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In undrained triaxial tests, membrane penetration varies with the change of effective confining pressure. Membrane compliance correction is a necessary step to accurately obtain the dynamic characteristics of soils in geotechnical tests. In this paper, a real-time instrument compensation equipment was adopted to eliminate membrane compliance The influence of membrane compliance on pore pressure was explained from experimental perspective, and a two-stage formula was proposed to establish the relationship between the optimized membrane correction coefficient Cm and the normalized pore pressure U/σ0′. C, which is the constant value of Cm in the stationary segment, is analyzed under different confining pressures, relative densities, consolidation ratios, and gravel contents, and the power function of C with the characteristic particle size d50 is proposed. Based on Cm, a series of predictions of pore pressure development without membrane compliance are implemented, which are basically consistent with the actual experimental results. The method to predict the pore pressure development without membrane compliance based on Cm is accurate and effective, and a large amount of experimental workload can be reduced for practical engineering applications. [ABSTRACT FROM AUTHOR]

In undrained triaxial tests, membrane penetration varies with the change of effective confining pressure. Membrane compliance correction is a necessary step to accurately obtain the dynamic characteristics of soils in geotechnical tests. In this paper, a real-time instrument compensation equipment was adopted to eliminate membrane compliance The influence of membrane compliance on pore pressure was explained from experimental perspective, and a two-stage formula was proposed to establish the relationship between the optimized membrane correction coefficient Cm and the normalized pore pressure U/σ0′. C, which is the constant value of Cm in the stationary segment, is analyzed under different confining pressures, relative densities, consolidation ratios, and gravel contents, and the power function of C with the characteristic particle size d50 is proposed. Based on Cm, a series of predictions of pore pressure development without membrane compliance are implemented, which are basically consistent with the actual experimental results. The method to predict the pore pressure development without membrane compliance based on Cm is accurate and effective, and a large amount of experimental workload can be reduced for practical engineering applications. [ABSTRACT FROM AUTHOR]

Three-dimensional consolidation testing, scanning electron microscopy, and undrained triaxial testing of granite residual soil (GRS) in Shenzhen, China, were conducted. The effect of the structural strength of GRS was quantitatively analyzed using the method of comprehensive structure potential parameter description. The failure strength ratio of the soil decreases with increasing confining pressure, and it is approximately 1 when the confining pressure is higher than the yield stress (σs). The undrained shear strength (Su) of the undisturbed soil varies in stages with confining pressure σ3. The relationship between Su and σ3 for the reconstituted soil is approximated as a straight line through the origin. The soil's comprehensive structure potential parameters (the structural contribution rates of the tangential shear modulus, mE, and the deviatoric stress, md) decrease linearly with increasing σ3. The mE and md sample values are similar under the same confining pressure. It is reasonable to use structure potential parameters to quantitatively evaluate the structure contribution rate of GRS. The microstructure is constantly adjusted, and the soil microstructure form for different confining pressures strongly correlates with its shear characteristics. [ABSTRACT FROM AUTHOR]

In the geological repository of high-level radioactive wastes (HLRW), one of the most important issues is the stability of artificial barrier basically using bentonite. However, due to the exist gaps between bentonite block and surround rock mass, the bentonite can swell relatively freely, whose constraining condition is quite different from the normal swelling pressure test. As an extreme case, the bentonite may engage completely into the surrounding ground and become a complete normal consolidated state. Therefore, in order to investigate the thermo-mechanical behavior of saturated bentonite at normally consolidated state, a newly proposed static compaction method for preparing specimen of saturated normally consolidated bentonite was proposed first. Then, a series of undrained triaxial compression tests were conducted under different confining pressure and temperature, by which the thermo-mechanical behavior of bentonite, used as an artificial barrier of geological repository of HLRW, is investigated systematically. The tests results reveal that the specimen prepared by the proposed method is confirmed to have sufficient saturation (> 0.92), meanwhile the prepared specimen is at slightly overconsolidated state that roughly equal to a K0-line consolidation. The deformation of the bentonite in undrained triaxial compression tests changes from plastic to brittle as the temperature increases. Simulation based on a sophisticated constitutive model considering water compressibility at high confining pressure was also discussed. The results are useful for assessing the mechanical behavior of bentonite after hundreds years of geological repository of HLRW. • Mechanical behavior of bentonite under different temperature and confining pressure was studied. • A quick preparing method for saturated bentonite on normally consolidated state was proposed. • The deformation of the bentonite changes from plastic to brittle as the temperature increases. • Model simulation at high confining pressure considering water compressibility was discussed. [ABSTRACT FROM AUTHOR]

This study concerns the creep impact on strength parameters of the selected very cohesive soils (PI = 30–70%). The analysis refers to Neogene clays characterized by a complex structure, resulting directly from a complicated load history in the geological time scale and identified glacitectonic deformations. In the process of samples’ preparation for strength tests as well as during the interpretation of the post-failure state, particular attention was paid to the soil structure. The imaging methods (X-ray densitometry and computer microtomography) enabled the comparison of the soil structure and the selection of samples with similar characteristics. The completed program of strength tests consisted of two series of tests in the triaxial stress state, differentiated by the occurrence of the initial creep stage, preceding the typical strength test scheme under undrained conditions. This study allowed to obtain a quantitative assessment of the influence of the creep process on the strength parameters of tested soils. Constant stress lower than 60% of the shear stress deviator leads to the deceleration creep course (m parameter 0.64–0.89). As a result, higher values of internal friction angle (20% increase comparing to triaxial tests without creep stage) and cohesion reduction are obtained from triaxial creep tests. Creep parameter m is found to be a valuable indicator for differentiation of landslide activity trend. The tests proved low values of axial strains (1–5%) at failure, which was associated with lithogenesis. By the implementation of obtained strength parameters into the 3D finite element model of the slope, the potential influence of the creep process on the stability of an exemplary cross section of the Warsaw slope could be determined.

This paper highlights the effect of non-plastic fines on sand matrix by performing a series of undrained triaxial tests. The silty sand specimens were reclaimed by moist tamping method and two distinct density indexes of sand matrix were selected: IDmat=0.00 (<0.15) for very loose state and IDmat=0.50 (>0.40) for medium-dense state. The addition of non-plastic fines to the host sand matrix makes the specimens more resistant to liquefaction by inducing dilatant behaviour and reducing the excess pore water pressure Δu. As for instability line in the very loose state (IDmat=0.00), a rotation is observed on the p'-q plane with increasing non-plastic fines fraction. This trend is non-linear and becomes less remarkable after a certain threshold value in fines fractions. In the medium-dense state (IDmat=0.50), there is no significant effect of non-plastic silt particles on the phase transformation state. Regarding the ultimate steady state, the line is unified in the p'-q space, but a clockwise rotation is observed in the log(p')-e space. Furthermore, the exponential form shows a good fitting performance for silty sand. [ABSTRACT FROM AUTHOR]

This paper presents an experimental thorough study on geocell reinforced loose sand (Dr = 25%), focusing on the improvement of the shear strength in undrained conditions. The experimental program includes a series of undrained compressive tests (CU) performed on loose sand samples with and without reinforcement. The experimental tests are performed for various geocell height (Hg = 15, 25 and 35 mm) and at different level of effective confining pressure (σ'c = 50, 100 and 200 kPa). The obtained results show a significant effect of geocells height and initial confining pressure on the shear strength of the reinforced sand. The apparent cohesion of the reinforced soil increases linearly with the axial strain while the friction angle is rather constant and slightly dependent on the sample deformation. [ABSTRACT FROM AUTHOR]

In practice, strength of soils commonly is measured isotropically, while for accurate design of structure foundations and earth structures, it is essential to consider the effect of anisotropy phenomenon on the mechanical behavior of the soils. In previous research, the effect of anisotropy on shear strength of sandy soils has been known and there is some limited work on cohesive soil. Moreover, anisotropy impact is not known on shear-induced pore water pressure. In this research, a series of triaxial tests were carried out on the undisturbed samples of different fine-grained soils, taken at different directions in the sites. The samples were consolidated under effective isotropic stresses of 200, 300, 500 and 700 kPa, and loaded in undrained condition. The results revealed that the behavior of all soils significantly depends on the sample orientation in the field. As the direction of samples changed from "perpendicular to bedding orientation" to "parallel to bedding orientation", the shear strength decreased gradually and it has the minimum value at anisotropy angle of 72°, 59° and 90° for red clay, yellow marl and olive marl, respectively. The reduction rate completely depends on the soil plasticity and level of confining pressure. [ABSTRACT FROM AUTHOR]

Undrained instability in sandy soil containing fine particles such as liquefaction has remained a troublesome challenge in geotechnical research. Soil reinforcement could be taken as an approach to enhance its engineering properties like shear strength, cohesion, and bearing capacity. The present paper aims to evaluate the capability of geotextile reinforcement in improving undrained shear strength and diminishing undrained brittleness of loose clayey sands. A series of consolidated undrained triaxial tests were carried out on clayey sands containing 0, 5, 10, 15, 20 and 30% fines content (F.C.) under 100, 150 and 200 kPa confining pressures. To investigate the effect of geotextile reinforcement, experiments were performed in three reinforcement arrangements. The results indicate that geotextile reinforcement increases the steady state shear strength and decreases liquefaction potential and softening behavior. Geotextile reinforcement leads to decrease brittleness index of clayey sand specimens. It can also be pointed out that the presence of clay fines changes the dilative behavior of clean sand to contraction, and consequently reduces its undrained shear strength. Also, it is found that by increasing the clay content up to 10%, the undrained shear strength of reinforced and unreinforced clayey sand are decreased and brittleness index increased. With further rises in clay content, the undrained shear strength increases and brittleness behavior decreases. [ABSTRACT FROM AUTHOR]

A coupled computational fluid dynamics and distinct element method (CFD-DEM) is proposed to simulate undrained triaxial tests on granular soils. In this method, the compressible volume-averaged Navier–Stokes equations of continuity and momentum with an additional state equation are used for modelling liquid flows. In addition, the volume-averaged Navier–Stokes equations are modified by including the velocity of moving grid in their convective terms to settle the problem of variable boundaries in the process of undrained triaxial compression. Then the capability of the developed method is mainly examined by comparisons with the constant volume method of simulating undrained triaxial tests, which shows that the results of the CFD-DEM method can compare favourably with those obtained from the constant volume method. Finally, the influences of strain rate, initial pore pressure and fluid compressibility on the undrained behavior of granular soils are carefully studied. [ABSTRACT FROM AUTHOR]

The B-value, measured after setting up a specimen for triaxial testing, is used for evaluating the degree of saturation of a specimen. However, the stress conditions used when measuring the B-value have not been standardized. Although some experimental approaches have been pursued, no detailed analyses have evaluated the effects of the measurement conditions on the B-value results. In this study, the B-values of nearly saturated specimens under various stress conditions were investigated, using a smart triaxial test apparatus capable of elaborate stress control, and comparisons among measurement results were conducted. Moreover, by applying Henry׳s law, which governs the dissolution of the gas phase into the liquid phase, to an existing constitutive model for unsaturated soil, a new soil model was proposed. This new model accurately represents the behaviors of pore air even under nearly saturated conditions. In addition, the effects of the stress conditions on the B-values were investigated through numerical simulations of B-value measurements. [ABSTRACT FROM AUTHOR]

Soil treated with cement becomes brittle because its shear strength decreases rapidly in a post-peak state, which is why in recent years the inclusion of fibre into soil treated with cement has become an increasingly popular research area. This paper presents a constitutive model to simulate the behaviour of the fibre reinforced cement treated soil, referred to as the improved soil composite. In this model, a non-linear failure envelope was formulated to merge with the Critical State Line (CSL) of the reconstituted soil mixture at high levels of stress in order to capture the broken cementation bonds and ruptured fibre. A non-associated plastic potential function and a general stress strain relationship that includes the softening of the composite soil were also proposed to simulate the pre-and-post peak state. Moreover, many researchers focus on the addition of fibre into sand, soft clay, and sand treated with cement, whereas the behaviour of soft clay treated with fibre and cement requires further investigation. Hence, in this study a series of undrained triaxial tests were carried out on natural Ballina clay treated with cement and 0.3%–0.5% of fibre to determine how the amount of fibre and cement affects the behaviour of soft clay. SEM images were also analysed to study the structure of the improved Ballina composite at the micro-structural level. The laboratory results indicated that the combined effects of cementation and fibre reinforcement increased the shear strength and ductility of treated soft clay. Under triaxial conditions the peak shear strength of soft clay treated with cement and fibre increases dramatically due to the formation of cementation bonds and the bridging effect provided by the fibres, and the brittleness caused by the cementation bonds breaking also improves significantly due to the inclusion of fibre. However, when shearing at a high mean effective stress the cementation bonds break and the fibre ruptures due to the mean effective stresses and plastic deviatoric strain which caused major cracks to appear within the sample. The performance of the model was evaluated by comparing its predictions with the results of the undrained triaxial tests conducted on the improved Ballina clay composite. By capturing the main features of the composite soil the model provided reliable predictions that agreed with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims at investigating the influence of grading on the undrained behavior of granular materials. Series of undrained triaxial tests were carried out with two different materials, glass balls and Hostun sand. For each material, samples with different gradings and similar relative densities were prepared. Experimental results show that the undrained shear strength decreases when the coefficient of uniformity increases from 1.1 to 20. The conditions of instability for the selected granular materials were also analyzed, based on the sign of the second-order work during undrained triaxial loading. The results demonstrate a significant influence of the grading: increasing the coefficient of uniformity heightens the potential of static liquefaction and the materials become more unstable. Furthermore, numerical tests using the three-dimensional discrete element method (DEM) were conducted on assemblies of spheres. The DEM inter-particle parameters were derived from the experimental test results on glass balls. The DEM simulations showed similar behaviors compared to experimental results and confirmed the influence of the grain size distribution on the stress–strain relationship and instability phenomena. [Copyright &y& Elsevier]

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In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global densification in the sample is observed. The results of this paper will help assess the change in static slope stability after a single or multiple small to moderate events occurred without causing initial instability. • Investigation of post-cyclic monotonic undrained shear strength of Ottawa Sand. • The strengthening effect increases as the seismic intensity increases. • The strengthening effect does not increase with the number of seismic events. • The change of shear strength does not depend on the change of relative density. • Shaking may induce non-homogeneity in the sample that influences its shear strength. [ABSTRACT FROM AUTHOR]