AN ABSTRACT OF THE THESIS OF Michael W. Baillie for the degree of Master of Science in Civil Engineering, presented on January 28, 1998. Title: Scourability of Weak Rock in the Oregon Coast Range. Redacted for Privacy Abstract Approved: Stephen E. Dickenson The undermining of bridge foundations can lead to either costly repairs or a bridge collapse. These foundations must be designed to counter the effects of scour. Current practice does not allow for accurate estimates of scour in erodible rock. Scour in rock can be related to geotechnical and hydraulic properties. A field study of eleven bridge sites provided samples of the bedrock where the abrasive resistance of the rock was determined and hydraulic properties of the channel were calculated. Laboratory abrasion resistance values from a modified slake durability test and hydraulic variables such as stream power were compared to recent and past stream channel cross-sections. A preliminary model has been proposed wherein the degradation of the stream channel is related to the abrasive resistance of the bedrock and the area under the daily stream power. This method provides an estimate of the degradation of the stream bed due to abrasion by bedload and flood events, not necessarily local or contraction scour. SCOURABILITY OF WEAK ROCK IN THE OREGON COAST RANGE by Michael W. Bail lie A Thesis Submitted to Oregon State University In partial fulfillment of the requirements for the degree of Master of Science Presented January 28, 1998 Commencement June 1998 Master of Science thesis of Michael W. Baillie presented on January 28, 1998 APPROVED: Redacted for Privacy Major Professor, representing Civil Engineering Redacted for Privacy Chair of Defertment of Civil Construction and Environmental Engineering Redacted for Privacy Dean of Graduate chool I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for Privacy Michae W. Baillie, Author ACKNOWLEDGMENTS The author would like to thank Dr. Stephen E. Dickenson for his guidance and patience, Dave Bryson and Glen Thommen of the Oregon Department of Transportation Hydraulics and Geotechnical Units for providing several sites and valuable information, Steve Kramer at the Bureau of Mines for the coring drill, Michael Riemer of University of California Berkeley for use of laboratory equipment, Bob Young of Siuslaw National Forest for providing sites, Dr. Pete Klingeman of Oregon State University for hydraulic analysis ideas, Dr. George Annandale for contributing valuable insights on scour in bedrock, and my fellow graduate students, especially Bret Shipton, Nason McCullough, and Jason Brown, who assisted on the surveys. 11 TABLE OF CONTENTS Page 1. INTRODUCTION 1 1.1 Statement of the Problem 1 1.1.1 Rock Scour 3 1.1.2 Current Design Procedures 6 1.2 Scope of the Project 8 1.3 The Premise Behind the Research 8 1.4 Integrated Stream Power 9 2. STREAM STUDY SITES 12 2.1 Site Investigation 12 2.2 Site Selection 12 2.2.1 Mill Creek at Rosenbalm 14 2.2.2 Mill Creek at Highway 22 15 2.2.3 Yaquina River at Mile Post 2.4 16 2.2.4 Yaquina River at M.P. 4.9 16 2.2.5 Alsea River at Thissel Road 17 2.2.6 Alsea River at Missouri Bend 17 2.2.7 Five Rivers at Fisher 18 2.2.8 Middle Fork Coquille River at Mile Post 51 (M.F.C. 51) 18 2.2.9 Middle Fork Coquille River at Mile Post 53 (M.F.C. 53) 19 2.2.10 Nestucca River at Powder Creek 19 2.2.11 Luckiamute River at Grant Road 20 2.2.12 Other sites 20 2.3 Historical Cross Sections 21 GEOTECHNICAL STUDY 23 3.1 Obtaining Samples 23 3.2 Laboratory Tests 23 3.2.1 Unconfined Compression 24 3.2.2 Continuous Slake Durability 24 3.2.3 Continuous Slake Number 0 27 3.2.4 Density 30 111 3.2.5 LA Abrasion Test 30 3.3 Wetting and Drying Effects 31 4. HYDRAULIC INVESTIGATION 33 4.1 Stream Gages 33 4.2 Adjusting and Synthesizing Daily Flow Values 33 4.3 Evaluation of Hydraulic Variables 35 4.4 Effect of Slope on Stream Power 37 4.5 Discussion of Hydraulic Study 39 5. STATISTICAL ANALYSIS OF GEOTECHNICAL AND HYDRAULIC DATA 40 5.1 Introduction 40 5.2 Bedload 40 5.3 Comparison of Cross-Sections 42 5.4 Development of an empirical model 43 5.5 Discussion 47 5.6 Proposed Design Applications 48 5.7 Evaluation of Existing Structures 50 5.8 recommendations for further research 51 BIBLIOGRAPHY 53 APPENDICES 55 APPENDIX A Site Information 56 APPENDIX B Laboratory Information 69 iv LIST OF FIGURES Figure Page 1.1 Summary of Oregon Department of Transportation Bridge Foundations of Bridges over Water 2 1.2 Erodibility of Rock and Complex Earth Materials 5 1.3 Conceptual Stream Power Graphs for Different Floods 10 2.1 Site Locations 13 2.2 Evidence of Bedrock Erosion at Mill Creek - Rosenbalm 14 2.3 Evidence of Erosion under a Bridge Footing at Mill Creek HWY 22. 15 3.1 Average Erosion as related to Unconfined Compression 24 3.2 Standard Dimensions for Slake Durability Cage and Water Level 25 3.3 Plot of results of Continuous Slake Test for Alsea-Thissel 27 3.4 Semi-log plot of Continuous Slake Test for Alsea-Thissel 28 3.5 Relationship of Continuous Slake Number (13) and Saturated Density 29 3.6 Continuous Slake Results for Mill Creek @ HWY 22 31 4.1 Regression of Alsea River at Tidewater and Five Rivers Near Fisher Daily Flows35 4.2 Flow versus Stream Power for Five Rivers Near Fisher 36 4.3 Daily Flow at Five Rivers Near Fisher 37 4.4 Daily Stream Power for Five Rivers Near Fisher 37 4.5 Effect of Various Slopes on Stream Power at Luckiamute 38 5.1 Plot of Average Erosion Compared with Energy-Days 44 5.2 Plots of (a) Average Erosion versus 13 and (b) Average Erosion versus Integrated Stream Power 45 V Figure Page 5.3 Average Erosion versus Integrated Stream Power (C2) and Continuous Slake Number 03) 46 5.4 Estimating Average Erosion 50 vi LIST OF TABLES Table Page 1.1 Summary of Existing Geotechnical Parameters for Evaluating Scour Potential 8 5.1 Variables Used in the Statistical Study 41 5.2 Energy Model for Alsea-Thissel 44 SCOURABILITY OF WEAK ROCK IN THE OREGON COAST RANGE 1. INTRODUCTION 1.1 STATEMENT OF THE PROBLEM In a study of 823 bridge failures in New York between 1950 and 1991, hydraulics accounted for 494, or 60% of the failures (Shirhole and Holt, 1991). These include failures due to the impact of scour, debris and ice flows on footings and abutments. Scour can be defined as "the result of the erosive action of flowing water, excavating and carrying away loose material from the bed and banks of streams" (Richardson, et. al., 1993). On April 5, 1987, two spans of the 1-90 crossing over Schoharie Creek near Amsterdam, New York collapsed and fell about 24 m (80 ft) into the flooding stream. The cause of the collapse was an undermining of a pier due to local scour in cohesionless material. Ten people were killed as a result of the scour induced collapse. A short time after the first collapse, another pier was undermined and collapsed, bringing down the third span. After this event the Federal Highway Administration (FHWA) expressed a heightened interest in the research and improvement of the state-of-practice of bridge design and scour, and implemented a comprehensive bridge inspection program for existing bridges to determine their safety (Landers and Mueller, 1995). The ensuing research efforts led to the development of design methods for scour in cohesionless materials. At sites underlain by weak rock, foundation design has traditionally relied on deep foundations in order to obtain secure bearing beneath the potential zone of scour. Current FHWA guidelines for scour require assessments and/or monitoring programs. Prioritizing the potentially hazardous bridge sites could be made, provided a method existed for evaluating the scour potential in weak rock foundations. Current methods are highly overconservative when applied for rock formations such as shales and weak sandstones that erode due to water and bedload abrasion. 2 The Oregon Department of Transportation (ODOT) accounts for 2,640 bridges in Oregon, of which, about 65 °/o are over water. Figure 1.1 summarizes a survey of all the state bridges over water in Oregon. Of these bridges, 44% are pile supported, 16% are spread footings on non-erodible material, and 40% are spread footings on erodible material (Bryson, 1998). Spread Footings on Erodible Pile Material Supported 40% 44% Spread Footings on Non-Erodible Material 16% Figure 1.1: Summary of Oregon Department of Transportation Bridge Foundations of Bridges over Water From Figure 1.1 it is evident that a large percentage of bridges have spread footings on erodible material. Spread footings on erodible material could fail before the design life of the structure due to problems such as undermining of foundations and subsequent collapse. This type of failure is not immediate, which makes the problem of scour time dependent. Scour in non-cohesive material is relatively immediate in comparison to scour in an erodible rock formation such as shale. Under constant flow conditions, maximum scour depth in sands and other non-cohesive materials takes hours, while rocks will reach the same scour depth in years or centuries depending on hardness (Richardson, et. al., 1993).