Abstract:

[Objectives] Large-scale urban underground space development has led to numerous anti-floating problems. Groundwater level is a key parameter in the anti-floating design of underground structures, but it is inherently dynamic and influenced by various factors. This study aims to investigate how groundwater level in fractured rock layers dynamically responds to rainfall. [Methods] Field monitoring was conducted along a subway line, with seven groundwater level monitoring points and two meteorological monitoring points installed. Real-time data of groundwater level in fractured rock layers and rainfall at the site were collected. Based on these data, the annual variation patterns of groundwater level and rainfall were analyzed. Groundwater level increments under moderate to heavy rainfall conditions (daily rainfall ≥10.0 mm) were extracted, and a linear fit was performed between rainfall and groundwater level increments. [Results] Groundwater level variations were closely related to rainfall, rising during wet periods and falling during dry periods, with peak-to-valley amplitudes ranging from 3.34 to 17.55 meters. Additionally, the slope of the linear fitting between groundwater level increment and rainfall ranged from 0.006 to 0.025, indicating varying response speeds of groundwater level changes to rainfall. These differences were mainly influenced by rainfall intensity, site topography, surface water systems, and excavation activities. Based on the permeability of the rock layers, recommendations for anti-floating design were proposed: when the permeability coefficient of fractured rock exceeds 20 m/d, underground structures should strengthen passive anti-floating measures or increase active drainage and pressure relief measures; when the permeability coefficient ranges from 10 to 20 m/d, the anti-floating safety factor should be appropriately increased; when the coefficient is below 10 m/d, standard design practices are sufficient. [Conclusions] The study identifies the main factors influencing groundwater level fluctuations, quantifies the response of groundwater level to rainfall, and proposes an anti-floating design method that accounts for the permeability coefficient of rock layers. This approach addresses the limitations of traditional anti-floating designs that assume a uniform design water level and provides practical guidance for the anti-floating design of subway stations.

Key words: fractured rock layers, groundwater level, rainfall, linear fitting, permeability coefficient