Loose State Research Articles

Dilatancy for cohesionless soils

Dilatancy is often considered a unique function of the stress ratio η = q/p′, in terms of the triaxial stress variables q and p′. With this assumption, the direction of plastic flow is uniquely related to η, irrespective of the material internal state. This obviously contradicts the facts. Consider two specimens of the same sand, one is in a loose state and the other in a dense state. Subjected to a loading from the same η, the loose specimen contracts and the dense one dilates. These two distinctly different responses are associated with a single η but two different values of dilatancy, one positive and the other negative. Treating the dilatancy as a unique function of η has developed into a major obstacle to unified modelling of the response of a cohesionless material over a full range of densities and stress levels (before particle crushing). A theory is presented that treats the dilatancy as a state-dependent quantity within the framework of critical state soil mechanics. Micromechanical analysis is used to justify and motivate a simple macroscopic constitutive framework. A rudimentary model is presented, and its simulative capability shown by comparison with experimental data of the response of a sand under various initial state and loading conditions.

Fundamental factors affecting liquefaction susceptibility of sands

Liquefaction phenomena in saturated sands are examined. Fundamental factors that influence liquefaction susceptibility are considered from the background of comprehensive experimental evidence from test results on reconstituted specimens. These include those related to the loading system characteristics which do not enable measurements of the true post-peak behaviour. In particular, several issues related to the influence of initial state variables, on which there appears to be a lack of consensus in the literature, are dealt with. It is shown that at identical initial void ratio - effective stress state, undrained (constant volume) behaviour is profoundly affected by the fabric that ensues upon sample reconstitution. Very loose moist-tamped states are unlikely to be accessible to in situ sands. The susceptibility to liquefaction, both static and cyclic, is not only dependent on the initial state variables, but is also strongly affected by the effective stress path during undrained shear. Comparative tests on undisturbed samples retrieved by in situ ground freezing and their reconstituted counterparts show that water-pluviated specimens closely mimic the behaviour of in situ sands. Very small expansive volumetric strains due to pore-pressure gradients during short-duration loading, or after its cessation, could transform a sand into a strain-softening type, which otherwise would be dilative if completely undrained.Key words: anisotropy, laboratory tests, liquefaction, sampling, sands, shear strength.

Intergranular state variables and stress–strain behaviour of silty sands

Relative contributions by the coarser and finer grains in a silty sand to its stress–strain response are affected by the intergranular matrix structure. The nature of this contribution is illustrated using an intergranular matrix phase diagram in terms of void ratio (e), fines content (FC), and intergranular and interfine void ratios (es and ef). New intergranular state parameters (ψs, ψf) and (es, ef) are introduced as state variables to characterize silty sands; es and ef dictate the steady-state characteristics of silty sand at low and high fines contents, respectively; ψs and ψf reflect the plastic compressibility characteristics at low and high fines contents, respectively. Using these state variables, the anticipated stress–strain–strength behaviour of silty sand in comparison to that of the host sand is presented. Similar stress-strain behaviour is expected at the same es and initial confining stress σ′c, with a few exceptions. At a constant void ratio e, es increases while ef decreases with addition of fines; a silty sand passes through different states. First, at low fines content, es (< emax,HS, the maximum void ratio of the host sand, case 1), and high ef the stress–strain behaviour is primarily governed by intergranular friction between the coarser grains. The steady-state line (SSL) is primarily dependent on es and is fairly independent of σ′c; When compared at the same es and at the same σ′c(or at the same ψ or ψs), the silty sand and the host sand show similar (not identical) stress–strain behaviour; with addition of fines, as es increases, the collapse potential increases and the stress–strain response becomes weaker. Second, with further addition of fines, when es approaches or exceeds emax,HS, the SSL is influenced by es and σ′c. When es is near emax,HS (case 2), the stress–strain curves for silty sands are similar at the same es and the same σ′c (or at the same ψ or ψs), but different from and stronger than that of the host sand. At very loose states (es > emax,HS; case 3), the stress–strain curves, normalized with respect to σ′c, are similar at the same es (or at the same ψ or ψs). Third, with further addition of fines beyond a threshold value, ef becomes sufficiently low (case 4); the fines impart a significant influence, while the role of intergranular (coarser-grain) friction diminishes; the silty sand is expected to behave similarly (not identical) to the host fines at the same ef and ψf ; at this stage, with further addition of fines, the collapse potential decreases.

Nucleotide binding of an ADP analog to cooperating sites of chloroplast F1-ATPase (CF1).

Pre-steady state nucleotide binding to the chloroplast F1-ATPase (CF1) was measured in a stopped-flow apparatus by monitoring the change of fluorescence intensity of TNP-ADP upon binding. The analysis of the time courses led to the proposal of a mechanism of nucleotide binding with the following characteristics. (a) It involves three sites binding nucleotides in a concerted manner. (b) Each binding site is able to undergo a conformational change from a loose binding state into a state refraining from any direct release of the bound nucleotide into the medium. Only the reverse reaction via the loose binding state enables release out of the tight binding state. (c) Due to very strong negative cooperativity, a maximum of two of the three sites can be found in the state of tight binding. (d) Cooperativity between the three sites leads to a slower nucleotide binding of the second nucleotide compared to the first nucleotide. Furthermore, the conformational change from the loose binding state to the tight binding state is slowed down if one of the other sites already is in the tight binding state. Three-sites mechanisms in which rotation leads to an exchange of the properties of the binding sites failed to simulate the observed time courses of nucleotide binding. However, as the experimental set up was designed to prevent catalysis taking place, our results entirely agree with the current finding that rotation requires catalytic turnover of the enzyme.

Inhibition of SNARE Complex Assembly Differentially Affects Kinetic Components of Exocytosis

In chromaffin cells, an increase in intracellular Ca 2+ leads to an exocytotic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of two separate pools of vesicles. To investigate how these components relate to SNARE complex formation, we introduced an antibody that blocks SNARE assembly but not disassembly. In the presence of the antibody, the sustained component was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form before Ca 2+-triggered membrane fusion and exist in a dynamic equilibrium between a loose and a tight state, both of which support exocytosis. Interaction of the antibody with preformed SNARE complexes favors the loose state.

State-dependant dilatancy in critical-state constitutive modelling of sand

A bounding-surface hypoplasticity model is modified to incorporate the basic premises of critical-state soil mechanics and cover both dense and loose sand behavior. The modification consists of rendering the phase-transformation line a function of the state parameter, which measures the difference between the current and critical void ratios at the same value of p, such that when the state parameter is zero, the phase-transformation line becomes identical to the critical-state line in q-p space. As a result the dilatancy depends on the state in a way that yields a zero value at critical state. This dependence allows a realistic modelling of the response of a sand in either loose or dense state, or in the transition from one state to another state. A comparison between model simulations and a sequence of experimental results for drained, undrained, monotonic, and cyclic loading conditions shows that the proposed concept and modelling technique work effectively over a wide range of densities and confining pressures using a unique set of parameters (or parameter dependence) for a given sand.Key words: bounding surface, critical state, dilatancy, phase transformation, soil plasticity, state parameter.

Vacuum consolidation of soda-ash tailings

The Solvay process, widely used in industrial production of soda ash, generates a large quantity of tailings. The tailings consist of fine solids suspended in alkaline water that remain in a loose state, causing environmental concerns. An alternative solution for the management of sodaash tailings is presented in this paper, that is, the use of a vacuum pre-loading technique to consolidate the tailings so that they can be used as a foundation material in industrial sites. This paper presents the procedure and results of vacuum pre-loading consolidation of soda-ash tailings in a field trial. A vacuum pressure of approximately 90 kPa was applied to a 45 × 45 m area to a depth of 8 m via prefabricated vertical drains. The results showed that after 66 days of vacuum pre-loading treatment, the water content of the soda-ash tailings was reduced by 12·3% to 42·4%. Also, densification was shown by changes in void ratio, unit weight and settlement. The tailings were consolidated to a material with the undrained shear strength ranging from 22·3 to 34·4 kPa, compared with the initial value of 7·1–14·5 kPa. The allowable bearing capacity of the tailings reached 120 kPa as tested from the static loading plate test. The vacuum pre-loading method has shown to be a fast, effective and economically feasible alternative for in situ consolidation of soda-ash tailings.

Experimental Evidence for the Anisotropic Ostwald Ripening of β‐Silicon Nitride

A powder mixture of α‐Si3N4, Y2O3, and SiO2 was heat‐treated in a loose powder state in the temperature range of 1750°–1900°C for 2 h; then, the mixture was acid‐rinsed to remove the glassy phase. The widths and lengths of the resulting β‐Si3N4 crystals were analyzed quantitatively. The width–aspect‐ratio distribution of the β‐Si3N4 crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width‐aspect‐ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the β‐Si3N4 crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model.

State-dependant dilatancy in critical-state constitutive modelling of sand

A bounding-surface hypoplasticity model is modified to incorporate the basic premises of critical-state soil mechanics and cover both dense and loose sand behavior. The modification consists of rendering the phase-transformation line a function of the state parameter, which measures the difference between the current and critical void ratios at the same value of p, such that when the state parameter is zero, the phase-transformation line becomes identical to the critical-state line in q-p space. As a result the dilatancy depends on the state in a way that yields a zero value at critical state. This dependence allows a realistic modelling of the response of a sand in either loose or dense state, or in the transition from one state to another state. A comparison between model simulations and a sequence of experimental results for drained, undrained, monotonic, and cyclic loading conditions shows that the proposed concept and modelling technique work effectively over a wide range of densities and confining pressures using a unique set of parameters (or parameter dependence) for a given sand.Key words: bounding surface, critical state, dilatancy, phase transformation, soil plasticity, state parameter.

Experimental investigation of shell foundations on dry sand

Shells are usually used as structural elements in buildings. In Germany they showed remarkable resistance to the effects of bombing during World War II. About 1 decade later, the possibility of employing shells in foundation engineering was explored. Surveys of the literature indicate that shell foundations have been employed effectively in different parts of the world and were proven to provide an overall economical alternative to the conventional flat foundations. However, the geotechnical design of these footings remained the same as for their respective flat ones. Accordingly, the advantages of shell geometry in foundation engineering has not yet been explored in the design of these footings. The objective of the present study is to examine the overall geotechnical behavior of three types of shell foundations resting on sand under axial loading conditions, namely, triangular, conical, and pyramidal shells. Furthermore, the resulting bearing capacities and settlements will be compared with conventional strip, circular, and square flat foundations. The present paper presents an experimental study on nine foundation models tested on loose, medium, and dense sand states. The influence of shell configuration and embedment depth on the ultimate bearing capacity and settlement will be presented. The results of the present experimental investigation have shown the admirable performance of shell foundations with respect to ultimate bearing capacity and settlement characteristics. Shell foundations provide higher resistance to lateral loading as compared with flat ones, and thus they will perform better in earthquake regions.Key words: shell foundation, experimental investigation, bearing capacity, settlement, sand, geotechnical engineering.

Inhibitor binding changes domain mobility in the iron-sulfur protein of the mitochondrial bc1 complex from bovine heart.

We have analyzed crystal structures of cytochrome bc1 complexes with electron transfer inhibitors bound to the ubiquinone binding pockets Qi and/or Qo in the cytochrome b subunit. The presence or absence of the Qi inhibitor antimycin A did not affect the binding of the Qo inhibitors. Different subtypes of Qo inhibitors had dramatically different effects on the mobility of the extramembrane domain of the iron-sulfur protein (ISP): Binding of 5-undecyl-6-hydroxy-4, 7-dioxobenzothiazol and stigmatellin (subtype Qo-II and Qo-III, respectively) led to a fixation of the ISP domain on the surface of cytochrome b, whereas binding of myxothiazol and methoxyacrylate-stilbene (subtype Qo-I) favored release of this domain. The native structure has an empty Qo pocket and is intermediate between these extremes. On the basis of these observations we propose a model of quinone oxidation in the bc1 complex, which incorporates fixed and loose states of the ISP as features important for electron transfer and, possibly, also proton transport.

Recombination of radicals in the high-pressure and high-temperature limit Part 1Reaction CH3+CH3

The reaction CH3 + CH3 –(k)→ C2H6 is used in a theoretical study of radical recombination in the gas phase. Long-range attractive forces are studied with the use of adiabatic curves that describe weak reactant interactions. Provided that the kinetic rotational energy is sufficiently large, so that the orbital momentum remains smaller than the boundary value llim, the radicals form loosely bound states CH3···CH3. Such loose states are of crucial importance in the postulated reaction mechanism. At higher pressures and temperatures (T > 200 K) the maximum asymptotical rate constant for the recombination of methyl radicals is given by the expression krec(T) = Lge√[h/(RT)](ℏ/μ)3/2〈llim〉2where L = 1/2 is the ratio of the symmetry coefficients for the reagents and transition state, ge = 1/4, µ is the reduced mass and 〈lmin〉 is the threshold value for the orbital quantum number l, averaged over all the states. For two methyl radicals 〈llim〉 = 39 ± 2. This simple analytical formula reproduces experimental results very well. The contribution of direct recombination involving a strongly bound complex is smaller by about an order of magnitude than that involving loosely bound CH3···CH3 complexes exclusively.

Ground Improvement at Chek Lap Kok: Review of Vibrocompaction at Hong Kong’s New Airport

The new Hong Kong airport at Chek Lap Kok is being constructed on a 1248 hectare site of which 928 hectares is reclaimed land. Part of the reclamation consists of marine sand fill. Site investigation, including the CPT and shear wave velocity testing, indicated that the sand was in a relatively loose state state below the water table, with relative densities in the order of 20 to 40%. Vibrocompaction was carried out at three different areas of the reclamation in order to increase the reiative density of sand fill, thereby increasing its’ stiffness and strength, and creating an improved and more uniform material for follow on construction. The method for acceptance testing for the vibrocompaction was based on the use of the CPT, with the acceptance criterion set as a tip resistance value of either 8 MPa or 15 MPa, depending on required degree of compaction. This paper presents the results of the vibrocompaction and discusses some of the factors encountered during the compaction that can lead to apparent no...

Using Geogrids for Base Reinforcement as Measured by Falling Weight Deflectometer in Full-Scale Laboratory Study

A model road was constructed in a laboratory. The road consisted of asphalt over a crushed rock base and a silty sand and gravel subbase. The silty sand and gravel were placed in a very loose state to simulate a thaw-weakened, poor-quality subbase. The water table was kept at 152 mm (6 in.) below the bottom of the asphalt. The model road was divided into three sections. A geogrid was installed at the bottom of the base course material in two of the test sections, and the third was left as a control section. A falling weight deflectometer was used to measure the dynamic response of the pavement structure. The traffic benefit ratio is defined as the expected life (equivalent single-axle loads) of one section divided by the expected life of another section. The Alaska Department of Transportation and Public Facilities asphalt pavement design procedure and the NCHRP pavement design procedure were used to compare the test sections. The results from the two procedures were very similar. By using either procedure, the life of the pavement with respect to reinforcement was on the order of 2 to 4, depending on the type of grid and the depth of base course material.

Segregation phenomena in a shaker

The segregation phenomena of different binary mixtures were studied experimentally in a vertical shaker. The larger particles tend to move upwards in the shaker in a certain range of vibrational accelerations. The segregation effect was more significant for the mixture with a larger size difference. The segregation coefficient was measured and found to increase with vibrational acceleration until reaching a maximum and then to decrease with acceleration. The bed expansion height was also measured. It was found that the greatest segregation effect occurred when the bed transformed from a dense state to a loose state.

Preliminary evaluation of the influence of surfactant injection on the mechanical behaviour of a compacted till

One-dimensional consolidation tests were performed with till samples compacted dry of optimum to 85% of the modified Proctor density. When the applied pressure reached 200 kPa, the samples were saturated (submerged) and then flushed with two types of surfactant solutions, under a hydraulic gradient varying from 35 to 40. Significant compression occurred during submergence and as a result of flushing with a 2% solution of an anionic surfactant. Only minor additional compression resulted from flushing with a 2% solution of a nonionic surfactant. While only preliminary conclusions could be drawn from the limited test results, this study highlighted the importance of considering, in the early stages of remediation procedure design (including the selection of the surfactant), the possible deleterious effects of surfactant injection on the mechanical properties of the soil mass, particularly when it is in a loose to moderately loose state. Key words : surfactants, adsorption, hydrocarbon remediation, compression, mechanical properties, soil behaviour.

Unified 3D critical state bounding‐surface plasticity model for soils incorporating continuous plastic loading under cyclic paths. Part II: Calibration and simulations

AbstractA systematic calibration procedure for the constitutive model gUTS using conventional triaxial data for all but one of the material constants is described. Typical ranges for the constants for clay and sand are specified together with default values. When all default values are adopted, just eight material constants need to be determined (gUTS‐lite). A comprehensive series of 49 simulations on clays, silt and sands in loose and dense states under a wide range of monotonic and cyclic stress paths, initial states and drainage conditions provide very satisfactory agreement with experimental results.

Unified Critical‐State Bounding‐Surface Plasticity Model for Soil

An isotropic plasticity model for soil is presented that introduces a number of significant modifications into an existing bounding‐surface model for clay to extend the range of application to sand, offering a simple unified treatment for soil. The paper focuses on the modeling of sands. The new features are both radial and deviatoric mapping rules to define the loading surface, a nonassociated flow rule incorporating a subelliptic plastic potential surface, and the presence of a kinked critical‐state line. Unlike many existing macroscopically based models, loose and dense states of a specific sand are not given different sets of material constants since the two states are linked by means of an “effective” normal consolidation line. Thus, for instance, sand in a dense state is considered very heavily overconsolidated in the normal mean effective‐stress range. After presenting the normalized triaxial formulation in full and discussing the calibration procedure, comparisons are made between the model's simu...

ビン遷移点付近の壁圧変動の緩衝法 シリンダー部の拡大効果

This paper outlines an approach to reducing the pulsating wall pressure near the transition point in a bin which is generated during the discharge of a dense bed achieved by peripheral feeding. The authors present a method for controlling the flow of the dense bed by means of installing a device that widens the cylindrical section near the bin transition point, thus changing the flow from a dense bed to a loose bed state. The stress states in the hopper section before and during discharge are explained by Mohr circles.In particular, this method can reduce the pulsating wall pressure in the hopper that maintains the dense bed in the cylindrical section of the bin.

繊維まわりに形成させたＺｎ‐Ａｌ合金相の軟化を利用したステンレス鋼繊維／アルミニウム複合材料板の曲げ加工とその強化

Stainless steel fiber reinforced aluminum with using zinc to form the interface layer was fabricated, and bending of the composite as secondary forming was tried. That composite was fully strengthened at 643K which was just below eutectic temperature of Zn-Al alloy (655K). Over the eutectic temperature, the fiber-matrix interfacial stress τ of the composite was remarkably reduced. Taking advantage of loose state of the interface layer, bending of the composite became possible without fiber breakage and buckling which was impossible in the case of the composite at lower temperatures or without the interface layer. The once remarkably reduced τ could be increased by mutual diffusion between the interface layer and the matrix caused by heat treatment, and the composite was fully strengthened even at 673K which was beyond the eutectic temperature. Accordingly, the possibility of realizing long fiber reinforced metal which could be secondarily formed was clarified.