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Geotechnical Evaluation.pdf9110 Olympic View Drive Edmonds, Washington Project No. 1751.00 January 25, 2017 Prepared for: Jan and Bill Munson Prepared by: ZGA Zipper Geo Associates, LLC Geotechnical and Environmental Consultants 1902336 th Avenue West, Suite D Lynnwood, WA 98036 Zipper Geo Associates LLQ Geotechnical and Environmental Consulting Project Number 1751.00 January 25, 2017 Jan and Bill Munson 18130 Sunset Way Edmonds, Washington 98026 Subject: Geotechnical Evaluation of Proposed Tree Removal Lane Residence Site 9110 Olympic View Drive Edmonds, Washington 98026 Dear Mr. and Mrs. Munson, In accordance with your request and written authorization, Zipper Geo Associates, LLC (ZGA) has completed the subsurface exploration and geotechnical evaluation of the Proposed Tree Removal at 9110 Olympic View Drive in Edmonds, Washington. This report presents the findings of our site reconnaissance and subsurface exploration, and our geotechnical recommendations for the project. Our work was completed in general accordance with our Short Form Service Agreement (Proposal No. P16306) dated October 26, 2016. 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 James P. Georgis, L.E.G. Principal Copies: Addressee (1) Thomas A. Jones, P.E. Principal 19023 36th Avenue West, Suite D Lynnwood, WA 98036 (425) 582-9928 TABLE OF CONTENTS INTRODUCTION............................ ........................... ............................................... ......., 1 SITE DESCRIPTION ....................... ......... ............ ................ .................. ........................... 1 PROJECTUNDERSTANDING ............................................................................................ 1 SITE RECONNAISSANCE ................................ .....,,.......,..,... ,,....,.. ,........ ........ 2 SUBSURFACE CONDITIONS ........ ...........................................p.......,......,...,................... 2 Published Geologic Mapping .....................................ww..,. ...........,...,....,........,,.......,......... 2 SoilConditions ............................ ......... ....... ....... ....................................................... 3 Groundwater Conditions ................................. ..,..................,.......... ............................ ,..,., 4 CONCLUSIONS AND RECOMMENDATIONS.... ......., ............................ ........ ....... 4 General................................................................,.........,..,................,.........,....,,.,,,........,. 4 Geologic Hazard ECA Considerations.....................................M..,.,...,..., .............. ..... ...,. 4 Slope Stability Considerations................................................................................................ 5 Temporary Erosion Control Considerations ..... ......................................,....,.......... ......., 6 General Steep Slope Considerations ..... ........ ....... ........ ............................. .-.,,, 6 CLOSURE................................................ ....... ............... ............................. ........ 7 FIGURES PREPARED BY ZGA Figure 1 — Site and Exploration Plan APPENDICES Appendix A — Subsurface Exploration Procedures and Logs GEOTECHNICAL EVALUATION OF PROPOSED TREE REMOVAL LANE RESIDENCE SITE 9110 OLYMPIC VIEW DRIVE EDMONDS, WASHINGTON Project No. 1751.00 January 25, 2017 INTRODUCTION This report documents the surface and subsurface conditions encountered at the site and our geotechnical evaluation of the proposed tree removal at 9110 Olympic View Drive in Edmonds, Washington. The project description, site conditions, and our geotechnical conclusions and recommendations are presented in the text of this report. Supporting data including detailed exploration logs, field exploration procedures, and other supporting information are presented as appendices. Our geotechnical scope of services for the project included a document review, site reconnaissance, subsurface evaluation, geotechnical analysis, and preparation of this report. The subsurface evaluation consisted of completing two hand auger explorations (designated HA -1 and HA -2) located near the tree proposed for removal. The hand auger explorations extended about 3 to 13 feet below the existing ground surface. SITE DESCRIPTION The site is located at 9110 Olympic View Drive in Edmonds, Washington. The site is a single- family residential property supporting a centrally located, one-story, wood -framed residence with a daylight basement and attached garage. A concrete driveway is located on the east side of the residence and a concrete basement level patio is located on the west side of the residence. The site is bounded to the north by a single-family residential development, to the south and east by Sunset Way, and to the west by Olympic View Drive. The residence is generally surrounded by trees and shrubs with several large diameter cedar trees located in the northwestern portion of the site. The approximate layout of the western portion of the site is presented on the enclosed Site and Exploration Plan, Figure 1. PROJECT UNDERSTANDING The site is mapped by the City of Edmonds as being within Erosion and Landslide Geologic Hazard Areas. Specifically, a majority of the site is mapped as having a Severe Erosion Hazard with 15 to 40 percent slopes, while a portion of the back yard (between the residence and Olympic View Drive) is mapped as being in a Landslide Hazard Area due to slopes of 40 percent or more. We understand that you are proposing the removal of one cedar tree with a trunk diameter on the order of 2'/2 to 3 feet from the northwestern portion of the site. The approximate location of the tree proposed for removal is shown on Figure 1. This report has been prepared to evaluate potential risks to Geologic Hazard Areas and the site relative to the removal of the identified cedar tree. Zipper Geo Associates, LLC Proposed Tree Removal, 9110 Olympic View Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 SITE RECONNAISSANCE We completed a reconnaissance of the property and immediate vicinity during a site visit completed on December 30, 2016. The purpose of the reconnaissance was to identify surficial features of site conditions such as the presence or absence of groundwater seepage, the presence or absence of surface erosion, indications of past slope instability, indications of past land use, the type, extent, and condition of existing vegetation, and the location of the tree identified for removal, and other site features that may be affected by the planned tree removal. The enclosed Site and Exploration Plan, Figure 1, shows the approximate location of some of the observed site features. A summary of our site observations is presented below. The site is located on a west -facing slope with an estimated total property relief of about 15 to 20 feet. The backyard includes a relatively level bench which extends north from the backyard concrete patio and appears to be the result of past grading activities. What appears to be a cut slope extends down to the east side of the bench with a slope inclination of about 30 degrees (58 percent) and about 4 feet of topographic relief. Near the proposed tree removal area, a slope extends down from the west side of the bench to Olympic View Drive. This slope has two distinct slope inclinations. The upper portion of the slope, where the tree proposed for removal is located, has a slope inclination of about 15 to 20 degrees (27 to 36 percent) and about 5 to 7 feet of relief. Slightly west of the property line, the slope angle increases to about 40 to 60 degrees (84 to 173 percent) with about 6 to 8 feet of relief. This steeper portion of the slope is adjacent to Olympic View Drive and appears to be a cut associated with the road construction. The tree proposed for removal consists of a 2% to 3 -foot diameter cedar located in the back yard about midway between the residence and Olympic View Drive. This portion of the back yard supports several other cedar trees with trunk diameters ranging from about 1 to 3 feet and several tree stumps ranging from about 1 to 3 feet in diameter. The cedar trees are near vertical with no prominent lean or curved trucks (which is often indicative a past slope movement) and appear to be in reasonably good condition (ZGA staff are not license arborists). In addition, the orientation of the tree stumps is not suggestive of past slope movement. The area includes a moderately well-developed understory of ferns, rhododendron, grasses, and other low -growing ground cover, along with a surficial mantling of duff. We did not observe any surface water flow or indications of groundwater seepage at the time of our site visit, or hydrophilic vegetation that would indicate the presence of springs or seeps. We did not observe any significant indication of surficial erosion, such as rills or sediment deposits at the toe of the slope (eastern edge of Olympic View Drive). SUBSURFACE CONDITIONS Published Geologic Mapping We assessed the geologic setting of the site and surrounding vicinity by reviewing The Geologic Map of the Edmonds East and Portion of the Edmonds West Quadrangle, Washington, U.S. Page 2 Zipper Geo Associates LLC Proposed Tree Removal, 9110 Olympic View Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 Geological Survey Map MF -1541, 1983. The site is mapped near the contact between Quaternary Vashon Till (Qvt) deposits and the underlying Quaternary Advance Outwash (Qva) deposits. The Vashon Till is described as a compact, poorly sorted mixture of clay, silt, sand, pebbles, cobbles, and boulders in variable amounts. The Advance Outwash is described as mostly clean, compact, pebbly sand with increasing amounts of gravel higher in the stratigraphic section. We also reviewed the United Stated Department of Agriculture Natural Resources Conservation Service (NRCS) on-line soils mapping application. The NRCS maps the site as Alderwood gravelly sandy loam (15 to 30 percent slopes). The Alderwood series is described as being derived from glacial drift and/or glacial outwash deposits. Soil Conditions The subsurface exploration for this project included two hand auger explorations (HA -1 and HA- 2) located near the tree proposed for removal. The hand auger explorations extended approximately 3 to 13 feet below the existing ground surface. The approximate exploration locations are shown on the enclosed Site and Exploration Plan, Figure 1. Soils were visually classified in general accordance with the Unified Soil Classification System. Detailed, descriptive logs of the subsurface explorations and the procedures utilized in the subsurface exploration program are presented in Appendix A. Generalized descriptions of subsurface soil conditions observed at the exploration locations are presented below. The stratification and horizontal extent of the soil types observed in our explorations may vary between explorations. Stratification boundaries on the exploration logs represent the approximate depth of changes in soil types, although the transition between materials may have been gradual. Please refer to the hand auger logs in Appendix A for a more detailed description of the conditions encountered at the exploration locations. Hand auger HA -1 was completed on a relatively level bench extending from the backyard concrete patio and was about 15 feet southeast of the tree proposed for removal. In general, the exploration encountered about 3 inches of topsoil over very loose to loose sand with some silt and trace fine gravel interpreted as fill. The fill extended to a depth of about 13/ feet and was underlain by about 3 inches or relic topsoil consisting of very loose sand with silt, fine organics, and fine roots. Soil interpreted as Advance Outwash deposits were encountered below the relic topsoil (at a depth of about 2 feet) and extended to the total depth explored of 13 feet below existing grade. In general, the Advance Outwash deposits consisted of loose grading to dense sand with trace to some silt and trace fine gravel. Hand auger HA -2 was completed about 6 feet east of the tree proposed for removal. The exploration encountered about 7 inches of topsoil underlain by Advance Outwash deposits which extended to Page 3 Zimer Goo Asgociates, LLQ Proposed Tree Removal, 9110 Olympic View Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 the total depth explored of 3 feet below existing grade. In general, the Advance Outwash deposits consisted of loose grading to medium dense sand with trace to some silt and trace fine gravel. Groundwater Conditions Groundwater seepage was not observed in the hand auger explorations which extended to a maximum depth of about 13 feet below grade. We did not observe soil mottling that might be indicative a seasonal perched groundwater conditions within the exploration depth. These observations represent groundwater conditions at the time of the field exploration. Groundwater conditions should be expected to fluctuate due to changes in season, precipitation patterns, site utilization, on-site or off-site irrigation activities, and other on- and off-site factors. CONCLUSIONS AND RECOMMENDATIONS General Based on the results of our site reconnaissance, document review, subsurface exploration, and geotechnical analysis, it is our opinion that removal of the proposed cedar tree is feasible from a geotechnical perspective. Provided that the conditions and recommendations contained within this report are satisfied, it is our opinion that the proposed tree removal will not increase the potential for soil movement and the risk of damage to existing structures, the site, and to adjacent properties from soil instability will not be increased. The following sections provide a discussion regarding City of Edmonds regulated ECAs and recommendations to reduce risks associated with the planned cedar tree removal. Geologic Hazard ECA Considerations The site is mapped by the City of Edmonds as containing Erosion Hazard and Landslide Hazard Environmental Critical Areas (ECAs). We understand that the City has requested a geotechnical evaluation of the proposed cedar tree removal to assess the risk to regulated Geologic Hazard ECAs and their associated buffers, as defined in Chapter 23.80 of the City of Edmonds Municipal Code (EMC) that may result from the planned tree removal and provide recommendations to mitigate identified risks. Erosion Hazard Areas: City of Edmonds maps generally identify the majority of the site as an Erosion Hazard Area. Section 23.80.020A of the EMC identifies erosion hazard areas as those areas containing soils that may experience severe to very severe erosion hazard. This group of soils include, but are not limited to, the following when they occur on slopes of 15 percent or greater. • Alderwood Soils (15 to 25 percent slopes) • Alderwood/Everett series (25 to 70 percent slopes) • Everett series (15 to 25 percent slopes) The site is mapped by the NRCS as Alderwood soils (soils derived from glacial till and/or outwash) and slope inclinations near the proposed tree removal range from of about 27 to 36 percent. As Page 4 Zipper Geo ALssociates, LLC Proposed Tree Removal, 9110 Olympic View Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 such, it is our opinion that the tree removal area is consistent with the EMC criteria for an Erosion Hazard Area. Landslide Hazard Areas: City of Edmonds maps identify a portion of the back yard (between the residence and Olympic View Drive) as being in a Landslide Hazard Area. Section 23.80.020B of the EMC defines landslide hazard areas as areas potentially subject to landslides based on a combination of geologic, topographic, and hydrologic factors. These factors include any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a 25 -foot horizontal run. This includes modified slopes (where there are breaks in slope) that meet overall average steepness and height criteria. Based on our visual observations and field measurements, it appears that the modified slope (over -steepened cut associated with Olympic View Drive) that extends up to the proposed tree removal area with a grade break is consistent with the EMC criteria for a Landslide Hazard Area. Risk Evaluation Summary: Provided that the conditions and recommendations contained within this report are satisfied, it is our opinion that the removal of the identified cedar tree will not increase the potential for soil movement and the risk of damage to existing structures, the site, and to adjacent properties from soil instability will not be increased. Slope Stability Considerations Tree root systems provide erosion protection and bio -reinforcement for slope soils, thereby increasing the strength of the soil mass and reducing the risk of slope instability. However, the weight of the tree can increase the forces tending to drive slope instability and the rocking action of the root mass during high winds can result in loosening of the slope soils, resulting in a reduction in slope stability, particularly for large, heavy trees located on or near the crest of steep slopes. There is some debate in the scientific community regarding the relative contributions of these different factors to slope stability, but the general consensus appears to be that under most conditions the presence of trees (in particular the bio -reinforcement they provide) results in a net improvement to slope stability. Some species of trees readily sprout after cutting (often referred to as stump -sprouting) thereby maintaining the reinforcing properties of the root system and decreasing the weight of the tree on the slope. Cedar is listed in the 1993 publication Vegetation Management: A guide for Puget Sound Bluff Property Owners, produced by the Washington State Department of Ecology (Publication 93-31) as being capably of stump -sprouting. However, in our experience, cedar trees of the size proposed for removal at this site do not always survive the cutting process and may not stump -sprout. Based on our evaluation of the site conditions, it is our opinion that the decay of the tree root system if the tree does not stump -sprout could result in a minor reduction in the stability of the slope near the removed trees relative to shallow surficial sliding. Research indicates that it typically takes about 3 to 5 years to realize a significant reduction in the bio - reinforcement provided by a tree that does not survive cutting. Page 5 0 per G e O—A 39 cWltes LLC Proposed Tree Removal, 9116 aympic view Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 As a precautionary measure, we recommend that the cedar stump be left in place and that the tree removal plan include new plantings near the stump. We recommend that native woody shrubs or small trees be considered to reduce the need for watering and keep the weight and wind loading characteristics of the new growth to reasonable levels as it matures. The following table provides some plant species that could be considered to reduce the potential for erosion and increase resistance to shallow surficial sloughing. The information presented below is a relatively small sampling of plant species discussed in the 1993 publication Slope Stabilization and Erosion Control using Vegetation, produced by the Washington State Department of Ecology (Publication 93-30), The table below is by no means intended to be comprehensive or restrictive relative to the selection of appropriate plantings. Temporary Erosion Control Considerations We anticipate that the tree removal process will likely result in disturbance of the ground surface and a temporary increase in the potential for erosion. We therefore recommend that the tree removal service utilize means and methods intended to minimize ground disturbance. In addition, we recommend that soils exposed within Geologic Hazard Areas and their buffers be covered with a temporary erosion control material, such as sterile straw or arborist mulch. General Steep Slope Considerations Owners of properties containing or adjacent to steep slopes should recognize that soil loss or downslope displacement from steep slopes is a naturally occurring mass wasting process, and that some soil loss from steep slopes should be expected over time, particularly in response to Page 6 Zipper Geo Associates LLQ Proposed Tree Removal, 9110 Olympic View Drive, Edmonds, WA Project No. 1751.00 January 25, 2017 episodic severe storm events. The conclusions and recommendations presented in this report are intended to allow for the removal of the identified cedar tree while maintaining the existing stability of the site slopes. The findings presented in this report should not be interpreted as indicating that the site steep slopes will be stable under all possible future conditions. CLOSURE The analysis and recommendations presented in this report are based, in part, on the explorations completed for this study. The number, location, and depth of the explorations were completed within the constraints of budget and site access so as to yield the information to formulate our recommendations. This report has been prepared for the exclusive use of Jan and Bill Munson, and their agents, for specific application to this project and has been prepared in accordance with generally accepted geotechnical engineering practices. No warranties, express or implied, are intended or made. Site safety is the responsibility of others. In the event that changes in the nature, design, or location of the tree removal 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. Page 7 APPROXIMATEI PROPERTY-~ LINE LOCATION 2.5' STUMP 30° SLOPE 4' RISE 9110 OLYMPIC VIEW DRIVE CONC. PATIO 2 X 1' STUMPS 2' STUMP co HA -1 LEGEND HA -1 HAND AUGER NUMBER AND APPROXIMATE LOCATION O APPROXIMATE LOCATION OF TREE STUMP APPROXIMATE LOCATION OF LIVE TREE 1ST FLOOR DECK OVERHANG RELATIVELY LEVEL BENCH 3' CEDAR 3' STUMP 15-20° SLOPE - I 2.5 TO T CEDAR PROPOSED ��d�`""' HA -2 V_w—"`–FOR REMOVAL PROP. CORNER. o STAKE 1.5' CEDAR 2' CEDAR 1' CEDAR 20-40° 40-60° — • SLOPE CUT T CEDAR OYLMPIC VIEW DRIVE APPROXIMATE EDGE OF ASPHALT PAVEMENT 20 0 10 20 SCALE IN FEET APPENDIX A FIELD EXPLORATION PROCEDURES AND LOGS FIELD EXPLORATION PROCEDURES Field Exploration Description Our field exploration for this project included two hand auger explorations completed on December 30, 2016. The approximate exploration locations are presented on the enclosed Site and Exploration Plan, Figure 1. Exploration locations were determined in the field by measuring distances from existing site features with a fiberglass tape. As such, the exploration locations should be considered accurate only to the degree implied by the means and methods to establish them. The following sections describe our procedures associated with the explorations. Descriptive logs of the explorations are enclosed in this appendix. Hand Auger Procedures An engineering geologist from our firm advanced a 3.5 -inch diameter auger by hand, continuously observing the soil cuttings as they were retrieved. Representative portions of the soils retrieved were placed in moisture tight containers and returned to our laboratory for further visual classification and testing. The enclosed hand auger logs indicate the vertical sequence of soils and materials encountered in each exploration, based primarily on our field classifications and supported by our subsequent laboratory testing. Where a soil contact was observed to be gradational or undulating, our logs indicate the average contact depth. Our logs also indicate the approximate depths of any sidewall caving or groundwater seepage observed in the explorations, as well as all sample numbers and sampling locations. ZIPPER; GEO ASSOCIATES, LLC 19023 361 Avenue West, Suite D, Lynnwood, Washington 98036 Depth (ft) HAND AUGER HA -1 Location: See Site and Exploration Plan, Figure 1 Approximate Ground Elevation: NA Material Description Project: Lane Residence Tree Removal Project No: 1751.00 Date Excavated: 12-30-2016 Sample Nc %M Testing 1 Very loose,, wet, dark brown SAND, with silt, fine organics, and lige roots (T --,.ll) ,. ,...,.,., F---_____-..- ..----------------- Very ry loose to loose, moist, lig ht brown SAND, some silt, trace fine gravel (Fill) ----- -------.-,------------------------------- Very loose moist, dark brown SAND, with silt, fine ,• and fine roots JRSliE Topsoil---------------'�� Loose to medium dense, moist, light gray -brown SAND, trace to some silt, trace fine gravel. (Glacial Advance Outwash) Grades to medium dense at about 4 feet Grades to dense at about 8 feet ger completed at 13 feet on 12/37012016. ndwater obseryed at time of exploration. FNollg observed, 2_----. S-1 @ 1.5' 4 3 4 5 6 S-2 @ 4' 15 7 8 S-3 @ 8' 30 9 10 11 � 12 Note: Nc is the Dynainic Cone Penetrometer blow count per ASTM Special Publication 399. ZIPPER GEO ASSOCIATES9 LLC 19023 361 Avenue West, Suite D, Lynnwood, Washington 98036 Depth (ft) IIAND AUGER 1°l A-2 Location: See Site and Exploration Plan, Figure 1 Approximate Ground Elevation: NA Material Description Project: Lane Residence Tree Removal Project No: 1751.00 Date Excavated: 12-30-2016 Sample Nc %M Testing Very loose, wet, dark brown SAND, with silt, fine organics, and fine roots (Topsoil) Loose grading to medium dense, moist, light gray -brown SAND, trace to some silt, trace fine gravel (Glacial Advance Outwash) Hand auger completed at 3 feet on 12/30/2016. No groundwater observed at time of exploration. No caving observed. 1 S-1 @ 2.5' I 14 2 3 4 5 6 7 8 9 10 11 12 Note: Nc is the Dynamic Cone Penetrometer blow count per ASTM Special Publication #399. Zipper Geo Associates, LLC 19023 - 36th Avenue West, Suite D Lynnwood, WA 98036 425.582.9928 Federal Tax ID # 45-4956075 INVOICE Jan and Bill Munson January 27, 2017 18130 Sunset Way Edmonds, WA 98026 Invoice 1795 Re: Billing for Geotechnical Professional Services Proposed Tree Removal at 9110 Olympic View Drive, Edmonds, WA Zipper Geo Associates Project No. 1751.00 Invoice Period December 12, 2016 through January 27, 2017 Date Professional Hours Services Provided 12/12/2016 Stephen Miller 1.00 Mark site for utility locate 12/12/2016 James P. Georgis 1.00 Project startup & One -Call 12/28/2016 James P. Georgis 0.50 Review regional geology 12/30/2016 James P. Georgis 4.00 Site reconnaissance & hand augers 12/30/2016 James P. Georgis 0.50 Prepare exploration logs 01/03/2017 Stephen Miller 1.00 Drafting 01/03/2017 James P. Georgis 1.00 Review DNR data and City code 01/24/2017 James P. Georgis 5.50 Report Preparation 01/27/2017 Tom Jones 0.25 Report Review 14.75 Professional Classification Total Hours Rate/ Hour Amount Billed Stephen Miller Field Engineer 1.50 $85.00 $127.50 Tom Jones Principal 0.25 $170.00 $42.50 James Georgis Senior Geologist 8.00 $130.00 $1,040.00 James Georgis Staff Geologist5.00 $95.00 $475.00 14.75 $1,685.00 Mileage Date Description Miles Rate/Mile 12.30.2016 Mileage 15 $0.535 $8.03 Total Expenses $8.03 Amount Due This Invoice $1,693.03 Authorized Budget per Agreement: $1,710.00 Previously Vnvolcpd Amount; $0.00 Current Invoiced Amount; $1,693.0$ Authorized Budget Remaining: $16..97 /I ),, By. James Georgis, Zipper Geo Associates, LLC