This paper presents the results of an intensive laboratory study carried out on undisturbed soft Bangkok clay specimens to investigate the effect of temperature on shear strength and yielding behavior especially in states wetter than critical condition. Modified triaxial apparatus that can handle elevated temperature up to 100°C has been used in this study. The experimental program includes series of drained and undrained compression triaxial tests, and isotropic and anisotropic consolidation tests conducted at different temperature levels (25, 70, 90°C). The results of the experimental program were analyzed in light of the definitions of the critical state soil mechanics theory. The outcomes of this study provide useful test results and thorough understanding that can enhance the constitutive modeling of saturated fine grained soils behavior under elevated temperatures.

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The Deep Mixing Method (DMM), an in-situ soil stabilization technique using cement and/or lime as a binder, is often applied to improve soft soils. The group column type pattern is extensively applied to stabilize foundations of embankment or lightweight structures. An improved-ground design procedure in Japan assumes two failure patterns related to external and internal stabilities. For the external stability, a collapse failure pattern where the DM columns tilt like dominos could take place instead of a sliding failure pattern. For the internal stability, the DM columns show shear, bending and tensile failure mode, depending not only on the ground and loading conditions but also on the location of each column. However, the current design does not incorporate the effect of these failure modes, but only the shear failure mode. In this study, a series of centrifuge model tests was carried out to investigate the internal stability of group column type improved ground under embankment loading. This paper describes the failure modes of DM columns and a proposed simple calculation that takes into account the bending failure mode.

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A comprehensive series of triaxial compression (TC) tests have been performed on two air-dried poorly graded fine sands (Hostun and Toyoura sands) and on a moist mixture of sand and clay (Hostun sand—Kaolin clay). Tests were conducted by means of two high precision devices: a hollow cylinder apparatus (HCA, “T4C StaDy”) and a triaxial apparatus (TA, “Triaxial StaDy”). The elastic properties of these granular materials were systemically and carefully measured at different stress levels by both static and dynamic methods, i.e., by small cyclic loadings for strain levels below 0.001% and by shear (S) and compression (P) wave propagations using piezoelectric elements. It is found that the elastic properties measured by static and dynamic methods become very consistent if the stress-induced anisotropy is properly taken into account. For this, two different assumptions were considered in the back analysis of the dynamic test interpretation: an isotropic elastic behaviour and a cross-anisotropic (or transverse isotropic) elastic behaviour. The resulting differences in the determination of each of the elastic parameters are quantified and discussed. An hypo-elastic model (DBGS model), taking into account stress-induced anisotropy (including rotation of stress principal axes) is found to be relevant in the prediction of both static and dynamic measurements. This model was otherwise used to consider the cross-anisotropic elastic assumption.

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A number of previous experimental studies showed that polymer geogrid reinforcement as well as sand exhibit significantly rate-dependent behaviour. The viscous properties of polymer geogrids and Toyoura sand were independently evaluated by changing stepwise the strain rate as well as performing sustained loading and load/stress relaxation tests during otherwise monotonic loading in, respectively, tensile loading tests and drained plane strain compression (PSC) tests. The viscous properties of the two types of material were separately formulated in the same framework of non-linear three-component rheology model. The viscous response of geogrid-reinforced sand in PSC is significant, controlled by viscous properties of geogrid and sand. Local strain distributions in the reinforced sand specimen were evaluated by photogrametric analysis and used to determine the time history of the tensile strain in the geogrid. The time history of tensile load activated in the geogrid during sustained loading of reinforced sand specimen was deduced by analysing the measured time history of geogrid strain by the non-linear three-component model. It was found that the tensile load in the geogrid reinforcement arranged in a sand specimen subjected to fixed boundary loads could decrease with time. In that case, the possibility of creep rupture of geogrid is very low.

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Skirted foundations are becoming an increasingly prevalent offshore foundation solution for the oil and gas and renewable energies industries. Their capacity under combined horizontal, vertical and moment loading must be found in order to ensure their stability under environmental loadings. As part of this process, knowledge of the vertical bearing capacity is required. In this paper, the vertical bearing capacity of skirted foundations on normally consolidated undrained soil was investigated using numerical and physical modelling. Finite element analyses were carried to investigate the vertical bearing capacity of foundations with different geometries for various embedment ratios. Accompanying upper bound plasticity analyses highlighted the mechanistic reasons for the varying response and allowed examination of the effect of changing skirt interface friction. Analyses showed that skirted foundation capacity under vertical load may be considered normally as if the foundation is rigid with an embedment depth equal to the skirt depth. Based on the numerical analysis, a design method is proposed to calculate vertical bearing capacity. Finally, good agreement with results from a series of centrifuge model tests partially validated the design method.

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An analytical solution for the consolidation problem of an axially loaded triaxial sample including anisotropy in stiffness is presented. The solution shows that anisotropy in stiffness strongly influences the consolidation process. The influence of anisotropy in stiffness is found in the initial pore pressure reaction, in the Mandel-Cryer effect and in the consolidation coefficient. Measurements on conventional sized peat samples appear not to correspond to the analytical solution. Besides drain resistance, literature presents two other explanations for this fact. These explanations are tested in a large scale test set-up. It is found that induced permeability changes have a strong influence. The possibility of using the Mandel-Cryer effect for assessment of stiffness parameters of peat introduces extra parameters to describe the variations in permeability. For strongly over-consolidated samples, leading to a nearly constant permeability the analytical solution fits perfectly to measurements.

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The objective of this paper is to examine the influence of geogrid layer on the integrity of clay liners of landfills. A series of centrifuge model tests were performed on model clay liners subjected to non-uniform settlements with and without a geogrid layer embedded within the top one-third portion of the clay liner moist-compacted on the wet side of its optimum moisture content at 40 g. The model clay liner material has been selected in such a way that it envelopes the material characteristics of the clay liners, which are used for constructing an impermeable barrier in a lining system. By maintaining type and location of the geogrid within the clay liner as constant, the thickness of clay liner is varied to check the possibility of reducing the thickness of a geogrid reinforced clay liner. Digital image analysis technique was employed to ascertain the initiation of cracking and to compute strains both on the surface and along the cross-section of the clay liner with and without any geogrid layer. It was observed that clay liners compacted at moulding water content towards wet side of their OMC found to experience multiple cracking at the onset of non-uniform settlements. Contrary to this, geogrid reinforced clay liner was observed to sustain large distortions and experience only tiny cracks limited up to a location of a geogrid. With an increase in thickness of the clay liner reinforced with a geogrid, geogrid reinforced compacted clay liner was observed to retain its integrity and restrains cracking completely.

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For more than 60 years, vertical drains have been employed to promote more rapid consolidation of relatively thick deposits of fine-grained soils. Although the design of vertical drains is based on Barron's or Hansbo's theory considering the clay layer to be homogeneous, some field data from the improved grounds using the vertical drains have sometimes shown that the advantage of modifying the commonly used equations due to the presence of permeable laminations in the clay. So there is a necessity of thorough study in the effects of intermediate permeable layers to have an optimum design, which will save time and money. This paper discusses the effects of the presence of intermediate permeable layers and smear using the consolidation equations developed by considering the balance equations regarding the movement of water in a small element and non-linear relationships in order to take the change of permeability and effective stress due to the change in void ratio into account. Finite difference analysis has been done for the combined effect of vertical and radial consolidation and the results are given with two defined parameters, such as; K=[k_s/k_c]•[H_s/H_c] and α₉₈=t_98(2D)/t_98(radial). A new design methodology and a sediment classification considering both permeability and thickness of different layers are proposed here with a set of graphs and a design example.

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Behaviors of a liquefied ground are influenced by many factors, such as density, confining stress, permeability, particle size and grading, etc. Among these factors, the density plays the most significant role. It is known that when subjected to cyclic loading under undrained condition, there exist three types of liquefaction behaviors, i.e., loose sand will fail directly towards zero mean effective stress without cyclic mobility; medium-dense sand will enter into a cyclic mobility before being completely liquefied; very dense sand, however, will not liquefy at all. In liquefaction analysis, it is important to distinguish these three types of liquefaction behaviors according to different densities. In this paper, firstly, a constitutive model proposed by the same authors is briefly introduced. Secondly, experimental results of shaking-table tests on saturated sandy ground with repeated liquefaction-consolidation process are presented. Then, a finite element-finite difference method (FE-FD) based on the constitutive model and two-phase field theory is conducted. Comparisons between the experiment and the numerical simulation show that the numerical simulation is capable of reproducing almost all main characteristics of the repeated liquefaction-consolidation of sandy grounds with different densities, such as the mechanical behavior pre- and during liquefaction, the settlement in post-liquefaction consolidation and the influence of density on the accumulation of excessive pore water pressure (EPWP) in repeated strong motions.

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The dynamic behavior of lattice-shaped ground improvements by mixing with cement was investigated using numerical analysis. Three-dimensional effective stress finite-element analyses were conducted to examine the effects of dimension and strength of the improved ground on the potential for liquefaction mitigation. In these analyses, both elastic and elasto-plastic models were used for expressing the behavior of the improved ground, where the elasto-plastic model can describe the post-peak stress-strain behavior of cement-treated soils. The numerical results suggested that the improvement area ratio and the elastic modulus of the cement-treated soil affect the potential of the improved ground for liquefaction mitigation. Moreover, numerical analysis using the elasto-plastic model showed that partial failure of the improved ground causes no considerable reduction in the potential for liquefaction mitigation. Since such partial failure of improved ground can be taken into account appropriately, the analysis using the elasto-plastic model provides an adequate solution for the performance-based design of a lattice-shaped ground improvement.

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The effects of long-term curing on the strength and deformation characteristics of compacted cement-mixed soil were evaluated. A series of unconfined compression tests and drained triaxial compression (TC) tests were performed on moist cement-mixed sand compacted at various water contents, w_i, and cured at unconfined conditions for different periods up to more than eight years. TC tests were performed on cement-mixed gravel compacted at the optimum water content. The ageing effects on the compressive strength, q_max, from the present study were compared to those with various types of cement-mixed soils and concretes from the literature. An increase in q_max of cement-mixed soil continues for a very long period, up to several years, unlike ordinary concrete. This result indicates that the compressive strength at 28 days of cement-mixed soil, usually employed as the design strength, may largely underestimate the long-term strength. The increasing rate with time of the initial stiffness at small strains becomes continuously smaller than q_max with time. A large high-stiffness stress zone develops when monotonic loading is restarted at a certain high strain rate after some long sustained loading. This stress size is much larger than the one in the case without ageing effects. By positive interactions between the ageing effect and the inviscid yielding, q_max exhibits a larger extra gain when cured longer at more anisotropic stress states.

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Different construction sequences may be used in braced excavations. In the conventional or “ordinary method”, temporary retaining walls are initially placed and struts are used to brace them as the excavation proceeds downwards. In an “alternative method”, sometimes called “inverted excavation”, the tunnel walls are used to retain the surrounding earth, while the roof slab helps to brace the excavation. Laboratory model tests were devised to simulate these construction procedures. The tests were carried out under two-dimensional conditions, using aluminum rods to represent the soil mass. The walls were constructed with aluminum plates fully instrumented with strain gauges in both sides to measure the bending strains of the wall. Cylindrical bars with and without springs were used to brace the excavation in the ordinary method; while an instrumented aluminum block was used as the top slab in the alternative method. As excavation proceeded, all data relative to surface settlements, wall deflections and bending moments, as well as axial strut loads was carefully recorded. Later the laboratory tests were simulated with finite element analyses using the recently proposed subloading t_ij model (Nakai and Hinokio, 2004). The numerical results were compared with the experimental data obtained during the models tests for both construction methods. The overall recorded behavior in terms of displacements, deflection, bending moment and axial load could be reproduced with striking accuracy both qualitatively and quantitatively. This shows the capability of the model to represent the complex behavior of granular materials even under the low stress range used in the model tests. The results of model tests and numerical analyses show that the alternative method is viable and effective in controlling induced surface settlements, provided that the tunnel walls are constructed with an appropriate thickness.

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A theoretical mechanical shear model for single rock joints has been developed which can systematically express the shear behavior of rock joints by inputting the normal confining stress, the uniaxial compressive strength of the intact material, information on the geometry of the rough joint surface, and the basic friction angle. This model is able to show the process by which the shear stress increases, reaches the peak stress, decreases (strain softening), and then gradually arrives at the residual state. Moreover, variations in the joint surface roughness can be presented in the shear process. In some cases of comparisons between the experimental and the analytical results, errors have been found in the residual state. Therefore, the authors have given thought here to the reason for the occurrence of the bearing effect of the shaved powder material in relation to these errors. In this paper, the mechanical shear model is improved in terms of the bearing effect. Then, by comparing the experimental results with the analytical results, the validity of the modified model is discussed.

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To investigate the effects of reaction time on removal of heavy metals in acid-washing, we conducted acid-washing using hydrochloric acid on allophanic and smectitic soils contaminated with lead (Pb). The Pb removal percentage in the smectitic soil increased with reaction time. On the other hand, in the allophanic soil, it reached to a maximum at around 10 min, and then decreased monotonically. This suggests that effective removal of heavy metals from allophanic volcanic ash soils can be achieved by separating the washing solution from the soil as quickly as possible. The pH of the washing solutions of the two soils were similar just after the acid addition but it rose much more quickly in the allophanic soil than in the smectitic soil. Under comparable conditions, the pH of the washing solution and Al concentration were always far higher for the allophanic soil than for the smectitic soil, probably due to the higher solubility of allophane and related non-crystalline aluminum silicate in the former soil. These experimental results indicate that the Pb that was once released from allophane was re-adsorbed via surface complexation as a result of pH increase.

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