The present study was conducted to examine the behaviour of instrumented flexible piles in dry sand under lateral cyclic loading using centrifuged models. Considering load service conditions, the influence of the number of cycles of their amplitude and of the soil density on the pile cap displacement and the maximum bending moment of the pile is examined. An empirical law to evaluate pile head displacements at application point is proposed. From the bending moment profile recorded during loading, P-y reaction curves are identified. A reduction coefficient r (P-multiplier) is introduced to quantify the effects of cyclic loads on P-y curves.

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Piles have been used to support unsymmetrical surcharges due to embankments or backfills on soft grounds. The unsymmetrical surcharges can be transferred by embankment piles to a firm layer below soft grounds according to mobilizing soil arching in pile-supported embankments or backfills. Two kinds of model tests such as the soil arching test and the load transfer test were performed to investigate, respectively, the configuration of the soil arch and the loads transferred on piles in pile-supported embankments. In these model tests, model piles were installed in several rows below sand fills, and the heads of piles in each row were connected with cap beams. The soil arch showed a configuration of a semi hollow cylinder, whose diameter was equal to the space between the outer edges of two cap beams and thickness was equal to the width of the cap beams. Based on the configuration of the soil arch defined by the soil arching test, a theoretical analysis was carried out to predict the loads transferred on the piles according to mobilizing soil arching in pile-supported embankments. The equation presented by the theoretical analysis could consider the effect of various factors affecting on the loads transferred on the cap beams; the loads depended on space between cap beams, width of cap beams, height and strength parameters of embankment fills, etc. The loads predicted by the presented equation showed good agreement with those measured in not only the presented test but also the previous test. Finally, the presented theoretical analysis was compared with the previous theoretical analyses on soil arching and its differences from the previous theories were discussed.

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This paper presents direct shear testing data for interfaces between a nonwoven geotextile or two types of geosynthetic clay liners (GCL) (reinforced and unreinforced) and two types of geomembranes (smooth and textured). In this study, the effect of moisture on interface shear behavior was investigated by performing shear tests in both dry and wet (or hydrated) conditions because the geosynthetic interfaces in a landfill are easily exposed to rain, leachate and groundwater beneath the liners. The degree of strength reduction with increasing displacement and the effect of the normal stress level on friction angles were examined, and the modified hydration method applied for the GCL was also validated. The test results showed that the normal stress level, interface water presence and hydration methods dominated the interface shear strength and behavior. The relationship between the peak secant friction angle and the normal stress demonstrated that the friction angle decreased with increasing normal stress, implying that the shear strength for safe design should be determined by using the maximum value of the normal stress applied in landfills. Finally, comparisons with a few published test results were presented and some design implications for the geosynthetic-installed landfills were discussed.

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Records and analyses have shown that, apart from soil stratigraphy, the geomorphic conditions (such as those characterising an alluvial valley) tend to modify the amplitude, the frequency content, the duration, and the spatial variability of seismic ground shaking. As most of the related records and studies to date refer to weak motions (and thereby to linear soil response), the question that has been raised is whether and by how much the unavoidably nonlinear soil behaviour during strong shaking may reduce the unavoidable “valley amplification” effects. The paper aims at shedding some light on this important issue by analysing numerically the effects of the sub-surface geomorphic conditions of a valley on its ground surface seismic motion, with emphasis on the influence of soil nonlinearity. Two-dimensional linear and equivalent-linear ground response analyses are performed to study an alluvial valley in Japan, the behaviour of which had been monitored during many earthquakes in the early 1980's. Then, using the geometry of this valley as a basis, a parametric investigation is performed on the effects of potential soil nonlinearity arising from the increased intensity of base excitation and/or decreased “plasticity” index^* of the clayey soil material. It is shown that strong soil nonlinearity may depress the amplitude of the multiply-reflected and, especially of the horizontally propagating Rayleigh waves, leading to substantially lower valley amplification.

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Stone columns, one of the most commonly used soil improvement techniques, have been utilized worldwide to increase bearing capacity and reduce total and differential settlements of structures constructed on soft clay. Stone columns also act as vertical drains, thus speeding up the process of consolidation. However, the settlement of stabilised bed is not reduced in many situations for want of adequate lateral restraint. Encasing the stone column with a geogrid enhances the bearing capacity and reduces the settlement drastically without compromising its effect as a drain, unlike a pile. The behavior of the encased stone column stabilized bed is experimentally investigated and analysed numerically. In the numerical analysis, material behaviour is simulated using Soft Soil, Mohr Coulomb and Geogrid models for clay, stone material and encasement respectively and is validated with experimental results. The parametric study carried out on varying the L/D ratio (L=length of the column; D=diameter of the column) of column, stiffness of geogrid and angle of internal friction of stone material gives a better understanding of the physical performance of the encased stone column stabilized clay bed.

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This study investigates the liquid limit (Casagrande's method) and plastic limit (rolling and thread method) of six inorganic soils and their respective mixtures with fine silica sand. It was observed that the liquid limit and plastic limit values of the mixtures tested, except those with a low clay percentage, are linked to the respective clay size contents by a linear relationship. The Atterberg limits were subsequently recalculated using the equations of the regression lines of the mixtures governed by linear law with the clay percentages. The plotting of the plastic limit as a function of the liquid limit of these data made it possible to determine the relationship among the liquid limit, the plastic limit and clay fraction valid for inorganic soils that contain platey clay minerals and for clay size contents that are not too low. Hence, on the basis of the interdependence among the parameters considered (W_L, W_p, I_p, CF, A), for a given inorganic soil, knowing only two of three parameters (W_L, W_p, CF) that are measurable using standard tests, the values of other three parameters can be obtained.

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Tests on soft sedimentary rock under conventional triaxial and plane-strain compression and/or creep have been conducted. The purpose of the tests is to investigate the influence of intermediate principal stress that may affect the strength and dilatancy of the soft sedimentary rock under different loading paths. Meanwhile, theoretical simulations by an elasto-viscoplastic constitutive model were also conducted. Influence of membrane and filter paper was investigated using finite element analysis based on the elasto-viscoplastic model. Furthermore, particular attention was paid to the migration of excessive pore water pressure within a specimen, by continuous monitoring of the inner excessive pore water pressure of specimens and calculation with soil-water coupled finite element-finite difference (FE-FD) method. The shear strength, inherent dilatancy and creep behavior of soft rock were explained experimentally and numerically in detail.

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This paper describes the tensile strength measured for three kinds of statically compacted unsaturated soils; mixtures of clay~silt~sand, Narita-sand and Kanto loam. Specimens were directly prepared either under controlled compaction stress or under controlled dry density by statically compacting them within the tensile mold of the apparatus. Image analysis was done to show the normality of tensile force to the tensile failure plane. Tensile strengths (q_t) were compared with the unconfined compressive strengths (q_u) for silt~sand mixture, clay~sand mixture, clay~silt mixture and Narita sand, respectively. Increment in tensile strength (also q_u/q_t ratio) with the increase in the percentage and decrease in the size of finer soils could be seen. Effects of number of compaction layers and tensile pulling rates on the q_t were also examined. Increase in the tensile strength with the increase in the number of compaction layers was observed; and it was suggested to prepare the unsaturated compacted specimen by 3 to 4 layers compaction. Increase in tensile strength of 0.3 kPa and 0.003 kPa per one cycle of logarithm of tensile pulling rate was observed for clay~sand-4 (1:3) and clay~sand-5 (3:1) for the pulling rate of 0.01 to 1.0 mm/min.

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This paper describes an elastoplastic model for rockfills and simulations of laboratory test by using the model. Rockfills express remarkable shear strength reductions and compressive deformations due to saturation. The authors have already developed elastoplastic models to represent the mechanical properties of unsaturated soils. Two suction effects were taken into account in the models. However for rockfills, one suction effect: an increase in suction enhances yield stresses and affects resistance to plastic deformations, remarkably appears. The effect may be evaluated by formulating state surfaces. In this paper, a model proposed by authors is modified for rockfills. To verify the elastoplastic model for rockfills, simulations of oedometer and triaxial compression tests were carried out. The simulation results could well express the volume change and shear behavior of the rockfill, especially the influence of water contents.

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The trapdoor problem is a useful model for providing a clearer understanding of stress distribution around basic geotechnical engineering structures such as tunnels, conduits and anchor plates. The interest is mainly directed towards the determination of soil load on the trapdoor, which can be substantially different from the initial geostatic loads, when the trapdoor is moved even slightly. Development of shear bands around the yielding trapdoor is also of interest. A series of centrifuge and 1g model tests were conducted to study active arching in dry granular soil on circular trapdoors. The study was undertaken mainly because of two reasons: (i) limited success in earlier centrifuge modeling studies of a trapdoor problem, and (ii) a lack of understanding of the load-displacement characteristics under axisymmetric conditions. A trapdoor assembly and in-flight precompression technique were developed to perform a series of tests involving different overburden soil thickness on circular doors of different diameters. Correct initial geostatic loads were measured. Greater confidence in the experimental results was obtained because modeling of models type experiments were also successful. A parametric study involving different overburden soil thickness to trapdoor diameter ratios (H/D) ranging from 0.67 to 6 was conducted. The pattern of shear bands in sand above the trapdoor observed in a centrifuge model under a high gravity field differed considerably from that in a 1g model. Maximum arching was completely mobilized at movements of only 1.5% of trapdoor diameter. The minimum load on the trapdoor became constant at H/D equal to 5 regardless of the initial overburden pressure or height of the soil model.

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This paper presents mechanical properties of two unsaturated low quality rockfills (Materials O and S) with almost the same water absorption values of particles and comprehensive experimental data in order to establish an elastoplastic model for rockfills. The data were obtained from soil water retention, oedometer, triaxial compression and rock particle compression tests. The soil column method used here may give certain suction ranges to obtain soil water retention curves for rockfills. In oedometer and triaxial tests, there is quite a difference in the deformation and shear strength properties between Materials O and S. The properties for Material S are strongly affected by initial water contents of specimens but there was little affection on those of Material O. In compression tests for rock particles, the breakage strength values for particles of Material S were strongly affected by the water content conditions, but the affection for those of Material O was remarkably smaller than that for Material S. The difference in mechanical behavior between Materials O and S may arise from the difference in breakage characteristics for particles of both materials.

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This paper describes a foundation design method in which the ground is improved around the heads of pile foundations in soft ground or loose sandy ground and its practical effectiveness. The shear strength increased due to ground improvement is reflected in the horizontal resistance of piles. In this design method, the influence range of the horizontal resistance of piles and the necessary range of ground improvement are determined by taking account of three-dimensional domain formed with the gradient of the surface of passive failure. The horizontal subgrade reaction of piles is evaluated by converting the shear strength of improved ground to the modulus of deformation. In this study, the validity of design method for the pile foundation with ground improvement was confirmed through an in-situ horizontal loading test. The dynamic behavior of pile foundation constructed in improved ground was also investigated through a series of centrifuge model tests and numerical analyses. The influence of the difference in strength between the original and improved grounds on piles during an earthquake was also confirmed based on the numerical analyses. The cost performance of the proposed method was discussed by comparing with the case without ground improvement.

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On May 27, 2006, the Mid Java Earthquake hit around Jogjakarta City in Java Island, Indonesia. In this report, results from field investigations on geotechnical issues conducted by the Japanese Geotechnical Society survey team are summarized. At several sites where liquefaction occurred in lowland areas, subsurface soil conditions consisting of high groundwater table with soft soil deposits were identified. The liquefaction caused frequent plugging of wells. In these lowland areas, high collapse ratio of houses was reported, which may be linked to the directivity of earthquake motions affected by the earthquake fault location and/or their amplification in the soft soil deposits. In mountainous areas, a large scale landslide and a large number of slope failures and rock falls occurred. Earth fill dams suffered from cracking at the crest, while the rate of the leakage water from the toe of the downstream slope was not changed. Some recommendations are made in conducting detailed analyses and executing rehabilitation works on the damaged dams.

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The stiffness characteristic of artificially frozen soils subjected to K₀ consolidation at small strains are investigated at different initial confining pressure and frozen temperature by secant Young's modulus representing the soil stiffness. The influence of freezing on stiffness is investigated by comparing the behavior of frozen and unfrozen soils with similar conditions. It is observed that the frozen soils secant Young's modulus may decay with increasing strain after keeping a stage of constant value, the threshold yield strain was found to be approximately 0.05% for all tested conditions. On the other hand, the stiffness of studied frozen soils and its degradation was heavily response to the initial confining pressure amplitude and temperature fluctuation.

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