The offshore wind farm industry has recently experienced significant global growth. This study presents a thorough site investigation and analysis of the cyclic resistance of marine clay for offshore foundation design, using the Shaba wind farm in southern China as a case study. In-situ cone penetrometer (CPTu) tests and borehole sampling are conducted to explore the geotechnical characteristics of the soils. However, the soil conditions are characterized by multiple layers and complex sedimentary components. The classification and mechanical properties, such as water content and cyclic resistances, are compared through CPTu interpretation and laboratory tests. The findings indicated that a single physical indicator cannot determine cyclic resistance. In addition, the well-established method in existing literature proved unsuitable for marine clay. Consequently, multiple regression analysis shows that a linear relationship exist between cyclic resistance and depth-corrected CPTu index [EXP(q_E/f_s)^0.3/H], hence a new evaluation method is developed to predict the cyclic resistance of marine clay based on CPTu data. This research aims to provide more reliable guidance for geotechnical investigations, supporting the rapid expansion of offshore wind farms.

In this article, steel slag was added into the marine silt to improve its dynamic properties. The characteristics of marine silt that can be studied by XRD, BET surface area, SEM, and FTIR were first described. Then, the influence of the sample density, water content, waste steel slag content, and the confining pressure on the dynamic shear modulus G and damping ratio λ was discussed by conducting resonant column tests. The results illustrated that the dynamic characteristics of the marine silt-steel slag admixtures were considerably improved. Additionally, G increased with the increasing steel slag content and λ increased with the decreasing steel slag content. Moreover, a dynamic model of the admixtures was developed based on the Hardin–Drnevich model. Furthermore, by comparing the dynamic characteristics of the admixtures with Nanjing sand, Fujian sand, Kiyohoro silty clay, and Wenzhou marine clay, it was found that the improved admixtures may be a more suitable geofiller in geotechnical engineering.

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.

To explore the characteristics of the dynamic shear modulus of structural soft soil, undisturbed structural soft from the floodplain of the Yangtze River in Nanjing was subjected to strain-controlled cyclic triaxial tests to investigate how the initial effective confining pressure (σ'_m), consolidation ratio (k_c), and degree of consolidation (U) influence the maximum dynamic shear modulus G_max and dynamic shear modulus (G) and damping ratio (λ). The results show that for this soil, G decreases with increasing shear strain amplitude (γ), and for a given γ, G increases with increasing σ'_m, k_c, and U. Compared with soil from the Yangtze estuary, k_c has a greater effect on G_max of the floodplain structural soft soil. Finally, a modified Martin-Davidenkov model is proposed for predicting G/G_max of floodplain structural soft soil under different σ'_m, k_c, and U.

This study conducted experimental tests on the undisturbed Nanjing Yangtze River floodplain soft soil using the bender element instrument to determine the maximum dynamic shear modulus of the Yangtze River floodplain overconsolidated soft soil. The G_max of floodplain soft soil with different overconsolidated ratio OCR, initial effective confining pressure σ_3c′, and void ratio e are discussed. The results indicated that G_max reduced as e rose for given σ_3c′ and OCR. In addition, an increase in OCR contributed to a gradual decrease in the decay rate of G_max, while the G_max decay rate is insensitive to the change of σ_3c′. The void ratio-normalized maximum shear modulus G_max/F(e) improved with the increase in the stress-normalized initial effective confining pressure σ_3c′/P_a, whereas the growth rate gradually drops, and a power relationship is then obtained between G_max/F(e) and σ_3c′/P_a. Based on the regression analysis, a G_max prediction method is established for reasonably characterizing Yangtze River floodplain soft soils with various over-consolidation states, initial stress conditions, and compactness levels, with a prediction error of less than 10%.