PREDICTION OF GEOTECHNICAL PROPERTIES OF COAL SLURRIES USING CONE PENETRATION TEST

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PREDICTION OF GEOTECHNICAL PROPERTIES OF COAL SLURRIES USING CONE PENETRATION TEST

ABSTRACT

This thesis presents an experimental research for studying liquefaction potential of coal slurry under seismic motions by cone penetration test and correlations of coal slurry physical and dynamic properties such as shear modulus, pre-consolidation stress, and soil classification. Total of eight cone penetration tests are performed, six before simulated earthquake shaking and two after shaking. Tip resistance, sleeve friction and pore pressure of coal slurry are collected during CPT and used to analyze coal slurry. Friction ratio, as one of the basic parameters of coal slurry, is calculated directly from tip and sleeve resistance. The ratio of cyclic resistance ratio and cyclic stress ratio is used to evaluate liquefaction potential of coal slurry. CPT at different locations in laminar shear box yield varying cyclic resistance ratio based on the tip resistance.

Three soil properties of coal slurry are correlated with CPT as the main findings of this research. Shear modulus and shear wave velocity of dried coal slurry are measured from resonant column test and compared with shear modulus correlated from CPT. Preconsolidation stress of coal slurry is measured from oedometer test of in-situ undisturbed coal slurry samples and compared to the correlated pre-consolidation stress derived from CPT. Soil classification is determined by lab testing such as sieve analysis, hydrometer test, and Atterberg limit test and also compared to the correlated soil classification from CPT.

The coal slurry is anticipated to liquefy with an arbitrary ground acceleration estimated based on cyclic resistance ratio. The shear modulus of coal slurry measured from resonant column test is less than correlated shear modulus from CPT results, but in an acceptable range. The correlated pre-consolidation stress from CPT is close to the preconsolidation stress measured from oedometer test. The correlated soil classification from

CPT also agrees with the classification based on laboratory testing.

 

TABLE OF CONTENTS

LIST OF FIGURES ………………………………………………………………………………………………….. vii

LIST OF TABLES …………………………………………………………………………………………………… x

ACKNOWLEDGEMENTS ………………………………………………………………………………………. xi

Chapter 1  Introduction ……………………………………………………………………………………………… 1

1.1 Problem Statement and Research Motivation …………………………………………………… 1

1.2 Research Objectives …………………………………………………………………………………….. 4

1.3 Thesis Outline ……………………………………………………………………………………………… 4

Chapter 2  Literature Review …………………………………………………………………………………….. 5

2.1 Tailings Dams of Coal Slurry ………………………………………………………………………… 5

2.1.1Tailings Dams Construction Types ………………………………………………………… 5

  • 2Tailings Properties and Impoundment ……………………………………………………. 6
    • Field Testing Methods of Soil ……………………………………………………………………….. 9
    • Coal Slurry Liquefaction Potential …………………………………………………………………. 10
    • Review of Correlation Between Pre-Consolidation Stress and CPT ……………………. 12
    • Review of Correlation Between Shear Wave Velocity and CPT Testing …………….. 14
    • Review of Correlation Between Soil Classification and CPT …………………………….. 16

 

Chapter 3  Liquefaction Potential Evaluation By Cone Penetration Test …………………………. 20

3.1 Methodology of Liquefaction Potential Characterized by CPT  …………………………. 20

  • Deposition Method of Soil Specimen …………………………………………………… 22
  • Analysis of Liquefaction Potential by Cyclic Resistance Ratio and Cyclic

Stress Ratio  ………………………………………………………………………………………………. 22

3.2 Results and Discussion  ………………………………………………………………………………… 24

  • Tip Resistance and Sleeve Resistance ………………………………………………….. 25
  • Liquefaction of Coal Slurry ………………………………………………………………… 27

3.3 Liquefaction Potential Conclusions  ……………………………………………………………….. 28

 

Chapter 4 Correlations between CPT Results and Coal Slurry Properties ………………………… 30

 4.1 Resonant Column Testing   …………………………………………………………………………… 30        4.1.1 Soil Sample Preparation  …………………………………………………………………….. 30    4.1.2 Soil Parameters and Resonant Column Testing Results  …………………………. 33

  • Coal Slurry’s Dynamic Properties Derived from CPT ……………………………. 38
  • Compare and Contrast of Coal Slurry Dynamic Properties between

Laboratory Testing and Correlation  …………………………………………………………….. 40

  • Conclusions and Suggesstion ……………………………………………………………… 42
  • Consolidation Testing ………………………………………………………………………………….. 42
  • Soil Classificaion ………………………………………………………………………………………… 48

 

Chapter 5  Conclusions and Recommendations ……………………………………………………………. 53

References ………………………………………………………………………………………………………………. 55

CHAPTER 1 INTRODUCTION

 

1.1 Problem Statement and Research Motivation

Tailings dam failure is one of the most catastrophic events in the world. A catastrophic release of tailings can lead to long-term environmental damage with significant cleanup costs (Chambers, 2015). Mining industries yield significant amount of tailings after washing the minerals each year. These wastes are usually stored in an impoundment built near the mining site. However, many coal mining sites are surrounded by villages and rivers. Failure of the tailings dam can discharge toxic wastes to the nearby villages and rivers, causing irreversible damage to the communities and environment. Soil liquefaction is one of the most common reasons that cause failure of tailings dams. It is triggered by the seismic motions such as earthquakes or blasting. Soil liquefaction happens when the vertical effective stress on the soil decreases to zero by the increased pore pressure. This makes soil particles free to move and results in increased lateral earth pressure on the tailings dam and could potentially cause tailings dam failure.

There are more than 1300 mine tailings impoundments in the United States that allow the mining and processing of coal and other minerals, and over 200 of them are classified as having high hazards potential by the Federal Emergency Management

Agency’s hazard rating system (FEMA, 2019). A corpus of 147 cases of worldwide tailing dam disasters is compiled in a database and fifteen percent of them is caused by seismic liquefaction (Rico, 2007). There are 17 tailings dam failures since 1980 where the volume of the waste released is significant (WISE, 2019). For example, the Kingston fossil plant coal fly ash slurry spill released 4-million m3 waste in 2008 (Tetra Tech EM, 2009). The released coal slurry composes heavy metals, which can cause ecological and environmental hazards in rivers and lakes, and pollute nearby villages. In 2014, Mount Polley tailings dam failure (Figure 1-1) happened in the British Columbia, Canada. According to the government-ordered report, 24 million cubic meters of silt and water had flooded to Polley lake and continued to nearby Quesnel lake and Cariboo river. The negative environmental impact caused by the overflow of mining waste is substantial. It takes years even decades to recover from the damage with billions of dollars spent and huge amount of efforts (Marshall, 2018). A recent tailings dam failure occurred in Brazil on Nov 5, 2015 known as the Fundão Tailings Dam failure as shown in Figure 1-2. Eighty percent of its stored waste had been released and 19 people were killed. Buffalo Creek flood and disaster resulted in 125 fatalities and 4000 people homeless (Davies et al. 1972). Another collapse of a Brazilian dam controlled by miner Vale happened in the end of January 2019, with 110 people confirmed dead and another 238 missing (Stargardter, 2019).

 

(a) B.C.’s Mount Polley, waste flow into lake   (b) B.C.’s Mount Polley, breakage of wall Figure 1-1. Examples of tailings dam failures: B.C.’s Mount Polley tailings dam failure,

photo courtesy of Marshall 2018

 

 

(a) before dam failure                                     (b) after dam failure

Figure 1-2. Examples of tailings dam failures: Fundão Tailings Dam failure, photo by

Fundão Tailings Dam Review Panel.

To ensure the stability of coal slurry impoundment, in-situ testing of soil properties becomes especially important. This research aims at finding the liquefaction potential using cone penetration test and correlating the analyzed data to soil properties. Research on tailings dam structure and properties of waste coal slurry is a significant step to prevent any environmental and economical disasters caused by tailing dam failure. Using in-situ methods such as cone penetration test is highly efficient in time and cost. It also reduces the inaccuracy of results caused by disturbance of soil samples. Furthermore, correlations between the CPT results and certain soil properties can be established for advanced stability analysis of the impoundment.

 

 

1.2 Research Objectives

The first objective of this research is to analyze the occurrence of liquefaction of coal slurry by performing cone penetration tests after shake table testing. The second objective is to correlate the tip and sleeve resistance from CPT data with coal slurry properties such as shear modulus, pre-consolidation stress, and soil classification for future reference if in-situ data are available. A laminar shear box on the shake table is used to deposit coal slurry and simulate earthquake motion. Acceleration of soil particles and pore pressure built up in the coal slurry during the simulated motion are recorded and used for liquefaction analysis.

 

1.3 Thesis Outline

This thesis consists of five chapters. Introduction of the research is given in chapter 1. Literature review of coal slurry impoundments, field testing methods of soil, liquefaction potential of coal slurry, correlations between CPT results with pre-consolidation stress, shear modulus, and soil classification are presented in chapter 2. Chapter 3 presents the methodology of calculating the liquefaction potential of coal slurry using CPT data and the analysis of the results. Chapter 4 elaborates three correlations that can be made based on the CPT data. Chapter 5 presents the summary and conclusions derived from this study with recommendations of future work.

PREDICTION OF GEOTECHNICAL PROPERTIES OF COAL SLURRIES USING CONE PENETRATION TEST

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