On July 5, the Balance Center organized experts to conduct a final acceptance review of the project "Decay and Regulation of Performance of Soil Inside and Outside the Pit, Pile Foundations, and Overall Structure Induced by Ultra-Deep Foundation Pit Excavation," undertaken by Zhejiang University.

The utilization of underground space in China has developed rapidly in recent years, such as urban underground complexes and underground transportation networks, and has become an important direction for sustainable urban development. Underground space excavation inevitably brings about a large number of foundation pit excavation projects, with increasingly deeper and larger foundation pit excavations and increasingly higher requirements on the surrounding environment. Foundation pit projects are often adjacent to existing structures such as elevated bridges and high-rise buildings, which commonly use pile foundations and are subjected to relatively significant dynamic actions, such as the vertical dynamic action of vehicle loads and the horizontal action of wind loads; in addition, some large dynamic equipment placed underground also requires pile foundations to resist the vibration loads of large equipment, and the pile foundation construction is completed before the foundation pit excavation. During foundation pit excavation, the stress in the soil inside and outside the pit is gradually released, causing soil disturbance, which in turn changes the constraint effect of the soil on the pile foundation, changes the deformation and internal forces of the pile shaft, and alters the pile-soil interface, ultimately leading to decay of the soil-pile dynamic performance and affecting the dynamic service performance of the superstructure. However, current research lacks effective theoretical means and methods to evaluate the decay of dynamic performance of surrounding piles and in-pit piles caused by foundation pit excavation.

This project relies on the ultra-deep foundation pit engineering of the Centrifugal Hypergravity and Interdisciplinary Experiment Facility. Starting from actual engineering, it first explores the mechanism of stress redistribution in the soil behind the wall under the influence of the soil arching effect, proposes a supporting calculation method, analyzes the correlation between excavation parameters and the distribution law of non-limiting active earth pressure behind the wall, and constructs an "excavation→soil" interaction model.

Subsequently, based on field measurements and hypergravity model tests, the project conducts a comparative analysis of the decay laws of dynamic characteristics of piles inside and outside the pit before and after foundation pit excavation, establishes a quantitative analysis method for excavation-induced decay of pile dynamic characteristics, and ultimately forms a complete"excavation→soil→pile"system coupling model. At the engineering practice level, this model provides key support for the design optimization of deep foundation pit retaining structures and the protection of the surrounding environment; at the design stage, it enables more accurate and convenient calculation of earth pressure parameters and provides a basis for environmental impact assessment. At the theoretical level, it systematically reveals the "excavation→soil→pile" dynamic interaction mechanism, providing a theoretical basis and methodological reference for the design practice and risk prevention and control of similar projects.

The project adopts a multi-scale research method combining theoretical analysis, numerical simulation, hypergravity model tests, and field tests.

This method reveals the mechanism of excavation-induced soil disturbance and the evolution law of the soil arching effect, establishes decay models for the horizontal and vertical static and dynamic stiffness of piles inside and outside the pit considering foundation pit excavation disturbance, and proposes analysis methods for the evolution of static and dynamic performance of pile foundations under different working conditions. The project results propose a soil stress calculation model considering the soil arching effect, solving the difficulty of calculating earth pressure behind the wall in soft soil areas; and establish a static and dynamic response analysis model for piles inside and outside the pit, filling the gap in research on the decay of pile performance under excavation unloading.