🔬 condensed matter

Investigating frictional instability due to pressurization in granular media: insights from coupled computational fluid dynamics discrete element method

This study employs 3D coupled CFD-DEM simulations to demonstrate that fluid-induced instability in granular media is governed not by pore pressure alone, but by complex hydromechanical feedbacks involving drainage conditions, granular fabric evolution, and the transition from dilatancy-dominated strengthening to pore-pressure-driven weakening.

Original authors: Bimal Chhushyabaga (Department of Civil and Environmental Engineering, University of Houston), Behrooz Ferdowsi (Department of Civil and Environmental Engineering, University of Houston)

Published 2026-06-05

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CC BY 4.0

Original authors: Bimal Chhushyabaga (Department of Civil and Environmental Engineering, University of Houston), Behrooz Ferdowsi (Department of Civil and Environmental Engineering, University of Houston)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a pile of sand, like a sandbox, but this one is soaked with water and squeezed tightly between two heavy plates. This is the setup for a study on how wet, grainy materials (like soil, sand, or rock dust) suddenly fail or slide when water pressure builds up inside them.

The researchers used a super-powerful computer simulation to watch this happen particle-by-particle. Think of it as a high-definition movie where every single grain of sand and every drop of water is tracked individually.

Here is what they found, explained through simple analogies:

1. The Setup: The "Sandwich" and the "Pressure Cooker"

Imagine a sandwich where the bread is a heavy plate and the filling is wet sand.

The Stress: The top plate is pushing down (gravity) and trying to slide sideways (shear stress).
The Trigger: They started pumping water pressure from the bottom of the sandwich, like inflating a balloon inside the filling.
The Goal: They wanted to see at what point the water pressure would make the sand lose its grip and slide uncontrollably.

2. The Two Scenarios: The "Sealed Box" vs. The "Open Window"

The researchers tested two different ways the water could behave, which changed the outcome dramatically:

The "Sealed Box" (Undrained): Imagine the top of the sandwich is sealed tight. When they pump water in, the pressure has nowhere to go. It builds up fast, pushing the grains apart and making them float slightly.
- The Result: This caused a sudden, violent collapse. The sand lost its strength instantly, and the top plate slid off quickly. It was like a sudden earthquake.
The "Open Window" (Drained): Imagine the top of the sandwich has a tiny hole or a sponge that lets water escape. When they pump water in, some of it leaks out.
- The Result: The pressure didn't build up as high. The sand didn't collapse all at once. Instead, it weakened slowly and steadily, like a slow slide down a hill rather than a sudden fall.

3. The Secret Ingredient: It's Not Just Water Pressure

A common misconception is that water pressure alone causes the slide. The paper says no.

Think of the sand grains as a team of people holding hands in a circle (a "force chain").

The Teamwork: As long as they hold hands tightly, the team is strong.
The Water: The water pressure tries to push the people apart.
The Real Cause: The slide happens only when the water pushes hard enough AND the people (grains) have already let go of each other's hands.
- If the team is already weak (the sand was already close to sliding), a little water pressure causes a crash.
- If the team is strong (the sand is far from sliding), you need a lot of water pressure to break the grip.

The study found that the "letting go" happens because the grains start to rotate and rearrange themselves, breaking the strong chains that hold the structure together.

4. The "Traffic Jam" of Grains

Inside the sand, things aren't moving evenly.

Dilation and Compaction: Some parts of the sand tried to expand (dilate) like a crowd of people trying to spread out, while other parts got squeezed tighter (compacted).
The Permeability Shift: This rearrangement changed how easily water could flow through the sand. It's like a traffic jam suddenly clearing up in one lane while another lane gets clogged. The water found new paths to travel, which changed where the pressure was highest.

5. The "Creep" vs. The "Crash"

The researchers looked at how fast the sand moved.

Slow Creep: At first, the sand moved very slowly, almost like it was hesitating. During this time, the sand actually got stronger temporarily because the grains were jostling into a tighter, more stable position (like people finding their footing).
The Crash: Eventually, the water pressure overwhelmed this stability. The grains stopped finding footing and started spinning wildly. The "force chains" (the hand-holding) collapsed, and the friction dropped, leading to a fast slide.

6. The "Viscous Number" (The Speed Limit)

The researchers tried to compare their sand to a thick soup or a dense suspension (like a smoothie). They used a math concept called the "viscous number" to see if the sand behaved like a fluid.

The Finding: It didn't quite fit the standard "soup" rules. The sand was too crowded and the grains were too solid. It behaved more like a slow-moving traffic jam where cars (grains) occasionally bump into each other and rearrange, rather than a smooth flow of liquid. The standard rules for thick fluids didn't perfectly predict what happened here.

Summary

The paper concludes that when wet sand fails, it's not just about the water pressure pushing. It's a complex dance:

Water pressure tries to push the grains apart.
The grains try to rearrange themselves to hold on.
If the water can't escape (sealed box), the pressure wins, and the grains lose their grip all at once.
If the water can escape (open window), the pressure is released, and the failure happens slowly.
Crucially, the failure only happens when the internal "hand-holding" (the contact network) has degraded enough that the grains can no longer support the weight pressing down on them.

Technical Summary: Investigating Frictional Instability Due to Pressurization in Granular Media

Problem Statement
Fluid pressurization is a primary trigger for deformation and instability in saturated geomaterials, including fault gouge, rainfall-affected slopes, and injection-perturbed reservoirs. While macroscopic failure criteria (e.g., Mohr–Coulomb) describe the reduction of effective normal stress due to pore pressure, they fail to explain the grain-scale feedbacks linking pressure diffusion, drainage, and contact-network degradation to the transition from stable creep to dynamic slip. Specifically, the mechanisms governing how subcritically stressed granular layers reactivate under prescribed shear stress, and how drainage conditions regulate this process, remain difficult to resolve at the particle scale.

Methodology
The study employs three-dimensional, two-way coupled Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) simulations to investigate pore-pressure-induced reactivation in a confined, fluid-saturated granular shear layer.

Numerical Framework: The fluid phase is solved on a fixed Eulerian grid using volume-averaged mass and momentum balance equations, while individual grains are tracked as Lagrangian particles obeying Newton's laws. Interphase momentum exchange is modeled via pressure-gradient forces and Syamlal–O'Brien hindered drag. Particle interactions utilize a soft-sphere Hertzian contact law with Coulomb friction.
Loading Protocol: The study utilizes a two-stage approach. First, strain-controlled simulations under servo-controlled normal stress establish the reference frictional strength and Mohr–Coulomb failure envelope. Second, stress-controlled simulations impose subcritical shear stresses ( $\tau = 0.46\tau_{ss}$ to $0.90\tau_{ss}$ ) while basal pore pressure is progressively increased.
Boundary Conditions: Simulations compare undrained conditions (impermeable top boundary, retaining excess pore pressure) with drained conditions (atmospheric pressure at the top, allowing vertical pressure dissipation).
Diagnostics: The analysis integrates macroscopic metrics (apparent friction, slip velocity, excess pore pressure) with micromechanical diagnostics (particle rotation, force-chain reorganization, coordination number, porosity redistribution) and second-order fabric anisotropy metrics.

Key Results

Role of Drainage: Instability is not governed by pore pressure alone but by its coupled evolution with effective stress and drainage.
- Undrained: Excess pore pressure is retained, leading to rapid effective-stress reduction, abrupt frictional weakening, and dynamic slip acceleration.
- Drained: Vertical pressure gradients are maintained, suppressing excess pressure buildup. Weakening is smoother and more progressive, as fluid escape buffers the conversion of pore-pressure forcing into effective-stress loss.
Stress Proximity and Fabric Degradation: The pressure increment required to trigger failure depends on the initial stress state relative to the shear strength.
- Layers closer to failure ( $\tau \approx 0.9\tau_{ss}$ ) fail after modest pressure increases with less extensive fabric degradation.
- Layers farther from failure ( $\tau \approx 0.46\tau_{ss}$ ) require larger pressure buildups and undergo more significant contact-network degradation, including stronger force-chain collapse and coordination-number reduction.
Micromechanical Evolution: Failure is characterized by the transition from distributed deformation to localized slip.
- Force Chains: Macroscopic weakening coincides with the collapse and reorganization of the strong-force backbone. Post-failure, the lower-shear case exhibits a substantial loss of directional force-chain organization, whereas the high-shear case retains a more coherent anisotropic structure.
- Porosity and Permeability: Failure involves alternating bands of dilation and compaction. These volumetric changes reorganize the porosity-derived effective permeability, indicating that hydraulic pathways evolve dynamically during deformation.
Rheological Scaling: The frictional response partially organizes with the viscous number ( $I_v$ ) in the low- $I_v$ regime ( $10^{-8} \lesssim I_v \lesssim 10^{-5}$ ). However, the data do not collapse onto a unique local rheology, consistent with a creeping regime dominated by intermittent rearrangements and nonlocal stress transmission rather than a simple local $\mu(I_v)$ law.

Significance and Claims
The paper claims to provide grain-resolving insights into how drainage-controlled hydromechanical feedbacks and fabric degradation convert pore-pressure forcing into frictional instability. Key contributions include:

Demonstrating that the transition to instability is regulated by the competition between pressure generation, hydraulic diffusion, and deformation-induced pore-volume change.
Showing that drainage conditions fundamentally alter the failure pathway: undrained systems favor abrupt weakening via pressure retention, while drained systems fail progressively via pressure gradients.
Establishing that the degree of contact-network degradation required for failure is dependent on the initial stress state; proximity to failure dictates the extent of fabric reorganization needed before the granular skeleton can no longer support the imposed shear load.
Clarifying that macroscopic weakening is the system-scale expression of contact-network degradation, specifically the loss of directional load-bearing structure, rather than merely a function of bulk pore pressure or contact loss.

The authors conclude that these findings offer a mechanistic basis for interpreting pressurization-driven instability in fault gouge, rainfall-triggered slopes, and injection-perturbed systems, highlighting that poorly drained layers are prone to abrupt weakening when local fabric degradation occurs within a broader context of pressure retention.