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The long-term deformation characteristics and damage degree of soft marine-deposited clay must be evaluated to safely design seawalls. According to abundant field evidence, soft marine-deposited clay tends to experience coupled dynamic loading and seepage loading. Determining the critical load and damage index for these complex loading conditions is imperative. With Tianjin marine-deposited clay as an example, we employed an improved cyclic triaxial instrument to analyse the cumulative plastic strain, water discharge and stress–strain relationship of the clay to understand its long-term deformation development. Based on shakedown theory, the critical cyclic stress ratio (CSR) was determined. A multi-factor damage variable method was proposed to evaluate the damage degree. Its comparison with a traditional mono-factor damage variable method illustrated that this method is effective for damage analyses of marine clay around seawalls under coupled dynamic-seepage loading. [ABSTRACT FROM AUTHOR]

The properties of a soil, either in situ or at a hydraulic reclamation site, may change if influenced by the chemical status of the surroundings. Acidity, indicated by the pH, and magnesium (Mg2+), sometimes used to reinforce soil, are good indicators of the chemical environment. It is therefore worth investigating how acidity and Mg2+ affect the settling of dredged fill. In this study, a cylinder model test and ethylenediaminetetraacetic acid (EDTA) titration were used to investigate the mechanisms behind self-weight consolidation settlement. The results indicated that the settlement process of the dredged fill occurred in three stages and that the consolidation settlement process of the dredged fill varied as the pH and mass percentage of Mg2+ varied. As the pH increased from an initial low value, the settlement amount and settlement rate of the dredged fill approximated to a quadratic function trend and increased first and peaked when the pH was 4 and then decreased, reflecting the influence of the mineral composition of the soil and the Zeta potential of the soil particles. Under medium pH conditions, the settlement amount and settlement rate increased in an approximately linear manner, reflecting increases in the mass percentage of Mg2+ and changes in the double electric layers on the clay mineral surface. [ABSTRACT FROM AUTHOR]

The stability of the shield tunneling face is an extremely important factor affecting the safety of tunnel construction. In this study, a transparent clay with properties similar to those of Tianjin clay is prepared and a new transparent clay model test apparatus is developed to overcome the "black box" problem in the traditional model test. The stability of the shield tunneling face (failure mode, influence range, support force, and surface settlement) is investigated in transparent clay under active failure. A series of transparent clay model tests is performed to investigate the active failure mode, influence range, and support force of the shield tunneling face under different burial depth conditions, whereas particle flow code three-dimensional numerical simulations are conducted to verify the failure mode of the shield tunneling face and surface settlement along the transverse section under different burial depth conditions. The results show that the engineering characteristics of transparent clay are similar to those of soft clay in Binhai, Tianjin and satisfy visibility requirements. Two types of failure modes are obtained: the overall failure mode (cover/diameter: C/D ⩽ 1.0) and local failure mode (C/D ⩾ 2.0). The influence range of the transverse section is wider than that of the longitudinal section when C/D ⩾ 2.0. Additionally, the normalized thresholds of the relative displacement and support force ratio are 3%–6% and 0.2–0.4, respectively. Owing to the cushioning effect of the clay layer, the surface settlement is significantly reduced as the tunnel burial depth increases. [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]

The causes, prevention, and control of clay disasters are of great concern in practical engineering applications. Recently, due to unprecedented economic development, increasing numbers of tunnels and subgrades of tidal flat areas have been built in Tianjin, China. Soft soil ground not only bears the vibration load during operation and construction but also receives seepage effects caused by the bursting of pipelines and variations in groundwater levels. Under the coupling action of dynamic and seepage loads, large settlement can occur in soft soil since the deformation is related to both cyclic stress and seepage. Therefore, it is significant to understand the dynamic deformation characteristics of soil to ensure the safety of engineering applications. In this study, a series of laboratory cyclic triaxial tests were conducted to study the deformation behaviours of Tianjin clays under coupled cyclic stress and seepage fields. To simulate the seepage field, water pressure is applied at the bottom of the specimen, and the water can be drained from the top of the specimen to form a head difference in the specimen during the tests. The effects of the seepage pressure, cyclic stress ratio (CSR), and a number of cycles (N) on the hydraulic conductivity, axial deformation, hysteresis curve and dynamic modulus of Tianjin clays were systematically investigated. The laboratory results show that the hydraulic conductivity (k) decreases with the increasing seepage pressure, CSR, and N, and its range is from 4.6 × 10−8 cm/s to 7.4 × 10−8 cm/s. The axial deformation of the soil increases with increasing CSR and seepage pressure. With an applied water pressure of 150 kPa at the bottom of the specimen and a CSR of 0.3, the maximum cumulative plastic strain reaches 4.78%, increasing by 53.2% compared with a CSR of 0.1. The hysteresis curve gradually becomes denser or thinner with increasing CSR and N; the hysteretic curve tends to a straight line for N of 5,000. The dynamic modulus increases with increasing N and depends on the CSR and seepage pressure. Empirical equations are suggested for calculating the dynamic modulus with different CSR values and seepage pressures. The results of this study can provide reference parameters for the construction and design of tunnels and subgrades of tidal flat areas in clay strata under the influence of water environments. [ABSTRACT FROM AUTHOR]

The vibration caused by moving traffic loading can create additional shear stress in the soil element, thereby resulting in the rotation of the principal stress axis, which can, in turn, accelerate soil softening and increase the cumulative deformation of soft clay. Nevertheless, the designed parameters are often obtained by the cyclic triaxial test (i.e., the rotation of the principal stress axis cannot be considered), or the cumulative axial plastic strain prediction is inaccurate, and these are some of the main reasons for geotechnical engineering problems. In this study, cyclic triaxial and cyclic torsional shear tests were performed to understand the softening and deformation characteristics of soft clay. The influences of the cyclic stress ratio, shear stress ratio, and loading frequency on the softening index and cumulative axial plastic strain of soft clay were analyzed. When the rotation of the principal stress axis was considered, the softening index decreased by 19.57%–51.09%, or the cumulative axial strain increased by 35.49%–74.39%, and failure occurred under higher cyclic stress ratios and shear stress ratios. Additionally, increasing the cyclic stress ratio and shear stress ratio and reducing the loading frequency will cause a surge in the softening index and cumulative axial plastic strain of soft clay. A cumulative axial plastic strain prediction equation considering the influences of cyclic stress ratio, shear stress ratio, and loading frequency was established and verified on the basis of literature. The availability of the prediction equation was verified by comparison with monitoring data. The relevant results can provide a theoretical reference for the prevention of urban engineering geological disasters. • The cyclic triaxial and cyclic torsional shear tests were run to understand softening and deformation properties of clay. • A cumulative axial plastic strain prediction equation considering was established and verified based on the literature. [ABSTRACT FROM AUTHOR]

Chemical conditions have significant effects on the engineering properties and microscopic characteristics of the dredged fill. In this study, a series of laboratory tests were conducted to investigate the engineering properties and microscopic characteristics of the dredged fill under different chemical conditions. In addition, the micro-mechanism of the effect of chemical conditions on the engineering properties of the dredged fill is revealed. Under alkaline conditions, the undrained shear strength and structural yield stress increased by 50.33 and 145 kPa, respectively. The particle morphology was transformed from a granular and flaky structure to an agglomerated structure, and the surface-to-surface connection was the main form of intergranular connection. These changes reflect the enhancement of the intergranular cementation strength of the dredged fill by the generated insoluble substances and suggest that adding a certain amount of alkaline materials is an effective method to improve the engineering properties of dredged fill. Under acidic conditions, the undrained shear strength was significantly decreased, while the compressibility was notably increased. The cementation between soil particles was severely damaged due to the corrosion of hydrogen ions, leading to an increase in the number of large-diameter pores in the dredged fill and a deterioration of the microstructural characteristics. Under saline conditions, the undrained shear strength was reduced by 5 kPa, and the compressibility was increased. No distinct agglomerated structures were formed due to the dispersion of the internal particles of the dredged fill, and the soil skeleton effect was not notable. • The basic chemical elements and mineral composition of the dredged fill were determined by ICP-OES test and XRD test. • The relevance between chemical conditions and engineering properties was investigated by a series of laboratory tests. • The microscopic characteristics of dredged fill under different chemical conditions were investigated by MIP and SEM tests. • The bonding strength of cemented granules is enhanced due to the insoluble substances generated in alkaline condition. [ABSTRACT FROM AUTHOR]