In this study, to explore the microstructure deformation mechanism of marine soft marine soil under cyclic loading, we analyzed the dynamic properties of soft marine soil under cyclic loading via dynamic consolidation compression testing. Then, using Image-Pro Plus (IPP) 6.0 image analysis software, and according to the dynamic consolidation compression test results and the images from a scanning electron microscope (SEM), we determined the weakening effect of soft soils under different consolidation confining pressures, different cyclic stress ratios, and different over-consolidation ratios. After dynamic consolidation and compression, the pore structure of undisturbed soft marine soil tends to compact, the degree of soil particle fragmentation intensifies, small pores increase, large pores decrease, the pores become more regular, and the distribution of pores is directional. Subsequently, for undisturbed soft marine soil, the higher the consolidated confining pressure, cyclic dynamic stress ratio, and over-consolidation ratio, the greater the damage to the pore structure, and the more obvious the structural weakening effect exhibited under cyclic loading.

Development of underground space in urban areas may be obstructed by the aboveground facilities that are in good condition. Instead of building new aboveground structures, an alternative solution is to add new basements under the existing structures. One of the key issues arising from this complicated technique is the behavior of the underpinning piles. This paper considers this issue in terms of a real case recently addressed in China. The retrofit construction procedure was carefully monitored. Observations included the axial-loading and excavation-unloading responses of the underpinning piles, the axial forces transmitted from the piles to the replacing pillars, and the variations in building settlement. The results indicated that the piles acted well as an underpinning role in transferring structure load into surrounding geomaterials. The mobilization degree of the pile-shaft friction increased as the shallow strata were excavated further. Abrupt increase in pillar force and building settlement were detected when the exposed portions of the piles were cut, and therefore the cutting operation should be careful for safety control. The performance-based analyses support the pile-pillar load transmission method and offer a fairly successful reference to future applications.

The mechanism of drilling borehole and its influence on the bearing behavior of existing foundation piles are unclear, hindering the application of excavation techniques. This paper proposes a new excavation scheme for high‐rise buildings in downtown and develops theoretical formulas to calculate the skin friction and end resistance of existing foundation piles affected by adjacent borehole drilling for installing underpinning piles. These underpinning piles are installed and connected with the existing pile by the head cap before excavation. In a case study, the parameter analysis was then performed to understand the effect of underpinning construction on the bearing behavior of existing foundation piles in terms of skin friction and end resistance. The results show that borehole diameter, depth of drilling borehole, distance between the existing foundation pile and drilling borehole, and number of boreholes have a negligible influence on the end resistance of existing foundation piles. The effect of drilling parameters on the skin friction of existing foundation pile, as well as the influence extent, varies for different parameters. The depth of drilling borehole and the number of boreholes have significant influence and thus should be considered in real engineering design while the borehole diameter and the pile‐pile distance have a negligible effect. The proposed new excavation technique could be potentially adopted for real engineering design of underneath excavation projects for high‐rise buildings in downtown.

In the context of deep foundation pit excavation, the settlement of surrounding buildings is a critical indicator for safety assessment and early warning. Due to the non-stationary and nonlinear characteristics of settlement data, traditional prediction approaches often fail to achieve satisfactory accuracy. To address this challenge, this study proposes a hybrid prediction model integrating the Grey Wolf Optimizer (GWO), Variational Mode Decomposition (VMD), and Long Short-Term Memory (LSTM) networks, referred to as the GWO-VMD-LSTM model. In the proposed framework, GWO is employed to optimize the key hyperparameters of VMD as well as LSTM, thereby ensuring robust decomposition and prediction performance. Experimental results based on settlement monitoring data from four typical points around the Yongning Hospital foundation pit in Taizhou, China, demonstrate that the proposed model achieves superior predictive accuracy compared with five benchmark models. Specifically, the GWO-VMD-LSTM model attained an average coefficient of determination (R²) of 0.951, mean squared error (MSE) of 0.002, root mean square error (RMSE) of 0.033 mm, mean absolute error (MAE) of 0.031 mm, and mean absolute percentage error (MAPE) of 1.324%, outperforming all alternatives. For instance, compared with the VMD-LSTM model, the proposed method improved R² by 26.56% and reduced MAPE by 45.87%. These findings confirm that the GWO-VMD-LSTM model not only enhances the accuracy and generalization of settlement prediction but also provides a reliable and practical tool for real-time monitoring and risk assessment of buildings adjacent to deep foundation pits in soft soil regions.

The behavior of underpinning piles during the soil excavation is critical for constructing new basements under the existing structures in urban areas; however, none of the existing studies has systematically evaluated their performance and interaction with existing piles under loading. This study aims to experimentally evaluate the impact of soil excavation on the bearing capacity of pile group foundation composed of underpinning piles and existing piles under loading. A model pile foundation with four model piles, one pile cap, and retaining structures was first constructed in our laboratory. Different soil excavation depths at 10 cm, 20 cm, and 30 cm (soil type: clayey silt) were used to explore their influence on the pile side friction, the axial force of the pile shaft, and pile settlement. The results were compared with the bearing behavior of the pile group foundation composed of four model piles and five underpinning piles at different pile lengths (500, 600, and 700 mm) and diameters (5, 10, and 15 mm). The results showed that soil excavation activity could immediately activate the different interaction responses between piles and soils and cause foundation settlements. The underpinning piles with sufficient length (more than 67 % of the existing model pile length) and diameter (more than 25 % of the existing model pile diameter) as well as proper pile’s embedded depth significantly reduced the foundation settlement, particularly at high soil exaction depth. The pile-soil interaction mechanism caused by soil excavation is revealed. The work could bring insights to the underpinning pile design for basement excavation beneath existing buildings.

In the present paper, the pile-soil system’s total potential energy equation of underpinning piles was established based on the Winkler elastic foundation beam theory. This energy equation was used to explore the effect of basement excavation beneath existing buildings on the underpinning pile’ buckling stability. Utilizing the minimum potential energy theory, the expression of the critical buckling load for underpinning piles’ stability during the excavation project was obtained. Moreover, the influencing factors of the critical buckling load were investigated. It was found that the underpinning pile’s critical load converged with the augment of half-wave number. Moreover, the pile skin friction and deadweight had an insignificant influence on it. In addition, the critical load of underpinning piles decreased sharply with the increasing excavation depth and gradually increased with the augment of pile diameter. The results of this study provides a basis for the design of adding piles in similar projects and reduces the hidden danger of excavation instability.