Assessment of Groundwater Depressurisation due to Mining Considering Flow Nonlinearity in Connected Fractures

Abstract Longwall mining is a productive method for underground coal resource recovery, but usually leads to extensive fracture networks that facilitate groundwater drainage into mined-out areas. Accurately assessing the coverage and magnitude of induced groundwater depressurisation is crucial for sustainable mining practices, but is often constrained by simplified representations of fracture flow. This paper investigates the nonlinear flow behaviour in connected fractures and develops a governing equation applicable to field-scale models for improved groundwater impact assessments. Laboratory tests on intersected pathways demonstrate that inflows interact and redirect along curved paths with localised turbulence. Numerical upscaling reveals a negative correlation between pressure drop and pathway geometry, identifying the laminar flow threshold as Reynolds numbers ( $$Re$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Re</mml:mi> </mml:mrow> </mml:math> ) ≤ 10. A modified cubic law (MCL) is derived from the best-fit linear relationship between hydraulic gradient and flow rate, defining a super-cubic correlation where the exponent increases asymptotically from 3.15 to 3.77 with apertures. Applying the MCL equation to a field-scale model reveals that the depressurised zone has a height of 0.86 times the panel width, aligning with the upper limit of horizontal stress relief and the significant outflow horizon measured by borehole acoustic scanning. The depressurisation remains confined below the claystone unit, which is interpreted as being associated with clay swelling upon water absorption. Comparisons with empirical equations suggest that integrating fluid flow analysis into numerical simulations allows more direct and comprehensive assessments for groundwater and sustainable purposes. The paper improves the understanding of flow nonlinearity in connected fractures and offers a practical numerical method for evaluating the hydrogeological impacts of mining activities.