial test data obtained for MZF and alumina powders using constitutive parameters such as the initial voids ratio ( e 0 ), compression index (λ), and spring-back index (κ). In the case of MZF powder, 8 out of 12 back-prediction cases had average relative difference (ARD) values below 20%. In the case of alumina powder, 7 out of 11 back-prediction cases had ARD values below 20%. Based on the back-prediction results, it was concluded that the PSU-EVP model gave fairly good results for most triaxial test data collected at 0.62 MPa/minute and 6.21 MPa/minute. However, the back-prediction results obtained at 20.7 MPa/minute had high ARD values. A sensitivity analysis was done to study the effect of changes in parameter values on the hydrostatic triaxial compression (HTC) and conventional triaxial compression (CTC) back-prediction results. From the sensitivity analysis,±10% (standard deviation variation from ±0.8σ to ±2.3σ) changes in λ and e 0 mean values had marked effect on the HTC results. However, changes in the λ, κ, and e 0 mean values do not produce any noticeable effect on the CTC prediction results. Overall, the PSU-EVP model can be considered to be the first step towards the development of a more robust and accurate model for prediction of stresses and strains in a dry powder compression process. [ABSTRACT FROM AUTHOR]

The rate-dependent mechanical behavior of a dry industrial powder (MZF powder) was studied using a cubical triaxial tester (CTT) within the context of a new elasto-viscoplastic model (PSU-EVP model). The compression and shear properties of the powder were quantified at compression rates of 0.62, 6.21, and 20.7 MPa/minute with pressures up to 11 MPa. Test results demonstrated that the compression and shear responses of the powder were nonlinear, consistent, and reproducible (coefficient of variation or COV = 15%). Also, MZF powder exhibited varying elastic and plastic deformation at different pressure levels that were quantified using statistical correlations (R 2 > 0.90). For example, the average bulk modulus and shear modulus values for MZF powder increased linearly with pressure (R 2 > 0.90) at all compression rates. The failure stress values also increased with the increase in mean pressure. For instance, at a compression rate of 0.62 MPa/minute, failure stress increased from 5.0 to 13.3 MPa as the confining pressure increased from 2.2 to 8.5 MPa. Similar effects were noted at compression rates of 6.21 and 20.7 MPa/minute. Overall, failure stress decreased with increasing compression rate. From the data collected, it was demonstrated that compression rate does have substantial effect on the compressibility and shear behavior of powders that can be quantified using the CTT and is suitable for use in the PSU-EVP model. [ABSTRACT FROM AUTHOR]

The behaviour of loose gassy sand was investigated with the use of a constitutive model. The constitutive model was modified from an existing model that focused on assessing the liquefaction of loose sands over a wide range of states and loading conditions. The modifications involved taking into account the compressibility and solubility of the pore gas and liquids. Hilf's equation, which calculates the pore-pressure change in a gassy or unsaturated soil subjected to an applied total stress, was incorporated into the model formulation. The initial degree of saturation is needed for model prediction, and the coefficient of volumetric solubility (Henry's constant) was introduced as a new model parameter. The modified model was used to predict the effect of gas on the undrained static behaviour of loose sand. The laboratory results of saturated specimens were modeled, and the predicted and observed behaviours were found to agree well. Results from gassy specimens were also predicted, and again the model predictions matched the test results. The model was used to confirm that gas has the effect of decreasing, but not eliminating, the susceptibility of loose sand to flow liquefaction. The major shortcoming of the modified model was its inability to predict the slight increase in effective normal stress that was observed in the initial stages of all the undrained triaxial tests. This shortcoming resulted in differences between the predicted and observed behaviour especially of strain-hardening specimens. [ABSTRACT FROM AUTHOR]

A range of mechanical compression tests were conducted on frozen soil samples at temperatures close to the melting point of ice to determine both the creep and the strength properties under triaxial stress conditions. The samples were obtained from two rock glaciers in the Swiss Alps. For comparison, additional samples were prepared artificially for testing. The results showed that the minimum creep strain rate increases exponentially with increasing temperature and applied deviatoric stress. The applied strain rate is the main parameter influencing resistance. However, test results show that the micromechanical behaviour is similar for all tests and independent of the applied loading condition. Measurements of the volumetric strain showed that the sample composition has a major influence, so samples with high air void ratios resulted in continuous contraction (i.e., reduction of the sample volume), whereas samples with low volumetric air and ice contents demonstrated dilatant behaviour. [ABSTRACT FROM AUTHOR]

Tire shreds and tire shred – soil mixtures can be used as alternative backfill material in many geotechnical applications. The reuse of tire shreds may not only address growing environmental and economic concerns, but also help solve geotechnical problems associated with low soil shear strength. In this study, an experimental testing program was undertaken using a large-scale triaxial apparatus with the goal of evaluating the optimum dosage and aspect ratio of tire shreds within granular fills. The effects on shear strength of varying confining pressure and sand matrix relative density were also evaluated. The tire shred content and tire shred aspect ratio were found to influence the stress–strain and volumetric strain behaviour of the mixture. The axial strain at failure was found to increase with increasing tire shred content. Except for specimens of pure tire shreds and with comparatively high tire shred content, the test results showed a dilatant behaviour and a well-defined peak shear strength. The optimum tire shred content (i.e., the one leading to the maximum shear strength) was approximately 35%. For a given tire shred content, increasing the tire shred aspect ratio led to increasing overall shear strength, at least for the range of tire shred aspect ratios considered in this study. The shear strength improvement induced by tire shred inclusions was found to be sensitive to the applied confining pressure, with larger shear strength gains obtained under comparatively low confinement. [ABSTRACT FROM AUTHOR]

The Japan Trench forearc offshore Honshu Island in northeast Japan, where the 130-m.y.-old Pacific oceanic plate is presently subducted, was drilled during the Ocean Drilling Program Leg 186. Results from mechanical and sedimentological studies of claystones recovered from Sites 1150 and 1151 in the overlying erosional forearc wedge are reported in the present study. Although many physical properties are similar in the seismic (Site 1150) and aseismic portion (Site 1151) of the shallow forearc, Site 1150 displayed a higher abundance of open fractures, two prominent fault zones and enigmatic pore fluid signatures in the claystones. The abundance of weak mineral phases, together with high smectite contents (from X-ray diffraction), control the low friction coefficients of 0.33–0.39 of the claystones in ring-shear experiments. Results from triaxial testing proposed overall low magnitudes of in situ effective vertical stress, with somewhat lower values at Site 1150 than at Site 1151. Similarly, samples from Site 1150 displayed slightly higher pore fluid pressures than those at Site 1151. The high sediment porosities, which are in part also a result of intact diatom tests (from scanning electron microscope), together with the anomalous fluid signatures and elevated pore fluid pressures, could very likely result from upward migration and influx of deep-seated waters. Dewatering reactions at depth result in enhanced pore fluid pressure transients along out-of-sequence thrusts and consequently lower effective stress. At depths greater than that of Leg 186 drilling, elevated pressure–temperature conditions trigger mineral transformation and cementation, which result in increasing friction, unstable sliding and seismic rupture. Such earthquakes could have repeatedly disaggregated the consolidated claystone fabrics at the seismic site, and could be responsible for differences in yield strength and cementation when compared to the aseismic Site 1151. [ABSTRACT FROM AUTHOR]

A medium pressure (<21 MPa) flexible boundary cubical triaxial tester was designed to measure the true three-dimensional response of powders. In this study, compression behaviour and strength of a microcrystalline cellulose powder (Avicel[sup ®] PH102), a spray-dried alumina powder (A16SG), and a fluid-bed-granulated silicon nitride based powder (KY3500) were measured. To characterize the mechanical behaviour, three types of triaxial stress paths, that is, the hydrostatic triaxial compression (HTC), the conventional triaxial compression (CTC), and the constant mean pressure triaxial compression (CMPTC) tests were performed. The HTC test measured the volumetric response of the test powders under isostatic pressure from 0 to 13.79 MPa, during which the three powders underwent a maximum volumetric strain of 40.8 per cent for Avicel[sup ®] PH102, 30.5 per cent for A16SG, and 33.0 per cent for KY3500. The bulk modulus values increased 6.4-fold from 57 to 367 MPa for Avicel[sup ®] PH102, 3.7-fold from 174 to 637 MPa for A16SG, and 8.1-fold from 74 to 597 MPa for KY3500, when the isotropic stress increased from 0.69 to 13.79 MPa. The CTC and CMPTC tests measured the shear response of the three powders. From 0.035 to 3.45 MPa confining pressure, the shear modulus increased 28.7-fold from 1.6 to 45.9 MPa for Avicel[sup ®] PH102, 35-fold from 1.7 to 60.5 MPa for A16SG, and 28.5-fold from 1.5 to 42.8 MPa for KY3500. In addition, the failure stresses of the three powders increased from 0.129 to 4.41 MPa for Avicel[sup ®] PH102, 0.082 to 3.62 MPa for A16SG, and 0.090 to 4.66 MPa for KY3500, respectively, when consolidation pressure increased from 0.035 to 3.45 MPa. In addition, the shear modulus and failure stress values determined from the CTC test at 2.07, 2.76, and 3.45 MPa confining pressures are consistently greater than those from the CMPTC test at the same constant mean pressures. This observation demonstrates the influence of stress paths on material properties. The CTT is a useful tool for characterizing the three-dimensional response of powders and powder mixtures. [ABSTRACT FROM AUTHOR]

This paper provides new experimental evidence of the specific volume creep and deviator creep of cement-based materials. Creep tests conducted on calcium-leached cement pastes and mortars under various triaxial loading conditions provide conclusive evidence of a specific short-term creep and a specific long-term creep. It is found that at least two competing dissipative mechanisms are at work in short-term creep of cement-based materials: (1) a creep relaxation mechanism in the calcium–silicate–hydrate (C–S–H) solid phase activated by microstress concentrations in the heterogeneous microstructure; and (2) stress relaxation by microcracking beyond a relatively small stress threshold. If the first dominates over the second, the short-term volume creep is contracting, and in the inverse case, a short-term dilating behavior is found. Furthermore, provided that microcracking stabilizes during the short-term creep, the long-term creep occurs at constant volume. Based on these results, we argue that the concept of a creep Poisson''s ratio, which is commonly employed in engineering practice to extrapolate uniaxial creep tests to multiaxial stress conditions, should be abandoned when triaxial stress states affect the durability performance of concrete structures. [Copyright &y& Elsevier]

Critical state models for unsaturated soils have been proposed in recent years; however, the proposed models have been based on limited experimental data. Compacted specimens have generally been used for research, and the complications of soil fabric resulting from the compaction procedures have brought difficulties into the interpretation of fundamental soil behavior. There is a need to undertake further laboratory research studies on unsaturated soils, particularly to obtain a fundamental understanding of the shear strength and critical state of unsaturated soils by testing soil specimens with simple soil structures. Suction-controlled triaxial drained shear tests on an unsaturated silt were carried out as part of this research program. Specimens were prepared by gradually consolidating the initially slurried soil. The resulting specimens had a relatively simple soil fabric and stress history. The results showed that applying suction to an initially saturated specimen has a similar influence on the stress–strain behavior and critical state characteristics as does increasing its density by applying a higher confining pressure. The critical state lines for the unsaturated soil corresponding to different soil suctions are parallel to those for the saturated soil on the (q : p' ' ), (v : p' ' ), and (v[sub w] : p' ' ) planes.Key words: critical state, shear strength, unsaturated soils, volume change, triaxial testing.Des modèles d'état critique ont été proposés au cours des récentes années; cependant, les modèles proposés ont été basés sur des données expérimentales limitées. Des spécimens compactés ont généralement été utilisés pour la recherche, et la complexité de la fabrique du sol résultant des procédures de compactage ont présenté des difficultés dans l'interprétation du comportement fondamental du sol. Il est nécessaire d'entreprendre d'autres recherches en laboratoire sur des sols non saturés, particulièrement pour acquérir une compréhension fondamentale de la résistance au cisaillement et de l'état critique des sols non saturés en faisant des essais sur des spécimens de sols ayant des structures simples. Des essais de cisaillement triaxial drainé avec succion contrôlée sur un silt non saturé ont été réalisés dans le cadre de ce programme de recherche. Les spécimens ont été préparés en consolidant graduellement le sol initialement sous forme de boue. Les spécimens de sol qui en résultent ont une fabrique et une histoire des contraintes relativement simples. Les résultats montrent que l'application d'une succion à un spécimen de sol initialement saturé a une influence sur le comportement contrainte–déformation et sur les caractéristiques de l'état critique semblable à l'accroissement de sa densité par l'application d'une pression de confinement plus élevée. Les lignes d'état critique pour le sol non saturé correspondant à différentes succions sont parallèles à celles du sol saturé sur les plans (q : p' ' ), (v : p' ' ), et (v[sub w] : p' ' ).Mots clés : état critique, résistance au cisaillement, sols non saturés, changement de volume, essais triaxiaux.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

The mechanical behaviour of a weakly cemented silt was studied experimentally. The cementing agent was a cement and fly ash slurry, and the samples so formed were slightly cemented. In triaxial testing, both drained and undrained tests on saturated samples were conducted. Special zero effective stress tests were conducted to measure, directly, the contribution of bonding between grains to the enhanced strength and stiffness. The cemented soils were initially less dilatant than their respective parent soils but eventually became more dilatant than the parent soils. The shear strength data followed a curved failure surface that merged back, at high stress, into that of the parent soil. This feature can be captured by a failure function that models the contribution of cementing agent to strength as two parts, true bonding and increase in dilatancy rate at failure. Both parts degrade with an increase in confining stress, but at different rates.Key words: bonding, cemented soil, dilatancy, strength, triaxial testing.Le comportement mécanique d'un silt faiblement cimenté a été étudié expérimentalement. L'agent de cimentation était constitué d'une boue de ciment et de cendres volantes, et les échantillons ainsi constitués étaient légèrement cimentés. On a réalisé dans l'appareil triaxial des essais tant drainés que non drainés sur des échantillons saturés. Des essais spéciaux à contrainte effective nulle ont été réalisés pour mesurer directement la contribution de la cimentation entre les grains à l'accroissement de la résistance et à la rigidité. Les sols cimentés étaient initialement moins dilatants que leurs sols respectifs d'origine, mais devinrent éventuellement plus dilatants que leurs sols d'origine. Les données de résistance au cisaillement ont suivi une surface de rupture courbée qui venait se confondre, à forte contrainte, avec celle du sol d'origine. Cette caractéristique peut être reproduite par une fonction de rupture qui modélise la contribution de l'agent de cimentation à la résistance sous forme de deux composantes, soit la vraie cimentation et l'accroissement du taux de dilatance à la rupture. Les deux composantes se dégradent avec l'augmentation de la contrainte de confinement, mais à des taux différents. Mots clés : cimentation, sol cimenté, dilatance, résistance, essais triaxiaux.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

An experimental facility has been developed that is capable of generating any combination of tensile and/or compressive forces on three mutually orthogonal axes of a cruciform-shaped composite specimen. Using this computer-controlled test facility, any stress ratio in biaxial (σ1-σ2) or triaxial (σ1-σ2-σ3) stress space can be evaluated. A description of this electromechanical testing facility, including the reaction frame, test fixture, computer control system and instrumentation is included. Once fully assembled, uniaxial, biaxial and triaxial tests were performed on aluminum and on a carbon/epoxy composite laminate to evaluate the performance of the test facility. Using the data generated from these tests, the performance of many aspects of the triaxial testing facility, including the intra-axis alignment, automated computer testing algorithms, data acquisition algorithms, calibration values and machine compliance, was evaluated. The overall very acceptable performance of the triaxial testing facility is believed to have been demonstrated. [ABSTRACT FROM AUTHOR]

Multiaxial testing in composites may generate failure modes which are more representative of what occurs in a real structure submitted to complex loading conditions. However, some of its main handicaps include the need for special facilities, the correct design of the experiments, and the challenging interpretation of the results. The framework of this research is based on a triaxial testing machine with six actuators which is able to apply simultaneous and synchronized axial loads in the three space directions. Then, the aim was to design from a numerical point of view a triaxial experiment adapted to this equipment. The methodology proposed could allow for an adequate characterization of the triaxial response of a polymer-based composite with apparent isotropic behaviour in the testing directions. The finite element method (FEM) is applied in order to define the geometry of the triaxial specimen. The design pursues to achieve homogeneous stress and strain states in the triaxially loaded region, which should be accessible for direct measurement of the strains. Moreover, a fixing system is proposed for experimentally reproducing the desired boundary conditions imposed on the numerical simulations. The procedure to determine the full strain tensor in the triaxially loaded region is described analytically and with the help of FEM virtual testing. The hydrostatic component and the deviatoric part of the strain tensor are proposed for estimating the susceptibility of the polymer-based composite to fail due to the triaxial strain state imposed. Then, the loading scenarios that cause higher values of the deviatoric components in the triaxially loaded region are considered to be more prone to damage the region of interest. Nevertheless, the experimental failure is expected to be produced in the arms of the specimen which are uniaxially loaded, since in all of the loading cases the simulations show higher levels of stress concentration out of the triaxially loaded region. Thus, although the triaxial strength could not be accurately determined by the proposed tests, they can be utilized for observing the triaxial response before failure. [ABSTRACT FROM AUTHOR]

Chemical agents capable of sealing pores and micro-fractures can be used to solve wellbore instability in shales. Nanomaterials, as sealing agents, have been applied to test fluids, and their wellbore strengthening effects were evaluated by triaxial tests. A recommended workflow is given to first conduct pore pressure transmission tests, and then perform triaxial tests to evaluate the influence of drilling fluids (sealing agents) on the strength of shales. Due to the low permeability of shales, the small sample (0.25 in. height and one in. diameter) is used in pore pressure transmission tests. This research gives special loading to handle small samples in triaxial testing and innovative calculation procedure to produce stress-strain curves for the shales. After Pore Pressure Transmission (PPT) tests with nanoparticles of various sizes (10, 40 nm), concentrations (3%, 10%), and types (aluminum oxide, magnesium oxide), triaxial tests were performed on Mancos Shale and Eagle Ford Shale. The best combinations for increasing UCS values (Uniaxial Compressive Strengths) are 10% 40-nm Al 2 O 3 for Eagle Ford Shale and 3% 40-nm MgO for Mancos Shale, according to our test matrix. As shown by the experiments, the application of the appropriate nanoparticles on shales will improve their strength and reduce the risk of wellbore collapse. The methodology developed in this research can be extended to triaxial testing on cuttings-size samples, which provides useful rock characterization information for samples that do not meet standard testing conditions. • Conducting pore pressure transmission and triaxial tests can evaluate drilling fluids effect on shale strength. • Special loading handles small sample in triaxial testing and innovative method calculates stress-strain curves. • Compared with base fluids (3% KCl), nanoparticle suspensions of 3% 40 nm MgO can increase UCS by 53.3% for Mancos Shale. [ABSTRACT FROM AUTHOR]

Oil well cement paste is a key structural component in wells that should provide mechanical support to the casing and prevent uncontrolled flow of formation fluids along the wellbore and to the environment. Since the annular cement paste is subjected to both hydrostatic pressure and to loads from formation or through the casing, it is important to understand the mechanical behavior and strength of oil well cement pastes under confining conditions. We study cement specimens cored from test sections that were cemented using a full-scale batch mixer. The specimens were tested mechanically without confinement and under 10, 20 and 40 MPa confining pressures using a state of the art triaxial test system. Unconfined samples are found to exhibit linear elastic behavior up to axial strains of approximately 0.2% with an average Young's modulus of 14.9 GPa and a Poisson ratio of 0.21. At larger strains, the stress–strain response deviates from the initial slope, terminating with a brittle shear failure at axial strains of approximately 0.5%. The corresponding average uniaxial compressive strength is 58 MPa, in good agreement with previously published results. When subjected to confining pressures of 10 MPa or higher, the cement paste accommodated larger strains at a given level of deviatoric stress, and maintained its load-carrying capacity through the entire test cycle. The Young's modulus for the initial loading phase was found to decrease with increasing confining pressures, and the ultimate deviatoric stress approached approximately 80 MPa, independent of the magnitude of the confining pressures used in this study. The results suggest that the main effect of increasing confining pressure is to increase the sample ductility and the axial strain corresponding to the ultimate deviatoric stress. It is further found that the confined stress–strain behavior of the oil well cement paste can be described by a simple nonlinear constitutive equation. The more ductile and flexible response of well cement under relevant confining pressures is considered to be a positive characteristic of cement as a barrier material for zonal isolation. This study is based on a commercial well cement slurry that is mixed and placed using field equipment. The results are therefore considered novel and unique, and can contribute toward improved knowledge and evaluation of mechanical well cement behavior under realistic, confined conditions. • Mechanical testing of oil well cement paste at 10, 20 and 40 MPa confining pressures. • Confining pressure makes cement paste more ductile and reduces the Young's modulus. • Confining pressure results in an ultimate deviatoric stress of approximately 80 MPa. • A nonlinear stress–strain model agrees well with measurements at all confining pressures. [ABSTRACT FROM AUTHOR]