Hydro-Mechanical Modeling of Over-Pressured Mobile Shale: Insights into Shear Dilation Effects on the Uplift at Zhong Liao Tunnel, Taiwan

Mobile shales strongly influence deformation, uplift, and fluid migration in compressional sedimentary basins, yet the mechanical pathway from "normal" shale to mobile shale is still debated. This study tests the idea that shear-induced dilation under high pore-fluid overpressure can trigger a positive feedback among shear localization, porosity–permeability increase, and fluid flow, thereby promoting long-lived, ductile-like shale mobility. We focus on the Zhongliao Tunnel area in southwestern Taiwan, where rapid uplift and sharp spatial gradients in vertical motion have been reported near major faults.

We develop a two-phase hydro-mechanical numerical model that couples a poro–visco–elasto–plastic solid with Darcy fluid flow. Porosity evolves through competing compaction and a strain-rate–dependent dilation term that is activated under elevated overpressure, allowing fault-related shear zones to dynamically transform into high-permeability conduits. In the reference experiment, a high-pressure layer sealed beneath a low-permeability cap sustains overpressure within mudstone. Once shear localizes, dilation increases porosity and permeability along damage zones, enhancing focused fluid discharge. The resulting seepage forces and reduced effective strength further intensify shear localization, producing sustained fault creep and pronounced uplift of the block bounded by the principal fault systems. The modeled uplift pattern reproduces key first-order observations: a sharp vertical-velocity contrast across the main fault and a more gradual decay of uplift away from it, with peak uplift rates reaching the order of centimeters per year.

Sensitivity tests demonstrate that overpressure alone generates only modest uplift without dilation-enabled conduit formation, while shear compaction suppresses localization and distributes deformation. Permeability exerts a non-monotonic control: very low permeability limits fluid flux and seepage forcing, whereas very high permeability drains overpressure too efficiently and weakens sustained creep. Overall, the results provide a mechanistic framework for how overpressured mudstone can evolve into mobile shale through coupled dilation and fluid flow, and offer testable criteria for identifying similar processes in other shale-dominated orogenic settings.