End platen conditions are known to affect the global response of triaxial specimens during laboratory testing of soils. For example, these effects lead to barrel-shaped specimens in axial compression tests with associated difficulties in determining cross-sectional area and stress distributions. This paper presents the results of a study that used optical analysis of images captured from resin impregnated specimens at various global axial strain levels to study how internal microstructure varied as a function of the end platen conditions. Tests were performed on dilatant specimens with different slenderness ratios and different end platen configurations. The global response was quantified in terms of axial and radial strain behavior. The microstructure was quantified in terms of spatial void ratio distribution and local void ratio distribution. Analysis relating observed global response to internal microstructure evolution is presented. The results of this experimental study show that the end platen constraint is critical for microstructure evolution in dilatant specimens.

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The results of triaxial tests carried out on both uncemented sands and sands injected by a microfine cement grout are presented in order to highlight and compare the main featuers of their mechanical behavior. The device used to prepare homogeneous samples of grouted sands in the laboratory is first described. The effect of the grout injection on the strength properties of three granular soils is then shown. By use of bender elements, the different evolutions of the shear modulus G_{vh, max} in the small strain domain along isotropic and deviatoric stress paths are presented for both uncemented and grouted sands.

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Very small to large strains properties of a silty sand were measured at several water contents ranging from a few percents to saturation, using three triaxial devices with different types of transducers, In the analysis of the data, attention was focused on the influence of water content on the maximum (Young's) modulus, the decay of modulus with strain, the effect of the confining stress, etc. Isotropic and triaxial tests, with measurement of negative pore water pressure, were performed on the same material. The effective sterss concept, validated by a recent theoretical analysis by Coussy and Dangla (2002), was used to interpret the results. The predictions of a micromechanical model are compared with the experimental data in the elastic domain and on the failure criterion. The method highlights a simple way to relate the strength and stiffness of the soil to its negative pore water pressure.

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Railway ballast deforms and degrades progressively under heavy train (cyclic) loading. Ballast degradation is influenced by several factors including the amplitude and number of load cycles, density of aggregates, track confining pressure, angularity, and most importantly, the fracture strength of individual grains. The degraded ballast is usually cleaned on track, otherwise fully or partially replaced by fresh ballast, depending on the track settlement and current density. The disposed waste ballast may be cleaned, sieved and re-used in the track. however, due to the breakage of the sharp edges and the development of micro-cracks during the previous loading cycles, recycled ballast is generally subject to excessive settlement and particle breakage. Therefore, the settlement and degradation aspects of recycled ballast must be carefully examined before recycling to the track. This paper investigates the deformation and degradation characteristics of recycled ballast under field-simulated loading conditions. Cyclic triaxial tests were conducted on both fresh and recycled ballast. The potential of recycled ballast as an alternative track construction material has been examined using geosynthetic technology, and the results are discussed in detail.

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This paper aims to clarify static and cyclic deformation-strength properties and evaluation method for volcanic coarse-grained soils with particle crushing, based on site investigations which were conducted using standard penetration test (SPT), cone penetration test (CPT) and seismic cone penetration test (SCP). In addition to these in-situ tests, a series of laboratory tests on disturbed and undisturbed samples was also carried out to obtain the mechanical properties such as shear strength, cyclic undrained triaxial strength, pseudo elastic shear modulus and damping ratio. From the results of drained and undrained triaxial compression tests, plane strain compression test and cyclic undrained triaxial test on volcanic soils, it was understood that the effect of particle crushing and fabric anisotropy on the cyclic and static strength-deformation behavior of volcanic coarse-grained soils could not be ignored. Furthermore, the effect of particle breakage on results of dynamic penetration test was also discussed based on the correlation between the sounding data and triaxial test results.

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The paper investigates the anisotropic yielding behaviour of dense Toyoura sand from very small to large strain in a p'-constant shear plane by means of hollow cylinder torsional shear tests. Efforts were made to draw yield surfaces in the stress space. To pay special attention to anisotropic yielding due to solely the shear process, p' was kept constant to avoid yielding caused by the change of p'. σ'₁-direction from the vertical in the shear process was fixed to be from 0°to 90°at 11.25°interval. It was found that the sand starts yielding progerssively from the strain level as small as 0.002%. The simple way of interpreting elastic behaviour below the large-scale yield surface, which is determined to be much larger strain level, can not explain such behaviour. Two subyield surfaces, Y₁ and Y₂, were identified inside the large-scale yield surface, Y₃. It is shown that these surfaces can explain nonlinearity and degradation of stiffness moduli. Anisotropy and mobility of the subyield surface were also studied in the p'-constant shear plane. When the initial stress state was at the origin of the shear plane (isotropic stress state), the shape of the yield surface was approximately circular with the center shifted toward compression in the vertical direction. Anisotropy increased with the progress of shearing. Y₁ and Y₂ surfaces were mobile and moved with the current stress point in the p'-constant shear plane, though Y₃ surface was comparatively immobile. The shapes of yield surfaces also changed with the movement of current stress point.

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Soil near ground surface is usually unsaturated and the modulus value is affected by the seasonal variation of water content and the corresponding capillary pressure. The Stokoe-type torsional resonant column (RC) testing equipment was modified to control the pore air and pore water pressures separately and to measure the variation of water content due to capillary pressure. Subgrade soil was collected at a test road site where the crosshole and water content measurements were performed with depth. RC tests were performed at various water contents by controlling capillary pressures using a given specimen and by using specimens compacted at various water contents. As the capillary pressure increased, the shear modulus was increased at the whole strain range due to the increase of effective stress between soil particles. The relation between maximum shear modulus (G_max and water content was much flatter and G_max value is small at a given water content for specimens prepared by controlling compaction water contents compared with those tested by controlling capillary pressures. The normalized modulus (G/G_max) reduction curve was almost unique independent of compaction water content and density during sample preparation and variation of water content due to capillary pressure during testing. Field crosshole tests were also performed which showed that G_max values determined by crosshole tests matched well with those determined by controlling capillary pressures in the laboratory, but also showed substantial difference with results obtained by controlling compaction water contents.

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The shaft capacity of piles in sand subjected to cyclic (wave) loading has been observed to decrease significantly with loading cycles (Poulos, 1989). A number of researchers (Boulon and Foray, 1986 ; Tabucanon et al., 1995 ; Shahrour et al., 1999) have replicated the characteristics of the load transfer degradation behavior in the laboratory through cyclic interface shear testing with a constant normal stiffness confinement condition (Vesic, 1972). However, no consensus currently exists as to the primary microscale mechanisms that govern cyclic interface shear behavior and load transfer degradation. A research program was undertaken to quantify the contribution of soil properties, cementation, confinement condition, and displacement mode, in load transfer degradation. Monotonic and cyclic interface shear tests were performed using a modified interface direct shear device with Perspex side window. The specimen particle displacement fields were quantified during selected cycles by capturing high resolution digital images (1600×1200 pixels) and using Particle Image Velocimetry (White et al., 2001a). Results indicate that the confinement condition, which is intended to replicate the elastic response of the fa-field soil, is of primary importance as it allows for normal stress relaxation with soil contraction adjacent to the interface. The displacement magnitude, particle characteristics, and particle-particle cementation were also observed to affect the magnitude and rate of degradation. It is anticipated that these findings will provide a fundamental rationale to identify field conditions where shear stress degradation is lilely to occur and a basis from which more rigorous models may be developed.

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The importance of grain rotation during shear deformation has been widely recognized in the mechanics of granular materials, which has led to extensive use of the Cosserat continuum theory in localization problems. Strain gradient theory, which relates the macro deformation gradient to higher-order stresses, is another possibility to overcome the ill-posedness of governing equations. This paper attempts to show an experimental basis for applying strain gradient theory to granular media. LAT (Laser-Aided Tomography), a technique to visualize the interior of 3-D granular assembly, is used to detect the grain rotation as well as the continuum rotation. A Discrete Element simulation is also conducted to reinforce the experimental data. It is concluded that the average grain rotation is roughly identical to the continuum rotation, which supports the applicability of rotational gradient theory, a particular case of strain gradient theory from the micro-mechanical point of view.

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Understanding the mechanisms behind time-dependent behaviour of granular materials is important in assessing agein of dynamically improved sands and set-up displacement piles in sand. A series of triaxial tests was carried out to investigate the creep of dense granular materials. In addition to shear strain development, a complex volumetric strain response with rotation of the creep strain vector over time was measured and the effect of the material characteristics was examined. The change in microstructure of dense sands during one-dimensional creep was also investigated using resin injection and optical microscopy of sections. Upon application of load, particles aligned to be more perpendicular to the load direction. However, over time, the particles rotated in space. A change in the local void ratio distribution was also found. Initially, the particles appeared relatively evenly spaced. However, with time, they grouped or clustered together. A conceptual model, accounting for these changes in microstructure, is proposed. The model emphasises the importance of the bimodal load-bearing nature of granular materials, suggesting that creep begins in frictional slippage of weakly loaded particles, which allows strongly loaded columns of particles to gradually adjust. The model may help to explain the complex volumetric creep response of dense soils and why dynamically densified soils 'age' with no detectable change in relative density.

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The old alluvium formed in early Pleistocene in San Juan, Puerto Rico has undergone substantial post-depositional weathering in the tropical marine climate, resulting in a special combination of mineralogy and microstructure, The stiff intact material consists of interconnected silt-sized aggregates, which in turn comprise groups of clay platelets containing smectites. Finely divided Fe-oxides act as coatings over clay platelets and bridge connections between aggregates so that a stiff network of aggregates is formed in the intact material. Such microstructure with Fe-oxides cementation renders this soil unusual deformation characteristics. The consolidation behaviour differs significantly from that of common sedimentary soil : (1) the coefficient of consolidation decreases from 0.1 to 10^-5 cm² / s by four orders of magnitude as the soil is compressed to 20 MPa ; (2) the swelling strain increases significantly with the maximum past consolidation stress ; (3) the intact soil exhibits exceptionally high yield stresses (e. g., 0.8 MPa and 6 MPa for the upper and lower layers, respectively) ; and (4) the normal compression strain can be completely recovered upon unloading when samples are pre-compressed beyond the yield stress. Results of triaxial drained shear tests suggest that the intact soil possess isotropic Mohr-Coulomb strength parameters and a nearly constant cohesion over depth, despite variations in mineralogy and density with depth. Such abnormal deformation characteristics are the macroscopic exhibitions of microstructure alteration caused by stresses.

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In the North Sea Elofisk oilfield, oil is located in a 300 m thick layer of porous chalk (n=40-50%) at a 3000 m depth. After the initial phase of depletion an enhanced oil recovery procedure was carried out by injecting sea water (waterflooding). An unexpected consequence of this waterflooding has been the occurrence of a seafloor subsidence, corresponding up to now to a decrease of the seafloor level of approximately 10 m. It is now well recognised that hydro-mechanical coupling involving multiphase fluid interactions (oil and water) is determinant for the interpretation of the phenomenological aspects associated with the chalk compaction and the related subsidence observed in the North Sea oilfields (Ekofisk reservoir) when water flooded. The subsidence due to waterflooding is interpreted as a collapse phenomenon due to suction decrease, typical of loose and low plasticity unsaturated soils when wetted under load. On the other hand, time-dependent stress-strain behaviour of geomaterials is one of the major concerns in soil mechanics and, in effect, subsidence includes creep effects. A multiphase approach, including creep effects under controlled suction levels, is proposed in this paper ; the preliminary results of this study are presented and discussed. Attention will be focused at first on the theoretical approach, supplying the essential elements for the work, and on the interpretation of the experimental results. This will provide secondly the base for formulation and validation of the constitutive law proposed for the description of the time-dependent mechanical behaviour of the chalk.

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The possibility of evaluating undrained cyclic strength of natural gravelly deposits using shear moduli was investigated. Through this study, the following conclusions were drawn. 1) It was found that cyclic shear moduli at relatively large strain amplitude were closely related with undrained cyclic strengths, irrespective of the effective confining pressure, grain size distributions and whether or not the samples are intact, whereas there seems to no unique relationship between nitial shear moduli and undrained cyclic strengths. 2) It was found that the G/G_0-γ relation of an intact sample can be approximated by that of the reconstituted sample depending on the initial shear modulus of the intact sample. Based on this fact and the relationship mentioned in 1), a simplified method to evaluate undrained cyclic strength of gravelly soil by shear modulus of the ground, which was calculated using both initial shear modulus of the ground and the G/G_0-γ relation of the disturbed sample, was proposed. 3) It was revealed that reduction in shear modulus with an increase in shear strain is closely related to void ratio. Based on this fact and the relationship mentioned in 1), a more simplified method to evaluate undrained cyclic strength of gravelly soil by initial shear modulus and void ratio of the ground was proposed.

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A series of true triaxial tests on tell prismatic specimens of sand at three different void ratios from the basis for analyses, prediction and verification of the influence of shear banding on the three-dimensional failure conditions in granular material. The results of the true triaxial tests are reviewed and discussed in view of the shear banding that is observed in these tests. Theoretical conditions for shear banding, which are based on the behavior of the material near the observed shear banding phenomenon, are reviewed. The detailed procedure for shear banding analysis is explained in general and calculations are performed for a single hardening elasto-plastic constitutive model. The predicted shear banding events are compared with those observed in the experiments. It is shown that the predicted stress conditions for shear banding match those observed in the true triaxial tests, and it is concluded that shear banding plays an important role in the three-dimensional strength of granular materials. Thus, peak failure is caused by shear banding in the hardening regime in the approximate range of b=(σ₂-σ₃)/(σ₁-σ₃) form 0.18 to 0.85, while it occurs in the softening regime outside this range of b-values. A smooth, continuous 3D failure surface is therefore not generally obtained for granular materials.

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A one-dimensional consolidation model of the isotach-law type is proposed for normally consolidated clays which have neither interparticle bonding nor sensitive structure. At first, the mechanical quantities that can absolutely define a consolidation state without employing any arbitrary reference satate are examined, and it is concluded that specific volume, f(=1+e), effective stress stress, σ', and natural strain rate, ε^^'_n, are best. Next, the existence of a single relationship f-σ'-ε^^'_n after compressional yielding is verified by the results obtained from various types of laboratory consolidation tests with the measurement of long-term secondary settlement. Then, a way of conceptualizing how to incorporate this relationship into the new consolidation models is introduced, and numerical simulations based on the new model are shown for clay layers of field-size. And finally, the simulation results are used to discuss the application limits of the Terzaghi theory.

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Strain rate sensitivity is one of the typical time-dependent behaviors of soil as well as creep and stress relaxation. In particular, it is well known that a unique stress-strain curve exists for each different strain rate in clayey soil : the isotaches characteristics. Originally, the concept of isotaches was proposed in one-dimensional consolidation of clay ; however, we can also observe the isotaches characteristics in the stress-strain relation obtained by the triaxial compression test with various strain rates. The purpose of the present study is to reexamine the concept of isotaches through the detailed results of triaxial tests on reconstituted Fukakusa clay and a constitutive modeling. Undrained triaxial tests both of normally consolidated and overconsolidated clay were performed with various constant strain rates and step-changed strain rates to observe the isotaches characteristics in a wide range of axial strain. The results of the step-changed strain rates test show that the phenomenon of isotaches exists in the range of low level strain. It is also seen, however, that this phenomenon is not observed ina range of high level strain, in particular, around the critical state. The results of step-changed strain rate tests were numerically simulated by an elasto-viscoplastic model for clay. It was found that the model can well simulate the trend of the isotaches behavior in the range of low level strain observed in the experiment. In addition, the trend of stress-overshooting and stress-undershooting in the range of high level strain observed in the experiment can be described adequately using the model except for the stress-overshooting of overconsolidated clay.

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Isotache models are being used increasingly to describe the time-dependent compression of soft soils. Parameters are usually obtained from incremental loading compression tests and, in the case of 2D isotache models, triaxial tests as well. It is possible to determine all necessary parameters, including strength parameters, from a constant rate of strain oedometer which also measures lateral stress. This is described in the paper. Some effects of unloading on the form of the isotaches are also presented, and it is shown that to be completely effective, adaptations of existing isotache formulations are necessary.

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The mechanical response of rockfill is largely dominated by particle breakage, which is controlled by stress level and the prevailing relative humidity (RH) in the large rockfill voids. The paper presents a model for rockfill behaviour under triaxial stress states, which is based on : (a) existing knowledge on rockfill compressibility, (b) experimental and theoretical developments on sand behaviour and (c) a series of suction-controlled triaxial tests performed in a new large-diameter triaxial cell, in which samples of gravel were tested under two relative humidities : 36% ("dry") and 100% (saturated). Changes in grain size distribution were measured in all the tests performed. The models is formulated within the framework of work hardening plasticity and critical state concepts. Specimen stiffiness and limiting conditions depend on the accumulated plastic work and on the prevailing RH. The model reproduces in a satisfactory way the set of triaxial tests performed.

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In this Paper, a back-analysis of the behaviour of an oil tank during a loading test is presented, and the results of the analysis are compared with the actual performance of the tank resulting from field measurements. The tank is founded on stiff cohesive soils and on a clayey backfill improved with soil-cement columns, obtained with the deep-mixing technique. The constitutive model used in the analysis is capable to reproduce the soil no-linearity and has been calibrated using in-situ measurement of the stiffness at small strains. The backfill improved with stabilised columns is regarded as an equivalent homogenous material, characterised by a non-linear stress-strain behaviour. A coupled consolidation analysis is carried out, that provides a satisfactory simulation of the observed time-settlement curve and of the overall field of displacements monitored during the loading test. A less satisfactory agreement is obtained between measured and computed values of pore water pressure. The effectiveness of the soil improvement scheme adopted is finally evaluated, comparing the back-analysed performance with the results of additional analyses, in which no treatment or a different geometry of treatment are hypothesised.

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