Shear Strength and Pore-Water Pressure Characteristics during Constant Water Content Triaxial Tests

Shear strength parameters used in geotechnical design are obtained mainly from the consolidated drained (CD) or consolidated undrained (CU) triaxial tests. However in many field situations, soils are compacted for construction purposes and may not follow the stress paths in CD or CU triaxial tests. In these cases, the excess pore-air pressure during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. Under this condition, it can be considered that the air phase is drained and the water phase is undrained. This condition can be simulated in a constant water content (CW) triaxial test. The purpose of this paper is to present the characteristics of the shear strength, volume change, and pore-water pressure of a compacted silt during shearing under the constant water content condition. A series of CW triaxial tests was carried out on statically compacted silt specimens. The experimental results showed that initial matric suction and net confining stress play an important role in affecting the characteristics of the shear strength, pore-water pressure, and volume change of a compacted soil during shearing under the constant water content condition. The failure envelope of the compacted silt exhibited nonlinearity with respect to matric suction.     

Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.     

Laboratory Studies of Two Common Saprolitic Soils in Hong Kong

Two commonly encountered saprolitic soils in Hong Kong, weathered volcanic tuff (WT) and weathered granite (WG), were studied using high-quality intact samples. The intact samples exhibited quasi-preconsolidation pressure or yield stress under isotropic compression due to their bonded structures, but the yield was progressive and not abrupt. As the stress increased, significant volumetric changes were measured. These changes resembled clay-type behavior. The soils also exhibited anisotropic deformation under isotropic loading and unloading, which was associated with the features of their parent rocks. During the drained tests, shearing at the in situ stress-state produced peak strength and volumetric dilation. Undrained shearing showed complicated stress paths and dilatancy behavior in these soils. Phase transformation states and dilative shear failure were readily seen, which resembles typical sand-type behavior. Distinct shear band(s) appeared in the WT specimens during shearing, whereas a bulging type of failure appeared in the WG specimens. The soils ultimately approached the corresponding state guided by a unique critical state line, regardless of their complex initial states in relation to the bonded structure and drainage conditions.     

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.     

Undrained Fragility of Clean Sands, Silty Sands, and Sandy Silts

In this paper, intergranular (ec) and interfine (ef) void ratios and confining stress are used as indices to characterize the stress–strain response of gap graded granular mixes. It was found that at the same global void ratio (e) and confining stress, the collapse potential (fragility) of silty sand increases with an increase in fines content (FC) due to a reduction in intergranular contact between the coarse grains. Beyond a certain threshold fines content (FCth), with further addition of fines, the interfine contact friction becomes significant. The fragility decreases and the soil becomes stronger. The value of FCth depends on e and the characteristics of fines and coarse grains. At FCFCth), fine grain friction plays a primary role and dispersed coarse grains provide a beneficial, secondary reinforcement effect. At the same ef, the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by ef only. An intergranular matrix diagram is presented that delineates zones of different behaviors of granular mixes as a guideline to determine the anticipated behavior of gap-graded granular mixes. New equivalent intergranular contact void ratios, (ec)eq and (ef)eq, are introduced to characterize the behavior of such soils, at FCFCth, respectively.     

Effects of Silt Content and Void Ratio on the Saturated Hydraulic Conductivity and Compressibility of Sand-Silt Mixtures

The hydraulic conductivity, the coefficient of consolidation, and the coefficient of volume compressibility play major roles on the pore pressure generation during undrained and partially drained loading of granular soils with fines. This paper aims to determine how much these soil parameters are affected by the percentage of fines and void ratio of the soil. The results of a large number of flexible wall permeameter tests performed on 60 specimens of two poorly graded sands with 0, 5, 10, 15, 20, and 25% nonplastic silt are presented and discussed. Hydraulic conductivity measurements were done at effective confining stresses of 50–300 kPa. The evaluation of the data shows that the hydraulic conductivity and the coefficient of consolidation of sands with 25% silt content are approximately two orders of magnitude smaller than those of clean sands. The coefficient of volume compressibility of the sand-silt mixtures is affected in a lesser degree by void ratio, silt content, and confining stress. The influence of the degree of saturation on the laboratory-measured k values is also discussed.     

Effects of Consolidation History on Critical State of Sand

A series of drained biaxial compression tests were conducted on two sands to determine the effects of consolidation history on their critical states. Specimens of each sand were consolidated along at least two separate paths in void ratio-effective stress space, creating several unique consolidation histories. Because the sands were dilative, strains localized during shearing and the evolution to critical state occurred only within the shear band. Digital images were obtained through a plexiglass sidewall throughout each test. Digital image correlation techniques were used to quantify the displacements within the band, and a linear regression technique was used to formulate a displacement function from which strains were computed. The critical state was achieved within the shear band in each test, but the critical state line was found to depend on the initial state and subsequent consolidation history of the sand specimens.     

Interpretation of Pressuremeter Tests in Sand using Advanced Soil Model

This paper describes a numerical study of drained pressuremeter tests in sand using a one-dimensional finite-element method in conjunction with an advanced soil model MIT-S1, and input parameters corresponding to Toyoura sand. This soil model is capable of describing realistically the transitions in peak shear strength parameters of cohesionless soils that occur due to changes void ratio and confining pressure. The predicted peak shear strengths can be normalized, at least approximately, by introducing a state parameter that references the initial (preshear) void ratio to the value occurring at large strain critical state conditions at the same mean effective stress. The numerical analyses idealize the pressuremeter test as the expansion of a cylindrical cavity and ignore disturbance effects caused by probe insertion. This idealization is relevant to self-boring pressuremeter tests. Results confirm that there is a linear correlation between the in situ (i.e., preshear) state parameter of the soil and the gradient of the log pressure-cavity strain expansion, as first suggested by Yu in 1994 using a much simpler soil model. Indeed, the linear coefficients derived for Toyoura sand differ only slightly from those obtained previously by Yu for six other sands.     

Constitutive Model of Soil Based on a Dynamical Systems Approach

A soil when sheared ultimately reaches a steady-state condition at which it deforms at a constant shear stress, effective normal stress, and void ratio. Various systems in nature dynamically evolve similarly from some initial condition, to a final steady-state condition. Such systems have been studied using dynamical systems theory. This technical note uses this theory to model monotonic shear of soil as a dynamical system. The principle proposed is simple—the rates of change of the shear stress, effective normal stress, and void ratio are proportional to the applied values of the shear and effective normal stress with the proportionality values decaying with strain until ultimately these proportionality values become zero at the steady-state condition. It provides a well-formed qualitative principle that fits closely the stress-strain-void ratio curves of undrained shear tests on uncemented, resedimented clays at various over consolidated ratios (OCRs), and drained shear tests on sands and silts at various relative densities, for various stress paths including compression, extension from standard triaxial, and true-triaxial tests. For the undrained shear of resedimented clay, these proportionalities and their decay rates vary smoothly with OCR. For drained shear of sand and silt, the model parameters show orderly variation with relative density. Its value lies in that a well-formed qualitative principle derived from the steady-state condition provides an alternate approach to current complex elastoplastic models based on critical state theory.     

State Pressure Index for Modeling Sand Behavior

The effort to model sand behavior within the framework of critical-state soil mechanics would benefit from a state variable that relates the current void ratio and mean pressure of the soil to its void ratio and mean pressure at the critical state. In this paper we propose a state pressure index, Ip, which is defined as the ratio of the current mean pressure to the mean pressure at the critical state that corresponds to the current void ratio. Using this state pressure index, a bounding surface hypoplasticity model for sand is modified so that the phase transformation and failure stress ratios both depend on Ip and merge into the critical-state stress ratio at failure. The Ip dependency introduced enables use of a single set of model constants in modeling sand behavior for various initial confining pressures and densities under both undrained and drained conditions. Dilatancy, strain softening, and strain hardening are simulated for both loose and dense sands. Simulations from the modified model are compared with results of laboratory tests of drained and undrained triaxial compression.     

Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas

The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0–7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly with suction, though linearization can be reasonably assumed for suction below around 30 kPa. Prediction of shear strength based on soil-water characteristic curves agrees better with ultimate than peak values. A simple equation is proposed for the minimum ultimate strength that can be expected in an unsaturated residual soil with a suction lower than about 30 kPa.     

Monotonic and Cyclic Behavior of Two Calcareous Soils of Different Origins

The behavior of two calcareous soils—Goodwyn (GW) and Ledge Point (LP)—is studied through a series of monotonic and cyclic triaxial tests. These two soils are selected because they represent two extreme formation conditions in terms of their depositional environments, physical characteristics, and grain strength. The experimental investigation included isotropic compression tests to high stress levels, undrained monotonic shearing tests, and undrained cyclic shearing tests under one-way and two-way loading conditions. Tests were performed on samples with different initial conditions. The experimental results show that, although the overall qualitative stress-strain behavior of both GW and LP soils is similar to that of other silicious soils, significant quantitative differences are observed between the two soils and also between calcareous and silicious soils, especially in terms of volumetric reduction during compression, monotonic and cyclic shear strength, and the strain required to mobilize the strength. This paper explores the mechanical behavior of the two calcareous soils and highlights the similarities and differences between their behavior and also between calcareous and silicious soils.     

Engineering Properties of Fibrous Peats

This state-of-the-art paper presents an interpretation of the permeability, compressibility, and shear strength of fibrous peats using data from laboratory tests on undisturbed block samples of two fibrous peats, as well as extensive laboratory and field data from the literature on fibrous peat deposits. Engineering properties of fibrous peats are significantly different from those of most inorganic soils. However, the same fundamental mechanisms and factors determine behavior of both inorganic soils and fibrous peats. Fibrous peat deposits possess very high initial permeability, typically 1,000 times the initial permeability of soft clay and silt deposits. Upon compression, the permeability of fibrous peats decreases dramatically, with a ratio of permeability change index to in situ void ratio equal to 0.25, as compared to 0.50 for soft clay and silt deposits. Fibrous peats display extreme compressibility to the increase in effective vertical stress, with compression index values right after preconsolidation pressure 5 to 20 times the corresponding compressibility of typical soft clay and silt deposits. Among geotechnical materials, fibrous peats display the highest ratios of secondary compression index to compression index, in the range of 0.05 to 0.07. The values of coefficient of earth pressure at rest for normally consolidated young fibrous peat deposits are in the range of 0.30 to 0.35, as compared to 0.45 to 0.65 for inorganic soils. The values of friction angle from triaxial compression tests for fibrous peats are in the range of 40 to 60°, as compared to less than 35° for soft clay and silt compositions. For fibrous peats, the ratios of undrained shear strength in compression to preconsolidation pressure are usually in the range of 0.50 to 0.75, as compared to 0.32 for soft clay and silt deposits. For surficial fibrous peat deposits the ratio of vane shear strength to preconsolidation pressure is near 1.0, as compared to 0.12 to 0.35 for inorganic soft clay and silt deposits. For fibrous peats, the ratio of undrained Young’s modulus to undrained shear strength is in the range of 20 to 80.     

Shear Strength of Compacted Soil under Infiltration Condition

Landslides in residual soil slopes are commonly induced by rainfall infiltration. These residual soils are typically in an unsaturated state with negative pore-water pressures or matric suctions since the groundwater tables in steep slopes are often deep. The net normal and shear stresses of the soil remain essentially constant during rainwater infiltration into the slope. Failure of the slope during rainfall can be primarily associated with the decrease in the matric suction of the soil. The objective of the study was to investigate the strength and deformation characteristics of a residual soil of the Bukit Timah Granitic Formation during infiltration that leads to slope failure. There were two modified direct shear apparatuses used. One apparatus was used for the determination of shear strength under controlled suction conditions while the other apparatus was used for shearing-infiltration tests. The shearing-infiltration test results were compared with the shear strength values obtained from the shearing tests under constant suction. The shearing-infiltration test results indicate a close relationship between the decreasing matric suction and the increasing displacement rate of the soil specimen. At the initial part of the infiltration process, there is a rapid reduction in matric suction that is accompanied by little movement in the soil. When failure of the soil is imminent, the soil movement will accelerate.     

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.     

Unified Catastrophic Model for Collapsible Loess

A unified model considering the microstructure instability and a set of complete constitutive relations, which describes the volumetric collapse and shearing collapse simultaneously, is developed for collapsible loess in the framework of the catastrophe theory. The validity of the model is verified by comparing the results calculated using the model with the data of triaxial compression tests performed on Lanzhou loess. The following characteristics of collapsing deformation are explained and reproduced successfully using the model: (1) the existence of two turning points on each deformation curve, (2) the shear curves under various stress ratios intersect one another, (3) the ratio of the shear strain to the volumetric strain is a function of the stress ratio, and (4) the initial collapse condition is approximately a circle.     

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.     

Induced-Partial Saturation for Liquefaction Mitigation: Experimental Investigation

The technical feasibility of a new liquefaction mitigation technique is investigated by introducing small amounts of gas/air into liquefaction-susceptible soils. To explore this potential beneficial effect, partially saturated sand specimens were prepared and tested under cyclic shear strain controlled tests. A special flexible liquefaction box was designed and manufactured that allowed preparation and testing of large loose sand specimens under applied simple shear. Partial saturation was induced in various specimens by electrolysis and alternatively by drainage-recharge of the pore water. Using a shaking table, cyclic shear strain controlled tests were performed on fully and partially saturated loose sand specimens to determine the effect of partial saturation on the generation of excess pore water pressure. In addition, the use of cross-well radar in detecting partial saturation was explored. Finally, a setup of a deep sand column was prepared and the long-term sustainability of air entrapped in the voids of the sand was investigated. The results show that partial saturation can be achieved by gas generation using electrolysis or by drainage-recharge of the pore water without influencing the void ratio of the specimen. The results from cyclic tests demonstrate that a small reduction in the degree of saturation can prevent the occurrence of initial liquefaction. In all of the partially saturated specimens tested, the maximum excess pore pressure ratios ranged between 0.43 and 0.72. Also, the cross-well radar technique was able to detect changes in the degree of saturation when gases were generated in the specimen. Finally, monitoring the degree of partial saturation in a 151?cm long sand column led to the observation that after 442 days, the original degree of saturation of 82.9% increased only to 83.9%, indicating little tendency of diffusion of the entrapped air out of the specimen. The research reported in this paper demonstrated that induced-partial saturation in sands can prevent liquefaction, and the technique holds promise for use as a liquefaction mitigation measure.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

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.