In this study, a series of resonant-column experiments were conducted on marine clays from Bohai Bay and Hangzhou Bay, China. The characteristics of the dynamic shear modulus (G) and damping ratio (D) of these marine clays were examined. It was found that G and D not only vary with shear strain (γ), but they also have a strong connection with soil depth (H) (reflected by the mean effective confining pressure (σ_m) in the laboratory test conditions). With increasing H (σ_m) and fixed γ, the value of G gradually increases; conversely, the value of D gradually decreases, and this is accompanied by the weakening of the decay or growth rate. An intelligent model based on a back-propagation neural network (BPNN) was developed for the calculation of these parameters. Compared with existing function models, the proposed intelligent model avoids the forward propagation of data errors and the need for human intervention regarding the fitting parameters. The model can accurately predict the G and D characteristics of marine clays at different H (σ_m) and the corresponding γ. The prediction accuracy is universal and does not strictly depend on the number of neurons in the hidden layer of the neural network.

To investigate the static and dynamic characteristics of rubber–sand composite soil (RS soil) reinforced with cement, a series of triaxial compression tests and resonant column tests was performed by considering the influence of rubber content (10%, 20%, 30%, 40%, and 50%), cement content (0, 1.5, 2.5, 3.5 and 4.0 g/100 mL), and effective consolidation confining pressure (50, 100, and 150 kPa). Compared with the RS soil, the addition of cement significantly improved the shear strength of a cement–rubber–sand composite soil (RCS soil), based on an undrained shear test. The increase in cement content not only makes the elastic modulus and cohesion of the RCS soil increase but also reduces the internal friction angle of the RCS soil. With the increase in rubber content, the failure of the RCS soil samples changes from strain-softening to hardening, and the prediction equation of the initial elastic modulus of the RCS soil is given herein when the recommended cement content is 3.5 g/100 mL. The effects of rubber content, cement content, and effective confining pressure on the dynamic shear modulus and damping ratio of the RCS soil were studied via the resonant column test. The test results show that the increase in rubber content slows down the modulus attenuation of the RCS soil, but increases its damping ratio. The test results also show that the increase in cement content makes the bonding force between particles greater so that the modulus attenuation of the RCS soil becomes slower and the damping ratio is reduced. At the same time, according to the change rule of the maximum dynamic shear modulus of the RCS soil with the rubber content, when the recommended cement content is 3.5 g/100 mL, an empirical formula and recommended value of the shear modulus G_max of the RCS soil are proposed.

Cyclic threshold shear strain is a fundamental property of saturated soils under cyclic loading. To investigate the cyclic threshold shear strain for pore water pressure generation (γ_tp) and stiffness degradation (γ_td), a series of strain-controlled multistage undrained cyclic triaxial tests were carried out on in-situ saturated marine clay in the Yangtze estuary with different plasticity index I_p. The test results show that both γ_tp and γ_td increase with increasing I_p, and γ_tp is larger than γ_td for the same marine clay tested under the same conditions, with γ_tp = 0.017 ~ 0.019%, γ_td = 0.008 ~ 0.012% for I_p of 17, γ_tp = 0.033 ~ 0.039%, γ_td = 0.020 ~ 0.025% for I_p of 32, and γ_tp = 0.040 ~ 0.048%, γ_td = 0.031 ~ 0.036% for I_p of 40. Moreover, the development of stiffness degradation may not necessarily require the generation of pore water pressure but can be aggravated by it. Furthermore, the γ_tp and γ_td of marine clay are compared with terrestrial soils and marine clays cited from the published literature, the results indicate that the special marine sedimentary environment and the combined action of flow and tidal wave system cause the γ_tp and γ_td of marine clay in the Yangtze estuary to be smaller than that of the terrestrial clays and marine clays in other sea areas.

The shear modulus is an essential parameter that reflects the mechanical properties of the soil. However, little is known about the shear modulus of coral sand, especially under complex consolidation conditions. In this paper, we present the results of a multi-stage strain-controlled undrained cyclic shear test on saturated coral sand. The influences of several consolidation state parameters: effective mean principal stress ($p_0^{\prime }$), consolidation ratio (k_c), consolidation direction angle (α₀), and coefficient of intermediate principal stress (b) on the maximum shear modulus (G₀), the reference shear strain (γ_r) and the reduction of shear modulus (G) have been investigated. For a specified shear strain level, G will increase with increasing $p_0^{\prime }$ and k_c, but decrease with increasing α₀ and b. However, the difference between G for various α₀ and b can be reduced by the increase of shear strain amplitude (γ_a). G₀ shows an increasing trend with the increase of $p_0^{\prime }$ and k_c; on the contrary, with the increase of α₀ and b, G₀ shows a decreasing trend. To quantify the effect of consolidation state parameters on G₀, a new index (μ_G0) with four parameters (λ₁, λ₂, λ₃, λ₄) which is related to $p_0^{\prime }$, k_c, α₀, b is proposed to modify the prediction model of G₀ in literature. Similarly, the values of γ_r under different consolidation conditions are also evaluated comprehensively by the four parameters, and the related index (μ_γr) is used to predict γ_r for various consolidation state parameters. A new finding is that there is an identical relationship between normalized shear modulus G/G₀ and normalized shear strain γ_a/γ_r for various consolidation state parameters and the Davidenkov model can describe the G/G₀–γ_a/γ_r curves. By using the prediction model proposed in this paper, an excellent prediction of G can be obtained and the deviation between measured and predicted G is all within ±10%.

To investigate systematically how the temperature effect influences the measurement accuracy of a hydrostatic leveling system (HLS), ambient-temperature and temperature-gradient test platforms were built in the laboratory. The temperature of each measuring point of the HLS was collected by a high-precision temperature sensor, and the influences of the ambient temperature ${T}_{\mathrm {A}}$ , temperature gradient ${T}_{\mathrm {G}}$ , heating/cooling rate, communicating-pipe expansion coefficient, and measuring-point distance ${D}_{\mathrm {test}}$ on the HLS measurement accuracy were analyzed quantitatively. The results show that the error of the two-HLS setup was caused by the expansion of the communicating pipe and the liquid between the datum point and the measuring point both under ambient-temperature and temperature-gradient conditions. The relative ambient-temperature influence coefficient $\Delta {P}$ and temperature gradient influence coefficient $\Delta {P}_{\mathrm {t-G}}$ , respectively, had a linear correlation with the difference between the expansion coefficients of the communicating liquid and pipe ( $\gamma _{\mathrm {di}} - \gamma _{\mathrm {pi}}$ ). Moreover, $\Delta {P}_{\mathrm {t-A}}$ and $\Delta {P}_{\mathrm {t-G}}$ had a negative power relationship with ${D}_{\mathrm {test}}$ for given communicating pipe and liquid. A temperature-modified model was developed theoretically based on test results, and its reliability was verified via on-site tests.