This paper presents a modified finite element formulation of frictional contact for soil-pile interaction. This modified formulation is based on smoothed discretisation of the pile surface using BÉZIER polynomials. The finite element code based on this formulation is then used to analyse the loading of piles with enlarged ends of different shapes. Very simple material models are used to represent the behaviour of the soil and the pile. The numerical predictions are compared with the laboratory measurements. It is demonstrated that the new finite element formulation can produce reasonable results for the pile loading problem that involves large interfacial sliding and surface separation.

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The seepage and inertia effects on the rate-dependent subgrade reaction of a single pile in liquefied soil are clarified through numerical studies. The quasi-static (for seepage effect) and dynamic (for inertia effect) numerical analyses are performed with a soil-water coupled formulation and a simplified cyclic elasto-plastic constitutive model for liquefied sand. The constitutive model can explicitly deal with the liquefaction intensity by changing the lower bounds of the mean effective stress. The liquefied soil at a certain depth around a pile is modeled with finite elements under a plane stress condition. The results of the quasi-static analyses under monotonic loading conditions show that the apparent rate-dependency of the subgrade reaction is caused by the seepage of pore water around the pile due to soil positive dilatancy under large strain range. The results of the dynamic analyses under cyclic loading conditions show that the positive correlation in a p-v (subgrade reaction-pile relative velocity) relation can be explained with the phase difference in movement between a pile and the neighboring soil around the pile due to inertia under small strain range.

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Scoria deposit can be found around many volcanoes of the Quarternary epoch in the world. In Japan, Mount Fuji is one of volcanoes, where surface area is widely layered by scoria. From a geotechnical engineering point of view, scoria has many problems because of its physical characteristics by means of its collapsible properties. Since scoria is a non-cohesive material, it is vulnerable to water-induced change and fails easily due to rainfall or snow melting. These are the primary causes of debris flow which frequently occurs in the Mount Fuji area. Typically, this type of debris flow is called “Yukishiro” in Japanese. For better understanding of the shear behavior of scoria within low stress level condition, drained triaxial compression tests were performed on loose dry scoria under low confining pressures (10-80 kPa) which are assumed as representative of the field surface stress conditions. The effects of grain size, dry density ρ_d and effective confining stress σ′_c upon stress-strain behaviour and angle of internal friction φ_d were investigated. Attention was also paid to level of grain breakage B_g and reproducibility of the test. Grain breakage investigations were carried out on the particle breakage phenomenon, which occurs during testing. Additionally, initial Young's modulus of scoria was identified. As a result, reproducibility of the test is good in terms of stress-strain curves, however volumetric strain versus axial strain exhibits less reproducibility than stress-strain curves. At stress level of 10-80 kPa, it was found that the dependency of φ_d on σ′_c is almost negligible, while there is noticeable dependency of φ_d on grain size. The effect of grain size on stress-strain relationships was observed. It was also found that grain breakage was noticed at stress level of 10~80 kPa, which describes the dependency scoria parameters on confining stress and the effect of grain size upon B_g-σ′_c relationships was also observed.

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The viscous properties, or loading-rate effects on the stress-strain behaviour, of unbound and bound soils, in particular unbound granular materials, are summarised. The viscous properties were evaluated by stepwise changing the strain rate, ε, and performing sustained loading during otherwise monotonic loading (ML) at a constant ε and also by performing ML tests at different constant values of ε. Four basic viscosity types, Isotach, Combined, TESRA (or Viscous Evanescent) and Positive & Negative (P & N), which were recently found are described. The Isotach type is the most classical one and, in the case of ML, the current viscous stress component is a function of instantaneous irreversible strain, ε^ir and its rate, ε^ir. So, the strength during ML at a constant ε increases with ε. With the other three types, the viscous stress increment that has developed at a given moment, denoted as Δσ^v, decays with ε^ir towards different residual values during subsequent ML. With the TESRAtype, Δσ^v decays eventually totally and the strength during ML at constant ε is essentially independent of ε. With the Combined type, Δσ^v decays with ε^ir like the TESRA type, but it does not decay totally. So, the strength during ML at constant ε increases with ε like the Isotach type. With the P & N type, found latest, a positive value of Δσ^v decays towards a negative value. So, the strength during ML at constant ε decreases with an increase in ε. The viscosity type tends to change with ε^ir: e.g., from Isotach toward TESRA and from TESRA toward P & N. A general mathematical expression that can describe these four viscosity types and transitions among them is proposed. Numerical simulations of typical drained triaxial compression tests of geomaterials based on a non-linear three-component model incorporating the general expression of the viscous stress are presented. The viscosity type is controlled by at least, grading characteristics and particle shape.

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The small-strain stress-strain properties of expanded polystyrene (EPS) geofoam with densities of about 20 kg/m³ and 30 kg/m³ were evaluated by laboratory unconfined compression tests on specimens of 75 mm in diameter and 150 mm in height. Two series of tests were conducted, which were continuous monotonic loading (ML) tests and ML tests intervened by sustained creep loading and minute cycles of unload and reload. Relatively small vertical and horizontal strains were locally measured by means of a pair of local deformation transducers (LDTs) and a set of three clip gauges, respectively. The paramount importance of measuring local strains in compression tests on EPS to reliably evaluate its stress-strain properties, in particular those at relatively small strains, is demonstrated. The initial modulus, E₀, and Poisson's ratio, ν₀, were evaluated from initial stress-strain relations at small strains obtained by these ML tests. The tangent parameters, E_tan and ν_tan, were also evaluated from the ML stress-strain behaviour. The equivalent parameters, E_eq and ν_eq, were evaluated from the stress-strain behaviour during minute cycles of unload and reload. The stress-strain behaviour is essentially linear only at small strains, and it becomes highly non-linear and a significant drop of stiffness occurs as observed in the overall stress-strain behaviour. The Poisson's ratio for inelastic deformation is found to be negative.

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In order to directly evaluate the effects of soil improvement by the Sand Compaction Pile (SCP) method on the density, deformation, and static and liquefaction strength characteristics of sandy soils, a series of field and laboratory tests were performed. Laboratory tests were performed on high-quality undisturbed samples obtained from sandy soils both before and after soil improvement by the SCP method. The high-quality undisturbed samples were recovered by the in-situ freezing sampling method. The drained shear strength (internal friction angle, φ_d), liquefaction strength (R₁₅: cyclic stress ratio needed to cause 5% double amplitude axial strain in 15 cycles), and cyclic deformation characteristics (G~γ and h~γ relations) were determined by performing a series of laboratory tests on the undisturbed samples. Both the in-situ density and the relative density were measured on the undisturbed samples used in the laboratory tests. A standard penetration test (SPT) and a suspension-type P-S wave logging test were performed to investigate the soil profile of the test site before and after the sand compaction. Both the static and the liquefaction strengths of the sandy soils obtained in the laboratory tests were also compared with those estimated by empirical correlations used in practice based on the SPT N-value and soil gradations.

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A series of cyclic triaxial tests of unsaturated soils was conducted to get a better understanding of the general liquefaction state of unsaturated soils. In the tests, cyclic shear strain was applied to fine clean sand with the same dry density but different initial suction states under the undrained condition. During cyclic shear, the volume change of the soil particle skeleton, the pore air pressure and the pore water pressure were measured continuously. Having used the effective stress defined by Bishop (Bishop et al., 1963), where the net stress and suction contribute to the effective stress, our test results showed that unsaturated sand specimens with quite a low degree of saturation lose their effective stress due to cyclic shear. At a zero effective stress state, unsaturated specimens behaved similarly to liquids in much the same way as saturated specimens. From experimental and theoretical considerations, the zero effective stress state (i.e., liquefaction) for unsaturated sand was found to have been established when both the pore air and water pressures build up to the point where it is equal to the initial total pressure. A volume change of pore air under the undrained condition, if a volume change of pore water is negligible, is equal to that of the soil particle skeleton. Therefore, it can be concluded that the liquefaction of unsaturated soil generally depends on the volume compressibility of the soil particle skeleton and the degree of saturation. On the other hand, according to the ideal gas equation of Boyle-Charles law, the volume change required to bring about a zero effective stress state can be calculated from the initial pore air pressure (usually the atmospheric pressure) and the final pore air pressure (the initial confining pressure). Therefore, the liquefaction of unsaturated soils also depends on the initial confining pressure. Based on this concept, the liquefaction potential of unsaturated soil can be evaluated by comparing the volume compressibility of the soil particle skeleton and the volume change of the pore air required to bring about a zero effective stress state.

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A series of undrained, cyclic simple shear tests were performed on reconstituted specimens with various clay contents to study the effects of clay content on liquefaction characteristics of clayey sands based on a framework of an idealized binary packing model and intergrain state parameters. From observed liquefaction characteristics, clayey sands with different clay contents can be grouped as sand-like or clay-like soils depending on the clay content and the transitional fines content of the sand-clay mixture. A simple equation is derived and verified to correlate the transitional fines content with the void ratios of the clean sand and the pure clay consisting of the mixture. In addition, a new relationship for clay content correction is proposed based on the linear relationship between the cyclic resistance ratio and the clay content at the same intergranular void ratio. The cyclic resistance ratio of sand-like clayey sands can be divided into two components: (1) the resistance of the sand skeleton at the specific intergranular void ratio, and (2) the increment of cyclic resistance from clayey fines. The rate of increment for cyclic resistance varies with the properties of contained clay particles. Data from three independent studies have shown the proposed procedure is promising.

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Compaction curves of soils are essential for establishing practical and reliable criteria for an effective control of field compaction. This paper deals with the development of a practical method of assessing laboratory compaction curves of fine-grained soils. It is found that for a given fine-grained soil compacted at a particular compaction energy, the relationships between water content (w) and degree of saturation (S) are represented by power function, which are w=A_dS^B_d and w=A_wS^B_w for the dry and the wet sides of optimum, respectively (where A_d, A_w, B_d and B_w are constant). The B_d and B_w values and optimum degree of saturation (ODS) are mainly dependent upon soil type irrespective of compaction energy. The A_d and A_w values decrease with the logarithm of compaction energy and the decrease rates are practically the same for any compacted fine-grained soil. This leads to a simple and rational method to assess the compaction curve wherein the compaction energy varies over a wide range using a one point test (a single test). Assuming that fine-grained soils compacted under standard Proctor energy behave in agreement with Ohio's curves, the modified Ohio's curves for the other three compaction energy levels (296.3, 1346.6 and 2693.3 kJ/m³) are developed based on the proposed method. These curves can be used to assess the entire compaction curves at the required compaction energy based on a single set data of dry unit weight and water content.

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The influence of anisotropy on the elastic shear modulus of clays was addressed using test results obtained from a series of laboratory tests; i.e., triaxial test and Square oedometer. Under various stress stages imposing under triaxial condition, the deviator stress had an indiscernible influence on the elastic shear modulus. The path of vertically logged elastic shear modulus of clay could reasonably be expressed in terms of mean effective stress. Two pairs of bender elements were installed in the square oedometer so that shear wave velocities of samples which were trimmed parallel or perpendicular to their bedding direction can be measured in the vertical and horizontal directions. For sample trimmed parallel to the bedding direction (conventional one), the horizontal shear wave velocity was larger than the vertical one. The opposite observation, however, was observed for sample trimmed 90 degrees apart. The extents of different in vertically and horizontally logged shear wave velocities from both types of samples were almost similar. This clearly indicated the influence of inherent anisotropy rather than the stress induced. The mean effective stress can well represent the stage variable of elastic shear modulus.

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Seabed pipelines undergo temperature cycles that create axial load which can be relieved through controlled lateral buckling. The prediction of lateral buckling in design requires accurate assessment of the lateral breakout resistance. This Technical Note describes upper bound plasticity analysis of a partially-embedded pipe on undrained soil. The purpose is to generate failure envelopes for vertical and horizontal loading to provide a theoretical basis for estimating breakout resistance. The following cases have been considered: smooth and rough pipes, with and without separation at the rear face of the pipe. The envelopes are similar to those developed previously for surface foundations, but capture additional effects that are due to the curved geometry of the pipe surface. The breakout resistance and the movement of the pipe at failure are strongly influenced by the separation condition. Pipe roughness and soil self-weight have a relatively minor effect on breakout resistance. Existing empirical expressions usually assume a linear variation in breakout resistance with embedment and vertical load. This theoretical analysis demonstrates that these relationships are non-linear. The resulting envelopes provide a more rigorous basis for predicting the breakout resistance of partially-embedded pipelines.

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