A GEOTECHNICAL INVESTIGATION of the OCTOBER 2011 CEDAR CITY LANDSLIDE, UTAH a Thesis Submitted to the Kent State University Grad
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A GEOTECHNICAL INVESTIGATION OF THE OCTOBER 2011 CEDAR CITY LANDSLIDE, UTAH A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science by Ashley S. Tizzano May, 2014 Thesis written by Ashley S. Tizzano B.S. Kent State University, 2010 M.S. Kent State University, 2014 Approved by Dr. Abdul Shakoor, Advisor Dr. Daniel Holm, Chair, Department of Geology Dr. Janis Crowther, Dean, College of Arts and Sciences ii TABLE OF CONTENTS Page List of Figures……………………………………………………………………….……………vi List of Tables……………………………………………………………………………………...ix Acknowledgements…………………………………………………………..…………………….x Summary...………..………………………………………………….…………………………….1 CHAPTER 1 INTRODUCTION……………………………...………………………………...3 1.1 The Cedar City Landslide…………….………...…………………………..3 1.2 Landslide Hazards in the Cedar Canyon………….…………………….......8 1.3 Geology of the Area…................…………………..………………..……...8 1.4 Research Hypothesis……………………………….…………………...…11 1.5 Objectives....…………….…………………………………………...……11 2 METHODOLOGY…………………………………………………………...…13 2.1 Field Investigations……………………………………………...………...13 2.1.1 Discontinuity Mapping..................................................................13 2.1.2 Subsurface Investigations………………….……...……..…...….16 2.2 Laboratory Investigations……………………………………...………….18 2.2.1 Grain Size Distribution Analysis……………..………………….18 2.2.2 Natural Water Content Test……………………………………...19 2.2.3 Atterberg Limits Test……………………………………..……...19 2.2.4 Absorption Test……………………………….…………………20 2.2.5 Slake Durability Test…………………..………………………...21 iii 2.2.6 Unconfined Compression Test…………………………………..21 2.2.7 Direct Shear Test.………………………………..……………….22 2.3 Data Analysis…………………………………………………………...…24 3 DATA ANALYSIS AND INTERPRETATION…………………...…………...25 3.1 Field Observations…………………………………………………….…..25 3.2 Discontinuity Data…………………………………………………...……25 3.3 Subsurface Data………………………………………………………...…29 3.4 Engineering Properties of the Colluvial Soil and Bedrock Units.....…...…32 3.4.1 Grain Size Distribution…………………………………………...32 3.4.2 Natural Water Content and Bulk Density………………...………35 3.4.3 Atterberg Limits…………………………………………………..35 3.4.4 Dry Density, Absorption, Slake Durability, and Unconfined Compressive Strength for Bedrock Units.....…………………….35 3.4.5 Shear Strength Parameters………………………………………..40 4 STABILITY ANALYSIS OF THE CEDAR CITY LANDSLIDE….………….42 4.1 Landslide Type and Failure Plane Location Used for Stability Analysis....42 4.2 Input Parameters for Stability Analysis…..……………………………….43 4.3 Stability Analysis Using the SLIDE Software Program…………………..46 4.4 Causes of the Landslide…………………………………………………...59 5 REMEDIAL MEASURES……………………………………………………...61 6 AN OVERVIEW OF SLOPE STABILITY HAZARDS IN CEDAR CANYON………………..……………………………………………………...70 7 CONCLUSIONS………………………………………………………………..80 REFERENCES……………………………………………………………………………….......82 iv APPENDICES A: Discontinuities…………………………………………………………………..85 B: Borehole Logs, Inclinometer Data, Geophysical Survey, and Rainfall Data.…..90 C: Laboratory Data………………………………………………………………..115 D: Stability Analysis………………………………………………………………180 v LIST OF FIGURES Figure 1.1 Location map of the Cedar City landslide……………..…………………………4 Figure 1.2 An aerial view of the Cedar City landslide …………………………………..…..5 Figure 1.3 Close up view of the head area of the landslide ……..…………………………..5 Figure 1.4 Panoramic view of the landslide and the Cedar Canyon ………………………...6 Figure 1.5 Displaced portions of SR 14 on the east side of the landslide …………………...6 Figure 1.6 Landslide material covering SR 14 on the west side of the landslide ..………….7 Figure 1.7 Slope hazard history of Cedar Canyon…………………………………...………9 Figure 1.8 Stratigraphic column for the Cedar Canyon...…………………………………..10 Figure 2.1 Discontinuities within the Dakota Sandstone…………………………...………14 Figure 2.2 Discontinuities within the Straight Cliffs Sandstone…………………………....15 Figure 2.3 Location map of the boreholes drilled…………………………………………..17 Figure 2.4 Relation between Schmidt hammer rebound number and unconfined compressive strength of rocks ………………………………………………………..………23 Figure 3.1 Contouring of Straight Cliffs Sandstone discontinuities using the DIPS software ..…........................................................................................................................26 Figure 3.2 Contouring of Dakota Sandstone discontinuities using the DIPS software……..27 Figure 3.3 Contouring of discontinuities from both the Dakota Sandstone and the Straight Cliffs Sandstone using the DIPS software ………………..…………………….28 Figure 3.4 Evidence of water runoff over the slope face comprised of the Dakota Sandstone ..............................................................................................................................30 Figure 3.5 Cumulative displacement from the inclinometer data obtained from borehole 8-2 ..…………………………………………………………………………………33 Figure 3.6 Grain Size Distribution curve for sample 4 of the colluvial soil.........………….34 vi Figure 3.7 A plot of Atterberg limits on Casagrande’s plasticity chart showing that the fines are classified as low plasticity clay …………............................................……..36 Figure 4.1 Cross-section created for stability analysis...........................................................44 Figure 4.2 Geological Strength Index (GSI) chart…………………………………….........45 Figure 4.3 Stability analysis for dry colluvial soil-Tropic Shale parameters, using both Bishop and the Janbu Simplified methods……...…………………………...…..47 Figure 4.4 Stability analysis for dry colluvial soil-Dakota Sandstone parameters, using both Bishop and the Janbu Simplified methods……………………………………...48 Figure 4.5 Stability analysis for the averaged dry colluvial soil-bedrock parameters, using the Bishop Simplified method...………………………………………………...49 Figure 4.6 Stability analysis for fully saturated colluvial soil-Tropic Shale parameters, using both Bishop and the Janbu Simplified methods.....……………………………...50 Figure 4.7 Stability analysis for fully saturated colluvial soil-Dakota Sandstone and the average soil-bedrock parameters, using both Bishop and the Janbu Simplified methods………………………………………………………………………….51 Figure 4.8 Stability analysis with the maximum water table height at 3.3 ft (1 m) above the contact, Φrequired = 29°…………………………………………………………....53 Figure 4.9 Stability analysis with the maximum water table height at 11.7 ft (3.5 m) above the contact, Φrequired = 30°…………………………………………………...54 Figure 4.10 Stability analysis with the maximum water table height at 23.7 ft (7.2 m) above the contact, Φrequired = 32°….……………………………………..……………...55 Figure 4.11 Stability analysis with the maximum water table height at 39.4 ft (11.9 m) above the contact, Φrequired = 34°………………………………………………………..56 Figure 4.12 Variation of back-calculated friction angle for varying heights of water table, for a safety factor of one…………………………………………………………….57 Figure 4.13 Geologic cross-section used for stability analysis showing the level of water encountered in Borehole 8-2.................................................................................58 Figure 4.14 Daily precipitation outside Cedar City, UT from September 1st to October 31st, 2011……………………………………………………………………………..60 Figure 5.1 Regraded slope above the road and riprap-filled drainage ditch……………..…62 Figure 5.2 Scaling of the Straight Cliffs Sandstone above the road………………………..62 vii Figure 5.3 East side view of the modular block retaining wall .……………………………64 Figure 5.4 West side view of the modular block retaining wall.…………………………...65 Figure 5.5 Smooth wheel roller compacting subgrade material for reconstruction of SR 14………………………………………………………………………………..65 Figure 5.6 View of the riprap filled ditch and road from slope above………...……………66 Figure 5.7 Aerial view of the repaired landslide and SR 14 …..…………………………...66 Figure 5.8 Riprap filled drainage ditch and the newly paved SR 14 .……………………...67 Figure 6.1 Unfavorably oriented joints within the Straight Cliffs Sandstone in the Cedar Canyon showing the potential for a variety of slope movements ....……………71 Figure 6.2 Jointed nature of the Straight Cliffs Sandstone, resulting in a blocky nature of the rock mass and a potential for rockfalls and other types of slope movement……72 Figure 6.3 Potential for rockfall hazard along discontinuities in the Straight Cliffs Sandstone …………………………………………………………………...……………...73 Figure 6.4 Valley stress relief joints along the Cedar Canyon wall showing the potential for rockfalls and plane failures ……………………………………………………..74 Figure 6.5 Debris from previous rockfalls in the canyon ………...………………………...75 Figure 6.6 Source area for the 2012 rockfall above SR 14 ………………………………...76 Figure 6.7 Rockfall debris on SR 14.……………………………………………………….76 Figure 6.8 Damage to SR 14 from 2012 rockfall ………………...………………………...77 Figure 6.9 Kinematic analysis using the RockPack 3 software, based on measured discontinuities within the Straight Cliffs Sandstone ……...…………………….78 viii LIST OF TABLES Table 3.1 Average Atterberg limits from the colluvial soil………….…………………….34 Table 3.2 Summary of dry density, absorption, slake durability, and unconfined compressive strength data for the bedrock units involved in the Cedar City landslide…............................................................................................................37 Table 3.3 Two-cycle slake durability classification………………..……………………...37 Table 3.4 Engineering classification of intact rock…………..……………………………41 Table 3.5 Summary of the shear strength parameters for the colluvial