Influence of Nonplastic Fines on Shear Wave Velocity-Based Assessment of Liquefaction

Many false positives (no liquefaction detected when the normalized shear wave velocity-cyclic stress ratio (Vs1-CSR) combination indicated that it should have been) are observed in the database used in the simplified liquefaction assessment procedure based on shear wave velocity. Two possible reasons for false positives are the presence of a thick surface layer of nonliquefiable soil and the effects of fines on cyclic shear resistance (CRR) and Vs1. About 67% of the false positives that could not have been caused by an overlying thick surface layer are associated with silty sands with less than 35% fines. The effects of fines on the liquefaction resistance of silty sands and on the shear wave velocity are analyzed. Theoretical CRRfield?versus?Vs1 curves for silty sands containing 0 to 15% nonplastic fines are established. They show that the theoretical CRR-Vs1 correlations for silty sands with 5 to 15% nonplastic fines are all located to the far left of the semi-empirical curves that separate liquefaction from no-liquefaction zones in the simplified liquefaction potential assessment procedures. The results suggest the currently used shear wave velocity-based liquefaction potential curves may be overly conservative when applied to sands containing nonplastic fines.     

Drained and Undrained Strength Interpretation for Low-Plasticity Silts

The engineering behavior of low-plasticity silts is more difficult to characterize than is the behavior of clay or sand. Due to their tendency to dilate during shear, establishing a consistent and practically useful failure criterion for low-plasticity silts can be very difficult. Consideration of how the undrained shear strength of silt is related to changes in pore pressure provides a more useful and practical framework for understanding the undrained strengths of these materials and for characterizing undrained strengths for practical purposes. Using a value of Skempton’s as a failure criterion has been found to result in very reasonable values of undrained strength and to reduce scatter in the results as compared to using other criteria. Using a failure criterion based on an appropriate value of results in consistent values of Su/p, and tolerably small values of strain at failure. For low-plasticity, dilative silts that pose the greatest problems with respect to definition of “failure,” using = 0 as a failure criterion is an appropriate and simple choice.     

Shear Strength of Fiber-Reinforced Sands

Soil reinforcement using discrete randomly distributed fibers has been widely investigated over the last 30 years. Several models were suggested to estimate the improvement brought by fibers to the shear strength of soils. The objectives of this paper are to (1) supplement the data available in the literature on the behavior of fiber-reinforced sands; (2) study the effect of several parameters which are known to affect the shear strength of fiber-reinforced sands; and (3) investigate the effectiveness of current models in predicting the improvement in shear strength of fiber-reinforced sand. An extensive direct shear testing program was implemented using coarse and fine sands tested with three types of fibers. Results indicate the existence of a fiber-grain scale effect which is not catered for in current prediction models. A comparison between measured and predicted shear strengths indicates that the energy dissipation model is effective in predicting the shear strength of fiber-reinforced specimens in reference to the tests conducted in this study. On the other hand, the effectiveness of the predictions of the discrete model is affected by the parameters of the model, which may depend on the test setup and the procedure used for mixing the fibers.     

State-Dependent Strength of Sands from the Perspective of Unified Modeling

This paper discusses the state-dependent strength of sands from the perspective of unified modeling in triaxial stress space. The modeling accounts for the dependence of dilatancy on the material internal state during the deformation history and thus has the capability of describing the behavior of a sand with different densities and stress levels in a unified way. Analyses are made for the Toyoura sand whose behavior has been well documented by laboratory tests and meanwhile comparisons with experimental observations on other sands are presented. It is shown that the influence of density and stress level on the strength of sands can be combined through the state-dependent dilatancy such that both the peak friction angle and maximum dilation angle are well correlated with a so-called state parameter. A unique, linear relationship is suggested between the peak friction angle and the maximum dilation angle for a wide range of densities and stress levels. The relationship, which is found to be in good agreement with recent experimental findings on a different sand, implies that the excess angle of shearing due to dilatancy in triaxial conditions is less than 40% of that in plane strain conditions. A careful identification of the deficiency of the classical Rowe’s and Cam-clay’s stress–dilatancy relations reveals that the unique relationship between the stress ratio and dilatancy assumed in both relations does not exist and thereby obstructs unified modeling of the sand behavior over a full range of densities and stress levels.     

Dilatancy and Shear Strength of Sand at Low Confining Pressures

Sand dilates with shearing at a rate that increases with increasing relative density (DR) and decreases with increasing effective confining stress (σc′). The peak friction angle of a sand depends on its critical-state friction angle and on dilatancy. In this paper, we develop a simple correlation between peak friction angle, critical-state friction angle, and dilatancy based on triaxial compression and plane-strain compression test data for sand for a range of confining pressures from very low levels to approximately 196 kPa.     

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.     

Postcyclic Recompression, Stiffness, and Consolidated Cyclic Strength of Silt

Low plasticity silts are liquefiable and the dissipation of pore pressures after an earthquake will be accompanied by densification and compression of the soil skeleton. Anisotropic rather than isotropic stress distributions are commonly found to exist in slopes or silty fills placed under K0 conditions and this can be enhanced further by the weight of overlying structures. Compression after an earthquake generally increases soil resistance but it can still be liquefied by aftershocks. The postcyclic recompression of silt, and postdrainage monotonic and cyclic strength and stiffness have therefore been investigated with respect to the effect of initial anisotropic consolidation. The compressibilities during postcyclic recompression were similar to those for isotropic consolidation. Samples with a greater initial anisotropy had less volumetric strain but larger axial strains during postcyclic drainage. Under stress reversal conditions failure occurred as a result of the development of double amplitude cyclic strains, whereas under nonreversal conditions compressive axial plastic strain was accumulated. Postdrainage second loading cyclic strength increased with increasing anisotropy. For isotropically consolidated samples failure under reversal cyclic loading resulted in a weaker soil structure even after postcyclic reconsolidation.     

Correlation between Cyclic Resistance Ratios of Intact and Reconstituted Offshore Saturated Sands and Silts with the Same Shear Wave Velocity

The cyclic liquefaction resistance of intact medium dense specimens of sands and silts obtained from offshore platform sites was compared to that of specimens reconstituted to the same values of shear wave velocity. The shear wave velocity was measured using a new system that is comprised of torsional piezoelectric ceramic ring transducers mounted in a triaxial cell, a multiwave measuring device, and special watertight connectors. The relationship between cyclic resistance ratio and the number of cycles to liquefaction Nf of intact and reconstituted specimens was compared at the same values of consolidation pressure and shear wave velocity. There was good agreement between cyclic resistance ratios of intact and reconstituted specimens with similar values of shear wave velocity if liquefaction is defined as ? 6% peak-to-peak axial strain. The results of this study support the hypothesis that the cyclic liquefaction resistance of reconstituted specimens may be restored to in situ conditions when their shear wave velocity is restored to in situ values.     

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.     

Influence of Mineral Composition on the Simple Shear Response of Sands: Experimental Study

Experiments were conducted to evaluate the role of mineral composition on the direct simple shear response (i.e., volume change during shear) of uniform sands composed of “strong” quartz grains and “weak” calcareous grains. The dry specimens were loaded in a custom-made direct simple shear apparatus and separated after testing with respect to grain size by sieving and with respect to mineral composition by dissolving the calcareous grains retained on each sieve. The results indicate that the presence of weak grains affects the direct simple shear response of predominately strong-grained sands: at low stresses, the response is controlled by particle shape; at high stresses, the response is controlled by weak particle breakage.     

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.     

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.     

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.     

Influence of Shear Rate on Undrained Vane Shear Strength of Organic Harbor Mud

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors μ are proposed using the test results and the observed time to failure in the harbor basin. The proposed correction leads to significant lower design undrained shear strengths than the classical Bjerrum correction and would have predicted the failure during the construction.     

Undrained Shear Strength of Pleistocene Clay in Osaka Bay

This study presents the undrained shear characteristics of Holocene and Pleistocene clay samples from depths of 20–200 m under the seabed in Osaka Bay. Automated triaxial K0 consolidation tests and anisotropically consolidated-undrained triaxial compression and extension tests are conducted using the recompression method. The average undrained strength ratio (su/σv0′) is 0.33 (SD = 0.03) when the extension strength is defined as the peak strength or the strength at an axial strain of 15%, while su/σv0′ is 0.29 (SD = 0.04) when the extension strength is defined as the shear stress at the axial strain corresponding to the peak compression strength. Circular arc stability analyses are carried out with the modified Fellenius and Bishop methods for the design cross section of the seawall structure of the Kansai International Airport to study the effects of different definitions of shear strength. The seawall is founded on 19 m of soft Holocene clay and 10 m of Pleistocene sand overlying the Pleistocene clay. The stability analyses show that the factor of safety and depth of the critical circle (i.e., above versus below the sand layer) are sharply affected by both the value of su/σv0′ (0.33 versus 0.29) and the method of slices (Fellenius versus Bishop). The marginal stability calls for careful monitoring of construction with field instrumentation.     

Particle Shape Effects on Packing Density, Stiffness, and Strength: Natural and Crushed Sands

The size and shape of soil particles reflect the formation history of the grains. In turn, the macroscale behavior of the soil mass results from particle level interactions which are affected by particle shape. Sphericity, roundness, and smoothness characterize different scales associated with particle shape. New experimental data and results from published studies are gathered into two databases to explore the effects of particle shape on packing density and on the small-to-large strain mechanical properties of sandy soils. In agreement with previous studies, these data confirm that increased angularity or eccentricity produces an increase in emax and emin. Furthermore, the data show that increasing particle irregularity causes a decrease in stiffness yet heightened sensitivity to the state of stress; an increase in compressibility under zero-lateral strain loading; an increase in the critical state friction angle ?cs; and an increase in the intercept Γ of the critical state line (there is a weak effect on the slope λ). Therefore, particle shape emerges as a significant soil index property that needs to be properly characterized and documented, particularly in clean sands and gravels. The systematic assessment of particle shape will lead to a better understanding of sand behavior.     

Plastic Limit, Liquid Limit and Undrained Shear Strength of Soil—Reappraisal

The concept that plasticity index of soils can be defined as a range of water contents producing a 100-fold variation in undrained shear strength has been experimentally verified with the help of a large number of tests on soils of diverse nature. This has led to the redefinition of the plastic limit as the water content at which undrained shear strength is around 170 kN/m2. Undrained shear strength of a soil at the liquid limit can be considered to be around 1.7 kN/m2. Accordingly, both the liquid limit and the plastic limit have been determined in the present work by a single consistent method, i.e., the Swedish fall cone method. The undrained shear strength-water content relationship has been found to be log-linear for a wide range of water contents beginning from lower than the plastic limit to higher than the liquid limit. This resulted in the formulation of an expression for predicting undrained shear strength of a remolded soil at any water content based solely on its liquid limit and plastic limit.     

Relationship between the Undrained Shear Strength, Water Content, and Mineralogical Properties of Fine-Grained Soils

The relationship between the undrained shear strength of fine-grained soils and the water content can be described with a nonlinear function in which the type of soil is determined by two parameters. It is well known that these parameters depend mainly on the mineral compositions of soils; these relationships, however, have not yet been investigated. The findings described in this paper define those mineralogical properties of soils which determine the values of both parameters. Experimentally obtained results suggest that the parameters primarily depend on the size of the clay minerals, their quantity in soil composition, and the interlayer water quantity in the expanding clay minerals. As this dependence is well defined, the parameters, and thus the undrained shear strength at different water content, can be defined from knowledge of these mineralogical soil properties.     

Comparison in Undrained Shear Strength between Undisturbed and Remolded Ariake Clays

Triaxial consolidation undrained shear tests are performed on both undisturbed and remolded Ariake clays to investigate the undrained shear strength behavior. When the applied confining stress is larger than the triaxial consolidation yield stress, the strength envelopes expressed in the plot of undrained shear strength versus confining stress of both the undisturbed and the remolded Ariake clays are straight lines through the origin. The strength envelope of the remolded Ariake clays lies above that of the undisturbed Ariake clays when the applied confining stress is larger than the consolidation yield stress. This difference is caused by the difference in water content between undisturbed and remolded states. When the data obtained from triaxial consolidation undrained shear tests of both the undisturbed and the remolded Ariake clays are plotted in the plot of undrained shear strength versus water content, it is found that the undrained shear strength decreases uniquely with the increase in water content.     

Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress Paths

The results of drained triaxial tests on fiber reinforced and nonreinforced sand (Osorio sand) specimens are presented in this work, considering effective stresses varying from 20 to 680?kPa and a variety of stress paths. The tests on nonreinforced samples yielded effective strength envelopes that were approximately linear and defined by a friction angle of 32.5° for the Osorio sand, with a cohesion intercept of zero. The failure envelope for sand when reinforced with fibers was distinctly nonlinear, with a well-defined kink point, so that it could be approximated by a bilinear envelope. The failure envelope of the fiber-reinforced sand was found to be independent of the stress path followed by the triaxial tests. The strength parameters for the lower-pressure part of the failure envelope, where failure is governed by both fiber stretching and slippage, were, respectively, a cohesion intercept of about 15?kPa and friction angle of 48.6?deg. The higher-pressure part of the failure envelope, governed by tensile yielding or stretching of the fibers, had a cohesion intercept of 124?kPa, and friction angle of 34.6?deg. No fiber breakage was measured and only fiber extension was observed. It is, therefore, believed that the fibers did not break because they are highly extensible, with a fiber strain at failure of 80%, and the necessary strain to cause fiber breakage was not reached under triaxial conditions at these stress and strain levels.