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Geotechnical investigation and design show that when evaluating the settlement of ground with soft soil ground, the use of a single stress path results in a larger error than in the case of a multistress path. To understand the influence of a multistress path on the deformation properties and microstructure characteristics of soil, a two-stage stress path (corresponding to three stress states: the in situ state, unloading-loading state, and grouting pressure state) of soil was examined by analytical solution. Then, a series of triaxial tests was conducted to analyze macroscopic deformation properties considering four influencing factors, i.e., the relative distance between the soil and tunnel, stress paths, type of silty clay, and drainage conditions. The results show that axial strain of silty clay caused by the two-stage stress path increases by 31.6% with the relative distance decreasing from 1.67R (R means tunnel radius) to 1.25R on tunnel side, while the volume strain increases by 80% with the relative distance decreasing from 1.21R to 1.06R on tunnel bottom. In addition, the deformation caused by the two-stage stress path (or reconstituted soil or undrained condition) is larger than that caused by the one-stage stress path (or undisturbed soil or drained condition). Finally, through scanning electron microscopy and mercury intrusion porosimetry, the pore size, distribution, shape, and arrangement of silty clay on the bottom and sides of the tunnel were investigated. The research shows that under the two-stage stress path, the number of large pores decrease and that of small pores increase. The probability entropy of pore orientation ranges from 0.834 to 0.971 when silty clay is subjected to a two-stage stress path. This illustrates that the pore order of the soil on tunnel side is better than that on the tunnel bottom. [ABSTRACT FROM AUTHOR]

The deformation of soft clays under cyclic loading is controlled by microfabric changes. Cyclic triaxial tests, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted to investigate the deformation and the corresponding micromechanism under cyclic loading. The correlations between the microparameters and deformation of Tianjin soft clay are discussed. The deformation increases linearly in the initial compression stage (number of cycles, N < 500), at a decreasing rate in the later shearing stage (N < 5000), and eventually stabilizes when the cyclic stress ratio (CSR) is 0.30. The probability entropy of both the soil particles and pores have a weak correlation with the deformation on the whole, reflecting a fluctuation within a small range in the initial compression stage. The correlations between the median pore diameter, median particle diameter, and specific surface area and the deformation are validated by using the gray correlation method. More specifically, the specific surface area and the deformation are highly correlated in the initial compression deformation stage. The median pore diameter and median particle diameter are highly correlated with deformation in the later shearing deformation stage. [ABSTRACT FROM AUTHOR]

The compressibility and microstructural changes of four types of reconstituted clays, Tianjin, Shanghai, kaolin, and montmorillonite clays, were studied to investigate the effects of mineral composition and clay content on the compressibility and microstructure evolution of clays during one-dimensional (1D) compression. The scanning electron microscopy test results indicated that the clays were characterized by an aggregated structure with inter- and intraaggregate pores. A preferential orientation of the particles toward the horizontal plane was induced during the 1D compression. The mercury intrusion porosimetry test results showed that the cumulative pore volume clearly decreased and that the volume and size of the widely distributed interaggregate pores decreased with the increase of the vertical effective stress. For montmorillonite clay, particle reorientation was obvious due to the more loosened microstructure, and the volume of mesopores was much larger compared to other clays, which resulted in the largest compressibility. For kaolin clay, a distinct preferential orientation of the particles toward the horizontal plane was induced due to the more planar microstructure even under the lower vertical effective stress, and the decrease of the pore volume mainly arose from micropores compared to mesopores in other clays. For the tested Tianjin and Shanghai clays, the main minerals were the same, but the compressibility of Tianjin clay was larger. It seems that the higher the clay content, the larger the particle reorientation and the cumulative pore volume, the higher the compressibility. [ABSTRACT FROM AUTHOR]

Marine clays often need improvements before construction and how to stabilize such kind of soft soil has been a great challenge. To address this issue, a newly proposed additive, which is named anionic polyacrylamide (APAM), with lime was applied to the clay stabilization. The main objective of this study is to figure out the effect of APAM on lime-treated marine clay and the micromechanism of stabilization. The unconfined compressive strength (UCS) tests and oedometer tests were performed to investigate the mechanical properties of stabilized clays; the X-ray diffraction (XRD) tests were conducted for mineralogical analysis; the mercury intrusion porosimetry (MIP) tests and scanning electron microscopy (SEM) tests were carried out for microstructural study. The experiment results show that with APAM dosage increasing, the stabilization effect first enhanced and subsequently weakened. The mineralogical analysis reveals that no new compounds were detected as APAM was added. The microstructural study shows that the physical adsorption of APAM to clay particles was conducive to form a more stable clay structure. The Ca2+ produced by lime served as the cationic bridge, facilitating the APAM adsorption. However, the electrostatic repulsion between APAM and negatively charged clay particles and the inhibitory effects of APAM on pozzolanic reactions were the main factors weakening the stabilization effect. This study indicates that the APAM/lime treatment method is efficient for marine clay stabilization as long as a rational APAM dosage is selected. [ABSTRACT FROM AUTHOR]

Soft marine clay is susceptible to deformation under cyclic loading, controlled by microstructure variations. To investigate the microscopic characteristics of soft clay with the progression of deformation under cyclic loading, a detailed study has been conducted using consolidated undrained cyclic triaxial tests, as well as mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The relationships between the micro parameters and accumulated plastic strain of soft marine clay are discussed. Cyclic triaxial test results show that low cyclic stress ratio (CSR) induces stable deformation, while for greater CSR the deformation of soft marine clay presents a closely linear or exponential increase, which leads to the destructuration of the soil structure. The microanalysis shows that with vibration cycles under 0.15 CSR, the content of pore obtained by SEM decreases from 37% to 25%, and large aggregates are visibly crushed into smaller ones, leading to a 24% increase in the pore numbers, which shows the slow increase of the deformation. When the CSR is greater than 0.25, large diameter pores generate resulting in divergent development of accumulated plastic strain. An empirical model of accumulated plastic strain involving micro parameter is proposed for assessing the deformation of soft marine clay. [ABSTRACT FROM AUTHOR]