Geologically Hazardous Area Assessment Guemes Channel Trail Extension Anacortes, Washington for Herrigstad Engineering, PS May 16, 2014 Earth Science + Technology Geologically Hazardous Area Assessment Guemes Channel Trail Extension Anacortes, Washington for Herrigstad Engineering, PS May 16, 2014 600 Dupont Street Bellingham, Washington 98225 360.647.1510 Table of Contents INTRODUCTION AND SCOPE ........................................................................................................................ 1 DESIGNATION OF GEOLOGIC HAZARD AREAS AT THE SITE ..................................................................... 1 SITE CONDITIONS .......................................................................................................................................... 2 Surface Conditions ................................................................................................................................. 2 Geology ................................................................................................................................................... 3 Subsurface Explorations ........................................................................................................................ 4 Subsurface Conditions .......................................................................................................................... 4 Soil Conditions ................................................................................................................................. 4 Groundwater Conditions ................................................................................................................. 4 GEOLOGICALLY HAZARDOUS AREA SITE ASSESSMENT .......................................................................... 5 Erosion Hazard ....................................................................................................................................... 5 Landslide Hazard ................................................................................................................................... 5 Shallow Slides ................................................................................................................................. 5 Deep Seated Landslide ................................................................................................................... 6 Seismic Hazard ...................................................................................................................................... 6 Tsunami Hazard ..................................................................................................................................... 7 GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS .................................................................... 7 General ................................................................................................................................................... 7 Riprap Revetment Design and Repair .................................................................................................. 7 General ............................................................................................................................................ 7 Rock Size and Revetment Thickness ............................................................................................. 7 Filter Recommendations ................................................................................................................ 8 Construction Considerations .......................................................................................................... 8 Gravity Block Retaining Walls ................................................................................................................ 9 Redi-Rock Materials and Construction Considerations .............................................................. 10 Trail Surfacing Considerations ............................................................................................................ 10 Drainage Considerations ..................................................................................................................... 10 Earthwork ............................................................................................................................................. 11 Site Preparation ............................................................................................................................ 11 Excavation and Temporary Slope Considerations ....................................................................... 11 Permanent Slopes ......................................................................................................................... 12 Structural Fill ................................................................................................................................. 13 LIMITATIONS ............................................................................................................................................... 14 May 16, 2014 | Page i File No. 0382-022-00 LIST OF FIGURES Figure 1. Vicinity Map Figures 2A through 2C. Site and Exploration Plan Guemes Channel Trail Phase VII – Sheets C1 through C12 APPENDICES Appendix A. Field Explorations and Laboratory Testing Figure A-1. Key to Boring Logs Figures A-2 through A-5. Logs of Hand-Augered Explorations Figures A-6 through A-9. Wildcat Dynamic Cone Logs Figures A-10 through A-11. Previous Project Logs of Test Pits Appendix B Field Explorations. Report Limitations and Guidelines for Use Page ii | May 16, 2014 | GeoEngineers, Inc. File No. 0382-022-00 GUEMES CHANNEL TRAIL EXTENSION Anacortes, Washington INTRODUCTION AND SCOPE This report presents the results of geotechnical engineering services for the proposed Guemes Channel Trail Extension project in Anacortes, Washington. A vicinity map showing the project location is provided in Figure 1. We understand that the proposed trail alignment will consist of an asphalt paved path that will vary from 10 to 12 feet wide, and extend a total of approximately 3,250 lineal feet at the base of the north-facing bluff along Guemes Channel. An additional 800 lineal feet of 12-foot-wide gravel ballast surfaced temporary access road will be constructed at the east end of the site. The trail alignment is an old railroad grade. The trail will have a final elevation of approximately 13.5 feet (NAVD88) to minimize cutting into the steep bluff. Low-height gravity block retaining walls, 4 feet high or less, are planned for the limited cuts (260 lineal feet of retaining wall total). The project will consist primarily of additional trail fill embankment and repairing the revetment on the waterside of the trail. A number of small landslides or sloughs from the oversteepened bluff have occurred along the trail alignment and the existing rock protection has toppled in numerous locations. GeoEngineers performed geotechnical engineering services for the previous phase of trail construction to the west. The results of our previous study were presented in our report titled “Geotechnical Engineering Services, Proposed Guemes Channel Trail, Anacortes, Washington” dated May 8, 2008. We have also completed numerous other studies in the project vicinity. Pertinent information from our previous studies is included in this report where appropriate. The purpose and scope of our services includes a reconnaissance-level site evaluation as a basis for providing geotechnical conclusions and recommendations to support the new trail. Our specific scope of services included two days of geologic reconnaissance with hand probes and hand-augured explorations along the trail alignment and slope above the trail, and providing geotechnical conclusions and recommendations for site preparation, preparation of subgrade soils, and providing conclusions regarding the stability of the slopes. Our specific scope of services is described in our proposal dated September 13, 2013. Our scope of services did not include detailed retaining wall design or layout, or design of the revetment repair. DESIGNATION OF GEOLOGIC HAZARD AREAS AT THE SITE The methods of designating specific geologic hazard areas are presented in the City of Anacortes Critical Areas Ordinance (CAO), Chapter 17.54. We provide the following discussion of the general hazard in the CAO, and then identification of hazards at this site. ■ Erosion Hazard. Erosion hazard areas are designated in the CAO using U.S. Department of Agriculture’s Soil Conservation Service Soil Survey Data. The site is mapped as having a severe water erosion hazard. Further discussion of this hazard and mitigation strategies are discussed in this report. ■ Landslide Hazard. The CAO lists several methods of designating landslide hazard areas. Landslide hazard area designations pertinent to the site include areas that have shown movement during the Holocene epoch (from ten thousand years ago to the present) or that are underlain or covered by mass wastage debris of that epoch, any area with a slope of May 16, 2014 | Page 1 File No. 0382-022-00 GUEMES CHANNEL TRAIL EXTENSION Anacortes, Washington 40 percent or steeper and with a vertical relief of 10 more feet, and areas potentially unstable because of undercutting by wave action. The site has several landslide hazards associated with it including shallow slough features onto the trail from the oversteepened bluff above the trail, and mass wasting of the trail itself as a response to undercutting by wave action. Further