Consolidated-undrained Triaxial Tests Research Articles

Modeling of flexible coupling boundary combing discrete element method with finite difference method for drained/undrained triaxial test

A simulation framework combining the coupling fluid method (CFM) and flexible coupling boundary method was developed to simulate drained and undrained triaxial tests. The discrete element method (DEM) model was calibrated using consolidated drained (CD) triaxial tests on saturated silt and then used to calculate the drained response of specimens with different initial porosity in both rigid and flexible boundaries. Subsequently, consolidated undrained (CU) triaxial tests were simulated and compared to the constant volume method. The bulk modulus of water was determined, and the reliability of the CFM was verified. On this basis, the DEM model was used to calculate the undrained response of the specimens with different initial porosity by simulating CU triaxial tests. The computed drained/undrained responses, including stress path, deviatoric stress, and shear-induced volumetric strain/pore water pressure, were consistent with the known behavior of silt. This study has promising potential to further enhance the understanding of granular behavior.

Step-changed strain rate effect on the mechanical properties of undisturbed expansive clay

Up to now, most of the research on rate effects has investigated the behavior of soft soils, much less is known for the stiff clays. In particular, the method of stepwise changing strain rate has rarely been used in studies of soil rate effects, but many studies have well demonstrated that this method is beneficial to overcome the problems of variability between samples and the time requirements associated with strain rates. Isotropic consolidated-undrained triaxial tests were conducted on specimens with the same void ratio at step-changed axial strain rates under four different confining pressures in this study. We carried out the effect of axial strain rates from 0.09 to 300%/hour on the mechanical properties of naturally expansive soils. Experimental results showed that the higher the strain rate, the higher the effective stress for a given strain. The shear strength increases from 5.9% to 11.4% per log cycle of axial strain rate for saturated undisturbed expansive soil specimens by introducing the strain-rate parameter ρ 0.09, which is similar to that of soft soils. The excess pore water pressure gradually decreases after reaching a peak at lower cell pressures and increasing at higher ones. In addition, the failure mode of the specimens is different from soft clay with both the main shear band and multi-fissures band. The new findings are helpful for better understanding the time-dependent mechanical properties of stiff clays.

Effects of initial stress anisotropy on the onset of undrained instability for Guamo sand

Undrained instability is a failure mode that has been observed in many sandy slopes. However, the highly anisotropic stress states caused by the geometry of the slope have not been duly considered from the experimental point of view. This article studies the influence of the initial stress anisotropy of laboratory samples on the static undrained instability of the Colombian reference sand. Forty-five consolidated undrained triaxial tests were performed, prescribing combinations of different initial void ratios, mean confining pressure, and initial stress ratios. Experimental results allow concluding that: a. the size of the instability zone on the p′ − q plane becomes smaller for larger initial anisotropic stress ratios; b. a novel undrained instability plane can be proposed in the p′ − q − e space to set a boundary between stable and unstable undrained loading conditions; c. mean confining pressure plays a vital role in the estimation of undrained instability for Guamo sand samples under low and medium confining stresses; d. despite multiple factors that influence the onset of undrained instability, a relation can be derived between the undrained instability susceptibility of sandy slopes and the initial stress ratio η0.

Constitutive Behaviour of a Clay Stabilised with Alkali-Activated Cement Based on Blast Furnace Slag

Alkaline cements have been extensively tested for soil stabilisation in the last decade. However, only a few studies have focused on the assessment of such performance by establishing the constitutive behaviour of the cement. In this paper, we focus on the mechanical behaviour, using triaxial testing of a clay with high water content stabilised with an alkali-activated binder and the subsequent prediction of the experimental stress–strain response using a kinematic hardening constitutive model initially developed for natural clays. Monotonic consolidated undrained triaxial tests were conducted on reconstituted and stabilised clay specimens cured for 28 days to evaluate the effects of cementation on the overall shear behaviour. Alkali-activated binder was synthetised from blast furnace slag and sodium hydroxide. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were performed to study the microstructure, whereas leachate analyses were performed after 28 and 90 days of curing to investigate the contamination potential. The main product formed was calcium aluminosilicate hydrate (C-A-S-H) with a low CaO/SiO2 ratio, and no risk of soil contamination was found. The compressibility and undrained shear strength in the pre-yield state was found to be independent of the initial mean effective stress (p’0), unlike what was observed in the post-yield state, where the shear strength seemed to be affected by p’0. The model provided reliable predictions of the experimental results and captured the main features of the artificially cemented clay for the tested p’0 range. Such studies are fundamental to establish adequate confidence in such alternative binders—an essential aspect if their use is to become widespread in the near future.

Geotechnical characterization of deep Shanghai clays

To date, investigations of soils in Shanghai have mainly focused on upper clays, namely, Layer 2–6, generally within 30–40 m depth. Knowledge of the deep clays of Layer 8 and 10, at depths of approximately 50 m and 100 m, respectively, is very limited. In this study, the detailed geotechnical properties of the deep Shanghai clays of these two layers were investigated through a series of field and laboratory tests. Physical property tests showed that both clays can be classified as low-plasticity silty clay. For the first time in Shanghai, high-quality intact clay samples were retrieved from the deep soil using a double-tube rotary sampler. High-pressure oedometer tests and consolidated undrained (CU) triaxial tests were conducted to evaluate the compressibility and shear strength of intact samples. The oedometer tests indicate that Layer 8 is an overconsolidated clay with an overconsolidation ratio (OCR) of 2–3, and Layer 10 is a normally consolidated clay with a light degree of structure. CU triaxial tests further confirmed the above findings. The compressibility of the Layer 8 clay is lower, and its undrained shear strength and effective friction angle are larger than those of Layer 10, with the latter being deeper. The sensitivity framework, based on Burland's intrinsic compression line (ICL) and sedimentary compression line (SCL), can be used to characterize the overconsolidation/structure nature of the deep Shanghai clays. The strain energy method was found to be more suitable than the Casagrande method for determining the preconsolidation pressure of the deep Shanghai clays.

Study on the Shear Strength of Root-Soil Composite and Root Reinforcement Mechanism

This study investigates the effects of root distributions and stress paths on the shear strength of root-soil composites using a consolidated-undrained (CU) triaxial test. On the basis of the limit equilibrium, two root reinforcement coefficients (n and m) are proposed for characterizing the effects of shear strength parameters on the principal stress considering different root distribution angles and root diameters. Then, n and m are introduced into the conventional limit equilibrium equation to develop a new limit equilibrium equation for root-soil composites. The results demonstrate that the root distribution angles (α) and root diameters (d) affect the shear strength of the root-soil composites. Under a consolidated-undrained condition, the effective cohesion (crs′) of the rooted soil is high and decreases in the order of 90°, 0°, 30° and 60°. For the same root distribution angle, crs′ increases with the increasing root diameter. Meanwhile, the effective internal friction angle (φrs′) changes slightly. The failure principal stress of the root-soil composites is positively correlated with n and m. Furthermore, the deformation of the samples indicates that the run-through rate of α = 90° and α = 0° are both 0. Meanwhile, the lateral deformation rate declines from 17.0% for α = 60° to 10.9% for α = 90°.

Characterizing Influence of Salt and Freeze–Thaw Cycle on Strength Properties of Clay

Subsea tunnel construction is usually associated with high risk due to high water pressure. To ensure the safety of the tunnel during construction, artificial ground freezing (AGF) technology is typically adopted to freeze the surrounding soil of the tunnel. Previous researches reveal that the strength properties of clay vary significantly due to the coupling effect of salt and freeze–thaw. For this reason, the consolidated-undrained triaxial compression test was carried out on clay saturated with NaCl solution. The influences of freeze–thaw cycle, salt content, confining pressure, and strain rate on the strength were comprehensively evaluated. A statistical analysis was performed to distinguish the influence of these factors and their coupling effect. Test results reveal that a critical salt content exists at which the soil strength before and after a freeze–thaw cycle is the lowest. Similarly, instead of linearly increase with increasing strain rate, a critical strain rate exists at which the soil strength is the highest under undrained loading. In addition, a damage model with consideration of the coupling effect of freeze–thaw cycle and salt content was specially proposed to predict the stress–strain characteristic of clay with different salt contents before and after a freeze–thaw cycle.

Liquefaction assessment of silty sands: Experimental characterization and numerical calibration

The main goal of this paper is to assess the reliability of a nonlinear hysteretic model to capture the behavior of saturated silty sand mixtures under cyclic loading conditions. The parameters governing the liquefaction triggering are identified using the results of standard laboratory tests (monotonic and cyclic consolidated undrained triaxial tests as well as resonant column tests) undertaken for reconstituted soil samples. Consequently, the uncertainty related with collected samples is reduced. The numerical results, obtained after calibration of the constitutive model, are in good agreement with the experimental ones and they are analyzed through the effects of relative density, fine content, and confining pressure in the liquefaction resistance of silty sands.

Strength characteristics of geosynthetic reinforced mechanically biologically treated waste by triaxial test

The reinforcement strength characteristics of mechanically biologically treated (MBT) waste were studied by conducting consolidated undrained triaxial tests with MBT waste collected from the Hangzhou Tianziling landfill pilot project. In the tests, the effects of the reinforcement material used (geomembrane, geotextile, and geogrid) and the number of reinforcement layers used (one, two, and three layers) were assessed. The results showed the following: (1) even through the axial strain increases up to 25%, the deviator stress of MBT waste could not reach a well-defined peak; (2) the reinforcement effect is related to the type of reinforcement material, with geogrid exhibiting the best reinforcement effect and geomembrane the worst; (3) the strength ratio of reinforced MBT waste is related to the confining pressure and the number of reinforcement layers, with a greater strength ratio in the MBT waste attained with a lower confining pressure in a logarithmic relationship and a greater strength ratio in the MBT waste attained with a greater number of reinforcement layers in a linear relationship; (4) the reinforced MBT waste shear strength parameter variation ranges for the cohesion (c), internal friction angle (φ), effective cohesion (c'), and effective internal friction angle (φ') are 3.92-13.69kPa, 19°-29°, 10.10-27.94kPa, and 24°-45°, respectively; and (5) the deviations in the test values from the apparent cohesion method and the semi-empirical formula method are less than 15%, indicating that these two theories of reinforced sand can also be applied to MBT waste. The results of this study are useful as a baseline reference for the stability assessment of MBT waste landfills.

Effect of initial water content on undrained shear strength of K0 consolidated clay

This study investigates the effect of initial water content on the pore pressure response and undrained shear behavior of K0 consolidated reconstituted clay. A series of K0 consolidated undrained triaxial compression tests were conducted on reconstituted Lianyungang clay. Results were compared to those obtained by isotropic consolidated undrained triaxial tests. The testing results showed that the K0 consolidated undrained strength envelope of reconstituted soil content is a straight line passing through the origin regardless of the initial water content. The initial water content would affect the undrained strength of K0 consolidated clay as decreased normalized undrained shear strength was observed with clay at higher initial water content. The slope C of normalized pore pressure and stress ratio is affected by the consolidation method, where C is found to be a soil constant for K0 consolidated clay and the value would be higher with clay under K0 consolidation. The pore pressure increases with increasing initial water content at a certain axial strain under given consolidation pressure, and the difference in excess pore pressure increases with the increasing consolidation pressure. Pore pressure coefficient at failure (Af) increases as the initial water content increases, where a trendline can be well fitted between the pore coefficient at failure and the ratio of initial water content to the liquid limit of clay. The undrained strength indexes, i.e., effective cohesion and effective internal friction angle have decreasing tendency with increasing initial water content; however, changes in the total strength indexes of soil in this study are insignificant with varying initial water content.

Influence of the microstructure and roughness of weakness planes on the strength anisotropy of a foliated clay-rich fault gouge

Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric.Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.

Analysis of different failure criteria to evaluate bauxite tailings mechanical behavior through numerical modelling

Abstract In recent years several dam failures have been reported throughout the world, generating a social concern on mine tailings. Along these lines, it became essential to understand the mechanical behavior of these materials in order to refine current design technologies and prevent more tragedies. In this context, this research had two main goals: (i) to analyze bauxite tailings mechanical behavior through isotropically consolidated-undrained triaxial tests and consolidation tests; and (ii) to compare triaxial tests and numerical simulations results. Confining stresses of 75kPa, 150kPa, and 300kPa were applied in the triaxial tests. Numerical modelling was performed through ABAQUS software, in which three different failure criteria were analyzed, Mohr-Coulomb Model (MCM), Drucker-Prager Model (DPM), and Modified Cam-Clay Model (MCCM). Results indicated that all studied criteria showed satisfactory results, however, DPM was the best criterion to simulate bauxite tailings mechanical behavior and respective strength parameters.

Mechanical Behavior and Constitutive Modeling of Cement–Bentonite Mixtures for Cutoff Walls

Cement–bentonite mixtures are commonly used to build cutoff walls, which limit water flow and underground transport of pollutants. These artificial materials are employed due to their very low permeability and adequate shear strength and ductility. In this paper, experimental results about the microstructure and mechanical behavior of three different cement–bentonite mixtures are presented. Specimens of these mixtures were subjected to oedometer and consolidated-undrained triaxial tests. These results were then used as a basis for the definition of a suitable constitutive framework. A quite good reproduction of the experimental results up to the peak strength was obtained using the classical Modified Cam Clay model, which could then be used satisfactorily when conventional analyses aimed at assessing the stability of cutoff walls are required. The reproduction of the strength degradation and the strains occurring in the postpeak stage requires, however, a more advanced constitutive model. To this extent, the Modified Cam Clay framework was enhanced by introducing some features commonly employed to reproduce the mechanical behavior of granular materials. This model may be useful for the real-scale analysis of more critical cases when local failure mechanisms are likely to occur and may influence the functionality of the wall.

Shear behavior and microstructural variation in loess from the Yan'an area, China

The shear behavior of loess is closely related to its microstructural variation. Their relationship is of great significance to better understand the loess landslide mechanism. Consolidated-undrained triaxial tests were performed on natural loess from Yan'an, China, with various initial water contents under different confining pressures to investigate the shear behavior. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were conducted on the specimens before and after the triaxial tests to study the variations in microstructure and pore characteristics and their relationship with the shear behavior. The triaxial tests revealed three different failure modes (shearing, homogeneous and plastic failure) and the corresponding stress-strain response of specimens depending on confining pressure level and initial water content. The cohesion decreases exponentially with increasing initial water content whereas the internal friction angle decreases slightly. This effect is closely related to the weakening effect of water on the inter-particle cementation and water-air interface in loess. The macroscopic shear behavior of loess is essentially the result of the continuous adjustment and change in its microstructure system under loading and wetting. This variation process is related to the confining pressure, initial water content and failure mode of the specimen. This condition is manifested by the softening, dispersion, disintegration and reassembly of cementations, particle movement and rearrangement, the reduction and mutual transformation of the inter-aggregate pore populations larger than 0.05 μm in diameter (macropores and abundant mesopores decrease, small pores increase), and the generation and development of cracks under certain conditions.

Behaviour of fine-grained gypsum-rich soil under triaxial tests

This study presents the unconsolidated undrained (UU) and consolidated undrained (CU) behaviour of a compacted gypsum-rich sandy lean clay with a total soluble salt content of 35% and a gypsum content of 33%. The compaction curve of the soil exhibits a double peak indicating two maximum dry densities and two optimum moisture content (OMC) values for the standard American Association of State Highway and Transportation Officials (Aashto) compaction effort. For triaxial testing, soil specimens were compacted at the wet OMC. UU triaxial tests on both unsoaked and 120 d soaked specimens in fresh water indicate a significant decrease of soil cohesion and angle of internal friction due to long-term soaking. An increase of compaction effort from that of standard Aashto to that of modified Aashto improved both soil strength parameters. CU triaxial tests were carried out on specimens compacted at standard Aashto compaction in a manufactured mould to prepare the triaxial soil specimens. The test results revealed that the stress paths for the tested samples differ markedly from those displaying the behaviour of overconsolidated soils, as discussed in detail in this paper.

Determination of Initial-Shear-Stress Impact on Ramsar-Sand Liquefaction Susceptibility through Monotonic Triaxial Testing

Liquefaction risk assessment is critical for the safety and economics of structures. As the soil strata of Ramsar area in north Iran is mostly composed of poorly graded clean sand and the ground water table is found at shallow depths, it is highly susceptible to liquefaction. In this study, a series of isotropic and anisotropic consolidated undrained triaxial tests were performed on reconstituted specimens of Ramsar sand to identify the liquefaction potential of the area. The specimens are consolidated isotropically to simulate the level ground condition, and anisotropically to simulate the soil condition on a slope and/or under a structure. The various states of soil behavior are studied by preparing specimens at different initial relative densities and applying different levels of effective stress. The critical state soil mechanics approach for identifying the liquefaction susceptibility is adopted and the observed phenomena are further explained in relation to the micro-mechanical behavior. As only four among the 27 conducted tests did not exhibit liquefactive behavior, Ramsar sand can be qualified as strongly susceptible to liquefaction. Furthermore, it is observed that the pore pressure ratio is a good indication of the liquefaction susceptibility.

Suitability of fly ash and cement for fabrication of compressed stabilized earth blocks

This study is aimed at utilizing industrial waste Fly ash (FA) in combination with cement for production of environment friendly and economically viable Compressed Stabilized Earth Blocks (CSEBs). CSEBs are prepared with four cement contents (4%, 6%, 8% and 10%) and four FA contents (0%, 10%, 20%, 30%) and its performance are assessed in terms of strength and durability. Strength was measured using unconfined compression test, and consolidated undrained (CU) triaxial test; and durability was assessed in terms of water absorption test, wet compressive strength test, and accumulated loss of mass (ALM). Unconfined compression test shows that with the increase of cement, strength of the blocks increases for a definite FA content, however, for a fixed amount of cement, strength increases with addition of certain amount of FA, which is defined as the optimum FA content beyond which strength begins to drop. For 4% and 6% cement, optimum FA content was found 10%, for 8% cement, 20% FA was found to be the optimum content, whereas for 10% cement the optimum FA content is 30%. Deformation characteristics of CSEBs with different cement-FA combinations are analyzed in terms of stress-strain response and modulus of elasticity. Addition of proper amount of FA is found to improve modulus of elasticity, peak and failure strain response for a definite cement content. Moreover, microstructural investigation is carried out to investigate the arrangement of soil matrix for different contents of stabilizers. Optimum content of cement-FA obtained from unconfined compressive strength provide wet-to-dry strength ratio >0.33 which is an indicator of better durability performance. CU triaxial test results show that at a confining stress of 200 kPa and 300 kPa, maximum stress ratio and secant modulus occurs at a mix composition of 8% cement and 20% fly ash, which was considered optimum from compressive strength test scheme. Finally, test results were compared with the specifications of various existing standards and it can be concluded that with addition of optimum amount of cement-FA, CSEBs with acceptable strength and durability response can be produced.

Macroscopic and Mesoscopic Mechanical Properties of Mine Tailings with Different Dry Densities under Different Confining Pressures

Particle flow numerical simulation software (PFC3D) was utilized to establish the consolidated-undrained triaxial compression test numerical models of mine tailings with different dry densities to deeply investigate the macroscopic and microscopic characteristics of mine tailings in a tailing pond in Hunan Province. Comparing the results of the simulation and the laboratory experiment, the mesoscopic parameters of the particle flow numerical simulation were obtained through continuously adjusting the mesoscopic parameter with the higher degree of agreement between the stress-strain curve, the peak strength, and the elastic modulus as the determining standard. The macroscopic and microscopic characteristics of mine tailings were studied from the perspectives of stress-strain, axial strain-volume strain, coordination number, particle velocity vector, and contact force between particles. After numerous numerical tests, it was found that the PFC3D simulation results are consistent with experiment results of the dry density tailing samples under different confining pressures; compared with the high confining pressure, the simulation test results at lower confining pressures were more with that of the laboratory tests; low density and high confining pressure both have inhibitory effect on the dilatancy characteristics of triaxial samples; with the same confining pressure, the dilatancy tendency of low dry density samples is suppressed comparing with the high dry density samples. The initial coordination number of the numerical model is large, which proves that the contact degree of the model is good to some extent.

Shear Strength Parameters of Cement Stabilized Amorphous Peat of Various Water Additive Ratios at Different Natural Moisture Contents under Consolidated Undrained Triaxial Test

Peat is a problematic soil which has a low shear strength characteristic. Addition of cement can improve the properties and strength of peat soil. This paper presents the findings of the shear characteristic of cement stabilized amorphous peat under consolidated undrained (CU) triaxial test. Three different natural moisture contents of peat which are 1210%, 803% and 380%, were stabilized using cement with water to additive (W/A) ratio of 2.0 and 3.0. CU triaxial test was conducted to the specimens after cured for 90 days. The shear parameters and characteristics were investigated towards the change of W/A ratio of the samples with different moisture contents. The result shows that the stabilized peat specimens exhibited ductile behavior and were sensitive to the over consolidation. The total and effective cohesion (ccu, c′) of the stabilized peat were found to be greater at W/A ratio of 2.0 compared to W/A ratio of 3.0, and greater at lower initial moisture content specimens. The total and effective friction angles (φcu, φ′) are ranged from 14o to 27o and 36o to 47o consecutively and found to be increased upon the increase of W/A ratio except for the specimens with moisture content 1210% and 803% in term of total friction angle.

Effect of Mono Ethylene Glycol Solution on Mechanical Behavior of a Clay Soil

Abstract This article presents the results of an investigation into the behavior of a clay soil contaminated with mono ethylene glycol (MEG) through a program of experimental tests. Soil specimens were prepared with water or different concentrations (10, 25 and 40 %) of MEG by the slurry method. One-dimensional consolidation and consolidated undrained (CU) triaxial tests were performed on the specimens. The results of the consolidation tests showed that the preconsolidation pressure is increased and the value of λ (slope of normal consolidation line) is decreased with an increase in the concentration of MEG. In addition, the results of the CU triaxial tests indicated that the friction angles (in term of total and effective stresses) are greater for the MEG solution than for water as pore fluid, and their values are increased with an increase in the concentration of the MEG solution. It was also shown that Roscoe surface exists for contaminated soil in q, p′, v space, and its position in this space is dependent on the concentration of pore fluid.