Loading...
HomeMy WebLinkAboutRS_Geotechnical_Report_181001_v1 GEOTECHNICAL ENGINEERING REPORT MCCARTHY RESIDENCE INVASIVE SPECIES REMOVAL AND RESTORATION 3403 BURNETT AVENUE NORTH RENTON, WASHINGTON PROJECT NO. 2041.01 August 22, 2018 Prepared for: Mary McCarthy Prepared by: 19019 36th Avenue W., Suite E Lynnwood, WA 98036 19019 36th Avenue West, Suite E Lynnwood, WA 98036 (425) 582-9928 Project No. 2041.01 August 22, 2018 Mary McCarthy 3403 Burnett Avenue North Renton, Washington 98105 Subject: Geotechnical Engineering Report McCarthy Residence Invasive Species Removal and Restoration 3403 Burnett Avenue North Renton, Washington Dear Ms. McCarthy, In accordance with your request and written authorization, Zipper Geo Associates, LLC (ZGA) has completed the subsurface exploration and geotechnical engineering evaluation for the McCarthy Residence Invasive Species Removal and Restoration project. This report presents the results of the subsurface exploration and document review, as well as our geotechnical engineering recommendations for the project. Our services were completed in general accordance with our Proposal for Geotechnical Engineering Services (Proposal No. P18105) dated June 20, 2018. Written authorization to proceed on our proposed scope of services was also provided by you on June 20, 2018. We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us. Sincerely, Zipper Geo Associates, LLC Ryan M. Scheffler, P.E. Robert A. Ross, P.E. Senior Geotechnical Engineer Principal TABLE OF CONTENTS Page INTRODUCTION ........................................................................................................................................... 1 SURFACE CONDITIONS AND PROJECT UNDERSTANDING ................................................................. 1 SUBSURFACE CONDITIONS...................................................................................................................... 1 Mapped Geology ............................................................................................................................................. 1 Soil Conditions ................................................................................................................................................. 2 Groundwater Conditions ................................................................................................................................. 2 CONCLUSIONS AND RECOMMENDATIONS ............................................................................................ 2 General Considerations ................................................................................................................................... 2 Geologically Hazardous Areas ......................................................................................................................... 3 Steep Slopes ............................................................................................................................................ 3 Landslide Hazards ............................................................................................................................................ 3 Seismic Hazards ............................................................................................................................................ 3 Coal Mine Hazards ........................................................................................................................................... 3 Hazard Mitigation ............................................................................................................................................ 4 Drainage Tight Lines ........................................................................................................................................ 4 Erosion Control Measures ............................................................................................................................... 4 CLOSURE ..................................................................................................................................................... 5 FIGURES Figure 1 – Site and Exploration Plan APPENDICES Appendix A – Subsurface Exploration Procedures and Logs Cover Page Photo Credit: King County Department of Assessments Page 1 GEOTECHNICAL ENGINEERING REPORT MCCARTHY RESIDENCE INVASIVE SPECIES REMOVAL AND RESTORATION RENTON, WASHINGTON Project No. 2041.01 August 22, 2018 INTRODUCTION This report documents the surface and subsurface conditions encountered at the project site and our geotechnical engineering recommendations for the current proposed McCarthy Residence Invasive Species Removal and Restoration in Renton, Washington. Our geotechnical engineering scope of services for the project included a review of published geologic maps, review of a site topographic survey prepared by Tyee Surveyors, site reconnaissance, subsurface exploration, geotechnical engineering analysis, and preparation of this report. The observations and conclusions summarized herein are based in part upon conditions observed in our subsurface explorations and site observations. In the event that site conditions or elements of the project change, it may be necessary to modify the conclusions and recommendations presented in this report. This report is an instrument of service and has been prepared in general accordance with locally accepted geotechnical engineering practice. This report has been prepared for the exclusive use of Mary McCarthy, and their agents, for specific application to the subject property and stated purpose. SURFACE CONDITIONS AND PROJECT UNDERSTANDING The project site consists of a developed single-family property located at 3403 Burnett Avenue North in Renton, Washington. The property is bordered to the north and south by single family residences, to the east by Burnett Avenue North, and to the west by Lake Washington Boulevard North. The property sits on a regional slope descending from east to Lake Washington in the west. The two-story house is accessed from Burnett Avenue North on the east side of the property at an elevation of approximately 70 feet and includes a daylight basement on the west side of the house with a floor elevation of roughly 64 feet. Beyond the west edge of the existing home, a steep slope descends to Lake Washington Boulevard with a total relief of approximately 19 to 20 feet. The slope is currently covered with invasive deciduous plant and shrub species including English Ivy and Himalayan Blackberry. The project will consist of removal of these invasive species on the slope and restorative landscaping of the slope. The project will also include tight lining downspouts from the home to the toe of the slope. SUBSURFACE CONDITIONS Mapped Geology According to the USGS Geologic map of surficial deposits in the Seattle 30’ by 60’ quadrangle, Washington (Yount, Minard, & Dembroff, 1993) the site is underlain by Vashon recessional outwash deposits (Qvr). These deposits are described as consisting of stratified gravel and sand with minor amounts of silt and McCarthy Residence Invasive Species Removal and Restoration ZGA Project No. 2041.01 August 22, 2018 Page 2 clay deposited in outwash channels behind receding glaciers. The soil conditions encountered in our hand auger borings were generally consistent with the mapped conditions. Soil Conditions The subsurface evaluation for this project included advancement of three hand auger borings (HA-1 to HA-3) completed on and above the slope. The hand auger borings were extended to depths of about 5½ to 8 feet below the existing ground surface (bgs) and their approximate locations are shown on Figure 1, the Site and Exploration Plan. Soils were visually classified in general accordance with the Unified Soil Classification System. Descriptive logs of the subsurface explorations and the procedures utilized in the subsurface exploration program are presented in Appendix A. A generalized description of soil conditions encountered in the hand auger borings is presented below. Please refer to the hand auger logs in Appendix A for a more detailed description of the conditions encountered at the exploration locations. Soil conditions observed in the hand auger borings generally consisted of sandy silt with layers of sand and silty sand. The silt generally ranged in consistency from medium stiff to stiff and the sand layers appeared to be in a loose to medium dense condition, with densities generally increasing with depth. Isolated areas of loose surficial soil were observed surrounding two exposed plastic pipe outlets near the north and south portions of the top of the slope. Groundwater Conditions We encountered minor groundwater seepage between about 3½ and 4 feet below the ground surface within hand augers HA-1 and HA-2 while digging, but did not encounter seepage within hand auger HA-1 atop the slope. The seepage appeared to be located within the upper, granular sand layer near the interface with an underlying, more fine-grained silts. Fluctuations in groundwater levels will likely occur due to seasonal variations in the amount of rainfall, runoff, and other factors not evident at the time the explorations were performed. Therefore, groundwater levels during construction or at other times in the life of the property may be higher than indicated on the logs. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project. CONCLUSIONS AND RECOMMENDATIONS General Considerations Based on soil and groundwater conditions observed in our borings, it is our opinion the proposed slope revegetation plan is feasible from a geotechnical perspective. Geotechnical engineering discussion and recommendations relative to erosion control and Geologically Hazardous Areas are included in the following sections. The recommendations contained in this report are based upon the results of our field explorations (which are presented in Appendix A), engineering analyses, and our current understanding of the proposed project. McCarthy Residence Invasive Species Removal and Restoration ZGA Project No. 2041.01 August 22, 2018 Page 3 Geologically Hazardous Areas As part of our services, we evaluated the presence of regulated geologically hazardous areas (GHAs) at the site as defined by Section 4-3-050 of the Renton Municipal Code (the Code), including steep slopes, landslide hazards, seismic hazards, and coal mine hazards. Steep Slopes: Section 4-3-050G-5(a) of the Code provides definitions for two types of steep slopes. Sensitive slopes are defined as a hillside, or any portion thereof, having an average slope of twenty five percent to less than 40 percent or a slope of 40 percent with a total rise of less than 15 feet. Protected slopes are defined as having an average slope of greater than 40 percent and a minimum vertical rise of 15 feet. Based on our review of the topographic survey by Tyee Surveyors, the slope west of the house extending down to Lake Washington Boulevard appears to meet the definition of a protected slope. The slope is also mapped as a protected slope between 40 percent and 90 percent on the City of Renton GIS online interactive mapping application. Landslide Hazards: Section 4-3-050G-5(b) of the Code defines four levels of landslide hazards. Low hazards (LL) are defined as areas with slopes less than 15 percent. Medium hazards (LM) are areas with slopes between 15 percent and 40 percent underlain by sand, gravel, or glacial till. High landslide hazards (LH) include slopes greater than 40 percent or slopes between 15 percent and 40 percent that are underlain by silt and clay soils. Lastly, very high landslide hazards (LV) include areas of known mapped or identified landslide deposits. The site slope is mapped as a moderate landslide hazard on the City of Renton GIS application. Based on the provided definitions, however, the western slope appears to meet the Code definition of a high landslide hazard area, in our opinion. Erosion Hazards: Section 4-3-050G-5(c) of the Code defines low erosion hazard areas (EL) as being underlain by soils characterized by the Natural Resource Conservation Service (NRCS) as having slight or moderate erosion potential and a slope less than 15 percent. High erosion hazard areas (EH) are defined as being underlain by soils classified by the NRCS as having severe or very severe erosion potential and a slope of more than 15 percent. According to the NRCS online Web Soil Survey, the site is underlain by Alderwood gravelly sandy loam (AgC) which have a slight to moderate erosion potential. Though the NRCS mapped soil descriptions describe the site soil as only having a slight to moderate erosion potential, in our opinion the steepness of the slope results in a high erosion hazard classification. Seismic Hazards: Section 4-3-050G-5(d) of the Code defines low (SL) and high (SH) seismic hazard areas by International Building Code (IBC) site classifications. IBC site classifications of A through D correspond to low seismic hazard areas and IBC site classifications of E and F correspond to high seismic hazard areas. Based on the soil conditions encountered in our hand auger borings and the mapped geologic conditions, in our opinion the site would be classified as Site Class D and represents a low seismic hazard. Coal Mine Hazards: Section 4-3-050G-5(e) of the Code defines coal mine hazards as low (CL), medium (CM), or high (CH). Low hazard areas are areas with no known mine workings and no predicted subsidence. Medium hazard areas are areas where mine workings are deeper than 200 feet for steeply McCarthy Residence Invasive Species Removal and Restoration ZGA Project No. 2041.01 August 22, 2018 Page 4 dipping seams or deeper than 15 times the thickness of the seam or workings for gently dipping seams. High hazard areas are areas with abandoned and improperly sealed mine openings and areas underlain by mine workings shallower than the maximums defined for medium hazard areas. We did not observe mapped coal mine hazards on or near the site on the City of Renton GIS mapping application. Hazard Mitigation: As noted above, the western slope is classified as a protected steep slope, a high landslide hazard area, and a high erosion hazard area. The proposed project is limited to removal of existing invasive species and restoration of the slope surface with native plants and landscaping and does not include construction of structures or impermeable surfacing on or near the slope. Slope restoration will include removal of loose surficial soils currently susceptible to erosion and replacement with boulders and new vegetation. Exposed plastic drain pipes currently daylighting near the top of the slope appear to be significantly contributing to surficial erosion and the observed areas of loose surficial soil. The restoration project will include extending the outlet of these pipes to the base of the slope and removing their impact on erosion of the slope. Slope landscaping restoration and temporary erosion control plans were completed and submitted under a separate cover. Provided the temporary erosion control measures discussed below and on the plans are properly implemented during the project and the slope is restored in accordance with the landscape restoration plan, in our opinion the stability of the slope will be improved and the risk of landslides and erosion will be reduced in the completed condition. Drainage Tight Lines During our site visit, we observed two exposed plastic drain lines of unknown origin daylighting in the northern and southern areas of the upper portion of the slope. We observed signs of minor erosion and loose surficial soils surrounding the pipe outlets. We recommend new non-perforated tight line pipes be attached to the existing exposed pipe ends and extended to outlet at the base of the slope. Additionally, we recommend the loose soil surrounding the existing pipe outlets be removed and replaced with boulders as shown on the landscape restoration plans. The approximate locations of the existing pipe outlets and loose soil areas are shown on the Invasive Species Removal and Restoration plans submitted under separate cover. If additional pipe outlets are encountered on the slope during clearing operations, we recommend they be tight lined to the bottom of the slope as described above. Erosion Control Measures Stripped surfaces are typically a source of runoff sediments. We recommend that silt fences be installed on the downslope side of stripped areas in order to capture runoff water and sediment. We also recommend that a sediment filter insert be placed in the catch basin below the southwest corner of the slope. If earthwork occurs during wet weather, we recommend that all stripped surfaces be covered with straw or plastic sheeting to reduce runoff erosion. Approximate erosion control measure locations and details are provided on the Invasive Species Removal and Restoration plans submitted under separate cover. McCarthy Residence Invasive Species Removal and Restoration ZGA Project No. 2041.01 August 22, 2018 Page 5 CLOSURE The analysis and recommendations presented in this report are based, in part, on the explorations completed for this study and review of additional information described in the text. The number, location, and depth of the explorations for the current phase of the project were completed within the constraints of budget and site access so as to yield the information to formulate our recommendations. Project plans were in the preliminary stage at the time this report was prepared. We therefore recommend Zipper Geo Associates, LLC be provided an opportunity to review the final plans and specifications when they become available in order to assess that the recommendations and design considerations presented in this report have been properly interpreted and implemented into the project design. This report has been prepared for the exclusive use of Mary McCarthy, and their agents, for specific application to the project discussed and has been prepared in accordance with generally accepted geotechnical engineering practices. No warranties, express or implied, are intended or made. Site safety, excavation support, and dewatering requirements are the responsibility of others. In the event that changes in the nature, design, or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless Zipper Geo Associates, LLC reviews the changes and either verifies or modifies the conclusions of this report in writing. APPENDIX A SUBSURFACE EXPLORATION PROCEDURES AND LOGS APPENDIX A SUBSURFACE EXPLORATION PROCEDURES AND LOGS Field Exploration Description Our field exploration for this project included advancing three hand auger borings on and atop the slope behind the residence on July 16, 2018. The approximate locations of the explorations are presented on Figure 1, the Site and Exploration Plan. Exploration locations were determined in the field based on hand measurements from existing site features. As such, the exploration locations should be considered accurate only to the degree implied by the measurement method. Descriptive logs of the hand auger borings are enclosed in this appendix. Surface elevations at the exploration locations were estimated based on the existing conditions plan provided by the civil engineer. The hand auger borings were advanced by an engineer from our firm using hand tools. The engineer continuously observed the spoils, logged the subsurface conditions encountered, and obtained representative soil samples of each discrete layer. All samples were stored in moisture-tight containers and transported to our laboratory for further evaluation. The enclosed hand auger logs describe the vertical sequence of soils and materials encountered in the hand auger borings, based primarily upon our field classifications. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also graphically indicate the sample type, sample number, and approximate depth of each soil sample obtained from the hand auger borings. If groundwater was encountered, the approximate groundwater depth, and date of observation, are depicted on the logs. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sample Nc % M Testing SAND, some gravel gravel, moderate mottling Medium dense, wet, light brown, SAND, some silt, trace Stiff, wet, light brown, sandy SILT, some gravel Groundwater not encountered while digging. Note: Nc is the Dynamic Come Penetrometer blow count per 1.75-inch interval measured in accordance with ASTM Special Technical Publication #399. S-1 @ 1 to 1 1/2 ft Depth (ft)Material Description S-5 @ 6 1/2 to 7 ft Loose to medium dense, moist, tan, silty SAND, trace S-2 @ 2 1/2 to 3 ft Approximate Ground Surface Elevation:N/A ft Date Excavated:7/16/2018 3 inches of red woodchips over moist, dark tan, silty Project:McCarthy Residence Project Number:2041.01 Hand Auger HA-1 See Attached Site & Exploration Plan, Figure 1Location: S-3 @ 4 1/2 to 5 ft S-4 @ 5 1/2 to 6 ft Stiff, wet, light brown, sandy SILT, some gravel gravel Boring terminated at approximately 8 feet. 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sample Nc % M Testing Stiff, wet, light brown, sandy SILT, trace gravel Grades to some gravel Boring terminated due to refusal on gravel at Minor groundwater seepage observed at approximately Note: Nc is the Dynamic Come Penetrometer blow count per 1.75-inch interval measured in accordance with ASTM Special Technical Publication #399. Depth (ft)Material Description Medium dense, moist, tan, SAND, with silt, some gravel Approximate Ground Surface Elevation:N/A ft Date Excavated:7/16/2018 10 inches of top soil S-1 @ 1 1/2 to 2 ft Project:McCarthy Residence Project Number:2041.01 Hand Auger HA-2 See Attached Site & Exploration Plan, Figure 1Location: S-2 @ 3 1/2 to 4 ft S-3 @ 4 1/2 to 5 ft approximately 5 1/2 feet. 3 1/2 feet while digging. 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sample Nc % M Testing gravel Stiff, wet, gray, sandy SILT, some gravel Minor groundwater seepage observed at approximately Note: Nc is the Dynamic Come Penetrometer blow count per 1.75-inch interval measured in accordance with ASTM Special Technical Publication #399. S-1 @ 1 to 1 1/2 ft Depth (ft)Material Description Loose to medium dense, wet, brown, silty SAND, some Approximate Ground Surface Elevation:N/A ft Date Excavated:7/16/2018 8 inches of gray SILT with organics Project:McCarthy Residence Project Number:2041.01 Hand Auger HA-3 See Attached Site & Exploration Plan, Figure 1Location: S-2 @ 4 1/2 to 5 ft S-3 @5 1/2 to 6 ft Stiff, wet, brown, SILT, some sand and gravel Boring terminated at approximately 8 feet. 4 feet while digging. 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036