Strain Localization in Sand: Plane Strain versus Triaxial Compression

A comprehensive experimental investigation was conducted to investigate the effects of loading condition and confining pressure on strength properties and localization phenomena in sands. A uniform subrounded to rounded natural silica sand known as F-75 Ottawa sand was used in the investigation. The results of a series on conventional triaxial compression (CTC) experiments tested under very low-confining pressures (0.05–1.30) kPa tested in a microgravity environment abroad the NASA Space Shuttle are presented in addition to the results of similar specimens tested in terrestrial laboratory to investigate the effect of confining pressure on the constitutive behavior of sands. The behavior of the CTC experiments is compared with the results of plane strain experiments. Computed tomography and other digital imaging techniques were used to study the development and evolution of shear bands.     

Factors Affecting Mechanical and Creep Properties of Silicate-Grouted Sands

Factors affecting the strength, modulus, stress-strain, and time-to-failure relationships of moist-cured silicate-grouted sands were investigated from short-term and creep tests. Variables included in the short-term tests were curing time, sand gradation and mineralogy, rate of loading, curing time, and confining pressure. Confining pressure was varied up to 550 kPa, and the stress and strain loading rates were varied from 0.05 to 5.0 Pa∕min and from 0.01 to 1.0%∕min, respectively. The shear strength and failure strain of moist-cured grouted sands were independent of the confining pressure, but they were affected by all other variables investigated. Compressive failure strains for silicate-grouted sands were less than 0.4% and the limitation in improving the compressive strength of sand has been quantified. Grouted limestone sand had the highest strength. The creep behavior of grouted sand was also investigated. Stress-strain and time-to-failure relationships for grouted sands have been developed.     

Experimental Characterization of Dynamic Property Changes in Aged Sands

This study investigates the aging effects on the small-strain shear modulus and damping ratio of sands and offers explanations for the measured results based on the concept of contact-force homogenization. Resonant column tests of aged sands under various aging conditions were conducted. The results show that loose sands exhibit greater aging effects than dense sands do at a confining pressure of 35?kPa and the effects are completely opposite when the aging pressure is increased to 100?kPa. The aging effects can be partially erased by unloading-reloading; the remaining effects can be restored when the applied pressure is the same as the original pressure used during aging and cannot be further erased by additional unloading-reloading cycles. The stress history is also a factor that affects aging behavior: unloading reloading and overconsolidation can reduce the aging rate in terms of the shear-modulus increase. The aging effects, however, can be wiped out by large strain shearing. An addition of fines (dry kaolinite powder) in the sand samples can increase the aging rate because of higher creep made by the kaolinite.     

Effects of Particle Crushing in Stress Drop-Relaxation Experiments on Crushed Coral Sand

Stress relaxation and stress drop-relaxation tests have been performed to complement a test series performed to study strain rate, creep, and stress drop-creep effects on crushed coral sand. Drained experiments with constant effective confining pressure of 200 kPa were performed in which triaxial specimens of crushed coral sand were loaded to initial stress differences of 500, 700, and 900 kPa, followed by stress drops of 0, 100, 200, 300, and 400 kPa at which points the axial strains were kept constant while the axial stress relaxation and the volumetric strains were observed. The stress drops produced delays in initiation of stress relaxation that were proportional with the magnitudes of the stress drops. The experiments show that sands do not exhibit classic viscous effects, and their behavior is indicated as “nonisotach,” while the typical viscous behavior of clay is termed “isotach.” Thus, there are significant differences in the time-dependent behavior patterns of sands and clay. A mechanistic picture of time effects in sands is proposed.     

Instability and Static Liquefaction on Proportional Strain Paths for Sand at Low Stresses

The behavior of Hostun RF sand on proportional strain paths at low confining pressures (20 to 100 kPa) is considered in this paper. In such paths, a constant dilation rate is imposed during shear. The usual features of pore pressure increase (contracting material) or decrease (dilating material) are here observed depending upon whether the imposed dilation rate is respectively greater or smaller than the “natural” dilation rate at failure (as measured in a drained test). Particular attention is given to the static liquefaction phenomenon, which is seen to occur for loose as well as dense sand provided the imposed dilation rate is large enough to lead to a continuous pore pressure increase during shear. Instability tests performed at low confining pressures on proportional strain paths show that the instability line is strain path dependent. It does not coincide with the peak deviator stress line in proportional strain paths tests, in general, but does coincide with the line d2W = 0 (nil second increment of total work).     

Effects of Silt on Three-Dimensional Stress–Strain Behavior of Loose Sand

An experimental study on the effects of nonplastic silt on the three-dimensional drained behavior of loose sand was performed employing a true triaxial testing apparatus. Laboratory experiments were performed on clean sand and on sand containing 20% nonplastic silt. The results indicate the failure stress levels and the overall trends of the stress–strain behavior were similar for both sands. However, the volume change behavior is significantly influenced by the presence of silt. The silty sand exhibited higher degrees of volumetric contraction during shearing than the clean sand. Relative density was used as the basis of comparison. The development of a shear band appears to have caused failure in all true triaxial testing performed, except in triaxial compression. This form of instability appears to increase its influence on the experimental results as the participation of intermediate principal stress increases. The formation of shear bands also appears to coincide with the cessation of contractive volumetric strain.     

Model for Dynamic Shear Modulus and Damping for Granular Soils

This paper presents a simple four-parameter model that can represent the shear modulus factors and damping coefficients for a granular soil subjected to horizontal shear stresses imposed by vertically propagating shear waves. The input parameters are functions of the confining pressure and density and have been derived from a generalized effective stress soil model referred to as MIT-S1. The predicted shear moduli and damping factors are in excellent agreement with high quality resonant column test data on remolded sands and confining pressures ranging from 30 kPa to 1.8 MPa. The proposed model has been implemented in a frequency domain computer code. By simulating the variations in stiffness and damping with confining pressure, the proposed model provides a more realistic simulation of ground amplification that does not filter out high frequency components of the base excitation.     

Numerical Study of the Effect of Foundation Size for a Wide Range of Sands

This paper presents a numerical investigation of the effect of foundation size on the response of shallow circular foundations on siliceous and calcareous sands. The study is based on the predictive capabilities of the MIT-S1 soil model for simulating both the compression and shear behaviors of natural sands over a wide range of densities, K0 values and confining pressures. The paper highlights the variations in the deformation mechanisms for the siliceous and calcareous sands cases. The assessment of the bearing capacity factor, Nγ, is examined, showing a dramatic decrease in the values with increasing foundation size for the case of footings on calcareous sands, eventually converging to a terminal Nγ value. At this stage the sand resistance is insensitive to variations in initial density and foundation size because the sand tends to loose its initial characteristics due to grain crushing, leading the material rapidly toward ultimate conditions. In the silicious sand case, it is found that, eventually, for extremely large footing diameters, the deformation mechanism progresses toward a punching shear mechanism, rather than the classical rapture pattern accompanied by surface heave as employed in current bearing capacity equations. A dimensional transition between the failure mechanisms can clearly be defined, referred to as a “critical size” in the Nγ–D relationship.     

Drained Cyclic Behavior of Sand with Fabric Dependence

The behavior of sand is characterized by dilatancy, an increase in overall volume as particles move over each other when the sand assembly is subjected to shear stresses. A stress-dilatancy model for sand in the cyclic loading regime, taking into account microstructural changes, is presented. The model is subsequently integrated into a constitutive model based on hypoplasticity so as to accurately calculate volume changes induced by fabric and dilatancy changes during cyclic loading. Furthermore, it is assumed that fabric evolution is a function of the ratio of deviatoric to mean principal stress. Some numerical examples that capture the effect of fabric changes on the drained cyclic behavior of sand are presented. Among others, it is found that initial fabric can drastically alter both the dilatancy response and net volume change at shakedown conditions even though the initial void ratio and confining pressure are kept unchanged. The void ratio here is defined as the volume of voids to that of solids.     

Pore Pressure Generation of Silty Sands due to Induced Cyclic Shear Strains

It is well established that the main mechanism for the occurrence of liquefaction under seismic loading conditions is the generation of excess pore water pressure. Most previous research efforts have focused on clean sands, yet sand deposits with fines are more commonly found in nature. Previous laboratory liquefaction studies on the effect of fines on liquefaction susceptibility have not yet reached a consensus. This research presents an investigation on the effect of fines content on excess pore water pressure generation in sands and silty sands. Multiple series of strain-controlled cyclic direct simple shear tests were performed to directly measure the excess pore water pressure generation of sands and silty sands at different strain levels. The soil specimens were tested under three different categories: (1) at a constant relative density; (2) at a constant sand skeleton void ratio; and (3) at a constant overall void ratio. The findings from this study were used to develop insight into the behavior of silty sands under undrained cyclic loading conditions. In general, beneficial effects of the fines were observed in the form of a decrease in excess pore water pressure and an increase in the threshold strain. However, pore water pressure appears to increase when enough fines are present to create a sand skeleton void ratio greater than the maximum void ratio of the clean sand.     

Measuring and Modeling Proportion-Dependent Stress-Strain Behavior of EPS-Sand Mixture

A geofoam was produced by blending expanded polystyrene (EPS) beads and sands in proportions. The formed mixtures, known as EPS-sands, were 26–63% lighter than general earth fills (e.g., sand). Consolidated-drained (CD) triaxial compression tests were conducted on EPS-sand mixture specimens to observe their stress-strain characteristics, specifically, the stress-strain responses in relation to the EPS contents (0.5, 1.5, and 2.5% by weight) used in the mixtures and confining pressures (100, 200, 300 to 400 kPa) loaded on the specimens. The EPS content and confining pressure were found to influence the stress-strain and volumetric strain behavior of the mixtures. Increasing EPS content led to decreased shear strength and increased volumetric strain. Increasing confining pressures enhanced the strength of the mixture. EPS-sand mixtures underwent a shear contraction throughout the CD tests. The optimum EPS bead content (i.e., the one reasonably balancing the unit weight, strength, and deformation) was in the order of 0.5% by weight. EPS content dependent strain increment equations were derived by compromising Cam-clay and modified Cam-clay, and used to model the stress-strain characteristics of EPS-sand mixtures. The established equations were verified being able to depict the stress-strain observations of EPS-sand specimens, at least for the ranges of EPS contents and confinements considered in this study.     

Discrete Element Modeling of Strength Properties of Johnson Space Center (JSC-1A) Lunar Regolith Simulant

This paper simulates the three-dimensional axisymmetric triaxial compression of JSC-1A lunar regolith simulant under lunar and terrestrial gravity environments under a wide range of confining pressures and relative densities. To accomplish this, the discrete element method (DEM), using Particle Flow Code In Three-Dimensional (PFC3D) software, was employed. The paper focuses on the peak and the critical state (CS) friction angles, which were predicted in the ranges of 35.4°–82.7° and 31.2°–79.8°, respectively, depending on the specimen density and confining pressure. A significant increase in peak and CS friction angles was predicted at near-zero confining pressure. The DEM results validated an empirical model that relates the peak friction angle with the CS friction angle, relative density, and mean effective stress at the CS. Comparison of DEM results with lunar in situ measurements of friction angle, from Apollo missions and other extraterrestrial laboratory experiments under a microgravity environment, shows a favorable agreement.     

含孔洞大理岩破坏特性的颗粒流分析

基于室内单轴压缩试验结果,利用颗粒流程序PFC2D,模拟含预制孔洞大理岩在单轴和双轴压缩条件下的破坏过程,分析预制孔洞形状、围压大小以及岩石非均质性对大理岩力学特性和裂纹扩展的影响.数值结果表明:与完整大理岩试样相比,含孔洞试样的峰值强度显著降低,降低程度与孔洞形状有关;围压对含孔洞大理岩试样的力学特性和裂纹扩展有显著影响,含孔洞试样的峰值强度随围压的增加而增加,但偏应力峰值随围压的增加呈先增大后减小的变化趋势;试样的破坏模式与孔洞形状相关,含圆形孔洞试样为类X型剪切破坏,含矩形孔洞或马蹄形孔洞试样为对角剪切破坏;岩石内部的矿物结核影响了裂纹的扩展路径,从而改变试样的宏观破坏模式.微观机理分析表明:孔洞周边裂纹的萌生与扩展过程伴随着应力集中区的释放与转移;含孔洞试样的宏观裂纹有3种模式:孔壁剥落、拉伸裂纹和压剪裂纹.      

Interaction of Foundry Sands with Geosynthetics

Excess foundry sands from gray-iron casting are a mixture of sand, bentonite, and additives that can have properties desirable for structural fills and hydraulic barriers, depending on their bentonite content. To facilitate beneficial reuse of foundry sands, typical strength parameters need to be available so that designers can make comparisons with designs employing virgin earthen materials. To provide typical design parameters, a testing program was conducted to characterize the strength of foundry sands and their interaction with geosynthetics. Small-scale direct shear tests, large-scale multistage interface shear tests, and pullout tests were conducted using foundry sands with bentonite contents representing the range normally found in the casting industry and three geosynthetics (geotextile, geogrid, and geomembrane). The results indicate that foundry sands can be used effectively in geotechnical construction. Friction angles of the as-compacted foundry sands generally ranged between 39° and 43°, and the as-compacted cohesions ranged between 17 and 28 kPa. Drained friction angles were similar to as-compacted friction angles except at high bentonite content. Typical interface friction angles ranged between 25° and 35°, with efficiencies ranging between 0.5 and 0.9. Interaction coefficients from the pullout tests ranged between 0.2 and 1.7.     

围压下混凝土动态损伤与能量耗散特征数值分析

为了研究围压对混凝土材料冲击破坏过程中损伤演化和能量耗散的影响,基于ANSYS/LS-DYNA有限元软件,模拟了不同冲击速度和围压级别下的混凝土SHPB实验. 结果表明:在不同的冲击速度下,峰值应力均随着围压的提高线性增大,最大可以达到混凝土静态抗压强度的3~4倍. 围压条件下混凝土的破坏形式为压剪破坏,试件的平均损伤度随围压的增大而非线性降低,相较于冲击速度,围压对损伤度的影响更弱. 随着围压的提高,混凝土对应力波的透射能力增强,反射波的能量非线性降低,而透射能随着围压的增大近似线性增加,混凝土的耗能随着围压的增大而近似线性降低;在不同的入射能下,反射系数、透射系数和试件耗能的变化趋势是一致的. 围压一定时,混凝土的损伤程度随入射能的增加线性增长;入射能一定时,试件损伤度随围压的增加而降低,变化幅度也降低.      

Monotonic and Cyclic Liquefaction of Very Loose Sands with High Silt Content

The results from an experimental study on sands with high nonplastic silt content are presented. Drained and undrained triaxial compression tests, undrained cyclic triaxial tests, and drained∕undrained instability tests were performed on specimens of loose Nevada sand with 40% silt content. The behavior was observed to be somewhat different from previously published tests with sands at lower silt content. The greater silt content appears to provide a more volumetrically contractive response throughout the entire stress-strain curve. However, some aspects of the response were similar to sands with lower silt content. Monotonic undrained tests indicated “reverse” behavior, i.e., static liquefaction occurred at low confining pressures and increasing dilatant volume-change tendency was observed with increasing confining pressure. Analyzing the results using the concepts of steady state resulted in a unique steady-state line only when undrained tests were sheared from the same isotropic compression line. When specimens of different initial densities were tested at the same initial confining pressures, the resulting steady-state points did not fall on the same steady-state line.     

Strain Rate, Creep, and Stress Drop-Creep Experiments on Crushed Coral Sand

The part of sand behavior that is affected by time, such as creep, relaxation, and loading rate effects are not similar to those observed for clay. To throw more light on the time effects in sand, many series of drained triaxial compression experiments have been performed on crushed coral sand. These tests were all performed with a constant effective confining pressure of 200?kPa. The test series included experiments with specimens loaded at five different strain rates with a 256-fold ratio between the extreme rates, tests with sudden changes in strain rate from slow to fast and vice versa, and tests in which axial and volumetric creep strains were observed at stress differences of 500, 700, and 900?kPa. Creep creates structuration and this has to be overcome to produce further plastic straining. Experiments were also performed in which the stress difference was dropped quickly from three different values of 500, 700, and 900?kPa followed by creep. In these stress drop-creep tests five different magnitudes of stress drops were employed: 0, 100, 200, 300, and 400?kPa. The results involving conventional creep effects and stress drop-creep effects are presented and analyzed.     

Liquefaction and Undrained Response Evaluation of Sands from Drained Formulation

A general approach has been established to assess the undrained stress-strain curve and effective stress path under monotonic loading from drained triaxial tests. An appropriate formulation of a drained and drained rebounded (i.e., overconsolidated) triaxial test response is developed that, in turn, allows the assessment of developing liquefaction and the undrained behavior of saturated sands. The formulation presented is based upon reported experimental drained test results that were obtained from different investigators using different testing techniques. This formulation is a function of the confining pressure and basic properties of the sand, such as relative density, uniformity coefficient, and particle shape (roundness), which can be obtained from visual inspection. The approach is verified by comparing predicted and reported (observed) undrained behavior. The developed formulas allow one to predict the potential of sand to liquefy, the type of liquefaction, the peak and residual strength values, as well as the whole undrained stress-strain curve and effective stress path. The simplicity of this approach makes it an attractive general method to characterize the undrained behavior of sands in a preliminary analysis with no need to run sophisticated experimental tests.