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.     

Shear Strength and Stiffness of Silty Sand

The properties of clean sands pertaining to shear strength and stiffness have been studied extensively. However, natural sands generally contain significant amounts of silt and∕or clay. The mechanical response of such soils is different from that of clean sands. This paper addresses the effects of nonplastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5–20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.     

Shear Band Formation in Plane Strain Experiments of Sand

A series of biaxial (plane strain) experiments were conducted on three sands under low (15 kPa) and high (100 kPa) confining pressure conditions to investigate the effects of specimen density, confining pressure, and sand grain size and shape on the constitutive and stability behavior of granular materials. The three sands used in the experiments were fine-, medium-, and coarse-grained uniform silica sands with rounded, subangular, and angular grains, respectively. Specimen deformation was readily monitored and analyzed with the help of a grid pattern imprinted on the latex membrane. The overall stress-strain behavior is strongly dependent on the specimen density, confining pressure, sand grain texture, and the resulting failure mode(s). That became evident in different degrees of softening responses at various axial strains. The relationship between the constitutive behavior and the specimens' modes of instability is presented. The failure in all specimens was characterized by two distinct and opposite shear bands. It was found that the measured dilatancy angles increase as the sand grains' angularities and sizes increase. The measured shear band inclination angles are also presented and compared with classical Coulomb and Roscoe solutions.     

Cyclic Critical Stress States of Sand with Nonfrictional Effects

New observations of experimental facts are made in this study to understand the physical essentials regarding the changes in mobilized maximum stress and phase-transformation stress states of saturated sands subjected to cyclic undrained shear applications. Saturated sands are not purely frictional materials that are governed only by frictional law (i.e., the shear-normal stress ratio). They are also characterized by the following effects of nonfrictional behavior: (1) Irreversible dilatancy effect; (2) viscous effect at large strain rate; and (3) coupling effect of viscous to frictional resistance. It was found that the first effect reduces the limiting shear resistance, whereas the second and third effects increase it. Based on this finding, 2D and 3D criteria that consider both the frictional and nonfrictional effects are developed by introducing several new concepts such as the “true effective stress,” “moving stress space,” and “moving spatially mobilized plane.” Their effectiveness is confirmed experimentally.     

Sydney Soil Model. II: Experimental Validation

This paper presents simulations of the mechanical behavior of reconstituted and natural soils using a new model presented in a companion paper and referred to as the “Sydney soil model.” It is demonstrated that the performance of the proposed model is essentially the same as that of modified Cam clay model when describing the behavior of clays in laboratory reconstituted states. The model has also been employed to simulate the drained and undrained behavior of structured clays and sands, including calcareous clay and sand. Five sets of conventional triaxial tests and one set of true triaxial tests have been considered. It is demonstrated that the new model provides satisfactory qualitative and quantitative modeling of many important features of the behavior of structured soils, particularly in capturing various patterns of the stress and strain behavior associated with soil type and structure. A general discussion of the model parameters is also included. It is concluded that the Sydney soil model is suitable for representing the behavior of many soils if their ultimate state during shearing can be defined by an intrinsic and constant stress ratio M* and a unique relationship between mean effective stress and voids ratio, i.e., a unique p′-e curve.     

Factors Affecting the Creep Behavior of Grouted Sand

The creep behavior of Fontainebleau sand grouted with a chemically unstable silicate grout was compared with the creep behavior of the same sand grouted with two chemically stable grouts, a microfine cement-based grout and a mineral-based grout. Unconfined and confined creep experiments were performed to highlight drained creep behavior. The duration of the tests was between 100 and 200 days. Results were compared in terms of creep slope, and creep limit strength. Test results have shown that the method with which sand was prepared plays an important part in creep strains of grouted sand. Creep strains were considerably reduced by applying a confining stress. The creep behavior of sands was modified by the grouting treatment: depending on the nature of the grout, the grouted sand has exhibited creep strains of different degrees. The creep slopes of grouted sand were proportional to the creep slopes of pure grout. Creep characteristics of grouted sand may also be deduced from the creep behavior of pure sand and the creep behavior of pure grout.     

Geotechnical Properties of Cemented Sands in Steep Slopes

An investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented. A case study from eroding coastal cliffs located in central California provides both the data and impetus for this study. Herein, weakly cemented sand is defined as having an unconfined compressive strength (UCS) of less than 100 kPa, and moderately cemented sand is defined as having UCS between 100 and 400 kPa. Testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear Mohr-Coulomb strength parameters, although for weakly cemented sands, curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement. Triaxial tests performed to simulate the evolving stress state of an eroding cliff, using a reduction in confinement-type stress path, result in an order of magnitude decrease in strain at failure and a more brittle response. Tests aimed at examining the influence of wetting on steep slopes show that a 60% decrease in UCS, a 50% drop in cohesion, and 80% decrease in the tensile strength occurs in moderately cemented sand upon introduction to water. In weakly cemented sands, all compressive, cohesive, and tensile strength is lost upon wetting and saturation. The results indicate that particular attention must be given to the relative level of cementation, the effects of groundwater or surficial seepage, and the small-scale strain response when performing geotechnical slope stability analyses on these materials.     

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.     

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.     

Influence of Stress Ratio and Stress Path on Behavior of Loose Decomposed Granite

This paper presents results from four series of triaxial compression tests of loosely compacted decomposed granite (DG) or silty sand on both isotropically and anisotropically consolidated specimens. These tests included undrained tests, drained tests with constant deviator stress, and a decreasing mean effective stress path. The silty sand possessed high compressibility during isotropic compression. The observed high compressibility is probably attributed to the loose soil structure created by using the moist tamping method and the presence of crushable feldspar in the soil. Static liquefaction behavior and the so-called “reversed” sand behavior were observed in all undrained tests. This “reversed” sand behavior can be readily explained by the high compressibility of DG leading to the nonparallel and converging nature of the initial state line and the critical state line. Preshearing resulted in a more brittle response in the postpeak behavior. The higher the initial stress ratio (ηc), the smaller the ductility. Structural collapse of DG was observed. This collapse is characterized by a sudden large increase in both the axial and contractive volumetric strains. The mobilized angles of friction at collapse range from 31.8° to 38.7°, which are smaller than the critical state angle (?col′), but higher than the mobilized friction angle of the instability line (28.1°) determined by the isotropically consolidated undrained tests. A trilinear approximate relationship can be found between ?col′ and ηc and a liquefaction potential index is introduced to provide a simple preliminary design parameter for static liquefaction and instability prone slopes.     

Double Strain Softening and Diagonally Crossing Shear Bands of Sand in Drained Triaxial Tests

A comprehensive understanding of the shear behavior of sand in the context of shear band development has not been achieved yet in spite of many detailed research works on each specified subject. In order to observe the entire drained shear behavior of Toyoura sand from the macromechanical point of view, conventional triaxial tests were performed and analyzed up to an axial strain of 30% for various void ratios, initial confining stresses, and stress paths, paying particular attention to volume changes. The strong correlation was found between “double strain softening” and “diagonally crossing shear bands” as a remarkable result. Finally, a qualitative explanation of relations among the stress–strain curve, the failure shape, the dilatancy index–strain curve and the strain localization, could be clearly made. Also, it is concluded that the dilatancy index is an indicator not only of the ratio of the volumetric strain increment to the axial strain increment but also the condition of the strain localization.     

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.     

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.     

Effect of Soil Permeability on Centrifuge Modeling of Pile Response to Lateral Spreading

This paper presents experimental results and analysis of six model centrifuge experiments conducted on the 150?g-ton Rensselaer Polytechnic Institute centrifuge to investigate the effect of soil permeability on the response of end-bearing single piles and pile groups subjected to lateral spreading. The models were tested in a laminar box and simulate a mild infinite slope with a liquefiable sand layer on top of a nonliquefiable layer. Three fine sand models consisting of a single pile, a 3×1 pile group, and a 2×2 pile group were tested, first using water as pore fluid, and then repeated using a viscous pore fluid, hence simulating two sands of different permeability in the field. The results were dramatically different, with the three tests simulating a low permeability soil developing 3–6 times larger pile head displacements and bending moments at the end of shaking. Deformation observations of colored sand strips, as well as measurements of sustained negative excess pore pressures near the foundations in the “viscous fluid” experiments, indicated that an approximately inverted conical zone of nonliquefied soil had formed in these tests at shallow depths around the foundation, which forced the liquefied soil in the free field to apply its lateral pressure against a much larger effective foundation area. Additional p-y and limit equilibrium back-analyses support the hypothesis that the greatly increased foundation bending response observed when the soil is less pervious is due to the formation of such inverted conical volume of nonliquefied sand. This study provides evidence of the importance of soil permeability on pile foundations response during lateral spreading for cases when the liquefied deposit reaches the ground surface, and suggests that bending response may be greater in silty sands than in clean sands in the field. Moreover, the observations in this study may serve as basis for realistic practical engineering methods to evaluate pile foundations subjected to lateral spreading and pressure of liquefied soil.     

Liquefaction Resistance of Sandy Soils under Partially Drained Condition

In order to simulate the effect of drainage on soils adjacent to gravel drains that are installed as countermeasure against liquefaction, several series of cyclic triaxial tests were performed on saturated sands under partially drained conditions. The condition of partial drainage under cyclic loading was simulated in the laboratory using triaxial testing equipment installed with a drainage control valve to precisely regulate the volume of water being drained from test specimens. Effects of both drainage conditions and loading frequencies on cyclic response were incorporated through the coefficient of drainage effect, α*. Experimental results showed that for sand exhibiting strain softening, the partially drained response was controlled by the critical effective stress ratio while for sand showing strain hardening behavior, the controlling factor was the phase transformation stress ratio. Moreover, test results indicated that the minimum liquefaction resistance under partially drained conditions can be used as a parameter to describe the liquefaction resistance of sands improved by the gravel drain method. From these results, a simplified procedure for designing gravel drains based on the factor of safety (FL) concept was proposed.     

Time Effects Relate to Crushing in Sand

Based on previously obtained experimental results, a mechanistic picture of time effects in granular materials is presented. Accordingly, time effects are caused by grain crushing, which in turn is time dependent, as indicated by static fatigue of brittle materials. Triaxial compression tests have been performed on Virginia Beach sand at high pressures, where grain crushing is prevalent, to study effects of initial loading strain rates on subsequent amounts of creep and stress relaxation. Grain size distribution curves were determined after each test and the amount of crushing, as characterized by Hardin’s breakage factor, is related to the energy input to the triaxial specimens. A pattern emerges that indicates the importance of crushing for the axial and volumetric strains, while rearrangement and frictional sliding between intact grains play much smaller roles in the stress-strain and volume change behaviors of granular materials at high stresses and shear strains. Because particle crushing is a time-dependent phenomenon described as static fatigue or delayed fracture, the close relation between time effects and crushing in granular materials is established.     

Piezocone Penetration Testing in Atlantic Piedmont Residuum

Piezocone soundings were performed in residual silts and fine sands of the U.S. Atlantic Piedmont geologic province to investigate penetration pore-water pressures and subsequent dissipation characteristics in these “nontextbook” geomaterials. Piedmont residuum has been formed by the in-place weathering of Paleozoic schist, gneiss, and granite. Pore-water pressures were measured at two positions: midface of the cone tip u1 and at the shoulder or behind the tip u2. At the standard rate of 2 cm∕s, penetration pore-water pressures on the midface element are fairly high positive values (u1 = +600 kPa ± 200 kPa), yet readings at the shoulder element were negative (u2 ≈ ?90+ kPa). At an increased penetration rate of 20 cm∕s, midface and shoulder readings increased in magnitude and were positive (u1 ≈ +800 kPa and u2 ≈ +200 kPa); however, measured tip resistance qc exhibited an unusual decrease, per comparisons with data available in the literature. Upon halt of penetration, both u1 and u2 decayed quickly and reached hydrostatic values u0 within 1–2 min.     

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.     

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).