Experimental and theoretical investigation of rheological behaviour of dry and saturated granular flows

Abstract

Geophysical granular and particle-laden flows, such as landslides and their subsequent mass flows on hillslopes or channels, are frequent mountain hazards triggered by rainfall, earthquakes, or other causes. The destructive nature of such flows demands reliable methods for predicting their flow dynamics. The dynamics of these flows is significantly affected by the rheological behaviour of involved soil-water mixtures. Such a particle-fluid mixture comprises a wide range of particles, from clay to boulders, and a wide range of particle concentrations. Given these wide ranges of variations in particle size and concentration, the rheological behaviour of soil-water mixtures may vary substantially. Previous studies on rheological behaviour of soil-water mixtures mainly focused on single-phase equivalent fluid-like behaviour of plastic fine mixtures, without considering the role of different interactions and multi-phase behavioral aspects of coarse non-plastic mixtures. How the flow dynamics of soil-water mixtures vary with the mixture properties is an open question that motivates a thorough experimental and theoretical investigation of rheological behaviour of dry and saturated granular flows in this doctoral thesis.

In this thesis, taking advantage of rotational rheometry, the rheological behaviour of dry and saturated granular flows was measured experimentally and evaluated theoretically, focusing on the contributions of different interactions such as particle-fluid, colloidal, collisional, and frictional interactions. Accordingly, a preliminary classification of different soil-water mixtures based on the dominant contribution of each interaction has been considered as dilute, moderately concentrated, and highly concentrated mixtures. For dilute and moderately concentrated mixtures, by using a small-scale viscometer, the contributions of particle-fluid, collisional, and colloidal interactions were evaluated. To distinguish the contribution of colloidal interactions, which are directly related to the clay fraction, a dispersion agent has been used. During measurements, the existing flow regime was also evaluated, and in the case of a turbulent flow regime, the effective rheological parameters were deduced using a torque-scaling method. For highly concentrated mixtures, the nature of frictional interactions and their transition to collisional interactions were evaluated first in the dry case, using a large-scale vane rheometer. Then the contribution of particle-fluid and frictional interactions of highly concentrated saturated granular flows was investigated considering their two-phase behavioral aspect.

For dilute soil-water mixtures, such as those involved in debris/mud floods or the interstitial fluid phase of highly concentrated mixtures, the Bingham model (τ = τ_col + η_m𝛾̇ ) can be used to describe the bulk rheological behaviour, where η_m governs the contribution of particle-fluid interactions, and τ_col governs the contribution of colloidal interactions from clay particles. For moderately concentrated mixtures, the shear-thinning rheology prevails for plastic mixtures such as those involved in mudflows because of the dominant effects of colloidal interactions, while the shear-thickening rheology prevails for non-plastic mixtures such as those involved in debris flows because of the dominant effects of collisional interactions.

A new classification for soil-water mixtures, distinguishing different types of rheological behaviour, was proposed based on sediment type, mass number (N_Mass, defined as the ratio of solid particles mass to that of the fluid matrix), and dominant interactions within the mixtures. Dilute soil-water mixtures are defined as those with N_Mass < 1 and C_V < 0.25 – 0.28, in which the particle-fluid interactions are dominant. By contrast, moderately concentrated soil-water mixtures are defined as those with N_Mass ≥ 1 and C_V > 0.25 – 0.28, in which interparticle interactions (collisional or colloidal) are dominant.

For dry granular flows, the measured flow characteristics revealed a transition from solid-to liquid-like behaviour with a descending-ascending pattern of effective friction parameter associated with a velocity-weakening behaviour in the quasi-static regime to a velocity-strengthening liquid-like behaviour in the intermediate regime. Besides, the visualized flow characteristics revealed the existence of a dynamic dilatancy. Accordingly, the governing rheological laws in terms of a new non-monotonic friction law and a dynamic dilatancy law were proposed. The friction law exhibits how the effective friction parameter varies with both pressure and shear rate, and the dynamic dilatancy law exhibits how the frictional interactions gradually become collisional interactions.

For saturated granular flows, the flow characteristics at the initiation and motion stages of flow revealed the transitional solid-fluid like behaviour and the transitional behaviour from one-phase to two-phase behaviour due to the relative motion of the fluid phase. Inspired by the µ(I) rheology, a new dimensionless number, namely the fluidisation number (F) that controls the two-phase behaviour of such soil-water mixtures was proposed theoretically and validated with experimental data. This number considers the role of two main interphase forces such as buoyancy and drag at particle scale. A mixture will behave as a solid when F < 1 while the mixture will behave as a fluid when F > 1. Finally, a new non-monotonic friction law was also proposed which describes the relation between µ and F and captures the two-phase rheological behaviour of soil-water mixtures.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Li Min ZHANG (Supervisor)