Soils become stiffer as they age. In this paper, attempts were made to assess the in-situ structure of natural clays by calculating the current quasi-elastic shear modulus from shear wave velocity measurement, G_max. The G_max value of reconstituted clays was employed as a yardstick against which the corresponding characteristic of natural sedimentary clays is examined, implying that the intrinsic properties of a non-aged reconstituted clay provide a basic frame of reference for assessing the in-situ structure of a natural clay and the influence of ageing (secondary compression and interparticle bondings) on its in-situ properties. In a series of bender element (BE) tests on a reconstituted clay, it was first manifested that the relationship between the void ratio and G_max as subjected to one-dimensional compression exhibited the state boundary (SB) curve applicable to the short-term migration of effective stress. Second, it was demonstrated that the SB curve was violated when the clay aged by drained creep, and that the state of the aged clay rejoined the SB curve in a gradual manner on subsequent stressing applied over a short period. In the light of this characteristic time-dependent behaviour of G_max, a measure termed metastability index MI (G_max) has been newly proposed for assessing the structuration/destructuration of in-situ natural sedimentary clays which undergo effects of a long-term insitu ageing. The performance of MI (G_max), together with the non-dimensional soil constant reflecting structure, S (Jamiolkowski et al., 1994), was carefully examined in the BE tests using various natural and reconstituted clays. It was successfully demonstrated that : i) the proposed index MI (G_max) as well as the S-value were both equally capable of expressing structuration/destructuration of clays as examined in tests on Ariake (Japan), Bangkok (Thailand) and Louiseville (Canada) clays, each comprising a pair of a natural and its reconstituted sample ; ii) two different formations of soil structure, i.e., an on-depositional structure formed at sedimentation and the evolving post-depositional structure enriched due to ageing, should both be addressed for a proper understanding of the current structure of natural clays ; iii) the S values were similar among a total of nine non-aged reconstituted clays, despite a wide spread of plasticity index as well as compresssibility ; and iv) the S-value of each reconstituted clay is the lower bound against which the in-situ structure of the natural clay can be quantified at the relevant effective stress. A time-dependent soil structuration was also predicted by introducing a soil constant for metastabilization, C_β.

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A three-dimensional elasto-plastic model for unsaturated soil is presented, using a transformed stress tensor based on the Extended SMP criterion. The influences of suction on the mechanical behavior of unsaturated soil are taken into account in the model by using a parameter named suction stress σ_o (s), which is added to net stress to form an effective stress, and associating a yield stress and model parameters with it. The proposed model needs few material parameters, which can be easily determined from the results of ordinary tests, for example, oedometer and direct shear tests or triaxial test. It can predict the stress-strain-strength behavior of unsaturated soil under the conditions of three-dimensional stresses and constant or variable suction. A considerable number of triaxial compression and extension tests on unsaturated compacted kaolin were performed by using a suction-controlled triaxial apparatus, to measure the stress-strain behavior under different stress paths with constant and decreasing suction. The prediction of the proposed model shows good agreement with the measured behavior of unsaturated soil not only in triaxial compression but also in triaxial extension under the conditions of constant suction and reduction in suction, which may induce an irrecoverable volumetric compression (collapse).

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In Japan, the undrained strength of soft clay deposits is determined by half of the average unconfined compressive strength q_u. One problem with the q_u method is that the validity of the method depends largely on the quality of the "undisturbed"soil sample, while there exists no accepted method to evaluate the sample quality. The strengths from the q_u method, Su (qu), were compared with those from the two proposed methods, SHANSEP and Recompression. For most"undisturbed"samples taken with a fixed piston sampler, the strengths obtained by the q_u method were almost the same as the Su (M. B.) that was obtained by the Modified Bjerrum's method (Recompression method). However, when the samples were disturbed for some reason, Su (qu) was smaller than Su (M.B.). The strength by the SHANSEP method is dependent on the consolidation yield stress, Pc. When Pc was obtained by conventional consolidation test, the strengths by the SHANSEP method were usually smaller than those by the q_u method. The q_u method seems to be valid for most Japanese marine clays, when the undisturbed samples are taken with a fixed piston sampler of Japanese Industrial Standard. For samples taken with a Shelby-tube, the q_u method seriously underestimates the undrained strength. A new practical method named "Advanced q_u method"is proposed to determine the quality of the sample and the strength to be used for design at a reasonable cost. The effectiveness of the new method has been ascertained in field test.

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This study examined the factors influencing the vibratory installation of piles in drained granular soils. Several variables were examined to assess their relative importance in pile installation by simulating the pile-hammer-soil system using the finite difference program, FLAC. The soil was modeled using an elasto-plastic hyperbolic stress-strain model. The variables investigated include : soil strength, pile-soil interface friction angle, lateral earth pressure coefficient, hammer operating frequency, bias weight, hammer centrifugal force, and pile embedment. A simple empirical relationship was introduced to estimate the ultimate axial load capacity of vibratory driven piles based on hammer, pile and penetration characteristics. The capacities estimated using the proposed relationship are compared to other formulas using the results of this study and to data from two field test projects.

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Plane strain compression tests were performed on large specimens that were either unreinforced or reinforced with 6 or 11 layers of geogrid, both 57.0 cm in height and 24.4 cm×21.4 cm in cross-section. It is shown that the effects of covering ratio for each grid layer is much more important than the total tensile stiffness of grid within the limits of the test conditions in this study. Numerical analysis of the test results by a plane strain non-linear elasto-plastic FEM was performed considering strain localisation as well as anisotropic stress-strain behaviour of sand and interface properties. The geogrid was modelled as a planar reinforcement. Not only the pre-peak stress-strain behaviour of the unreinforced and reinforced specimens, but also the peak strength, post-peak behaviour and dilatancy characteristics from the FEM analysis all compared well with those from the physical tests. The effects of reinforcement rigidity and covering ratio were also well simulated. The relationship between the reinforcement covering ratio in the physical tests and the equivalent interface friction angle for the FEM analysis that provides the same reinforcing effects is presented. The mechanism of tensile-reinforcing is analysed based on local stress paths within the reinforced sand obtained from the FEM analysis.

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The superloading yield surface concept applied to the original Cam-clay model is examined for the application to the one-dimensional consolidation problems of highly structured soils. When the proposed elasto-plastic constitutive model is taken to be the case for a structured soil, the one-dimensional consolidation computation clarifies the following : (1) Delayed consolidation should occur even under one-dimensional compression condition with a considerably low stress ratio when softening of the soil occurs with volume compression. The delayed consolidation is the consolidation process that has sometimes been referred to as "secondary compression". (2) The structure is always degrading as plastic deformation proceeds even within the period of "secondary compression". (3) The decay of overconsolidation to normal consolidation also proceeds with plastic deformation. In this sense, the degradation of structure can not be independent of the decay of overconsolidation. However, since the decay of overconsolidation is much faster than the degradation of structure in clay, then softening becomes possible with volume compression even under a considerably low stress ratio. Recovery of structure with time due to chemical bonding effects etc. in soil particles is beyond the scope of this study.

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Natural sedimentary clays exhibit ageing effects on their behaviour when subjected to both consolidation and shearing. Attempts to characterising the in-situ 'structure'of soft marine clays have been made in case studies performed in Ariake (Japan) and in Bangkok (Thailand), each showing a typical profile of aged clay-structure with depth reflecting its site-specific geological consolidation history. In each site, profiles of Atterberg limits, natural water content, overconsolidation ratio (OCR), quasi-elastic shear modulus G_max and the undrained shear strength c_u with depth were attained from various laboratory tests, together with the a comparable profile of quasi-elastic shear modulus from insitu seismic survey G_f. Quantitative assessment of the aged clay-structure was made by using two different measures, metastability index MI (G) and the soil constant reflecting structure S (Jamiolkowski et al., 1994). The profiles of MI (G) and the S-value with depth were both determined based on the corresponding G_max behaviour of the reconstituted sample. "Disturbance"of laboratory samples retrieved by using two different samplers (i.e., Laval and Japanese thin-walled samplers) with two different sampling techniques (i.e., pre-boring and displacement methods) was discussed. A new approach to estimate c_u"in the ground", termed by the authors the MILK (Metastability Index coupled with Laboratory K_o test) method, is proposed. A trial of the MILK-method is demonstrated in soft Bangkok clay, the texture of which is similar to "soft cheese".

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Undrained cyclic torsional shear and triaxial tests were performed on hollow cylindrical specimens of dense Toyoura sand, which were consolidated isotropically after preparation by air-pluviation and saturation. At several stress states, small strain quasi-elastic deformation properties were measured by applying very small amplitude cyclic torsional and vertical loads. During the liquefaction process, the observed degree of degradation of quasi-elastic shear modulus and vertical Young's modulus, was similar to that measured during isotropic consolidation. This suggests that the soil structure was damaged during the process of liquefaction. A larger extent of degradation was observed under triaxial extension or torsional shear than under triaxial compression condition. Such a change in the quasi-elastic deformation properties should be properly considered when analyzing the liquefaction process of sands based on elasto-plastic modeling. The behaviors observed during small amplitude cyclic vertical loading under undrained condition could be explained by considering inherent and stress state-induced anisotropy in the modeling of quasi-elastic deformation characteristics, and by correcting for the effects of membrane penetration. The latter correction was necessary, although the involved strain level was as small as 0.001%.

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The Hualien Large-Scale Seismic Test (HLSST) program is currently to investigating soil-structure interaction during large earthquakes in Hualien, a region in Taiwan of high seismicity. This paper summarizes the laboratory tests, including cyclic triaxial tests, conducted to obtain small strain characteristics of the soils for dynamic response analysis and to characterize the ground of the test site. Factors affecting accurate evaluation of small strain characteristics of soils are also investigated in this paper. The following conclusions are drawn. (1) According to the results of cyclic triaxial tests, shear modulus at small strain by local measurement is discernibly larger than that by external measurement because of the effect of bedding errors. However, there seems to be almost no effect of bedding error in hysteresis damping at small strain level. (2) For sands, the shear wave velocities measured by cyclic triaxial tests are almost the same as those by in-situ velocity logging, whereas shear wave velocities of gravelly soils measured by cyclic triaxial tests are much smaller than those by in-situ velocity logging. This difference might be attributable to the effect of heterogeneity in the gravelly layer.

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