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THE EXPANSIVE EFFECTS OF CONCENTRATED PYRITIC ZONES IN SHALES OF THE MARCELLUS FORMATION
ABSTRACT
Expansive pyritic shales have caused untold amounts of damage to civil infrastructure throughout the world. The traditional characterization of potentially expansive pyritic shales only considers the presence of finely disseminated microscopic pyrite and references to case histories. The research presented in this paper shows that concentrated pyritic zones can have a significant impact on establishing
microenvironments that are conducive to the production of heave inducing sulfates.
Oxidation tests are conducted in a laboratory environment to assess the potential to form heave inducing sulfates through swell measurements and geochemical markers. Simple swell modeling is simulated with the PHREEQC geochemical computer program and a regression analysis of shale with significant gypsum infilling verifies calcite and pyrite concentrations. A detailed case history of micropile underpinning over expansive pyritic shales highlights the challenges associated with extensive structural remediation.
The O2 diffusion controlled oxidation process in calcareous shales with finely disseminated pyrite is impractically slow under intense conditions and does not adequately explain how gypsum infilling can occur over time periods of less than a decade. Geochemical laboratory testing and theoretical modeling suggest that an acidic or low pH environment is not possible with a significant presence of calcite with zones of availability. Gypsum is most likely to crystallize in a low pH environment when highly oxidative conditions are present. The research shows that microenvironments within highly concentrated pyritic zones more adequately explain the acidic conditions necessary that lead to accelerated oxidation. Acidic capillary pore water, which is not subject to flushing by a fluctuating or flowing water table through the vadose zone, influences the calcareous microfractures and discontinuities resulting in the
crystallization of sulfates.
TABLE OF CONTENTS
LIST OF TABLES …………………………………………………………………………………….. viii
LIST OF FIGURES ……………………………………………………………………………………..x
LIST OF MATHGRAMS …………………………………………………………………………… xvi
ACKNOWLEDGEMENTS ……………………………………………………………………….. xvii
Chapter 1. INTRODUCTION ……………………………………………………………………….1
1.1 General ………………………………………………………………………………………1
1.2 Microenvironmental Considerations ………………………………………………3 1.3 Thesis Organization …………………………………………………………………….5
Chapter 2. LITERATURE REVIEW ……………………………………………………………..8
2.1 Introduction ………………………………………………………………………………..8
2.2 Pyritic Bedrock Geology……………………………………………………………..10
2.3 Pyrite Oxidation and Sulfate Precipitation Chemistry ……………………..15
2.4 Weathering of Pyritic Shales ……………………………………………………….20
2.5 Identification of Potentially Expansive Pyritic Shales ……………………..21
2.6 Pyritic Sulfur Identification Techniques ………………………………………..22
2.6.1 Munsell Color Guidelines ………………………………………………23
2.6.2 Static Laboratory Testing Techniques ……………………………..24
2.6.3 Kinetic Laboratory Testing Techniques ……………………………26
2.7 Laboratory Testing Methods – Swell Test ……………………………………..27
2.8 Remnant Stresses and Horizontal Fracturing ………………………………….28
Chapter 3. CHEMICAL AND PHYSICAL EXPLANATIONS ……………………….32
3.1 Introduction ……………………………………………………………………………….32
3.2 Materials and Methods ………………………………………………………………..33
3.2.1 Experiments Using 30% H2O2 ………………………………………..33 3.2.2 Experiments Using 10% H2O2 ………………………………………..34
3.3 Results ………………………………………………………………………………………36
3.3.1 Experiments Using 30% H2O2 ………………………………………..36
3.3.2 Experiments Using 10% H2O2 ………………………………………..39
3.4 Microscopic Observations …………………………………………………………..41
3.5 Conclusions ……………………………………………………………………………….43
Chapter 4. PHREEQC HYDROGEOCHEMICAL TRANSPORT MODEL ………45
4.1 Introduction ……………………………………………………………………………….45
4.2 Irreversible Reaction Model ………………………………………………………..46
4.2.1 Introduction ………………………………………………………………….46
4.3 Input Parameters ………………………………………………………………………..48
4.3.1 First Run (0.1% S2 and 5% CaCO3 ………………………………….48
4.3.2 Second Run (0.5% S2 and 5% CaCO3 ……………………………..48
4.3.3 Third Run (Concentrated FeS2 and 5% CaCO3 …………………48
4.4 Results ………………………………………………………………………………………49
4.4.1 Moles in Assemblage and Volume Change ………………………49
4.4.2 Swell Model …………………………………………………………………51 4.5 Conclusions ……………………………………………………………………………….53
Chapter 5. EMPIRICAL REGRESSION MODEL …………………………………………59
5.1 Introduction ……………………………………………………………………………….59
5.2 Observational Data ……………………………………………………………………..59
5.3 Experimental Data ……………………………………………………………………..65
5.3.1 Image Analysis……………………………………………………………..65
5.3.2 Measurements ………………………………………………………………68
5.4 Regression Analysis ……………………………………………………………………69 5.5 Conclusions ……………………………………………………………………………….70
Chapter 6. GEOTECHNICAL LABORATORY STUDIES …………………………….75
6.1 Introduction ……………………………………………………………………………….75
6.2 Materials and Methods ………………………………………………………………..76
6.2.1 Controlled Experiments with Bacterial Oxidation ……………..76
6.2.1.1 Bacterial Preparation Procedure ………………………..76
6.2.1.2 Swell Experiment Procedure ……………………………..78
6.2.2 Swell Experiments Using Kinetic Oxidation Techniques …..81
6.2.2.1 Swell Experiments Procedure ……………………………81
6.3 Results ………………………………………………………………………………………87
6.3.1 Controlled Experiments with Bacterial Oxidation ……………..87
6.3.2 Swell Experiments Using Kinetic Oxidation Techniques …..89
6.4 Conclusions ……………………………………………………………………………….96
6.4.1 Controlled Experiments with Bacterial Oxidation ……………..96 6.4.2 Swell Experiments Using Kinetic Oxidation Techniques …..97
Chapter 7. CASE HISTORY: MICROPILE UNDERPINNING OVER
EXPANSIVE PYRITIC SHALES ……………………………………………….100
7.1 Introduction ………………………………………………………………………………100
7.2 Shale Expansion History …………………………………………………………….105
7.3 Field and Laboratory Investigations …………………………………………….106
7.3.1 Field Investigation ……………………………………………………….106
7.3.2 Laboratory Testing ……………………………………………………….109
7.4 Observations During Demolition and Overexcavation ……………………113
7.4.1 Demolition and Removal of Floor Slab …………………………..113
7.4.2 Swelled Shale Observations …………………………………………..114
7.5 Micropile Design ………………………………………………………………………115
7.5.1 Structural Loading Information ……………………………………..115
7.5.2 Design Method 1 – Telephone Room ……………………………..117
7.5.3 Design Method 2 – Corridor ………………………………………….121
7.5.4 Design Method 3 – Isolated Spread Footings …………………..124
7.6 Construction Considerations ……………………………………………………….133
7.6.1 Methods and Equipment ……………………………………………….133
7.6.2 As-Built Challenges ……………………………………………………..134 7.7 Conclusions ………………………………………………………………………………136
Chapter 8. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS ……..137
8.1 Summary ………………………………………………………………………………….137
8.2 Conclusions ………………………………………………………………………………138
8.3 Recommendations ……………………………………………………………………..139
8.3.1 Changes to the State-of-Practice …………………………………….139 8.3.2 Future Research …………………………………………………………..140
CHAPTER 1
INTRODUCTION
1.1 General
The study of expansive pyritic shales is becoming a rapidly relevant geotechnical topic with the decrease in favorable building sites in highly populated areas. Our understanding of shale expansion has been limited to historical studies of building sites where there is documentation of structural damage. Other than case study reviews there has been relatively little study of expansive pyritic shales with the notable exception of Ballivy and Bellaloui (1999) and Cormier (2000). There are many gaps in the science of expansive pyritic shales where there needs to be better agreement between characterizing the potential for swelling and developing responsible geotechnical engineering
techniques.
A description of the expansion process in a typical construction project is modeled in Figure 1.1. During initial excavation of a construction site, pyritic shale that typically had been in an intact and chemically reduced condition fractures due to unloading expansion and oxidizes as a result of lowering of the water table. As a result of these initial microenvironmental changes, there is an increase in fracturing, beddingplane discontinuities, and permeability which allows for further availability of pyrite to oxidize. Capillary movement of groundwater within the vadose zone provides a transport mechanism for the migration of an acidic front. The pyrite oxidation leads to the formation of hydrous sulfates, and the entire shale cell heaves within the vadose zone.
The model shown in Figure 1.1 presents a visual description of the typical conditions that lead to heave in pyritic shale environments (Hawkins and Pinches, 1987 and Hoover and Pease, 2007).
INCREASE IN
REDUCED ZONE
EVAPORATION OF PORE LEGEND:
H0 = UNCHANGED PLAN DIMENSIONS AND ORIGINAL HEIGHT OF
WATER AND
OXIDIZED VADOSE ZONE.
PRECIPITATION OF HYDROUS SULFATES H1 = NEW HEIGHT OF OXIDIZED VADOSE ZONE DUE TO STRESS RELIEF FRACTURING.
H2 = NEW HEIGHT OF OXIDIZED VADOSE ZONE DUE TO GYPSUM INFILLING OF MICRO-FRACTURES AND DISCONTINUITIES.
Z1 = NEW HEIGHT OF REDUCED ZONE DUE TO STRESS RELIEF FRACTURING.
SWELLING AT
DISCONTINUITIES
REDUCED ZONE
ANAEROBIC BELOW WT
Figure 1.1 Expansive pyritic shale model as a step-wise progression during a typical construction project. (a) Elevated water table with overburden materials prior to construction. (b) Water table is lowered and overburden removed as part of the excavation process resulting in increased fracturing and oxidation of available pyrite (H1>H0). (c) Capillary water evaporates and hydrous sulfates crystallize resulting in expansion of the shale matrix (H2>H1>H0).
1.2 Microenvironmental Considerations
The cause of most swelling in pyritic shales is from the oxidation of pyrite and subsequent formation of hydrous sulfates, with accompanying increase in molar volume of the solid phases. The hydrous sulfate most commonly referenced in cases of heave and subsequent structural distress is gypsum or calcium sulfate dihydrate (CaSO4•2H2O). Although other metal hydrous sulfates result from oxidation of pyrite, these minerals are rarely responsible for documented cases of swelling and damage to civil infrastructure and are more likely to be responsible for the production of acid upon dissolution. The metal hydrous sulfates are often transitory, whereas the lower solubility of gypsum and stability of gypsum relative to competing phases over a broad pH range results in a more stable condition.
Because gypsum is the mineral of primary importance, we must also be interested in the presence of calcite in pyritic shales. Calcite or some source of Ca(II) must be available in order for gypsum to form, which is typically through the interaction with sulfuric acid from the oxidation of pyrite (see Equation 2.5 and Chapter 4 on the PHREEQC Geochemical Model). Pyrite may be present in a microscopic form within the shale matrix (framboidal or microcrystalline) or in a macroscopic or concentrated form (nodules, burrows and/or replacement fossils). Microscopic pyrite is generally defined as having a particle size less than 10 microns and macroscopic is anything greater than 10 microns but typically recognizable with a conventional hand lens (>10x).
This research for my dissertation provides empirical and theoretical evidence that macroscopic pyrite is a significant contributor to expansive pyritic shales that contain abundant calcite. The limited availability of microscopic forms of pyrite to oxidation is the leading cause for skepticism about the primary role of framboidal or microcrystalline pyrite to be the driving mechanism for the production of gypsum in pyritic shales with significant amounts of calcite. Although microscopic pyrite will more readily oxidize than larger forms due to increased surface area (Brady et al. 1998), the microscopic forms of pyrite are only physically available at the surface of fractures and other discontinuities. The availability to oxidize the microscopic pyrite within the shale matrix comes with weathering processes and breakdown of the shale structure. The crystallization of gypsum typically occurs in original discontinuities or small stress relief fractures that result from unloading of the shale during construction (see Section 2.6 on remnant
stresses).
The rate of pyrite oxidation increases significantly at low pH environments with abundant ferric iron (Rimstidt and Newcomb, 1993). The metabolism of Thiobacillus ferrooxidans is also high at low pH (Jaynes, 1984a and 1984b). Both the abiotic and biotic pathways are expected to be slow if the presence of calcite keeps the system at a near neutral pH. Groundwater monitoring data from the Hamilton Group of Pennsylvania’s Ambient and Fixed Station Network (FSN) Monitoring program (Reese and Lee, 1998) reports that the pH of the groundwater to be at an average of 7.26.
This reasoning leads to the importance of macroscopic forms of pyrite within the Devonian shales as a key indicator for swell potential. The release of overburden pressures associated with construction projects will likely produce an abundance of stress relief fractures at the interface of the concentrated macroscopic pyrite sources due to the discrepancy in hardness between pyrite and the surrounding shale mineralogy. These fractures allow for the oxidation process to begin around the concentrated pyrite sources.
The drop in pH from the production of acid around the microenvironments of the concentrated pyrite sources lead to favorable conditions that allow for ferric iron and bacteria to rapidly increase oxidation rate. These microenvironmental “hot spots” can influence the surrounding materials through the migration of capillary pore water that spreads from the concentrated source through fractures and discontinuities and into the surrounding calcareous shale. This acidic migratory front then reacts with the calcite or mobilizes dissolved calcium carbonate to form gypsum. The crystallization of gypsum leads to heave of the shale matrix.
1.3 Thesis Organization
Chapter 2 is a literature review describing case studies of construction projects where expansive pyritic shales have caused structural distress to civil infrastructure. The geology of pyrite formation in carbonaceous shales is briefly covered with a particular emphasis on Devonian Shales and current mapping techniques that describe specific formations having sulfides that could potentially cause acid rock or mine drainage problems. Acid production and sulfate precipitation chemistry is presented to give an overview of the important processes that cause expansion. Laboratory testing methods for determining the percent by weight of finely disseminated or microscopic pyrite in rock and soil samples is presented. Finally, the remnant stress theory highlights the potential for stress relief fracturing.
Chapter 3 gives chemical and physical explanations of the expansive shale process through the oxidation of pyritic shales. Experimental oxidation of pyritic shales with varying amounts of pyrite nodules is accomplished through the introduction of hydrogen peroxide of varying concentration. The resulting temperature and pH and sulfate ion, calcium, aluminum, potassium and iron concentrations are tracked and compared with theoretical predictions utilizing the Visual MINTEQ (Gustafson, 2007) hydrogeochemical program.
The PHREEQC (Parkhurst and Appelo, 1999) hydrogeochemical model is
presented in Chapter 4 to highlight the processes involved in the oxidation of pyrite and subsequent production of gypsum. A generalized expansion model is offered in an attempt to quantify the expected differences in heave that could result as calcite dissolves in the reaction with sulfuric acid resulting in gypsum precipitation.
An empirical regression model technique is shown in Chapter 5 as a means of uncovering clues to the origin of sulfate formation. Samples of shale that have undergone expansion beneath the Evangelical Hospital in Lewisburg, Pennsylvania (Hoover, 2004 and Hoover and Pease, 2007) are microscopically analyzed. Each thin section of expanded rock is analyzed to estimate the total amount of gypsum produced and then back calculated to estimate the amount of pyrite and calcite required in the reaction.
Two separate geotechnical laboratory studies are presented in Chapter 6 in an effort to produce an expansion test capable of determining the conditions responsible for heave related failures. The first test consists of two molds with known quantities of illite, calcite and pyrite. A bacteria rich solution is used to inundate the samples in order to establish oxidation of the pyrite that is present in various concentrations within the samples. The second test combines a revised version of the Acid Drainage Technology Initiative (ADTI-WP2) Leaching Column Method with the capability of measuring expansion of the pyritic shale samples.
Chapter 7 is a case history involving micropile underpinning over expansive pyritic shales. The Evangelical Hospital in Lewisburg, Pennsylvania has undergone significant structural distress due to expansive pyritic shales. Comprehensive field and laboratory testing was accomplished and is detailed in this chapter. Also, extensive micropile underpinning techniques were utilized to support the structure and protect against future heave related distress.
Chapter 8 summarizes the findings of the laboratory and theoretical exercises and conclusions are presented concerning the characterization of the physical and geochemical processes of pyritic shales of the Marcellus Formation. Finally, recommendations are offered for changes to the current state-of-practice and future research challenges are highlighted.
The appendices provide laboratory testing data, established testing procedures and other miscellaneous information that support this research into expansive pyritic shales of the Marcellus Formation.
THE EXPANSIVE EFFECTS OF CONCENTRATED PYRITIC ZONES IN SHALES OF THE MARCELLUS FORMATION
