The West Tidewater Earthflow, Northern Oregon Coast Range
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Portland State University PDXScholar Dissertations and Theses Dissertations and Theses Winter 2-14-2014 The West Tidewater Earthflow, Northern Oregon Coast Range Barry A. Sanford Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/open_access_etds Part of the Geology Commons, and the Geomorphology Commons Let us know how access to this document benefits ou.y Recommended Citation Sanford, Barry A., "The West Tidewater Earthflow, Northern Oregon Coast Range" (2014). Dissertations and Theses. Paper 2497. https://doi.org/10.15760/etd.2494 This Thesis is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. THESIS APPROVAL The abstract and thesis of Barry A. Sanford for the Mast~r of Science in Geology were presented March 1, 2002 and accepted by the thesis committee and the department. COMMITTEE APPROVALS: Kenneth M. Cruikshank Trevor D. Smith Representative of the Office of Graduate Studies DEPARTMENT APPROVAL: ABSTRACT An abstract ofthe thesis ofBarry A. Sanford for the Master of Science in Geology presented March 1, 2002 Title: The West Tidewater Earthflow, Northern Oregon Coast Range. The West Tidewater earthflow, one ofthe largest in Oregon's history, occurred in December of 1994. The earthflow is located approximately 15 km north ofJewel, Oregon near the summit ofthe Northern Oregon Coast Range Mountains. The earthflow is 900 m long and 250 m wide, giving it a surface area of9 ha, or 22 acres. 3 Volume is 3.5 million m • The earthflow occurred in low strength, well-bedded, tuffaceous, 'carbonaceous, micaceous, clay-rich mudstone, and very fme-grained, feldspathic, clay-rich siltstone ofthe lower Miocene age Northrup Creek Formation. The soil clay fractions contain up to 90% smectite with indications ofhalloysite. This earthflow is a reactivation ofa 650-year-old landslide (C-14 dating of uncovered buried trees). The failure mode is examined using a Janbu slope analysis and includes double wedge failure near the headscarp. High soil pore water pressure is one ofthe major causes ofthis slope failure. Rainfall levels for October, November, and December of 1994 were twice the previous five-year average. Present day groundwater level withinthe basin is less than one meter below ground surface. The earthflow is partially controlled by two faults ofregional extent that dissect the basin near the headscarp in NW-SE and NE-SW directions. The Inceptisol soils in the basin remain moist below 20 cm year around. Soil in the basin may have been further weakened due to loss ofroot strength following timber harvest on the site in 1991. Soil liquid limits range from 42% to 95%, with PI values ranging from 2% to 77%. Soil clay content ranges between 18% and 30%. Direct shear tests on the mudstone and siltstone bedrock in both drained and undrained conditions produced internal friction angles of 14-18°, with cohesion values of4 - 8 kPa. Back calculation of study area soil strength using the modified Bishop method results in a residual friction angle of 20.7°. The failure mode ofthe earthflow is from the headscarp downward and is modeled using Janbu methods. The study includes a detailed topographic map and a failure analysis of the earthflow basin. THE WEST TIDEWATER EARTHFLOW, NORTHERN OREGON COAST RANGE by BARRY A. SANFORD A thesis submitted in partial fulfillment ofthe requirements for the degree of MASTER OF SCIENGE In GEOLOGY Portland State University 2004 ACKNOWLEDGEMENTS Many thanks to Associate Professor Ken Cruikshank and the students ofhis Landslide Anatomy class of 1996 at Portland State University for their assistance in mapping the West Tidewater earthflow. Jared May provided extensive field assistance for the surveying, mapping, and soil investigation. Dave Michael and the Oregon Department ofForestry office in Forest Grove, Oregon provided access to their aerial photo library. Dr. Trevor Smith and the Civil Engineering Department of Portland State University allowed me to use their soils laboratory and test equipment. Dr. Christina Hulbe helped me organize my thoughts and edit my writing. Special thanks to Dr. Scott Bums, for his role as advisor, and his never-ending enthusiasm, encouragement, and dedication to education. TABLE OF CONTENTS ACKNOWLEDGMENTS 11 LIST OF TABLES IV LIST OF FIGURES V LIST OF PLATES V11 CHAPTER 1: INTRODUCTION 1 CHAPTER 2: STUDY AREA 3 Location 3 Description 3 Similar Earthflows in Oregon 8 Geology 11 Hydrology 15 Soils 15 CHAPTER 3: METHODS 19 CHAPTER 4: RESULTS 21 Atterberg Limits 21 Penetration Test 22 Particle Size Analysis 24 I Direct Shear Tests 25 Bulk Density 32 Clay Mineralogy 32 Groundwater 35 Precipitation 38 Base Map and Earthflow Features 41 Tree Raft 50 Soil Mounds 51 CHAPTER 5: STABILITY ANALYSIS 55 Pre-Failure Analysis 56 CHAPTER 6: DISCUSSION 63 Physical Strength ofSoil 63 Earthflow Models 65 11 Clay Mineralogy 66 Clay Content 74 Geometry ofthe Earthflow 75 Woody Debris Factor 78 Failure Mode 79 Influence ofStructural Features 80 Vegetation Removal Factor 86 CHAPTER 7: CONCLUSIONS 89 Future Work 92 REFERENCES 93 APPENDIX 98 Appendix A: Air Photos 99 Appendix B: Soils 107 Appendix C: Atterberg Limits 109 Appendix D: Gradation Data 115 Appendix E: Direct Shear Testing 122 Appendix F: X-ray Diffraction Analysis 153 Appendix G: Radiocarbon Dating 164 111 LIST OF TABLES Table 1 Soil horizons '18 Table 2 Atterberg Limits 21 Table 3 Penetration Test data 22 Table 4 Direct Shear tests 30 Table 5 Bulk density 32 Table 6 Clay percentages 33 Table 7 Water table levels 37 Table 8 Rainfall levels 39 Table 9 Soil strength calculations 59 Table 10 Janbu pre-failure spreadsheet 60 Table 11 Janbu Toe-failure spreadsheet 67 Table 12 Janbu undrained spreadsheet 70 iv LIST OF FIGURES Figure 1 Location Map 4 Figure 2 Study Area 6 Figure 3 Base Map 7 Figure 4 Earthflow drawing and classifications 10 Figure 5 Geologic map 13 Figure 6 Coast Range Stratigraphic section 14 Figure 7 View ofthe earthflow 16 Figure 8 Penetration test 23 Figure 9 Gradation curve sample STA-l 26 Figure 10 Gradation curve sample A3 27 Figure 11 Gradation curve sample AS 28 Figure 12 Mohr-Coulomb diagram 31 Figure 13 X-ray diffraction chart 34 Figure 14 Carbon bonding to clays 36 Figure 15 Precipitation bar chart 40 Figure 16 Colluvium deposition 42 Figure 17 Colluvium deposition and fault trace 43 Figure 18 Headscarp 44 Figure 19 Soil block 45 Figure 20 Structural feature 46 Figure 21 Sag pond 47 v Figure 22 Basalt exposure in headscarp 49 Figure 23 Tree raft 51 Figure 24 Soil mounds 52 Figure 25 Infinite slope model 57 Figure 26 Pre-failure slip circle 61 Figure 27 Pre-failure force diagram 62 -) Figure 28 Toe failure slip circle 68 Figure 29 Toe failure force diagram 69 Figure 30 Slip circle, undrained 71 Figure 31 Force diagram, undrained 72 Figure 32 Soil block force diagram 73 Figure 33 Clay reorientation 77 Figure 34 Soil crack with roots 82 Figure 35 Fault evidence, east margin 84 Figure 36 Fault evidence, west margin 85 Figure 37 Aerial photo offault 86 Figure 38 Aerial photo offault 87 Figure 39 Double wedge failure diagram 88 VI LIST OF PLATES Plate 1 West Tidewater Earthflow Base Map Pocket Vll CHAPTER 1 INTRODUCTION Landslide is the general tenn for various modes ofcatastrophic mass wasting including mudflow, earthflow, debris flow, and rock fall (Chassie and Goughnour, 1976). Although landslides occur naturally, human impacts can substantially increase their frequency and scope. The West Tidewater earthflow was remobilized in December of 1994. This was one ofthe largest landslides to occur in the Oregon Coast Range and one ofthe largest in Oregon's modem history. The duration ofthe actual movement has been estimated at between four and 24 hours (personal communication, Dave Michael, Oregon Department ofForestry, 1995). Small, one to two meter soil blocks have continued to separate from the main scarp sporadically since the remobilization. These smaller fail~res were observed in late winter, spring, and summerof 1996. The goals ofthis project are to detennine what happened to the soils ofthe Tidewater Summit region ofthe Oregon Coast Range Mountai!1s to cause them to (ail and to discover how they failed. AI: 3000 scale map ofthe earthflow describes the geometry and topography ofthe slide. The chemical and physical properties of the soils are examined using Atterberg limits,particle size gradations, and a clay mineralogy detennination. The soil cohesion and internal friction angles were quantified by direct shear testing on drained and undrained samples. Other workers 1 have examined the regional geology, structure, and fonnation relations through previously published material. The groundwater characteristics in the basin were examined using several monitoring wells installed during the course ofthis project. The data gathered by the above mentioned methods were used to perfonn a stability analysis ofthe earthflow basin. The purpose ofthis earthflow characterization and study is to map the earthflow, to examine engineering properties ofthe soils, to investigate soil behavior during a large earthflow event, and to perfonn a stability analysis ofthe slopes in order to detennine the overall failure mode. 2 CHAPTER 2 STUDY AREA LOCATION The Tidewater earthflow is located in Clatsop County, Oregon approximately 15 km northwest ofJewel, Oregon on State Highway 202, milepost 22 (Figure 1). This part of the crest ofthe Coast Range is known as the Tidewater Summit. Map coordinates ofthe earthflow study area are; SE 114, NE 1/4, Section 18, Township 6 North, Range 7 West, Wickiup Mountain, Oregon, U.S.G.S. 7.5 minute quadrangle, or 460 00' 42" north latitude, 123 0 35' 50" west longitude.