Initial Mean Effective Stress Research Articles

Effects of temperature on the shear strength of saturated sand

The effect of temperature on the shearing response of saturated, dense sand was investigated using a series of temperature-controlled, isotropically-consolidated, hollow cylinder triaxial compression tests, where specimens were heated in drained conditions followed by shearing in undrained conditions. As expected, the deviatoric stress at the peak state (i.e., the undrained shear strength) was observed to increase with increasing initial mean effective stress. However, it was observed to decrease linearly with increasing temperature. The effects of temperature on the deviatoric stress at the peak state were attributed to a linear increase in the magnitude of negative shear-induced pore water pressure at the peak state with temperature. The relationship between the undrained shear strength and the pore water pressure with changes in temperature was represented well by linear equations. When the shear strength was interpreted in terms of the critical state, no obvious changes in the critical state line in the p′-q plane were observed, and the critical state friction angle was unaffected by temperature. During drained heating, the dense sand specimens were observed to expand volumetrically, causing the normal consolidation line in the e-p′/pa0.5 plane to shift upward with increasing temperature without a change in the slope. The negative pore water pressure during undrained shearing caused the state paths of the dense sand specimens to move to the right. As the magnitude of negative pore water pressure increased with increasing temperature, no obvious effects on the critical state line in the e-p′/pa0.5 plane were observed.

Effect of Flyash on Post-Cyclic Behavior of Sand

ABSTRACT This study focused on investigating the effect of FA addition on the post-cyclic behavior of sand by performing a series of undrained post-cyclic monotonic triaxial compression tests. The effect of four FA contents (i.e., 0, 2, 4, and 6% by dry weight), three pre-consolidation relative densities (i.e., 20, 40, and 60%), and three initial mean effective stresses (i.e., 50, 70, and 90 kPa) were investigated. To conduct the post-cyclic tests, initially the cyclic loadings were applied until desired pore water pressure ratio (ru) values, and then the post-cyclic test was initiated. Also, a series of undrained monotonic triaxial tests were conducted to investigate the effect of zero pore water pressure ratio (ru = 0). The results showed that increasing the FA content is effective to increase the ultimate deviatoric strength (qu) of the sand. Also, investigations showed that the FA-treated specimens have a greater qu value at a higher relative density and lower initial mean effective stress.

DEM study of fabric features governing undrained post-liquefaction shear deformation of sand

In an effort to study undrained post-liquefaction shear deformation of sand, the discrete element method (DEM) is adopted to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles. The simulations are able to successfully reproduce the generation and eventual saturation of shear strain through the series of liquefaction states that the material experiences during cyclic loading after the initial liquefaction. DEM simulations with different deviatoric stress amplitudes and initial mean effective stresses on samples with different void ratios and loading histories are carried out to investigate the relationship between various mechanics- or fabric-related variables and post-liquefaction shear strain development. It is found that well-known metrics such as deviatoric stress amplitude, initial mean effective stress, void ratio, contact normal fabric anisotropy intensity, and coordination number, are not adequately correlated to the observed shear strain development and, therefore, could not possibly be used for its prediction. A new fabric entity, namely the Mean Neighboring Particle Distance (MNPD), is introduced to reflect the space arrangement of particles. It is found that the MNPD has an extremely strong and definitive relationship with the post-liquefaction shear strain development, showing MNPD’s potential role as a parameter governing post-liquefaction behavior of sand.

Cone penetration tests in unsaturated silty sands

Very little is known about how to interpret cone penetration tests (CPTs) when performed in unsaturated soils. The few published studies on CPTs in unsaturated soils have focused on either clean sands or silt. In this study, new results of laboratory-controlled CPTs in an unsaturated silty sand are presented. Silty sand exhibits hydraulic hysteresis and suction hardening. Suction is observed to have a pronounced effect on measured cone penetration resistance. For an isotropic net confining stress of 60 kPa, it is observed that higher suctions give rise to cone penetration resistances that are 50% larger than those for lower suctions. A semi-theoretical correlation is presented that links measured cone penetration resistances to initial relative density and mean effective stress. Suction has an influence on cone penetration resistances through suction hardening, as well as its contribution to effective stress. For this silty sand, it is shown that failing to account for suction may result in significant overestimations and unsafe predictions of soil properties from measured cone penetration resistances.

The cone penetration test in unsaturated silty sands

Very little is known about how to interpret the cone penetration test (CPT) when performed in unsaturated soils. The few published studies on the CPT in unsaturated soils have focused on either clean sands or a silt. In this study new results of laboratory-controlled CPTs in an unsaturated silty sand are presented. The silty sand exhibits hydraulic hysteresis and suction hardening. Suction is observed to have a pronounced affect on measured cone penetration resistance. For an isotropic net confining stress of 60 kPa it is observed that higher suctions give rise to cone penetration resistances that are 50% larger than those for lower suctions. A semi-theoretical correlation is presented that links measured cone penetration resistances to initial relative density and mean effective stress. For this silty sand it is shown that failing to account for suction may result in significant overestimations and unsafe predictions of soil properties from measured cone penetration resistances.

Stiffness of Clays and Silts: Normalizing Shear Modulus and Shear Strain

AbstractAn analysis is presented of a database of 67 tests on 21 clays and silts of undrained shear stress-strain data of fine-grained soils. Normalizations of secant G in terms of initial mean effective stress p′ (i.e., G/p′ versus log γ) or undrained shear strength cu (i.e., G/cu versus log γ) are shown to be much less successful in reducing the scatter between different clays than the approach that uses the maximum shear modulus, Gmax, a technique still not universally adopted by geotechnical researchers and constitutive modelers. Analysis of semiempirical expressions for Gmax is presented and a simple expression that uses only a void-ratio function and a confining-stress function is proposed. This is shown to be superior to a Hardin-style equation, and the void ratio function is demonstrated as an alternative to an overconsolidation ratio (OCR) function. To derive correlations that offer reliable estimates of secant stiffness at any required magnitude of working strain, secant shear modulus G is norma...

Liquefaction Potential Variations Influenced by Building Constructions

In this paper, a sand deposit with liquefaction potential was simulated in FLAC using Finn constitutive model. Equivalent loads of 5, 10, 15 and 20-story buildings were applied to the deposit. As a cyclic loading, acceleration time history of Bam earthquake was used. To examine pore water pressure variations in the soil mass during the earthquake loading, parameter ru (pore water pressure ration, which equals pore water pressure increment divided by the initial mean effective stress in the soil mass) was defined for the software by a Fish function. Static analyses show that by increasing the applied loading due to building construction, the values of effective and shear stress increase and it generally can be a factor to influence liquefaction potential. Furthermore, dynamic analyses show that there is a shallow longitudinal area beneath 15 and 20-story buildings in which liquefaction potential increases due to high confining effective stress.

Strength mobilization in clays and silts

A large database of 115 triaxial, direct simple shear, and cyclic tests on 19 clays and silts is presented and ana- lysed to develop an empirical framework for the prediction of the mobilization of the undrained shear strength, cu, of natural clays tested from an initially isotropic state of stress. The strain at half the peak undrained strength (gM=2) is used to normal- ize the shear strain data between mobilized strengths of 0.2cu and 0.8cu. A power law with an exponent of 0.6 is found to describe all the normalized data within a strain factor of 1.75 when a representative sample provides a value for gM=2. Multi-linear regression analysis shows that gM=2 is a function of cu, plasticity index Ip, and initial mean effective stress p 0 0. Of the 97 stress-strain curves for which cu, Ip, and p 0 were available, the observed values of gM=2 fell within a factor of three of the regression; this additional uncertainty should be acknowledged if a designer wished to limit immediate founda- tion settlements on the basis of an undrained strength profile and the plasticity index of the clay. The influence of stress his- tory is also discussed. The application of these stress-strain relations to serviceability design calculations is portrayed through a worked example. The implications for geotechnical decision-making and codes of practice are considered.

Numerical analysis of cylindrical cavity expansion in sand considering particle crushing and intermediate principal stress

Considering the effects of particle crushing and intermediate principal stress on material yielding strength, the spatial mobilization plane (SMP) yielding criterion and state parameter model including a general critical state line are selected in the analysis of cylindrical cavity expansion. Meanwhile, combining Rowe’s flow rule and Bolton’s simplification to stress-dilatancy relationship to reflect soil shear dilatancy and softening behavior, this paper analyzes the problem of cylindrical cavity expansion in sand by discretizing the plastic zone, which is applicable to cavity expansions from zero initial radius and finite initial radius simultaneously and can determine stress field, strain field and limit cavity pressure. A series of comparative analyses are made with the results ignoring crushing and based on Mohr-Coulomb criterion to examine the effects of crushing as well as the coupled effects of crushing and intermediate principal stress on cavity expansion. Results indicate that crushing causes a reduction in limit cavity pressure and void ratio, and results in less stiff response in expansion curves, the amounts of which increase with initial density and mean effective stress. The consideration of crushing can weaken the effects of intermediate principal stress on expansion, while the consideration of intermediate principal stress makes crushing effects become more prominent. The limit cavity pressure can be reduced by over 40% for dense sand with high initial stress based on SMP criterion when crushing is considered.

Steady State Strength Behavior of Yamuna Sand

The paper pertains to the study of steady state or residual strength of sandy soils (Yamuna sand lying in the Indo-Gangetic alluvial plains) by consolidated rebounded drained triaxial test with volume change measurements and strain-controlled consolidated undrained test as well. The observed behavior obtained from these two tests is then compared to check their comparative merit. The same was also compared with those of Ganga and Toyoura sand, and with the predicted behavior obtained by using a semi empirical model. The results obtained from rebounded drained and undrained tests are found to be in good agreement. The curvature of ultimate steady state line of Yamuna sand is similar in trend to Ganga and Toyoura sand in the initial mean effective principle stress range; but the experimental observations with reference to Yamuna sand is not in good agreement with the model predictions in the region of higher mean normal stress. A semi empirical general model has been developed fitting the data for better prediction of the steady state behavior.

Crack initiation in clay observed in beam bending

Tensile cracking in clay is an important phenomenon that affects the strength and permeability of clays in many facilities, such as dams, embankments and landfill liners, as well as being a precursor to slope failures in the natural terrain. This study investigates the stress–strain criteria for cracking in clays by performing four-point bending tests on consolidated kaolin clay beams. Load-controlled and strain-controlled tests were performed on clay beams with varying initial suction to understand the stress–strain criteria for crack initiation in clay. At no stage was the negative pore pressure permitted to exceed the air entry value of the clay, so the clay remained saturated throughout. Strains in the clay were obtained by particle image velocimetry analysis of digital images of the clay beam, and suction measurements were obtained from pore pressure and tension transducers installed within the clay beams. Results from this investigation showed that the threshold tensile strain to cracking in kaolin clay decreased from about 4% at an initial mean effective stress of 15 kPa to about 1·5% at an initial mean effective stress of 100 kPa. The extreme fibre stress at failure of the clay beams indicated an effective tensile strength that was a fraction, dropping from 0·27 to 0·11 of the preconsolidation pressure as the overconsolidation ratio increased from about 2 to about 20. Alternatively, the tensile strength could be regarded as 0·45 ± 0·15 of the geometric mean of the preconsolidation pressure in the plane of shear and the initial mean effective stress. Most significantly, evidence is presented of a new understanding of cracking in clays. Cracks were seen to open either as pure tension cracks at zero minor principal effective stress, or as mixed-mode shear-tension cracks when an effective stress path reaches the Hvorslev criterion of brittle shear rupture, but only if the minor total stress is also tensile.

Undrained monotonic response of clean and silty sands

A series of undrained monotonic triaxial compression tests were performed on Ottawa sand containing 0%, 5%, 10% and 15% of non-plastic silt. The paper focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using slurry deposition and moist tamping to investigate the effect of the initial sand fabric on these states. It was observed that an increase in the non-plastic fines content leads to a downward shift of the critical-state line in the void ratio–mean effective stress space and to an increase of the critical-state friction angle. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behaviour in the early stages of shearing, with its influence vanishing at large strains. Moist-tamped specimens demonstrate considerably larger undrained instability state strength than their slurry-deposited counterparts, and a unique critical state locus is reached by both moist-tamped and slurry-deposited specimens. A mathematical formula that allows the estimation of the mean effective stress at the phase transformation state based on the initial mean effective stress and void ratio is also proposed.

Effect of material properties on the behaviour of sand—cement—fibre composites

Saturated drained triaxial compression tests with local strain measurement were carried out to evaluate the effects of using three different randomly distributed fibres (polyester,polypropylene and glass fibres) and rapid hardening Portland cement to improve the engineering behaviour of a uniform fine sand. In addition to the nature of the fibres,also analysed have been the separate and combined effect of fibre content (up to 0·5% by weight), fibre length (up to 36 mm), cement content (from 0% to 7% by weight) and initial mean effective stress (20, 60 and 100 kN/m2) on the deformation and strength characteristics of the soil. The present work searches for the establishment of a relation between the properties of the fibres and the mechanical behaviour of the composite material, originated from their combination with soil and cement. The cementation itself notably increased modulus, peak cohesive intercept, peak friction angle and brittleness of the sand. Inclusion of polyester and glass fibres (both relatively stiff) slightly reduced the stiffness and increased the peak friction angle of both the cemented and uncemented sand, and also slightly reduced the peak cohesive intercept and brittleness of the cemented composite. On the other hand,relatively flexible polypropylene fibre reinforcement dramatically reduced the brittleness (changing the mode of failure of the cemented sand from brittle to ductile for longer fibres) and stiffness, while increasing the ultimate strength of the cemented composite. Nous avons fait des essais de compression triaxiale drainéeet saturée avec des mesures de déformation locales afind'évaluer les effets de trois fibres différentes réparties sansordre (polyester, polypropylène et verre) et du ciment dePortland àdurcissement rapide pour améliorer le comportement industriel d'un sable uniforme fin. En plus dela nature des fibres, nous avons également analysél'effet séparé et combiné du contenu en fibres (jusqu'à0,5% en poids), de la longueur des fibres (jusqu'à 36 mm),du contenu en ciment (de 0% à7% en poids) et de lacontrainte effective moyenne initiale (20, 60 et 100 kN/m2)sur les caractéristiques de déformation et de résistance dusol. Cette étude cherche àétablir une relation entre lespropriétés des fibres et le comportement mécanique du matériau composite, venant de leur combinaison avec dusol et du ciment. La cimentation elle-même a nettementaugmenté le module, l'interception cohésive de pointe,l'angle de friction de pointe et la friabilité du sable.L'inclusion de fibres de verre et de fibres de polyester (toutes deux relativement rigides) a légèrement réduit larigidité et a augmenté l'angle de friction de pointe dusable cimenté et du sable non cimenté et a égalementlÉgèrement réduit l'interception cohésive de pointe et lafriabilité du composite cimenté. Par ailleurs, un renforcement relativement flexible àla fibre de polypropyléne aréduit la friabilité (changeant le mode de défaillance du sable cimenté de friable àmalléable pour les fibres plus longues) et la rigidité, tout en augmentant la résistance ultime du composite cimenté

Engineering Behavior of a Sand Reinforced with Plastic Waste

Unconfined compression tests, splitting tensile tests, and saturated drained triaxial compression tests with local strain mea- surement were carried out to evaluate the benefit of utilizing randomly distributed polyethylene terephthalate fiber, obtained from recycling waste plastic bottles, alone or combined with rapid hardening Portland cement to improve the engineering behavior of a uniform fine sand. The separate and the joint effects of fiber content~up to 0.9 wt %!, fiber length ~up to 36 mm!, cement content ~ from 0t o 7 wt %!, and initial mean effective stress ~20, 60, and 100 kN/m 2 ! on the deformation and strength characteristics of the soil were investigated using design of experiments and multiple regression analysis. The results show that the polyethylene terephthalate fiber reinforcement improved the peak and ultimate strength of both cemented and uncemented soil and somewhat reduced the brittleness of the cemented sand. In addition, the initial stiffness was not significantly changed by the inclusion of fibers.

Characterization of Cemented Sand in Triaxial Compression

This work aims at studying the stress-strain-strength behavior of an artificially cemented sandy soil produced through the addition of portland cement. An analysis of the mechanical behavior of the soil is performed from the interpretation of results from unconfined compression tests, drained triaxial compression tests with local strain measurements, and scanning electron microscopy, in which the influence of both the degree of cementation and the initial mean effective stress was investigated. For cemented sandy soils, it was concluded that the unconfined compression resistance is a direct measurement of the degree of cementation. Consequently, the triaxial shear strength can be expressed as a function of only two variables: (1) the internal shear angle of the nonstructured material; and (2) the unconfined compression resistance. In addition, a logarithmic formulation is adopted to express the relationship between static deformation moduli and axial strain amplitude in axisymmetric conditions. Data from other reported investigation programs give to the proposed correlations a broader acceptance to general geotechnical applications.

Analysis of Undrained Cyclic Behavior of Sand under Anisotropic Consolidation

In order to examine the capability of a cyclic mobility model consisting of multiple shear mechanism defined in strain space (Iai et al., 1992), cyclic behavior of anisotropically consolidated sand is analyzed under an undrained and simple shear condition. This behavior of sand, though commonly encountered in earthquake engineering, was not easy to analyze with the conventional plasticity models because it involves effects of rotation of principal stress axis directions. Two analyses are conducted in the present study; one is cyclic simple shearing with constraining lateral normal strain as in the level ground, the other is without the constraint on the lateral strain but keeping the axial stress difference unchanged. The former analysis indicates that cyclic behavior of sand is not affected by initial anisotropic consolidation conditions if initial mean effective stress is kept the same. The latter indicates gradual growth in the axial strain difference causing gradual settlement with lateral bulging. Both of these results are consistent with the previous laboratory findings, suggesting reasonable applicability of the cyclic mobility model.