Measurement and Simulation of Triaxial Compression Tests for a Sandy Loam Soil By Mukta R. Nandanwar A thesis submitted to the Faculty of Graduate Studies of the University of Manitoba in partial fulfilment of the requirements of the degree of Master of Science Department of Biosystems Engineering University of Manitoba Winnipeg, Manitoba Copyright © 2015 by Mukta R. Nandanwar i Abstract In the past, most research on soil mechanical properties was carried out for cohesionless soils in the fields of civil and geotechnical engineering. Little research has been carried out for mechanical properties of agricultural soil, which are essential for designing soil engaging tools in agriculture. In this study, unconsolidated undrained triaxial compression tests were performed to study the effects of moisture level and confining pressure on a sandy loam soil. The soil specimens tested had three moisture levels, and they were high (27-29% d.b.), medium (19-21% d.b.) and low (9-11% d.b.). The confining pressures used for the triaxial tests were 50, 100, and 150 kPa. Soil specimen was loaded at a strain rate of 1%/min. Measurements from the tests included stress-strain curve, shear strength, Young’s modulus, Poisson’s ratio, angle of internal friction, and cohesion. A model was developed using the Discrete Element Method (DEM) and computed by Particle Flow Code in three dimensions (PFC3D), a common DEM software. The model simulated the triaxial compression tests, and the model specimen was an assembly of 5- mm spherical particles which were defined by a set of micro parameters. During simulations, soil shear strength was monitored under different confining pressures. Through sensitivity analysis, it was found that most of the micro parameters affected the simulated soil shear strengths and the stress-strain behaviours. The most influential micro parameter was particle friction coefficient. This micro parameter was calibrated with the data from triaxial tests for different combinations of soil moisture levels and confining pressures. The calibrated particle friction coefficients varied from 0.2 to 1.0. The calibrations were done through matching the shear strengths between simulations and measurements, and the relative errors ranged between 0 and 6 %. ii Acknowledgements I would like to thank my advisor, Dr. Ying Chen for her sound technical guidance and for her immense support and understanding. I am extremely grateful to Dr. Marolo Alfaro, committee member, for his expert comments which I was glad to receive for the geotechnical engineering aspect of my study. I am thankful to Dr. Neil McLaughlin, committee member, for his advice on statistical analysis of data and for the constructive criticism about the thesis. I would like to thank Kerry Lynch, technician, for helping me understand the nuances involved in conducting an experiment. I would like to express my gratitude to the Department of Civil Engineering for letting me use their laboratory. I am filled with gratitude for my family and friends for encouraging me and for providing me with such a supportive environment. iii Table of contents Measurement and Simulation of Triaxial Compression Tests ......................................................... i for a Sandy Loam Soil ..................................................................................................................... i Abstract ........................................................................................................................................... ii Acknowledgements ........................................................................................................................ iii Table of contents ............................................................................................................................ iv List of Tables ................................................................................................................................ vii List of Figures .............................................................................................................................. viii List of Symbols and Abbreviations................................................................................................ xi 1. Introduction ................................................................................................................................. 1 2. Literature Review .................................................................................................................... 3 2.1 Mechanical properties of soil ................................................................................................ 3 2.2 Factors affecting the shear properties of soil ........................................................................ 3 2.3 Methods to measure the mechanical properties of soil ......................................................... 5 2.3.1 Direct shear test .............................................................................................................. 5 2.3.2 Vane shear test ................................................................................................................ 7 2.3.3 Triaxial compression test ................................................................................................ 7 2.4 Discrete Element Method (DEM) .................................................................................... 8 iv 2.5. Simulations of soil tests using the DEM .............................................................................. 9 2.6 Particle Flow Code in Three Dimensions (PFC3D) ............................................................. 10 2.7 Modeling of triaxial test using the DEM ............................................................................. 12 3. Objectives .............................................................................................................................. 14 4. Materials and Methods .......................................................................................................... 15 4.1 Experimental Study: Unconsolidated undrained confined triaxial compression test on a sandy loam soil .......................................................................................................................... 15 4.1.1 Specific objectives ........................................................................................................ 15 4.1.2 Experimental design ..................................................................................................... 15 4.1.3 Equipment description .................................................................................................. 16 4.1.4 Soil specimen preparation ............................................................................................ 19 4.1.5 Unconsolidated undrained triaxial compression test .............................................. 21 4.1.6 Measurements ............................................................................................................... 21 4.2 Simulations of unconsolidated undrained triaxial compression test soil using PFC3D ....... 25 4.2.1 Specific objectives ........................................................................................................ 25 4.2.2 Model development ...................................................................................................... 25 4.2.3 Specimen preparation in PFC3D .................................................................................... 29 4.2.4 Working of the model ................................................................................................... 29 4.2.5 Model micro parameters ............................................................................................... 31 v 4.2.6 Sensitivity analysis ....................................................................................................... 33 4.2.7 Calibration approach ..................................................................................................... 35 4.2.8 Data analysis 5. Results and Discussion .......................................................................................................... 37 5.1 Results from laboratory experiments .................................................................................. 37 5.1.1 Failure pattern and stress-strain curves ............................................................................ 37 5.1.2 Soil shear strength ........................................................................................................ 39 5.1.3 Soil modulus of elasticity ............................................................................................. 40 5.1.4 Soil Poisson's ratio ........................................................................................................ 41 5.1.5 Soil cohesion and angle of friction ............................................................................... 42 5.2 Results from Simulations .................................................................................................... 44 5.2.1 Model general behaviours ............................................................................................ 44 5.2.2 Results for sensitivity analysis ..................................................................................... 45 5.3 Calibration Results .............................................................................................................. 58 6 Conclusions ...............................................................................................................................