Mean Net Stress Research Articles

Mechanical behaviour of unsaturated kaolin (with isotropic and anisotropic stress history). Part 2: performance under shear loading

Validation of a framework for unsaturated soil behaviour has frequently resulted in disagreement with basic propositions. A primary reason for this disparity is considered to be attributable to the anisotropic properties of the soil specimens tested as a result of preparation using one-dimensional compaction. As part of the work presented, comparison is made between tests on samples of unsaturated kaolin prepared at identical specific volumes and specific water volumes using isotropic compression and one-dimensional compression. The suctions in the samples were reduced to predefined values by wetting under low isotropic loading in a triaxial cell. The samples were then taken through various stress paths to failure, defined as the critical state strength, while the suctions were held constant. Stress path tests were also performed on samples without reducing the suction to predefined values. In the latter, constant water mass tests, the suctions were allowed to vary and were measured using a psychrometer. The results of the tests at critical state are compared with the propositions of Wheeler and Sivakumar. The shear strengths of samples with isotropic previous history are shown to be significantly greater than those of samples with one-dimensional stress history when plotted against the mean net stress. The normal compression lines, critical state lines and yield characteristics are also shown to be significantly influenced by the previous stress history and are shown to be different for isotropically and one-dimensionally prepared samples.

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus ( Go ) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil...

A framework for the construction of state surfaces of unsaturated soils in the elastic domain

This paper deals with volume changes of an unsaturated soil in the elastic domain. Two constitutive relations are established in the general framework of unsaturated porous media mechanics. They express the variations of the void ratio e and the water content w with respect to the mean net stress (σ − pg*) and the suction s. Associated with validated experimental results, these relations give access to the construction of the state surface e = f((σ − pg*), s) in the elastic domain. This surface shows two domains: the first, near saturation, is based on the hypothesis of a Biot's coefficient bw equal to 1; the second, for high suctions, is based on an experimental value of bw. Next, a preliminary construction of the state surface Gsw = g((σ − pg*), s) is proposed for a given soil. Although restricted to the elastic domain, these constructions give a new understanding of the suction role in important problems related to overconsolidation, shrinkage, or swelling of unsaturated soils.

Critical state for overconsolidated unsaturated silty soil

The critical state concepts have been investigated for an overconsolidated unsaturated silty soil through a comprehensive set of controlled suction triaxial laboratory experiments. The experimental tests were conducted in a specially designed double-walled Bishop and Wesley triaxial cell on samples of unsaturated silty soil. Isotropic loading, unloading, and triaxial drained shear tests were performed on samples of unsaturated silty soil with different overconsolidation ratios at various suctions. The data from the triaxial tests were used in the development of a critical state framework for overconsolidated unsaturated silty soil. The framework is defined in terms of four state variables: mean net stress (p'), deviator stress (q), suction (s), and specific volume (v). The results show that the critical state lines in q:p' space for different soil suctions are not parallel and merge with each other. The slopes and intercepts of these lines are functions of suction. However, the critical state lines are nearly parallel in the v – ln p' plane except under saturation conditions, and the slope and intercept of these lines are also functions of suction.

Determination of the anisotropic shear stiffness of an unsaturated decomposed soil

The shear modulus at very small strains (0·001% or less), G0, is one of the most important parameters for predicting ground movements and dynamic responses of many engineering earth structures. With advances in the use of bender elements for soil testing, anisotropic shear moduli G0(ij)can now be evaluated by measuring the velocities of shear waves propagating in a soil specimen in different planes and polarisations. This leads to the determination of the stiffness anisotropy at very small strains. While almost all previous studies of anisotropic soil stiffness have been conducted with saturated soils, experimental results from unsaturated soils have rarely been reported in the literature. A new computer-controlled triaxial apparatus equipped with three pairs of bender elements was developed to measure shear wave velocities and hence to investigate the degree of stiffness anisotropy of recompacted completely decomposed tuff (CDT) under both saturated and unsaturated conditions. The axis-translation technique was applied to control matric suction. New state-dependent equations describing aniso-tropic moduli of unsaturated soil were derived and verified by experimental results. Based on the derived equations and measurements, it was possible to identify that there was an inherent shear stiffness anisotropy of about 1·03–1·04, due to sample preparation by moist tamping, for isotropic net mean stress varying from 110 to 400 kPa. When suction was increased from 0 kPa to the air-entry value of 50 kPa, suction-induced anisotropy in terms of the shear stiffness ratio, G0(hh)/G0(hv), ranged from about 1·06 to 1·09 for isotropic net mean stress varying from 110 to 500 kPa. No further significant increase in suction-induced stiffness anisotropy was observed at suctions higher than the air-entry value.

Critical State Behavior of a Compacted Silt Specimen

Shear strength of unsaturated soil is commonly obtained from a consolidated drained (CD) triaxial test. However in many field situations, fill materials are compacted where the excess pore-air pressure developed during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. This condition can be simulated in a constant water content (CW) triaxial test. The critical states of an unsaturated soil obtained from the CW and the CD triaxial tests have not been extensively investigated and compared. A series of CW and CD triaxial tests was carried out on a statically compacted silt. The results from this study show that the critical state lines at different matric suctions on the (q-p) plane were parallel with a slope of 1.28 for both the CW and CD triaxial tests, indicating the unique relationship between deviator stress and mean net stress. The results also indicate the unique relationship between the specific volume, v, and mean net stress, p, on the (v-p) plane for both the CW and CD triaxial tests. The slope of the critical state lines on the (v-p) plane for both the CW and CD triaxial tests decreased with the increase in matric suction.

Collapse behaviour of unsaturated compacted soil with different initial densities

Very few experimental data on the mechanical behaviour of unsaturated soils exists, particularly on the collapse behaviour under general stress states, because of the technical difficulties and time-consuming nature of measuring suction and deformation. This paper presents the results of a series of controlled-suction triaxial tests on the collapse behaviour of an unsaturated compacted clay with different initial dry densities and suctions. The collapse behaviour here includes deformation characteristics, such as volume changes, and hydraulic characteristics, such as saturation changes. It is found that the wetting-caused collapse mainly depends on the mean net stress and the initial density, and that the volume decrease reaches a maximum when the specimen is under the initial yielding mean net stress. It is also found that the soil-water characteristic curve in terms of suction and degree of saturation shifts upwards with increasing specimen density. The soil-water characteristic curve of a compacted soil mainly depends upon the current density, not directly upon the stress state. In addition, experimental data show that the collapse occurs mainly in an intermediate range of suction levels, which are neither very high nor very low, and that the wetting-caused volume decrease is accompanied by an increase in the degree of saturation.Key words: unsaturated soil, density, triaxial test, suction, collapse, degree of saturation.

Modelling the influence of stress–strain history on the initial shear stiffness of an unsaturated compacted silt

This is the follow-up paper to Vassallo et al. (2007), which discussed the experimentally observed small-strain behaviour of an unsaturated compacted silt. The influence that suction and, more in general, mean net stress – suction history has on the initial shear stiffness was analysed and ascribed to the accumulation of irreversible volumetric strains. In this study, a model able to predict the observed behaviour is proposed, based on classical unsaturated soil volumetric hardening elastoplastic formulations. Starting from the interpretation of the results relative to simple stress paths, such as preliminary equalization and loading–unloading compression, the results of "complex" stress paths, such as those including drying–wetting cycles, have subsequently been modelled by introducing some additional parameters.Key words: unsaturated, compacted, small strain, stiffness, volumetric behaviour, stress history.

Effects of net stress and suction history on the small strain stiffness of a compacted clayey silt

An experimental study was carried out to investigate the effects of the mean net stress and suction history on the initial shear stiffness, G0, of a compacted clayey silt. Isotropic tests were performed using two suction-controlled devices, a triaxial cell and a resonant column torsional shear (RCTS) cell, so as to investigate the volumetric behaviour of this material. As for saturated soils, one can expect to find a strong correlation among stress history, volumetric state, and G0. Initial shear stiffness was measured almost continuously along various isotropic stress paths, including compressions and drying–wetting single stages or cycles, by using the RCTS cell. The collected data demonstrate a strong dependency of G0 on mean net stress (p – ua) and suction (ua – uw). Cycles of suction, in particular increasing suction beyond the past maximum value, induce significant accumulation of irreversible strains and increase of stiffness, confirming that G0 is not univocally related to the stress state (p – ua, ua – uw).Key words: unsaturated, compacted, small strain, stiffness, volumetric behaviour, stress history.

Effect of loading rate on the volume change behavior of unsaturated residual soil

Residual soils occur in most countries of the world but those that occur in greater area and depth are usually found in the tropical humid areas. This research examines the effects of loading rate of applied or net mean stress on the volume change behavior of an unsaturated granitic residual soil. Three loading rates were adopted: fast, slow and extra slow loading rate. It was observed that the loading rate of applied or net mean stress has a pronounced effect on the void ratio and degree of saturation but has an insignificant effect on the water content of residual soils subjected to constant matric suction.

Collapsibility and Volume Change Behavior of Unsaturated Residual Soil

Residual soils occur in most countries of the world but the greater areas and depths are normally found in tropical humid areas. In these places, the soil forming processes are still very active and the weathering development is much faster than the erosive factor. Most residual soil exhibit high suctions for most of the year. The absence of positive pore water pressure except immediately after rain, makes conventional soil mechanics for saturated soil not so relevant. Ignorance or lack of understanding of the geotechnical behavior of soil in the partially or unsaturated state has caused a lot of damages to infrastructures, buildings and other structures. For instance, the collapsibility and volume change of partially saturated soils in connection with the drying or wetting causes a lot of damage in foundation, roads and other structures. It is also observed that many shallow slope failures involve a slumping (collapse) type of failure. As such, the development of extended soil mechanics, which embraces the soil in the unsaturated state or subjected to soil suction, is essential. This study examines the collapsibility and volume change behavior specifically of an unsaturated residual soil under various levels of applied matric suction (ua-uw) and net mean stress (σ-ua) in a predetermined stress path. The volume change of the soil is found to be sensitive to both the applied matric suction and net mean stress. The soil is found to exhibit a collapsibility behavior upon a reduction in applied matric suction at constant net mean stress.

Hydraulic Behavior of Collapsible Compacted Gneiss Soil

The hydraulic behavior of a residual gneiss soil compacted at conditions that produce collapsible characteristics was studied using a triaxial permeameter system that allows independent control of net stresses and matric suction. Wetting paths under differing but constant loads were utilized to reflect field conditions associated with a collapsing soil earth fill dam. The experimental results showed that different stress paths produce different amounts of volumetric collapse and distinct variations in the coefficient of permeability, which at relatively high matric suctions was mainly affected by the increase in degree of saturation. However, as the soil approached saturation, the coefficient of permeability became dependent on void ratio and net mean stress, with the saturated coefficient of permeability being reduced at larger amounts of collapse. The results also suggested that entrapped air in the soil voids may produce a significant reduction in the coefficient of permeability. A constitutive relationship for the coefficient of permeability that takes net mean stress changes into account was proposed based on best-fit analyses.

Laboratory Study of Loose Saturated and Unsaturated Decomposed Granitic Soil

Weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as Hong Kong. The mechanical behavior of loose decomposed fill materials, particularly in the unsaturated state, has not often been investigated and is not yet fully understood. The objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state, the stress path and the soil suction on the stress-strain relationship, shear strength, volume change, and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens. It was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing. Strain-softening behavior is observed in loose saturated specimens. In unsaturated specimens sheared at a constant water content, a hardening stress-strain relationship and volumetric contractions are observed in the considered range of net mean stresses. The suction of the soil contributed little to the apparent cohesion. The angle of friction appeared to be independent of the suction. In unsaturated specimens subjected to continuous wetting (suction reduction) at a constant deviator stress, the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation. It was revealed that the dilatancy of the unsaturated soil depends on the suction, the state, and the stress path.

A possible elastic–plastic framework for unsaturated soils with high-plasticity

The paper proposes a new elastic–plastic framework for unsaturated, high-plasticity, clayey soils and sand–clay mixtures. The framework considers possible coupling of stress- and suction-induced hardening, leading to a yield surface that is closed or "capped" as suctions increase. This produces a stress state boundary surface in three-dimensional p–q–s stress space (where p is the net mean stress, q is the deviator stress, and s is the matric suction) which differs from that of other conceptual models of its kind. Yielding, a hardening law, and failure criteria for saturated soils are incorporated into the stress state boundary surface. Two parameters, equivalent pressure pe and stress ratio ηs, are introduced to form the basis of the proposed elastic–plastic framework for highly plastic soils with high suctions. This provides an alternative for the stress variables net mean stress and matric suction that are commonly used in modeling unsaturated soils with lower plasticity and lower suctions. This framework has allowed results of experiments on an unsaturated sand–bentonite mixture to be successfully described using elastoplasticity. Yield and failure envelopes associated with the proposed state boundary surface in p–q–s space can be normalized using pe and ηs in such a way that they agree with a comparable envelope for saturated specimens. Key words: unsaturated, elastic–plastic, triaxial, matric suction, state boundary surface, sand-bentonite.

Behavior of a Loosely Compacted Unsaturated Volcanic Soil

In this paper, the stress-strain relationsihp and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil (fill) were studied by conducting three series of triaxial stress path tests: (1) consolidated undrained on the saturated fill; (2) constant water content; and (3) a reducing suction under constant deviator stress on the unsaturated fill. The last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes, respectively. It was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear. For isotropically consolidated unsaturated specimens sheared under a constant water content, a hardening stress-strain and a nonlinear shear strength–suction relationship are observed. At relatively high suctions, both angle of friction and apparent cohesion appear to be independent of suction. Volumetric contraction during shear is observed in this series of tests. On the other hand, anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases. This observed volumetric behavior, unlike the shear strength, is stress path–dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils.

Deformation Characteristics of a Compacted Clay in Collapse Under Isotropic and Triaxial Stress State

Deformation behavior of an unsaturated soil in collapse was studied using a modified triaxial test apparatus. Three kinds of wetting test were conducted on compacted clay specimens, (i) wetting tests under isotropic stress state, (ii) wetting tests under constant shear stress, and (iii) repeated wetting during shear test under a constant mean net stress. Collapse occurred later in comparison to isotropic stress state conditions under triaxial stress state when shear stress was kept constant. This phenomenon is due to the decrease of the coefficient of permeability, which relates to the changes associated with bulk water to meniscus water. The relationship between void ratio change and increase in water content observed under the different stress states tested showed similar behavior. In the repeated wetting during shear test, a continuous decrease in void ratio was observed with water absorption and drainage. This phenomenon is also related to the change of the bulk water to meniscus water. The secondary wetting process is observed in the experimental testing program discussed in this paper. The presently available constitutive models do not take into account the secondary wetting process. The research studies presented in this paper provide more insight into the collapse behavior.

Yielding characteristics of unsaturated compacted soils

The elasto-plastic constitutive model for unsaturated soils postulates the existence of a loading collapse yield locus (LC) and a suction increase yield locus (SI) in the p:s stress plane. The characteristics of the LC yield locus have been reasonably well established in recent years but there is a much greater degree of uncertainty in relation to the SI yield locus. In the work reported in this paper a number of stress path tests in the p:s plane were conducted on samples of unsaturated compacted kaolin in a modified triaxial apparatus. Two series of tests were conducted on samples compacted to an identical initial condition so as to ensure a uniform soil structure prior to testing. Samples tested in Series 1 were taken through three stages: wetting at constant mean net stress, drying at constant mean net stress and a combination of isotropic compression and wetting. Samples tested in Series 2 were taken to pre-selected values of suction prior to isotropic compression. Data obtained from these tests indicate a possible form of single yield locus for unsaturated compacted soil. The data also indicate that the position of the yield locus, initially imposed by the compaction process will be influenced by the subsequent wetting process. Copyright © 2000 John Wiley & Sons, Ltd.

Long‐life torsion fatigue with normal mean stresses

Relatively simple fatigue tests have been performed on two common engineering materials, cast ductile iron and low‐carbon steel, using two stress states, cyclic torsion and cyclic torsion with static axial and hoop stresses. Tests were designed to discriminate between normal stress and hydrostatic stress as the most suitable mean stress correction term for high cycle fatigue analysis. Microscopy shows that cracks in low‐carbon steel nucleate and grow on maximum shear planes, while for cast iron pre‐existing flaws grow on maximum normal stress planes. The data illustrate that tensile normal stress acting on a shear plane significantly reduced fatigue life and is an appropriate input for fatigue analysis of ductile materials. Static normal stresses did not significantly affect the fatigue life for the cast iron because the net mean stress on the maximum normal stress plane was zero. Mean torsion significantly reduced the fatigue strength of the cast iron. A critical plane long‐life parameter for nodular iron which accounts for both stress state and mean stress is proposed, and is found to accurately correlate experimental data.

Overall volume change, water volume change, and yield associated with an unsaturated compacted loess

This paper presents the overall volume change, water volume change, and yield associated with an unsaturated, compacted, low-plasticity loess from China. Two types of tests were conducted using a modified triaxial apparatus, namely isotropic compression tests with controlled suction (ICT), and triaxial shrinkage tests (TST). In the triaxial shrinkage tests, the net cell pressure is controlled and suction is allowed to increase. The results show that the stress path has a significant effect on overall volume change and the water volume change. The soil parameters related to overall volume change and the water volume change for the isotropic compression tests vary significantly in the low suction range, whereas the same soil parameters related to the triaxial shrinkage tests exhibit gradual changes. The yield net mean stress for the loading-collapse yield curve increases with suction. The yield suction is not equal to the maximum suction experienced by the soil specimen for the suction increase yield locus. Based on the test data obtained from the low-plasticity loess, a modified criterion for yield associated with suction increase is proposed. The proposed modified yield criterion should be valid for loess and silts of similar densities and plasticities.

Overall volume change, water volume change, and yield associated with an unsaturated compacted loess

This paper presents the overall volume change, water volume change, and yield associated with an unsaturated, compacted, low-plasticity loess from China. Two types of tests were conducted using a modified triaxial apparatus, namely isotropic compression tests with controlled suction (ICT), and triaxial shrinkage tests (TST). In the triaxial shrinkage tests, the net cell pressure is controlled and suction is allowed to increase. The results show that the stress path has a significant effect on overall volume change and the water volume change. The soil parameters related to overall volume change and the water volume change for the isotropic compression tests vary significantly in the low suction range, whereas the same soil parameters related to the triaxial shrinkage tests exhibit gradual changes. The yield net mean stress for the loading-collapse yield curve increases with suction. The yield suction is not equal to the maximum suction experienced by the soil specimen for the suction increase yield locus. Based on the test data obtained from the low-plasticity loess, a modified criterion for yield associated with suction increase is proposed. The proposed modified yield criterion should be valid for loess and silts of similar densities and plasticities.