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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 tunnelling-induced ground deformation can be affected by the existing tunnel(s) in overlapped tunnels construction, especially when the existing tunnel is in operation. Therefore, the effect of subway train vibration loads on the ground movement induced by overlapped shield tunnelling is needed to further investigate. In this study, model tests and numerical simulations were conducted to investigate ground deformation caused by overlapped tunnelling while taking subway train vibration loads into account. The research results indicated that with increasing tunnel clearance, the settlement trough appeared to be a much steeper U-shape. At d/D = 0.5, the maximum ground settlement increased by 37.5% along the transverse section and 30.6% along the longitudinal section compared to the case of the vibration-free condition. As d/D increased from 0.5 to 1.0, the maximum value of ground settlement decreased by 34.1% along the transverse section and 31.25% along the longitudinal section. The ground lateral displacement appeared to be biconcave and flattened with increasing d/D. Based on the test results, a model was introduced to predict the ground settlement induced by overlapped tunnelling coupled with vibration loads in subway train. The findings in this study will help evaluate the ground deformation caused by overlapped tunnelling considering vibration loads of subway train in operation. [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]