Evaluation of Cyclic Shear Strength of Two Cemented Calcareous Soils

The mechanism controlling the cyclic shear strength of cemented calcareous soils was investigated based on the results obtained from monotonic and cyclic triaxial tests on two different types of calcareous soil. Undrained cyclic triaxial tests performed on artificially cemented calcareous soils with different loading combinations showed that the effective stress path moved towards or away from the origin, due to the generation or dissipation of pore pressure with progressive cycles. Previous investigations have shown that the Peak Strength Envelope or the State Boundary Surface or the Critical State Line forms a boundary beyond which effective stress paths during cyclic loading cannot exist. However, in this study it was observed that the maximum stress ratio (ηmax) obtained from monotonic tests defined the boundary for the cyclic tests. Based on the information obtained from this study, an approach for evaluating the cyclic shear strength is proposed. It was observed that the modified normalized cyclic shear strength had a strong linear relationship with the logarithm of the number of cyclic to failure irrespective of confining pressure, type of consolidation and stress reversal.     

Drained Deformation Behavior of Anisotropic Sands during Cyclic Rotation of Principal Stress Axes

A series of drained tests for sands with inherent fabric anisotropy were conducted with an automatic hollow cylinder apparatus. The samples were subjected to cyclic rotation of principal stress axes while the magnitudes of effective principal stresses were maintained constant. The evolution of strain components and the volumetric strain with number of cycles, the relationship between the shear stress and shear strain components, and the flow rule of sands were investigated. It is found that plastic deformation is induced due to principal stress axes’ rotation alone without variation in the magnitudes of effective principal stresses. The contractive volumetric strain accumulates steadily with the increasing number of cycles; however, its accumulation rate is lowered with its progressive accumulation. The results also exhibit obvious noncoaxiality between the directions of strain increment and stress, and the noncoaxiality shows segmentation characteristics during the rotation of principal stress axes. Meanwhile, special attention was paid to the significant role of the intermediate principal stress parameter b [b = (σ2′?σ3′)/(σ1′?σ3′)] in the deformation behavior of sands during cyclic rotation of principal stress axes. It is found that the volumetric strain and the shear modulus ratio of the jth cycle to the first cycle increase with the increase in the b value under otherwise identical conditions. The effects of the relative density, effective mean normal stress, and deviatoric stress ratio on sand deformation behavior are also addressed in this work.     

Fatigue crack initiation and strain-controlled fatigue of some high strength low alloy steels

Initiation and growth of fatigue microcracks were investigated in several Nb and V alloyed high strength low alloy steels, including conventional and dual phase microstructures. Fatigue microcracks initiated along prominent slip bands. Macrocracks formed by linking up of small microcracks. At low applied stress or strain, the number of cycles to crack initiation increased with the cyclic yield stress. Comparing the cyclic stress-strain curves to the monotonie stress-strain curves, cyclic hardening or softening occurred, depending upon strain amplitude. Plateau regions were observed in plots of cyclic stress amplitudevs cyclic plastic strain amplitude obtained by increasing the total strain amplitude in steps after 30 cycles at each step. In polycrystalline 0.03 pct Nb steel, the plateau region was identified with prominent slip band formation, as others have observed in single crystals of copper, C-doped iron, and other metals.     

Engineering Behavior of a Sand Reinforced with Plastic Waste

Unconfined compression tests, splitting tensile tests, and saturated drained triaxial compression tests with local strain measurement were carried out to evaluate the benefit of utilizing randomly distributed polyethylene terephthalate fiber, obtained from recycling waste plastic bottles, alone or combined with rapid hardening Portland cement to improve the engineering behavior of a uniform fine sand. The separate and the joint effects of fiber content (up to 0.9 wt?%), fiber length (up to 36 mm), cement content (from 0 to 7 wt?%), and initial mean effective stress (20, 60, and 100 kN/m2) on the deformation and strength characteristics of the soil were investigated using design of experiments and multiple regression analysis. The results show that the polyethylene terephthalate fiber reinforcement improved the peak and ultimate strength of both cemented and uncemented soil and somewhat reduced the brittleness of the cemented sand. In addition, the initial stiffness was not significantly changed by the inclusion of fibers.     

Dynamic Shear Behavior of a Needle-Punched Geosynthetic Clay Liner

An experimental investigation of the dynamic internal shear behavior of a hydrated needle-punched geosynthetic clay liner is presented. Monotonic and cyclic displacement-controlled shear tests were conducted at a single normal stress to investigate the effects of displacement rate, displacement amplitude, number of cycles, frequency, and motion waveform on material response. Monotonic shear tests indicate that peak shear strength first increased and then decreased with increasing displacement rate. Cyclic shear tests indicate that cyclic response was primarily controlled by displacement amplitude. Excitation frequency and waveform had little effect on cyclic shear behavior or postcyclic static shear strength. Number of cycles ( ≥ 10) also had little effect on postcyclic static shear strength. Shear stress versus shear displacement diagrams displayed hysteresis loops that are broadly similar to those for natural soils with some important differences due to the presence of needle-punched reinforcement. Secant shear stiffness displayed strong reduction with increasing displacement amplitude and degradation with continued cycling. Values of damping ratio were significantly higher than those typical of natural clays at lower shear strain levels. Finally, cyclic tests with increasing displacement amplitude yielded progressively lower postcyclic static peak strengths due to greater levels of reinforcement damage. Postcyclic static residual strengths were unaffected by prior cyclic loading.     

High-Pressure Isotropic Compression Tests on Fiber-Reinforced Cemented Sand

High-pressure isotropic compression tests were carried out on reconstituted sand samples that were reinforced with cement, randomly distributed fibers, or both, making comparisons with the unreinforced sand and conducting tests from a variety of initial specific volumes. The results indicated changes in the isotropic compression behavior of the sand due to the inclusion of fibers and/or cement. Cementitious bonds are sufficiently strong relative to the particles to allow the cemented samples to reach states outside the normal compression line (NCL) of the uncemented soil, but the effectiveness of cemented fiber-reinforced specimens is even larger due to the control of crack propagation in the cemented sand after the inclusion of fibers. Distinct NCLs were observed for the sand, fiber-reinforced sand, cemented sand, and fiber-reinforced cemented sand. Both fiber breakage and fiber extension were observed in fibers measured after testing indicating that fibers individually have worked under tension, even though in the macroscopic scale, isotropic compressive stresses were applied. Fiber reinforcement was found to reduce the particle breakage of both the uncemented and cemented sands.     

Use of = 0 as a Failure Criterion for Weakly Cemented Soils

There is considerable uncertainty in the determination of effective stress strength parameters of cemented soils from undrained triaxial tests. Large negative excess pore pressures are generated at relatively large strains (typically 4–5% for cemented silty sand) in isotropically consolidated undrained (CIU) tests, which results in gas coming out of solution during shear and significant variability in the measured peak deviator stress. In this study, different failure criteria for weakly cemented sands were evaluated based on the results of CIU and isotropically consolidated drained triaxial compression tests conducted on samples of artificially cemented sand. The use of = 0 as a failure criterion eliminates the variability between the undrained tests and also ensures that the mobilized failure strength is not based on the highly variable negative excess pore pressures. In addition, the resulting strains to failure are comparable to the strains to failure for the drained tests. Mohr-Coulomb strength parameters thus estimated from the undrained tests are generally lower than strength parameters obtained from drained tests, and the difference between the failure envelopes from undrained tests increases as the level of cementation increases. This divergence is attributed to differences in the stiffness of the cemented soil under the different loading conditions. The stiffness under undrained loading conditions decreases with increasing cementation due to an increase in the generation of positive excess pore pressure at low strains.     

颗粒刚度变化对胶结砂岩力学响应的影响

为分析胶结砂岩的力学响应和破坏机理,基于试验建立不同刚度比的三维颗粒流数值模型,验证数值模型的可行性,并分析不同胶结性状的砂岩力学响应,进一步说明胶结物质的重要作用及模型的适用性.分析颗粒接触刚度比和平行黏结刚度与颗粒接触刚度的比值变化时,砂岩的应力比、体应变、配位数和平行黏结破坏数的变化规律以及对模型的泊松比、初始刚度和延性的影响.结果表明:不同的颗粒刚度比对岩样宏观力学响应的影响不同,颗粒接触刚度比越小,且切向刚度越大时,胶结砂岩的脆性越强;平行黏结刚度与颗粒接触刚度的比值越大,脆性越强,黏结破坏越容易,剪切破坏越明显.颗粒刚度对胶结砂岩的力学响应和变形能力有重要的影响,是实际储层砂岩力学模拟选择有效细观参数和构建本构关系的关键.      

Influence of the Grain-Size Distribution Curve of Quartz Sand on the Small Strain Shear Modulus Gmax

The paper presents a study of the influence of grain-size distribution curve on the small strain shear modulus Gmax of quartz sand with subangular grain shape. The results of 163 resonant column tests on 25 different grain-size distribution curves are presented. It is demonstrated for a constant void ratio that while Gmax is not influenced by variations in the mean grain-size d50 in the investigated range, it significantly decreases with increasing coefficient of uniformity Cu = d60/d10 of the grain-size distribution curve. Well-known empirical formulas (e.g., Hardin’s equation with its commonly used constants) may strongly overestimate the stiffness of well-graded soils. Based on the RC test results, correlations of the constants of Hardin’s equation with Cu have been developed. The predictions using Hardin’s equation and these correlations are in good accordance with the test data. Correlations of the frequently used shear modulus coefficient K2,max with Cu and empirical equations formulated in terms of relative density, are also given in the paper. A comparison of the predictions by the proposed empirical formulas with Gmax-data from the literature and a micromechanical explanation of the experimental results are provided. Correction factors for an application of the laboratory data to in situ conditions are also discussed.     

Small-Strain Behavior of Granular Soils. I: Model for Cemented and Uncemented Sands and Gravels

A simple formulation is presented that predicts the nonlinear small strain behavior of cemented and uncemented granular soils. Its performance is evaluated through the comparison of model predictions to results from laboratory tests. A companion paper evaluates the performance of this model implemented in a site response analysis code through comparison with the measured response at two sites. The formulation for the maximum shear modulus, Gmax, which is selected through the evaluation of existing formulations and data, is presented with the hysteretic model developed to describe the shear modulus reduction and damping increase with increasing strains. Few parameters are needed to predict the small strain response, and correlations between model parameters and index properties of granular materials are presented when possible. The model, SimSoil, is shown to capture the cyclic response for sands and gravels with varying densities over a wide range of pressures measured in laboratory tests, including cases when cementation is present.     

Stress-Induced Anisotropic Gma x of Sands and Its Measurement

Anisotropy in elastic shear modulus Gma x exists in most soils as the result of either anisotropic soil fabric or anisotropic stress conditions. This paper presents a theoretical and experimental study on the anisotropy in a Gma x of sand due to a K0 stress condition. Elastic shear moduli of two types of sand in multiple stress planes under a K0 condition were measured using bender elements. Stress-induced anisotropy in Gma x of the sands during loading and unloading processes and the important influential factors were investigated. An empirical relationship for the estimation of K0 was proposed based on the experimental data. Shear moduli in nonprincipal stress planes were measured and compared with the results from the theory. The influence of stress cycles on Gma x in multiple stress planes was studied.     

Liquefaction, Cyclic Mobility, and Failure of Silt

It is known that the mechanical properties of low-plasticity silt are similar to those of sand, and yet silts are frequently used as coastal reclamation materials in many cities and industrial areas and will thus be susceptible to liquefaction. Samples of a low-plasticity silt have been tested under monotonic and cyclic loading under isotropic and anisotropic stress conditions to characterize liquefaction, cyclic failure, and to develop an empirical model describing its cyclic strength. A sedimentation technique produced samples that had the highest susceptibility to liquefaction. Contractive behavior of monotonically loaded samples was triggered when the stress path reached an initial phase transformation (IPT) in both compression and extension tests. The samples became dilative after reaching a phase transformation (PT) point. The cyclic shear behavior of the silt samples prepared using the sedimentation method and consolidated at various initial sustained deviator stress ratios was examined in terms of two different failure criteria: a double amplitude axial strain εa,DA = 5% for reversal conditions; or axial plastic strain εa,P = 5% for nonreversal. For isotropically consolidated samples the initial phase transformation determined from undrained monotonic extension tests was the boundary between stable and contractive behavior. For anisotropically consolidated samples failure was defined by a bounding surface formed by undrained monotonic compression tests. An empirical model was developed relating the number of cycles to failure under conditions of both liquefaction and cyclic mobility to the initial anisotropic sustained deviator stress and cyclic deviator stress ratio.     

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc, larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at γc<γt such changes are negligible even at large N. To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt = 0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.     

Effect of Wetting on Unconfined Compressive Strength of Cemented Sands

A weakly cemented sand and gravel has been partly or entirely used in the construction of earth structures such as dams and retaining walls. Such cemented soils that are usually highly permeable can undergo repetitive wetting and drying during curing due to temporary rainfall or a change in the groundwater table. In this study, weakly cemented sand specimens with four different cement ratios were compacted at optimum water content and cured for 28 days. When the cemented sand specimens were exposed to repetitive wetting and drying during curing, their 28-day unconfined compressive strength was evaluated. Wetting for one day on the last day was found to decrease the unconfined compressive strength of cemented sand, whereas wetting for one day in the middle of curing resulted in an increase in strength. The strength reduction due to wetting on the last day decreases as the cement ratio increases. For a specimen under repetitive wetting and drying over 28-day curing, the strength increases as the number of wetting increases up to three cycles. After three cycles of wetting and drying, the strength either becomes constant or slightly decreases due to insufficient water for hydration and/or washing cementitious materials.     

Undrained Shear Strength of Granular Soils with Different Particle Gradations

A series of undrained tests were performed on granular soils consisting of sand and gravel with different particle gradations and different relative densities reconstituted in laboratory. Despite large differences in grading, only a small difference was observed in undrained cyclic shear strength or liquefaction strength defined as the cyclic stress causing 5% double amplitude axial strain for specimens having the same relative density. In a good contrast, undrained monotonic shear strength defined at larger strains after undrained cyclic loading was at least eight times larger for well-graded soils than poorly graded sand despite the same relative density. This indicates that devastating failures with large postliquefaction soil strain are less likely to develop in well-graded granular soils compared to poorly graded sands with the same relative density, although they are almost equally liquefiable. However, if gravelly particles of well-graded materials are crushable such as decomposed granite soils, undrained monotonic strengths are considerably small and almost identical to or lower than that of poorly graded sands.     

In Situ Soil Response to Vibratory Loading and Its Relationship to Roller-Measured Soil Stiffness

An investigation was conducted to characterize and relate in situ soil stress-strain behavior to roller-measured soil stiffness. Continuous assessment of soil stiffness via roller vibration monitoring has the potential to significantly advance performance based quality assurance of earthwork. One vertically homogeneous and two layered test beds were carefully constructed with embedded sensors for the field testing program. Total normal stress and strain measurements at multiple depths reveal complex triaxial soil behavior during vibratory roller loading. Measured cyclic strain amplitudes were 15–25% of those measured during static roller passes due to viscoelasticity and curved drum/soil interaction. Low amplitude vibratory roller loading induces nonlinear in situ modulus behavior. Roller-measured stiffness and its dependence on excitation force is influenced by the stress-dependent modulus function of each soil, the varying drum/soil contact area, and by layer characteristics (modulus ratio, thickness) when layering is present. On vertically homogeneous clayey sand, roller-measured stiffness decreased with increasing excitation force, a behavior attributed to stress-dependent modulus reduction observed in situ. On the crushed rock over silt test bed, roller-measured stiffness increased with increasing excitation force despite the mild stress-dependent modulus reduction observed in the crushed rock. In this case, the stiffer crushed rock takes on a greater portion of the load, resulting in the increase in roller-measured stiffness.     

Undulation of W/matrix interface by resintering of cyclically heat-treated W-Ni-Fe heavy alloys

When liquid-phase-sintered W-Ni-Fe alloys were cyclically heat treated at 1100 °C and resintered at 1485 °C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method (FEM) also showed nonuniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.     

Oedometric Tests on Artificially Weathered Carbonatic Soft Rocks

Carbonatic rocks are subject to progressive degradation of their mechanical characteristics due to weathering. This phenomenon may have relevance for engineering since it can induce settlement of foundations, progressive failure of slopes or an increase of pressure on tunnel liners. To study weathering effects on a laboratory timescale, oedometric tests have been conducted on specimens of calcarenite and artificially cemented silica sand. Weathering was simulated by percolating an acid solution. Under constant load, the recorded progression of deformation over time ends with a horizontal asymptote when only bonds are destroyed (cemented sand), whereas it results in a steady increase when the grains are also degraded (calcarenite). To determine the variation in stress state induced by weathering, a special oedometer was designed. Allowing very small lateral deformation of the ring, its circumferential stretching was measured with strain gauges and correlated to the lateral pressure exerted by the specimen. In all cases considered, the horizontal stress increased remarkably during weathering at constant vertical load. Predictions of a chemoplastic constitutive model are then compared to experimental data, and then show good agreement, especially for cemented sand.     

Postcyclic Degradation of Strength and Stiffness for Low Plasticity Silt

Stress-controlled undrained cyclic triaxial tests followed by strain-controlled monotonic compressive shear tests were carried out on normally consolidated and overconsolidated reconstituted Keuper Marl silt to investigate the strength and stiffness degradation characteristics of a low plasticity silt. Special attention was paid to the changes in undrained strength and deformation modulus after undrained cyclic loading. It was observed that cyclic degradation in stiffness for low plasticity silt is more marked than that of strength, and this tendency increases with increasing overconsolidation ratio. It was found that a previously proposed model for predicting postcyclic degradation in strength and stiffness of normally consolidated fine-grained soils could be applied to that of overconsolidated silt but not however to the postcyclic degradation in Young’s modulus. Thus, an attempt was made to correlate postcyclic degradation of overconsolidated silt to the equivalent cyclic shear strain instead of the normalized excess pore pressure. It was concluded that cyclic shear strain was a better parameter than cyclic-induced excess pore pressure for correlating the postcyclic stiffness degradation not only for normally consolidated but also for overconsolidated silt.