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Accurate estimation of the buoyancy forces exerted on underground structures is a problem in geotechnical engineering that directly impacts the construction safety and cost of these structures. Therefore, studying the buoyancy resistance of underground structures has great scientific and practical value. In this study, an initial difference in the hydraulic head, Δ<i>h</i>₀, was discovered to be present in aquitards through analysis of water-level data collected from the observation of real-world structures and in laboratory control tests. That is, seepage occurs beyond a threshold Δ<i>h</i>₀. Analysis of test data reveals that a deviation from Darcy’s law is the theoretical basis for Δ<i>h</i>₀ and that Δ<i>h</i>₀ equals the initial hydraulic gradient multiplied by the length of the seepage path. The general consistency between the experimentally measured and theoretically calculated values of Δ<i>h</i>₀ validates the theoretical explanation for Δ<i>h</i>₀. The results of this study provide a basis for scientifically calculating the buoyancy resistance required for the construction of underground structures.