A Unified Pipe Network Model for coupled thermo-hydraulic transport in frozen fractured rock: Formulation, validation and application

Freeze and thaw processes in fractured permafrost couple heat transport, phase change, and groundwater flow, yet most numerical approaches either represent fractured rock as an equivalent continuum or do not explicitly link freezing to fracture hydraulic conductance. This study develops UPMFrost, a thermo-hydraulic numerical framework based on the Unified Pipe Network Method for the consistent representation of porous media and discrete fracture networks. The formulation combines Darcy flow in porous media, cubic-law flow in fractures, conductive and advective heat transport, a soil freezing characteristic curve, and an apparent heat-capacity treatment of latent heat. Freezing reduces the effective hydraulic aperture of fractures, while a mixed control-volume formulation preserves the physical fracture-fluid volume within the rock discretization. Shared intersection nodes allow flow to redistribute through connected fracture pathways as local conductance decreases. The formulation is verified against analytical solutions and benchmarked against the InterFrost thermo-hydraulic intercomparison. A separate comparison with published single-cleft laboratory data provides an external fracture-scale check on the magnitude of advective and latent-heat exchange. A sequence of controlled fracture-scale cases is then used to isolate the effects of fracture aperture, spacing, and network connectivity. The results show a clear hydraulic-gradient threshold separating open and frozen states in a single fracture, measurable thermal interaction between neighbouring fractures, and flow redistribution within intersecting fracture networks when individual branches become restricted. The framework is finally applied to a field-derived discrete fracture network surrounding an underground drift under permafrost conditions. The simulations identify a transition between sustained drainage and hydraulic cutoff under steady thermal forcing, while annual thermal cycling produces winter flow cutoff followed by partial summer recovery. These results show that hydraulic persistence in fractured permafrost depends not only on the amount of freezing, but also on where frozen segments develop relative to the hydraulically active fracture pathways.