Effect of Anisotropy and Destructuration on the Behavior of Murro Test Embankment

This paper investigates the influence of anisotropy and destructuration on the behavior of a test embankment on soft clay. The test embankment at Murro, Finland, was commissioned in 1993 by the Finnish Road Administration and has been monitored for over 10?years. The construction and consolidation of Murro test embankment is analyzed with finite element method using three different constitutive models to represent the soft soil. The results are compared with field observations. The constitutive models used include two recently proposed constitutive models, namely S-CLAY1 that accounts for initial and plastic strain induced anisotropy and its extension, called S-CLAY1S. The S-CLAY1S model accounts, additionally, for interparticle bonding and degradation of bonds. For comparison, the test embankment is also analyzed using the isotropic Modified Cam Clay model. The simulations demonstrate that for this type of problem, it is important to account for the anisotropy, whereas destructuration appears to have less influence on predicted deformations. However, only a model incorporating destructuration can explain the decrease in undrained shear strength during consolidation that was measured in field.     

Numerical Modeling of Nonhomogeneous Behavior of Structured Soils during Triaxial Tests

The nonhomogeneous behavior of structured soils during triaxial tests has been studied using a finite element model based on the Structured Cam Clay constitutive model with Biot-type consolidation. The effect of inhomogeneities caused by the end restraint is studied by simulating drained triaxial tests for samples with a height to diameter ratio of 2. It was discovered that with the increase in degree of soil structure with respect to the same soil at the reconstituted state, the inhomogeineities caused by the end restraint will increase. By loading the sample at different strain rates and assuming different hydraulic boundary conditions, inhomogeneities caused by partial drainage were investigated. It was found that if drainage is allowed from all faces of the specimen, fully drained tests can be carried out at strain rates about ten times higher than those required when the drainage is allowed only in the vertical direction at the top and bottom of the specimen, confirming the findings of previous studies. Both end restraint and partial drainage can cause bulging of the triaxial specimen around mid-height. Inhomogeneities due to partial drainage influence the stress–strain behavior during destructuring, a characteristic feature of a structured soil. With an increase in the strain rate, the change in voids ratio during destructuration reduces, but, in contrast, the mean effective stress at which destructuration commences was found to increase. It is shown that the stress–strain behavior of the soil calculated for a triaxial specimen with inhomogeneities, based on global measurements of the triaxial response, does not represent the true constitutive behavior of the soil inside the test specimen. For most soils analyzed, the deviatoric stress based on the global measurements is about 25% less than that for the soil inside the test specimen, when the applied axial strain is about 30%. Therefore it can be concluded that the conventional global measurements of the sample response may not accurately reflect the true stress–strain behavior of a structured soil. This finding has major implications for the interpretation of laboratory triaxial tests on structured soils.     

Experimental Study of the Behavior of a Lumpy Fill of Soft Clay

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported sand, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from 12.5?to?50?mm. The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about 100?kPa, the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of 100?kPa. When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue.     

Influence of Osmotic Suction on the Soil-Water Characteristic Curves of Compacted Expansive Clay

Unsaturated clays are subject to osmotic suction gradients in geoenvironmental engineering applications and it therefore becomes important to understand the effect of these chemical concentration gradients on soil-water characteristic curves (SWCCs). This paper brings out the influence of induced osmotic suction gradient on the wetting SWCCs of compacted clay specimens inundated with sodium chloride solutions/distilled water at vertical stress of 6.25 kPa in oedometer cells. The experimental results illustrate that variations in initial osmotic suction difference induce different magnitudes of osmotic induced consolidation and osmotic consolidation strains thereby impacting the wetting SWCCs and equilibrium water contents of identically compacted clay specimens. Osmotic suction induced by chemical concentration gradients between reservoir salt solution and soil-water can be treated as an equivalent net stress component, (pπ) that decreases the swelling strains of unsaturated specimens from reduction in microstructural and macrostructural swelling components. The direction of osmotic flow affects the matric SWCCs. Unsaturated specimens experiencing osmotic induced consolidation and osmotic consolidation develop lower equilibrium water content than specimens experiencing osmotic swelling during the wetting path. The findings of the study illustrate the need to incorporate the influence of osmotic suction in determination of the matric SWCCs.     

Microstructural Interpretation on Reliquefaction of Saturated Granular Soils under Cyclic Loading

It is well known that the resistance to liquefaction of a saturated sand decreases sharply when it has been presheared, either cyclically or quasi statically, beyond a threshold value. The possible mechanism is discussed in light of recent findings on the microstructural anisotropy developed in preshearing (induced anisotropy). A columnlike structure, through which applied stress is mainly transmitted, grows parallel to the major principal stress direction in the strain hardening process. Voids, randomly distributed at first, are also connected in series between the columnlike structures. The anisotropic structure can carry the increasing stress as long as the major stress is applied parallel to the elongation direction of the structure. However, it becomes extremely unstable when the major stress is rotated. The excess pore-water pressure increases markedly under undrained cyclic loading, particularly when the connected voids are stressed perpendicular to their elongation direction. This is the reason why once liquefied sand sharply loses liquefaction resistance in a subsequent reliquefaction test.     

Strain Rate, Creep, and Stress Drop-Creep Experiments on Crushed Coral Sand

The part of sand behavior that is affected by time, such as creep, relaxation, and loading rate effects are not similar to those observed for clay. To throw more light on the time effects in sand, many series of drained triaxial compression experiments have been performed on crushed coral sand. These tests were all performed with a constant effective confining pressure of 200?kPa. The test series included experiments with specimens loaded at five different strain rates with a 256-fold ratio between the extreme rates, tests with sudden changes in strain rate from slow to fast and vice versa, and tests in which axial and volumetric creep strains were observed at stress differences of 500, 700, and 900?kPa. Creep creates structuration and this has to be overcome to produce further plastic straining. Experiments were also performed in which the stress difference was dropped quickly from three different values of 500, 700, and 900?kPa followed by creep. In these stress drop-creep tests five different magnitudes of stress drops were employed: 0, 100, 200, 300, and 400?kPa. The results involving conventional creep effects and stress drop-creep effects are presented and analyzed.     

Prediction of Damage Behaviors in Asphalt Materials Using a Micromechanical Finite-Element Model and Image Analysis

A study of the micromechanical damage behavior of asphalt concrete is presented. Asphalt concrete is composed of aggregates, mastic cement, and air voids, and its load carrying behavior is strongly related to the local microstructural load transfer between aggregate particles. Numerical simulation of this micromechanical behavior was accomplished by using a finite-element model that incorporated the mechanical load-carrying response between aggregates. The finite-element scheme used a network of special frame elements each with a stiffness matrix developed from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. Continuum damage mechanics was then incorporated within this solution, leading to the construction of a microdamage model capable of predicting typical global inelastic behavior found in asphalt materials. Using image processing and aggregate fitting techniques, simulation models of indirect tension, and compression samples were generated from surface photographic data of actual laboratory specimens. Model simulation results of the overall sample behavior and evolving microfailure/fracture patterns compared favorably with experimental data collected on these samples.     

Effects of macroscopic pores on the damping behavior of foamed commercially pure aluminum

Experiments have been carried out to study the effects of macroscopic pores on the damping behavior of foamed commercially pure aluminum (FA). The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The FA specimens were prepared by an air pressure infiltration process. The size of the macroscopic pores was on the order of a millimeter and in large proportions, typically, up to 56 vol pct. The internal friction (IF) of the FA specimen was measured at low frequencies at room temperature. The measured IF shows that FA has a damping capacity, which is enhanced in comparison with bulk commercially pure aluminum, increases with increasing porosity, increases with decreasing macropore size, increases with increasing frequency, and increases with increasing strain amplitude. The microstructural analysis was performed using transmission electron microscopy (TEM). The TEM observations show that a moderate density of dislocation substructures exists near the grain boundaries in the FA. In order to explain these phenomena, the possible operative damping mechanisms in the FA are discussed in light of IF measurements and microstructural observations, and an approximate expression for IF is derived, which is based on the equation of plane waves in elastic material with voids.     

Undrained Shear Behavior of Cement Admixed Clay at High Water Content

Understanding of undrained shear behavior of cement admixed clay is of utmost importance for strength and deformation analyses in composite soft clay under short-term condition. From the critical analysis, the distinct difference in the responses of the same clay at uncemented and induced cemented states is brought out. The undrained shear behavior of uncemented clay is mainly dependent upon the clay fabric. The dismembering of the clay clusters in the fabric brings about the interlocking when the clay is in overconsolidated state. For the cement admixed clay, the clay is in meta-stable state. Hence, the strength and deformation characteristics are controlled by the clay fabric and cementation. The shear resistance is the sum of the shear resistance due to cementation qb and due to fabric qf. The term qb is practically constant with the increase in effective confining pressure at preyield state. The contribution from the clay fabric to the shear resistance qf comes into the picture at postyield state.     

Effects of Temperature and Strain Rate on Flow Behavior and Microstructural Evolution of Super Duplex Stainless Steel under Hot Deformation

Hot compression tests were carried out in the temperature range of 1 223-1 473 Kand strain rate range of0.01-30s-1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507(SDSS2507).It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization(DRX)and the flow stress increases with the decrease of temperature and the increase of strain rate.The apparent activation energy Qof SDSS2507 with varying true strain and strain rate is determined.As the strain increases,the value of Qdeclines in different ways with varying strain rate.The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter(Z).At a lower lnZ,dynamic recovery(DRV)and continuous dynamic recrystallization(CDRX)of the ferrite dominate the softening mechanism during the compression.At this time,steady state deformation takes place at the last stage of deformation.In contrast,a higher lnZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization(DDRX)of the austenite,which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.     

Effect of Microfabric on Mechanical Behavior of Kaolin Clay Using Cubical True Triaxial Testing

Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from K0 consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed strength behavior using cubical triaxial testing shows a similar pattern of variation with applied stress anisotropy; hence, only a marginal influence of fabric-induced anisotropy.     

Evaluation of Sample Quality of Sensitive Clay Using Intrinsic Compression Concept

A simple index, the degree of sample disturbance, is proposed in this study to quantitatively evaluate the quality of sensitive clay samples based on the concept of void index proposed by Burland in 1990. The degree of sample disturbance is defined as the ratio of the difference between the in situ void index and the void index of the undisturbed sample tested in the laboratory to the difference between the in situ void index and the void index of the completely remolded clay. All these indices are determined at the same effective overburden stress. Theoretically, the degree of sample disturbance varies from 0% (perfect undisturbed sample) to 100% (completely remolded sample). The proposed index is used in this study to evaluate the sensitive Ariake clay in Japan. Oedometer tests on undisturbed samples of natural Ariake clay obtained from the field using the current sampling practice in Japan show the degree of sample disturbance ranging from approximately 5 to 38%. The clay sample with an in situ void index closer to the intrinsic compression line has a lower degree of sample disturbance. In addition, a series of consolidation and unconfined compression tests were conducted on artificially disturbed samples in the laboratory to demonstrate the change of consolidation yield stress, unconfined compressive strength, and compression index with the degree of sample disturbance. A simple method is proposed in this paper to correct the mechanical parameters measured in a laboratory setting considering the degree of sample disturbance.     

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.     

Deformation consolidation of metal powders containing steel inclusions

Uniaxial consolidation experiments have been conducted at room temperature for two deformable metal powders (1100 Al and Pb5%Sb) containing various amounts of spherical steel inclusions. The experiments illustrate that the inclusion phase offers little constraint to matrix deformation at volume fractions <0.20, but produces a rapidly increasing constraint at larger volume fractions. Two constraining mechanisms have been identified through microstructural observations: (1) the matrix must be deformed more within the composite because of the excluded volume associated with the packing of particles and inclusions of different sizes, and (2) the inclusions form a continuous touching network (predicted with site percolation theory and direct observations of deformation flats on the steel spheres) which supports a portion of the applied stress and thus, partially “shields” the deformable phase from the total applied consolidation pressure. An analysis is presented to separate the contributions of each mechanism and shows that both mechanisms contribute comparable amounts to the constraint of matrix consolidation. It is suggested that because the inclusion network supports a significant portion of the applied pressure and releases its elastic strain in a non-linear manner (as described by Hertzian theory), the matrix is placed in tension when the applied pressure is released after consolidation.     

Volumetric Deformation of Natural Clays

A theoretical framework to describe the behavior of natural clay is proposed in a new four-dimensional space, consisting of the current stress state, stress history, the current voids ratio, and a measure of the current soil structure. A key assumption of the proposed framework is that both the hardening and the destructuring of natural clay are dependent on plastic volumetric deformation. Two different assumptions about how this destructuring occurs are proposed, based on which two versions of a complete constitutive model have been formulated. The behavior of reconstituted soil can also be simulated by the proposed model as a special case where the structure of soil has no effect on soil deformation. Characteristics of the proposed model are demonstrated through systematic simulations of the influence of soil structure on clay behavior. The simulated behavior of natural clay is compared qualitatively with widely available experimental data. It is seen that the proposed model successfully represents the main features of natural clays with various soil structures.     

Equivalent Effective Stress and Compressibility of Unsaturated Kaolinite Clay Subjected to Drying

Three-dimensional compressibility tests performed on unsaturated kaolinite clay subjected to drying showed that the volume change is a function of the equivalent effective stress (EES). The EES in the clay at different water contents was measured by performing direct tensile tests. When the clay has high water content (saturated funicular state), its volume decreases notably as the water content is reduced, i.e., the equivalent effective stress is increased. If the clay has a water content in an intermediate interval (complete pendular state), the volume is almost constant because the equivalent effective stress is almost constant. For the interval of low water contents (partial pendular state), the volume of the clay increases as the water content is reduced. This occurs because the equivalent effective stress is reduced when the moisture content in the clay is reduced, and contrasts with the saturated funicular state. The minimum volume in the clay was reached when the maximum equivalent effective stress was developed. A conceptual framework explains the influence of the different states of water distribution to the EES.     

Practical Model of Frost Heave in Clay

Freezing behavior of clay differs from that of silt. This difference stems primarily from the low permeability or hydraulic conductivity of clay, and the higher water content of saturated clay. Freezing effects include simultaneous heave and consolidation. Six small physical model columns of clay were frozen: one at 1g; and five on a centrifuge at various scales and with corresponding accelerations, to bring self-weight stresses into similarity with a full scale column of clay 4?m in height. The experimental results demonstrated the importance of replicating the prototype stress conditions in a model. They demonstrated the importance of local water content on development of heave in clay, and the relative insensitivity of heave to location of the phreatic surface. Low permeability caused the clay to behave essentially as a closed system with regard to water flow. A simple analytical model was developed to explain observed soil response. Further research is recommended to provide more guidance in selecting input parameters.     

Temperature-dependent void-sheet fracture in Al-Cu-Mg-Ag-Zr

Previous research showed that tensile fracture strain increases as temperature increases for AA2519 with Mg and Ag additions, because the void-sheet coalescence stage of microvoid fracture is retarded. The present work characterizes intravoid-strain localization (ISL) between primary voids at large constituents and secondary-void nucleation at small dispersoids, two mechanisms that may govern the temperature dependence of void sheeting. Most dispersoids nucleate secondary voids in an ISL band at 25 °C, promoting further localization, while dispersoid-void nucleation at 150 °C is greatly reduced. Increased strain-rate hardening with increasing temperature does not cause this behavior. Rather, a stress relaxation model predicts that flow stress and strain hardening decrease with increasing temperature or decreasing strain rate due to a transition from dislocation accumulation to diffusional relaxation around dispersoids. This transition to softening causes a sharp increase in the model-predicted applied plastic strain necessary for dispersoid/matrix interface decohesion. This reduced secondary-void nucleation and reduced ISL at elevated temperature explain retarded void sheeting and increased fracture strain.     

Simplified Method for Consolidation Rate of Stone Column Reinforced Foundations

Field observations and numerical studies demonstrated that stone columns could accelerate the rate of consolidation of soft clays. A simplified method for computing the rate of consolidation is presented in this paper by assuming that stone columns; (1) are free draining; (2) have higher drained elastic modulus than soft clay; and (3) are deformed 1D. The formats of the final solutions in vertical and radial flows are similar to those of the Terzaghi 1D solution and the Barron solution for drain wells in fine-grained soils, respectively. Modified coefficients of consolidation are introduced to account for effects of the stone column-soil modular ratio. The new solutions demonstrate stress transfer from the soil to stone columns and dissipation of excess pore water pressures due to drainage and vertical stress reduction during the consolidation. Comparisons between the results from this simplified method and the numerical study by Balaam and Booker in 1981 exhibit reasonable agreement, when the stress concentration ratio is in the practical range (2–6). The discrepancies in the results from these two methods are discussed. This paper also includes design charts and a design example.