Influence of Groundwater Level Rising on Mechanical Properties of Pile Foundations Under a Metro Depot in Loess Areas

The span of pile foundations beneath metro depots typically ranges from 10 to 20 m, exhibiting a notably large span. This structural characteristic results in the pile foundations bearing a more concentrated upper load, while the interstitial soil between the piles bears minimal force. Concurrently, global climate change and enhanced urban greening initiatives have led to a significant increase in rainfall in northwest China, a region traditionally characterized by arid and semi-arid conditions. This climatic shift has precipitated a continuous rise in groundwater levels. Furthermore, the extensive distribution of collapsible loess in this region exacerbates the situation, as the rising groundwater levels induce loess collapse, thereby adversely affecting the mechanical behavior of the pile foundations. In light of these factors, this study utilized the pile foundations of a metro depot in Xi’an as a prototype to conduct static load model tests under conditions of rising groundwater levels. The experimental results reveal that the load–settlement curve of the pile foundations in the absence of groundwater exhibited a steep decline with distinct three-stage characteristics, and the ultimate bearing capacity was determined to be 5 kN. When the groundwater level is situated below the loess stratum, the settlement of both the pile foundations and the foundation soil, as well as the axial force, skin friction, and pile tip force, remains relatively stable. However, when the groundwater level rises to the loess stratum, there is a significant increase in the settlement of the pile foundations and foundation soil. Negative skin friction emerges along the pile shaft, and the bearing type of the pile foundation transitions gradually from a friction pile to an end-bearing pile. The influence range of the pile foundation on the settlement of the foundation soil is approximately three times the pile diameter.