title: Characteristics and 3D Behaviour of Granular Geomaterials due to Shear
Creators: name: Peerun, Mohammud Irfaan
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Direct Shear Test (DST)
subject: Particle Image Velocimetry (PIV)
subject: X-Ray MicroCT Scan
subject: 3D Printing, Selective Laser Sintering (SLS)
subject: Discrete Element Modelling (DEM)
subject: Rocky DEM
subject: Realistic Particle Shape
subject: Pipe Jacking
subject: Microtunnelling
subject: Arching

description: Over the last century, numerous practitioners and researchers have investigated the particle behaviour of granular soils based on laboratory testing, to improve their knowledge of strength qualities. The discrete element modelling (DEM) technique has been commonly used to improve the understanding of granular soil behaviour. Since 1979 from the development by Cundall and Strack, spheres have commonly been used to represent soil particles or aggregates in DEM, despite increasing appreciation that particle shape significantly influences the strength properties of granular soil when subjected to shear. As such, it is widely acknowledged that results from spheroidal modelling do not accurately portray how particle edges and shapes affect a grain’s true strength, however there is an absence of a ‘more accurate’ alternative technique. Addressing this lack of accuracy in the DEM technique, this study investigated the characteristics and threedimensional behaviour of granular geomaterials due to shear. The first step for this study was to investigate the shear behaviour of poorly graded angular metasandstone particles. Particle Image Velocimetry (PIV) and the four-stage shearing model were used to interpret the shear behaviour of metasandstone. Large direct shear tests were conducted on gravel-size metasandstone at normal stress of 50 kPa, 100 kPa and 150 kPa. A novel process of X-Ray scanning was used to obtain the actual particle shape of metasandstone gravels. Micro-CT scanning was used to scan 20 types of metasandstone particles which were converted into a 3D mesh format known as Standard Tessellation Language (STL). Using ‘Rocky DEM’ software, the actual scanned particle shapes were incorporated into an ‘advanced-DEM’ technique to represent soil particles in simulations without the use of sphere clumping. These advanced-DEM results using realistic particle shapes were then compared with spheroidal modelling simulation results. To validate the advanced-DEM technique, 3D-printed synthetic particles were also produced from the X-Ray scans, and then sheared as a calibration and validation exercise. Using the known material properties and shear behaviour of the synthetic particles, the material interaction parameters were calibrated in DEM to ensure adequate simulation results were produced. Direct shear test simulations using sphere particles were found to overestimate the peak shear stress while an underestimate was observed at residual state across various normal stresses. The difference in shear stress when using sphere particles compared to laboratory results ranged from 8.9% to 42.8% in DEM. This broadly agreeable magnitude of shear stress was significantly improved by adopting the realistic particle shapes in DEM where the difference in shear stress was reduced to a range of 3.2% to 6.5% compared to the laboratory results. The research findings are immediately useful for the design and construction sector, where accurate soil characterisation and modelling can enable more streamlined design due to removing additional ‘factors of safety’ margins that have been necessary with ‘approximate’ spheroidal DEM results. Given the significant cost associated with bulk earthworks and soil stabilisation, there are significant gains to be made in spending more time ‘up front’ in project to accurately model the granular structure of major soil types present. In the pursuit of improved understanding of the effects of particle geometries on soilstructure interaction and composite material behavioural properties for designing geostructures, the research findings were implemented in the assessment of pipe-jacking forces. The shear strength properties of metasandstone aggregates were used to assess the vertical stress acting onto two pipe jacking drives to demonstrate the possible occurrence of arching phenomenon. Using the laboratory test results of metasandstone, the calculated jacking forces were found to match agreeably with the site records. With the improved DEM technique, it is hoped to simulate a complete pipe jacking process in the near future.