Micromechanics of Hong Kong Debris Flow Materials

香港泥石流材料的微觀力學研究

Student thesis: Doctoral Thesis

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Detail(s)

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Supervisors/Advisors
Award date21 Aug 2019

Abstract

The topography of terrain in Hong Kong makes landslides and debris flows a common occurrence. Understanding the mechanisms behind these soil/rock movements is challenging as it involves complex materials across varying scales. Numerical modeling tools such as DEM allows for the study of these complex movements. An important contribution of this thesis is the systematic laboratory-based study of scale effects via grain size, the role of material type on the tribological behavior at the grain contacts and also the behavior against impact in terms of coefficient of restitution. Material response and micromechanical parameters, as obtained via this advanced laboratory testing, can provide important inputs in numerical simulations and constitutive modeling.

This thesis focuses on the contact response between two discrete granules in order to obtain insights into the mechanical behavior during compression and shearing via monotonic and complex load-path tests. This was achieved by conducting laboratory tests using custom-made inter-particle loading apparatuses, which were designed and built at the City University of Hong Kong. An existing small-size apparatus was used to test the grains ranging between 0.5-5mm in diameter. During the course of this work, a new large-size inter-particle loading apparatus was built to test large grains ranging in size between 5-50mm. A new impact loading apparatus was also built to study the collision behavior of grains impacting on base blocks in terms of the coefficient of restitution and energy loss.

A wide range of diverse materials was characterized and tested. For the inter-particle loading tests, four natural sands from different geological environments were selected in addition to four landslide/potential landslide materials. Variously sized artificial grains were also included (chrome steel balls, glass balls) alongside three further materials (crushed limestone, fresh granite, lunar regolith simulant). For the impact tests, four different base blocks (polished granite, stainless steel, brass, rubber) and three types of grains were utilized (chrome steel balls, glass balls, Leighton Buzzard sand). Even though this thesis had a focus on landslide/debris flow materials, a broader range of grain types was also tested which helped to draw general conclusions on the micromechanical behavior of geological materials. This is mainly due to a limited number of works in the literature on the important aspects/factors which control the grain contact behavior of geological materials such as, the influence of surface roughness and Young’s modulus, the role of geological environment and degree of weathering, as well as the influence of grain.

Tests on variously sized glass grains showed that the grain size effects both the normal and tangential loading response. Effects of the surface condition in terms of smooth and rough interfaces were investigated. Rough surfaces exhibited low stiffness and high slip displacements compared with smooth surfaces. Interface/inter-particle friction increases with increasing surface roughness and it is independent of grain size.

Grains from landslide materials and lunar regolith with low Young’s modulus and high surface roughness exhibited high values of the inter-particle coefficient of friction compared to quartz sands. For potential landslide materials, such as weathered grains, immersion conditions affected their micromechanical response. Grains tested after the removal of clay coating exhibited lower values of inter-particle coefficient of friction compared with the natural grains. Ploughing mechanism could easier be observed for quartz sand grains through microscopic observations and an attempt was made to quantify the contact area based on the detected damaged regions.

Theoretical models were fitted to the experimental data both in the normal and tangential directions. In the normal direction, Yimsiri and Soga model could better fit the experimental data from the initial stages of loading, while Hertz theory could only fit well beyond the initial regime of plastic displacements. In the tangential direction, theoretical models over-predicted the experimentally derived tangential stiffness and under-predicted the slip displacement during shearing. An analytical expression of slip displacement proposed in the literature was modified, incorporating material micro-hardness in a normalized form to establish an expression which can be used in the micromechanical-based analysis of problems involving geological materials.

Impact loading tests showed that the engineered grains exhibited more consistent collision behavior compared with Leighton Buzzard sand by means of repeatable test results. For all the grain types, the lowest and highest values of the coefficient of restitution were observed for the impacts on rubber and granite base blocks, respectively. The effect of grain size and surface roughness on the impact loading behavior was investigated. Reduced values of coefficient of restitution were observed for the materials with high surface roughness.