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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 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]

Subgrade soft clays are subjected to intermittent changeable-frequency cyclic loading and exhibit complex dynamic reactions compared with those under single-frequency cyclic loading. This research proposed a novel testing scheme to take the influence of combined-frequency subway train speed and intermittency into account. The testing procedure consisted of five loading periods in which occurred undrained combined (low–high–low) frequency cyclic loading and drained static loading, and post-cyclic loading consolidation settlement. A series of triaxial tests on Tianjin soft clay were conducted under intermittent combined-frequency cyclic loading conditions. The pore pressure, deformation, shear modulus and damping ratio, and the post-cyclic-loading consolidation settlement were investigated. The pore pressure and deformation increment decreased greatly when N (the number of cycles in each loading period) = 1,000 (200 × 5), but smoothly when N = 500 (100 × 5) from the first to fifth loading period. The intermittency made the soft clays more resistant to degradation of shear modulus and the influence of subway train speed on the shear modulus and damping ratio should be considered in dynamic design. The post-intermittent combined-frequency cyclic-loading consolidation settlement presented a stable increasing pattern when the cyclic stress ration (CSR) = 0.10. The subway train speed had an evident influence on the post-intermittent combined-frequency cyclic-loading consolidation settlement of soft clays. [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]

Subgrade soils are subjected to coupled dynamic-static and changeable-frequency traffic loading. Considering different cyclic modes of single-frequency, coupled dynamic-static and intermittent combined-frequency loading, deformational characteristics of the Tianjin soft clay in each vibration period were analyzed by triaxial testing. Results show that the accumulative plastic strain and pore pressure have quite different development patterns under coupled dynamic-static and intermittent combined-frequency loading than single-frequency cyclic loading. The combined-frequency loading caused a considerable increase of pore pressure increment, but intermittency resulted in a decrease. During the loading of either coupled dynamic-static or intermittency, the static loading caused the soft clays to be more resistant to later undrained cyclic loading. The intermittency and combined-frequency loading have an integrated effect on the accumulative plastic strain development pattern and increment during each vibration period. The degradation index under the coupled dynamic-static loading decreased much less than that under single-frequency loading, especially with increasing cyclic stress ratio. • An experimental method to simulate the subway loading considering effect of intermittency and combined-frequency is proposed. • Deformation development pattern and increment of soft clays in each vibration period are investigated rather than the overall pattern under conventional cyclic loading. • The influence of intermittency and combined-frequency on soft clay pore pressure and deformation are compared with those under single-frequency cyclic loading. [ABSTRACT FROM AUTHOR]

In recent years, the artificial ground freezing (AGF) method has been widely used in subway construction, and the engineering properties of soil have changed significantly after freezing and thawing. In this paper, a series of dynamic triaxial tests were used to investigate the pore pressure development of freeze-thaw soft clay under different cyclic dynamic stress ratios, loading frequencies and freezing temperatures. The test results show that the types of normalization pore pressure development include developmental and stable. An empirical model, considering the effects of cyclic dynamic stress ratio, loading frequency, freezing temperature and number of cyclic loading, was proposed and compared with three representative models proposed by the predecessors to validate that the proposed method has the best estimation on the development of the pore pressure. • The type of pore pressure development in frozen-thawed clay contains two parts. • An empirical model for predicting pore pressure development is proposed. • Freeze-thaw cycle plays an important role on the pore pressure development. • The model can be used for stable and developmental type of pore pressure development. [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]

The artificial ground freezing (AGF) technique has been extensively employed in the construction of underground structures. Due to the existence of freeze-thaw cycle in the AGF technique, the properties of soft clay deteriorate and thus larger settlement is induced during its later operation phase. To investigate the strain behaviors of the soft clay undergoing freeze-thaw cycle and discuss the differences between the soft clay undergoing and without undergoing freeze-thaw cycle under dynamic cyclic loading, a series of dynamic triaxial tests with consideration of different influencing factors as dynamic stress amplitudes, freezing temperatures, and freezing temperatures were conducted. Based on the experimental data, an empirical model for predicting the accumulative plastic strain was proposed and validated. Besides, the microscopic structural characters of the artificially freeze-thaw soft clay were explored via mercury intrusion porosimetry (MIP) tests. The results show that the total accumulative plastic strain of specimens increases with the increase of the dynamic stress amplitude, while decrease with the increase of the loading frequency. The specimens undergoing freeze-thaw cycle produces larger plastic strain, and the lower the freezing temperature is, the larger the total accumulative plastic strain is. The empirical accumulative plastic strain model, which can synthetically reflect the effects of dynamic stress amplitudes, loading frequencies, freezing temperatures and number of cyclic loading is proposed and validated. The freeze-thaw cycle has a significant effect on the strain behavior of soft clay in a way decreasing the pore volume and increasing the pore size of the specimens. Results obtained in this paper may provide meaningful references for studying the deformation characteristics of the soft clay in engineering applications. • The effects of dynamic stress amplitude and loading frequency on the accumulative plastic strain of soft clay were discussed. • The effects of freeze-thaw cycle as well as various freezing temperature on the accumulative plastic strain behaviour of soft clay was investigated. • An empirical model for predicting the long-term strain of soft clay, can synthetically reflect the effects of dynamic stress amplitudes, loading frequencies, freezing temperatures and number of cyclic loading was proposed and validated. • The microscopic structural mechanism of dynamic stress amplitude, loading frequency and freezing temperature on affecting the accumulative plastic strain of soft clay was analyzed. [ABSTRACT FROM AUTHOR]