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Home My WebLink AboutD_Children’s Institute Hazardous Tree Removal Critical Areas Exemption_210222DEPARTMENT OF COMMUNITY AND ECONOMIC DEVELOPMENT D_Children’s Institute Hazardous Tree Removal Critical Areas Exemption_210222 PLANNING DIVISION CERTIFICATE OF EXEMPTION FROM CRITICAL AREAS REGULATIONS EVALUATION FORM & DECISION DATE: February 22, 2021 PROJECT NUMBER: LUA21-000058, CAR PROJECT NAME: Children’s Institute Hazardous Tree Removal Critical Areas Exemption PROJECT MANAGER: Angelea Weihs, Associate Planner OWNER: Children's Institute for Learning Differences 2640 Benson Road S Renton, WA 98055 APPLICANT/CONTACT: Ray Robinson Integrated Site Design, Inc 12743 26th Ave NE Seattle, WA 98125 PROJECT LOCATION: 2640 Benson Road S PROJECT DESCRIPTION: The applicant, Children's Institute for Learning Differences, is requesting approval of a Critical Areas Exemption in order to remove 21 high-risk hazardous trees and prune four (4) trees on the site located at 2640 Benson Road S (Parcel number 2923059102). Based on the provided site plan (Attachment A), two (2) of the trees proposed for removal (trees numbered 1906 and 2022) are located just off site to the north on parcel number 2923059061. This decision does not approve the removal of off site trees. A separate application will be required for the removal of trees numbered 1906 and 2022. The site is within the Residential-8 (R-8) Zone and is 175,111 square feet. The site is presently used as a private school. The site contains sensitive and protected slopes, high landslide hazards, moderate coalmine hazards, and high coalmine hazards within 50 feet of the site. An arborist report prepared by Rippey Arboriculture, LLC, was submitted with the project application (Attachment B). The purpose of the tree removal is to prevent damage by hazardous trees to residents, buildings, and other existing improvements within and nearby the subject property. The applicant states that, should the trees fail, the risk of impact to parking areas and drive aisle areas is high. Of the 19 on site trees recommended for removal by the arborist report, the the arborist states that nine (9) of these trees (trees numbered DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 City of Renton Department of Community & Economic Development Certificate of Exemption from Critical Areas Regulations Children’s Institute Hazardous Tree Removal Critical Areas Exemption LUA21-000058 DATE OF PERMIT: February 22, 2021 Page 2 of 4 D_Children’s Institute Hazardous Tree Removal Critical Areas Exemption_210222 1808, 1856, 1853, 1814, 1684, 1776, 1775, 1774 and 1755) are along the nature trail used by children of the school. The arborist report states that these trees, if safe to climb, can potentially be reduced to wildlife habitat snags of a height no taller than the distance of the tree to the trail. City Arborist, Ian Gray, concurs with the assessment of the trees proposed for removal and recommends retaining as wildlife snags where feasibile. The applicant proposed to prune four (4) trees identified as trees numbered 1888, 1869, 1889, and 1684 in arborist report. The proposed pruning includes removal of branches and trun ks that pose a risk to people and/or existing site improvements. The proposed tree removal has been assessed by a geotechnical engineer (Attachment C), and the report indicates that the tree removal will not have a significant impact on the slope/critical area stability, provided that the conditions and recommendations contained within the provided geotechnical report are satisfied. The geotechnical report recommended that the trees listed for removal within steep slopes, landslide hazards, and their buffe rs be cut off near the ground surface or reduced to wildlife snags , if safe to climb, and that the root systems be left in place to provide ongoing erosion protection and reinforcement during the decay period. In addition, the report recommended that the proposed removal include replacement plantings near trees located within landslide hazard areas, specifically native woody shrubs and small trees that are resistant to the fungal types adversely affecting the site. The applicant provided a tree replacement proposal with the application (Attachment A) that includes a proposed replacement of 2 trees for every 1 tree removed within the steep slope clitical area boundary as shown on the site plan. There are a total of five (5) trees located within the slope critical area boundary; therefore, the replacement planting would include a total of ten (10) replacement trees. The proposed replacement plantings (4-6’ in height) include Douglas fir, Western hemlock, and Western red cedar, and would be located in the vicinity of the trees to be removed. The geotechnical engineer evaluated the tree replacement proposal and determined it to be consistent with the recommendations provided in the geotechnical report. CRITICAL AREA: Sensitive and Protected Slopes, High Landslide Hazards, Moderate and High Coal Mine Hazards EXEMPTION JUSTIFICATION: Renton Municipal Code, Section 4-3-050C.3.c.iii. Dangerous Trees: Removal of non-native invasive ground cover or weeds listed by King County Noxious Weed Board or other government agency or dangerous trees, as defined in Chapter 4-11 RMC which have been approved by the City and certified dangerous by a licensed landscape architect, or certified arborist, selection of whom to be approved by the City based on the type of information required. Limited to cutting of dangerous trees; such hazardous trees shall be retained as large woody debris in critical areas and/or associated buffers, where feasible. FINDINGS: The proposed development is consistent with the following findings pursuant to RMC section 4-3-050.C.2.d: DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 City of Renton Department of Community & Economic Development Certificate of Exemption from Critical Areas Regulations Children’s Institute Hazardous Tree Removal Critical Areas Exemption LUA21-000058 DATE OF PERMIT: February 22, 2021 Page 3 of 4 D_Children’s Institute Hazardous Tree Removal Critical Areas Exemption_210222 i. The activity is not prohibited by this or any other provision of the Renton Municipal Code or State or Federal law or regulation; ii. The activity will be conducted using best management practices as specified by industry standards or applicable Federal agencies or scientific principles; iii. Impacts are minimized and, where applicable, disturbed areas are immediately restored; iv. Where water body or buffer disturbance has occurred in accordance with an exemption during construction or other activities, revegetation with native vege tation shall be required; v. If a hazardous material, activity, and/or facility that is exempt pursuant to this Section has a significant or substantial potential to degrade groundwater quality, then the Administrator may require compliance with the Wellhead Protection Area requirements of this Section otherwise relevant to that hazardous material, activity, and/or facility. Such determinations will be based upon site and/or chemical-specific data. DECISION: An exemption from the critical areas regulations is approved. DATE OF DECISION ON LAND USE ACTION: SIGNATURE: _______________________________________ ____________________________ Vanessa Dolbee, Planning Director Date The administrative land use decision will become final if the decision is not appealed within 14 days of the decision date. APPEALS: This administrative land use decision will become final if not appealed in writing to the Hearing Examiner on or before 5:00 PM on March 8, 2021. An appeal of the decision must be filed within the 14-day appeal period (RCW 43.21.C.075(3); WAC 197-11-680). Due to Governor Jay Inslee’s Proclamation 20-25 (“Stay Home, Stay Healthy”), the City Clerk’s Office is working remotely. For that reason, appeals must be submitted electronically to the City Clerk at cityclerk@rentonwa.gov. The appeal fee, normally due at the time an appeal is submitted, will be collected at a future date. Appeals to the Hearing Examiner are governed by RMC 4 -8-110 and additional information regarding the appeal process may be obtained from the City Clerk’s Office, cityclerk@rentonwa.gov. If the situation changes such that the City Clerk’s Office is open when you file your appeal, you have the option of filing the app eal in person. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 2/22/2021 | 2:33 PM PST City of Renton Department of Community & Economic Development Certificate of Exemption from Critical Areas Regulations Children’s Institute Hazardous Tree Removal Critical Areas Exemption LUA21-000058, CAR DATE OF PERMIT: February 22, 2021 Page 4 of 4 D_Children’s Institute Hazardous Tree Removal Critical Areas Exemption_210222 RECONSIDERATION: Within 14 days of the decision date, any party may request that the decision be reopened by the approval body. The approval body may modify his decision if material evidence not readily discoverable prior to the original decision is found or if he finds there was misrepresentation of fact. After review of the reconsideration request, if the approval body finds sufficient evidence to amend the original decision, there will be no further extension of the appeal period. Any person wishing to take further action must file a formal appeal within the 14-day appeal time frame. EXPIRATION: Five (5) years from the date of decision (date signed). Attachments: A) Site Plan, B) Arborist Report and C) Geotechnical Report DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 ATTACHMENT ADocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 1 Arboreal Report Date: January 5th, 2021 To: Carrie Fannin, Executive Director Children’s Institute for Learning Differences (CHILD) From: Chris Rippey, Rippey Arboriculture LLC Subject: Result of a tree risk survey Introduction This report is the summary of an October 9th, 2020 tree inventory at 2640 Benson Road South in Renton, Washington. This survey was performed to address a City of Renton requirement that a tree risk survey must be performed every two years. My scope of work was to: • Inspect all trees at Children’s Institute for Learning Differences at 2640 Benson Road South in Renton, Washington. • Using a two-year timeframe, identify any tree or tree part that is likely to fail and impact the zipline area, wooded trail, parking lot, playground, driveway, school buildings or school walkways. • Attach an identification tag to each identified tree that meets the before mentioned categorization. • Offer care recommendations for each identified tree. Limitations and Assumptions The site and tree assessments were performed on the subject property only. All tree assessments were performed from the ground only, no aerial or below soil level investigation techniques were used. No advanced assessment tools including resistance drilling, sonic tomography imaging or ground penetrating radar were used. Any information provided was assumed to be true. All trees will eventually fail. Tree failures are the result of a combination of factors. Predicting precisely when or how trees will fail is not possible. Unless fully removed, all trees pose some amount of risk to humans. My assessments and care recommendations do not consider targets or the occupancy rates of targets which the assessed trees could damage. These recommendations do not consider the risk tolerances of the tree owner. Tree assessments and recommendations are not meant to be relied upon as fact or promises of a result. These assessments and care recommendations are only considerations for the tree owner’s decision-making process. The tree owner, and not Rippey Arboriculture LLC is responsible and potentially liable for the assessed tree and damages that it may cause. Site Assessment The subject property is a 4.01-acre parcel on the southern tip of east side of Renton. This parcel on a western facing slope. The southern half of the property is developed with the school, a playground, and parking lot. The northern half of the property is relatively undeveloped with a zipline and a trail through a wooded area. Tree Assessment The trees on this site are a mixture of Pacific Northwest native trees. The dominant tree on the site is big leaf maple (Acer macrophyllum). The next most observed tree was black cottonwood (Populus trichocarpa), followed by red alder (Alnus rubra). Most of the trees on the property were declining in health or dead. There are few trees in good health. Most trees of this site declining in health or being dead is due to several factors including the trees having limited access to water due their location on slopes and the presence of aggressive understory plants like grass, blackberry and ivy taking up much of the rainwater. There are also several possible biotic disorders affecting the trees on the site. These include the parasitic fungal infections of Kretzchmaria duesta, Ganoderma sp., and Cryptostroma corticale. The identifications of these organisms are based on my ATTACHMENT B DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 2 22-year experience in identifying wood decay causing fungus. The presence of both organisms is concerning but is not unexpected. These organisms are common to our area and well known to cause death or tree failure in infected trees. Both organisms are untreatable but infected trees should be removed as soon as possible to reduce fungal spore presences and slow the spread of these organisms throughout the site. The timing of removing sooty bark disease infected trees is particularly important because the spores produced by the organism by the organism can infect humans with respiratory infections. To limit this hazard to users of the area, trees should be removed in winter during wet weather but not cold dry weather as this is when spores are most active. December is usually the best time to schedule work of trees infected with this disease. Site Assessment Results My site assessment has identified 24 trees that are likely fail within the next two years and impact the specified areas. Nine of these trees (1808, 1856, 1853, 1814, 1684, 1776, 1775, 1774 and 1755) are along the nature trail used by children of the school. These trees are all dead and can impact the trail and strike trail users when they fall. These trees should all be removed, or if safe to climb, reduced to wildlife habitat snags of a height no taller that the distance of the tree to the trail. Tree 1687 is in the natural area and is leaning towards the adjacent property. This tree needs to be removed because it is extensively decayed and will impact the adjacent property and apartment homes when it fails. There are six trees (1888, 1872, 1868, 1869, 1807, 1906, and 1889) located near a zipline area that is frequently used by children at the school. Three of these trees will be removed because they are dead and when they fail, they will likely impact the zipline area and could impact any users of that area. All other trees have large dead branches over hanging the area that must be removed before they fall on the zipline or the children using the zipline area. Tree 2495 is located on the school grounds. This tree is extensively decayed and is adjacent to a school walkway. When it fails, this tree will fail onto this frequently used walkway and can injure anyone using the walkway. All other trees (2022, 2615, 1978, 1977, 1976. 1926 and 1994) are dead or severely declining in health. These trees are all tree species that will start to shed dead branches soon after dying. These trees should all be removed because they are all adjacent to the parking lot and these dead branches or whole dead trees will impact the parking lot, users or the parking lot or cars in the parking lot when they fail. Specific recommendations for each identified tree can be found in the following Tree Inventory section of this report. Christopher Rippey Rippey Arboriculture, LLC PNW-ISA 2019, Arborist of the Year ASCA Registered Consulting Arborist #633 ASCA Tree and Plant Appraisal Qualified ISA Certified Arborist (WE-7672AUTM) ISA Tree Risk Assessor Qualified DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 3 Tree Inventory Tree ID Tree Species DBH Height Condition Present Decay Organism Work Notes and target 1888 Big leaf Maple (Acer macrophyllum) 68 80 Declining Kretzchmaria duesta (aggressive root decay organism) Pruning: remove upper portion of eastmost trunk. 5 trunks. Dead trunk will impact zipline when it fails 1872 Big leaf Maple (Acer macrophyllum) 17 80 Declining Kretzchmaria duesta(aggressive root and trunk decay organism) Remove Decayed at base. Will impact zipline when it fails. 1869 Big leaf Maple (Acer macrophyllum) 36 85 Declining Kretzchmaria duesta(aggressive root and trunk decay organism) Pruning: remove dead branch over zipline. 2 large parent branches at 5’. Dead branch will impact zipline when it fails. 1807 Big leaf Maple (Acer macrophyllum) 35 50 Dead Remove Tree will fail onto zipline area when it fails. 1868 Big leaf Maple (Acer macrophyllum) 6 20 Dead Remove Tree will fail onto zipline area when it fails. 1906 Big leaf Maple (Acer macrophyllum) 14 50 Dead Remove Tree will fail onto zipline area when it fails. 1889 Big leaf Maple (Acer macrophyllum) 126 80 Declining Sooty bark disease (can cause respiratory infection in humans) and Kretzchmaria duesta (aggressive root and trunk decay organism), sooty bark disease (can cause respiratory Pruning: remove 3 trunks, remove any dead wood over 4” diameter Dead tree branches will impact zipline area or driveway when they fail. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 4 1978 Red alder (Alnus rubra) 12 25 Dead Remove Dead snag. Will impact driveway when it fails. 1977 Black cottonwood (Populus trichocarpa) 31 100 Declining Remove Upper canopy is dead. Will impact parking lot when dead trunks or branches fail. 1976 Black cottonwood (Populus trichocarpa) 32 100 Declining Remove Upper canopy is dead. Will impact parking lot when dead trunks or branches fail 2615 Big leaf Maple (Acer macrophyllum) 20 45 Dead Remove Will impact parking lot when tree fails. 2495 Big leaf Maple (Acer macrophyllum) 12 30 Declining Remove Decay at parent branch connection. Will school walkway when tree fails. 2022 Big leaf Maple (Acer macrophyllum) 50 50 Dead Kretzchmaria duesta (aggressive root and trunk decay organism) Remove 7 trunks. Will impact parling lot when tree fails. 1926 Black cottonwood (Populus trichocarpa) 75 100 Dead Three trunks Remove Two trunks at grade. Will impact parling lot when tree fails. 1994 Big leaf Maple (Acer macrophyllum) 10 35 Dead Remove Will impact parling lot when tree fails. 1687 Big leaf Maple (Acer macrophyllum) 13 50 Declining Kretzchmaria duesta, Ganoderma sp. (aggressive root and trunk decay organisms) Remove or reduce to wildlife snag, if safe to climb Previous failure. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 5 1808 Red alder (Alnus rubra) 20 45 Dead Remove or reduce to wildlife snag, if safe to climb 2 trunks. Will impact parling lot when tree fails. 1856 Red alder (Alnus rubra) 14 40 Dead Remove or reduce to wildlife snag, if safe to climb Will impact nature trail frequented by school children when tree fails 1853 Red alder (Alnus rubra) 14 40 Dead Remove or reduce to wildlife snag, if safe to climb Will impact nature trail frequented by school children when tree fails 1814 Red alder (Alnus rubra) 10 40 Dead Remove or reduce to wildlife snag, if safe to climb Will impact nature trail frequented by school children when tree fails 1684 Big leaf Maple (Acer macrophyllum) 26 40 Dead Pruning: remove dead 10” trunk Will impact nature trail frequented by school children when tree fails 1776 Big leaf Maple (Acer macrophyllum) 55 60 Dead Sooty bark disease (can cause respiratory infection in humans) Remove or reduce to wildlife snag, if safe to climb 5 trunks. Will impact nature trail frequented by school children when tree fails 1775 Big leaf Maple (Acer macrophyllum) 10 40 Dead Remove or reduce to wildlife snag, if safe to climb Will impact nature trail frequented by school children when tree fails 1774 Big leaf Maple (Acer macrophyllum) 12 40 Dead Remove or reduce to wildlife snag, if safe to climb Will impact nature trail frequented by school children when tree fails 1755 Big leaf Maple (Acer macrophyllum) 24 40 Declining Kretzchmaria duesta (aggressive root and trunk decay organism) Remove or reduce to wildlife snag, if safe to climb Two trunks. Very decayed. Will impact nature trail DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 6 Exhibit Map The subject property. The approximate property line is indicated with red lines. The approximate locations of trees are desig nated with red X’s for those to be removed and green leaves for those to be pruned. Photo from King County Parcel Viewer, 2019. North is up. 2615 DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 7 Developed southern half of the property 2615 DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 8 Undeveloped northern half of the property. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 9 Photographic Evidence Tree 1888 with the to be removed dead branch encircled in red. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 10 Tree 1869 with the to be remove dead branch over the zipline identified by red arrows. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 11 Tree 1889 with the to be removed eastmost trunk and its hollow cavity. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 12 Tree 1889 with the to be removed dead trunk. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 13 . Tree 1889: Two westernmost to be removed trunks are identified by red X’s. Dead canopy of those trunks is encircled in red. Red arrows indicate to be removed dead branches. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 14 Tree 2495 with the decay at that connection of the two parent branches encircled in red. Decay on easternmost parent branch is identified with a red arrow. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 R i p p e y A r b o r i c u l t u r e L L C C H I L D T r e e I n v e n t o r y J a n u a r y 5 th, 2 0 2 1 P a g e | 15 Tree 2022 is dead and will impact the parking lot when it fails. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 19019 36th Avenue West, Suite E Lynnwood, WA 98036 (425) 582-9928 GEOTECHNICAL EVALUATION OF 2021 HAZARD TREE REMOVAL PLAN Children’s Institute for Learning Differences 2640 Benson Road South Renton, Washington Project No. 1601.01 February 2, 2021 Prepared for: Children’s Institute for Learning Differences Prepared by: SITE ATTACHMENT CDocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 19019 36th Avenue West, Suite E Lynnwood, WA 98036 (425) 582-9928 Project Number 1601.01 February 2, 2021 Children’s Institute for Learning Differences 2640 Benson Road South Renton, Washington 98055 Attention: Ms. Carrie Fannin Subject: Geotechnical Evaluation of 2021 Hazard Tree Removal Plan Children’s Institute for Learning Differences 2640 Benson Road South Renton, Washington 98055 Dear Ms. Fannin, In accordance with your request and written authorization, Zipper Geo Associates, LLC (ZGA) has completed the geotechnical evaluation of the 2021 Hazard Tree Removal Plan prepared by others for the Children’s Institute for Learning Differences (CHILD) campus located at 2640 Benson Road South in Renton, 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 Proposal for Geotechnical Engineering Services (Proposal No. P16151A) dated January 6, 2021. 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 2/2/21 2/2/21 James P. Georgis, L.E.G. Thomas A. Jones, P.E. Principal Senior Consultant Copies: Addressee (1) Ray Robinson with Integrated Site Design, Inc. (1) DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 TABLE OF CONTENTS INTRODUCTION ....................................................................................................................... 1 SITE DESCRIPTION.................................................................................................................. 1 PROJECT UNDERSTANDING .................................................................................................. 1 SITE RECONNAISSANCE......................................................................................................... 2 Southeast Tree Removal Area ............................................................................................... 2 Northeast Tree Removal Area ................................................................................................ 3 Northwest Tree Removal Area ............................................................................................... 3 Central Playground Slope and Main Building Tree Removal Area .......................................... 4 SUBSURFACE CONDITIONS ................................................................................................... 5 Published Geologic Mapping .................................................................................................. 5 Soil Conditions ....................................................................................................................... 5 Groundwater Conditions ......................................................................................................... 6 Historic Coal Mine Maps ......................................................................................................... 6 CONCLUSIONS AND RECOMMENDATIONS ........................................................................... 7 General .................................................................................................................................. 7 Geologic Hazard ECA Considerations .................................................................................... 7 Tree Stump and Root System Retention ................................................................................ 8 Replacement Plantings ........................................................................................................... 9 Temporary Erosion Control Considerations ...........................................................................10 Retention of Woody Material .................................................................................................10 General Steep Slope Considerations .....................................................................................10 CLOSURE ................................................................................................................................10 FIGURES PREPARED BY ZGA Figure 1 – Vicinity Map Figure 2 – Site and Exploration Plan FIGURES PREPARED BY OTHERS Tree Inventory Plan, Sheet L1.1 APPENDICES Appendix A – Subsurface Exploration Procedures and Logs DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Page 1 GEOTECHNICAL EVALUATION OF 2021 HAZARD TREE REMOVAL PLAN CHILDREN’S INSTITUTE FOR LEARNING DIFFERENCES 2640 BENSON ROAD SOUTH RENTON, WASHINGTON Project No. 1601.01 February 2, 2021 INTRODUCTION This report documents the surface and subsurface conditions encountered at the site and our geotechnical evaluation of the 2021 Hazard Tree Removal Plan prepared by Integrated Site Design for the Children’s Institute for Learning Differences (CHILD) campus located at 2640 Benson Road South in Renton, 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. SITE DESCRIPTION The site is located at 2640 Benson Road South in Renton, Washington. The CHILD facility is located in the south-central portion of the site and includes campus buildings and a perimeter access drive with a parking lot on the east side of the facility. The northern portion of the site is undeveloped and includes mixed evergreen and deciduous trees with a moderately well- developed understory of brush and groundcover. A zipline and pedestrian trails are located in the undeveloped, wooded area. The site is bounded to the north by a multi-family residential development, to the south by South 27th Street, to the east Benson Road South, and to the west by a middle school. The approximate site location is shown on the enclosed Vicinity Map, Figure 1. Existing site features, site topography, and nearby features are shown on the enclosed Site and Exploration Plan, Figure 2. PROJECT UNDERSTANDING We understand that the City of Renton requires the completion of periodic tree risk surveys of the subject site. ZGA completed a geotechnical evaluation of the Hazard Tree Removal Plan for the site prepared in 2016. Our findings and recommendations were presented in our Geotechnical Evaluation of Hazard Tree Removal Plan, dated March 25, 2016. This 2021 geotechnical report includes some site information collected in 2016 but has been prepared as an independent document and reference to our 2016 report is not required. The site is mapped by the City of Renton as containing Coal Mine Hazard, Steep Slope Hazard, and Landslide Hazard Environmental Critical Areas (ECAs). We understand the City has requested that a geotechnical evaluation of the 2021 Hazard Tree Removal Plan be completed to assess the risk to regulated Geologic Hazard ECAs and their associated buffers, as defined in Part 4-3-050-G-5 of the City of Renton Municipal Code (RMC) and provide recommendations to mitigate identified risks. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 2 Our understanding of the project is primarily based on a review of the following documents and discussions with the project landscape architect Integrated Site Design. • Arboreal Report, Children’s Institute for Learning Differences (CHILD), prepared by Rippey Arboriculture, LLC, dated January 5, 2021. • Children’s Institute, 2640 Benson Road South, Renton WA, Sheet L1.1, prepared by Integrated Site Design, dated February 2, 2021. Based on our review of the referenced documents, it appears that the 2021 Hazard Tree Removal Plan includes pruning 4 trees and removal of 21 trees located within the CHILD property. No off- site work is planned. The Arboreal Report indicates that 9 of the identified trees are declining in health and 16 of the trees are dead due to limited access to water and/or fungal infection. The Arboreal Report further indicates that the identified trees or tree parts are likely to fail within a two- year time frame and could impact the zipline area, wooded trail, parking lot, playground, driveway, school building, or school walkways. About 10 of the trees identified for removal are located within Steep Slope and Landslide Hazard ECAs or their buffers as shown on Plan Sheet L1.1 A Copy of the Tree Inventory Plan, (Plan Sheet L1.1) is enclosed with this report without modification to assist in report interpretation. SITE RECONNAISSANCE We completed a reconnaissance of the property during a site visit completed on January 19, 2021. The purpose of the reconnaissance was to identify surficial features or site conditions such as the presence or absence of groundwater seepage, indications of surface erosion, indications of past slope instability, indications of past land use, the type and extent of existing vegetation, the location of trees identified for pruning or removal, and other site features that may be affected by the planned tree pruning or removal. The enclosed Site and Exploration Plan, Figure 2, shows the approximate location of some of the observed site features. A summary of our site observations is presented below. Southeast Tree Removal Area Three trees are listed for removal in this area. The trees are located on a west-facing slope that extends down to a relatively new rockery wall constructed just within the property line. Based on the topographic site plan, the slope inclination above the rockery ranges from about 23 to 44 percent (13 to 23 degrees). The rockery appears to be in good condition, ranges from about 4 to 6 feet tall, includes a drainage zone of quarry spall backfill, and appears to include a perforated drainpipe. In addition to the listed trees, slope vegetation includes dense Laurel bushes, blackberry, fern, ivy, blue star creeper, and grasses. We did not observe any obvious indications of past slope movement or distress. Surficial erosion was slight and limited to several foot paths. We did not observe any obvious indications of groundwater seepage from the exposed slope surface. However, the rockery drainpipe daylights at the north end of the wall and we observed about ½ gpm discharge from this pipe at the time of our site visit. The discharge flowed down a roadside ditch to a dispersion area in the northwest portion of the site. The approximate rockery DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 3 groundwater discharge location and subsequent down-gradient surface water flow is shown on Figure 2. Northeast Tree Removal Area For the purposes of this report, the northeast tree removal area includes the central steep slope area located in the northern portion of the site and the gently sloping bench located between the central steep slope area and the east property line. Eight trees are listed for removal and four trees are listed for pruning in this area. The north-central, west-facing slope has inclinations ranging from about 23 to 44 percent (13 to 23 degrees). In addition to the trees listed for removal and pruning, slope vegetation includes mixed evergreen and deciduous trees which were considered by the arborist to be of low risk at this time and are not listed for removal, blackberry, fern, ivy, and grasses. Areas between vegetation were typically mantled by several inches of forest duff. We did not observe any obvious indications of past slope movement or distress. Surficial erosion was slight and limited to several foot paths. We did not observe any obvious indications of groundwater seepage from the exposed slope surface. We observed what appears to be a lobe of fill soils extending into the slope area from the eastern bench area. The fill appeared to be relatively old and may have been associated with mass grading for the CHILD development. The approximate location of the fill is shown on Figure 2. The bench area located between the north-central steep slope and the east property line slopes gently west with inclinations ranging from about 5 to 12 percent (3 to 8 degrees). We estimate that about 60 to 70 percent on the bench is surfaced with old wood chips. The remainder of the area consists of soil with scattered ferns and grass. We did not observe any obvious indications of past slope movement or distress associated with the bench or adjacent slopes. Surficial erosion was slight and limited to several foot paths. We observed slight surface water flow in the northern portion of the bench area. The flow appeared to be originating from a spring located near the top of the steep off-site slope which extends up to the adjacent middle school play field. We visually estimated a flow rate on the order of ¼ gallon per minute at the time of our evaluation. The spring had incised a small drainage feature about two feet wide and one foot deep into the surface of the off-site slope. Surface water from the spring was infiltrating into the bench soils at the time of our evaluation and was not running down the north-central steep slope. The approximate location of the spring and down-gradient surface water flow is shown on Figure 2 Northwest Tree Removal Area For the purposes of this report, the northwest tree removal area includes the north-central gently sloping bench area located in the northern portion of the site and the steep slope area located between the north-central gently sloping bench area and the west property line. Five trees are listed for removal in this area. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 4 The north-central bench area slopes gently west with inclinations ranging from about 10 to 17 percent (6 to 10 degrees). In addition to the trees listed for removal, vegetation includes mixed evergreen and deciduous trees which were considered by the arborist to be of low risk at this time and are not listed for removal, blackberry, fern, ivy, and grasses. Areas between vegetation were typically mantled by several inches of forest duff. We did not observe any obvious indications of past slope movement or distress. Surficial erosion was slight and limited to several foot paths. We did not observe any obvious indications of groundwater seepage. The west-facing slope located between the north-central bench and the west property line has typical inclinations ranging from about 30 to 56 percent (17 to 30 degrees). An isolated section of slope adjacent to the sidewalk has an inclination of about 100 percent (45 degrees) with a maximum slope height of about 6 feet, and appears to be the result of a cut completed to construct the sidewalk. Slope vegetation includes mixed evergreen and deciduous trees which were considered by the arborist to be of low risk and are not listed for removal, blackberry, fern, ivy, and grasses. Areas between vegetation were typically mantled by several inches of forest duff. We did not observe any obvious indications of past slope movement or distress. Surficial erosion was slight and limited to several foot paths. We did not observe any obvious indications of groundwater seepage, but standing water was present in two drainage ditches located near the west property line at the time of our evaluation. Central Playground Slope and Main Building Tree Removal Area A steep west-facing slope is located near the middle of the site between the playground and the main parking lot. Portions of the slope extend south to the north side of the main CHILD facility building. Slope inclinations range from about 42 to 80 percent (23 to 39 degrees). Five trees are listed for removal in this area. Slope vegetation east of the play area primarily consists of ivy and blackberry brush, with scattered patches of grass in the southern portions of the slope and several deciduous trees. In general, the trees located on the slope exhibited pistol-butted growth pattern (bent trunks near the ground surface) which is indicative of slope creep. We did not observe indications of deep- seated slope movement or shallow sloughing. Surficial erosion was slight and limited to non- vegetated portions of the slope. We did not observe any obvious indications of groundwater seepage. That portion of the slope located immediately north of the main building is primarily vegetated with a maintained lawn and two trees located about two feet apart. The northern tree is listed for removal. We did not observe indications of deep-seated slope movement or shallow sloughing. We did not observe indications of surficial erosion or any obvious indications of groundwater seepage. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 5 SUBSURFACE CONDITIONS Published Geologic Mapping We assessed the geologic setting of the site and surrounding vicinity by reviewing The Geologic Map of the Renton Quadrangle, King County, Washington, U.S. Geological Survey Map GQ-405, 1965. The geologic map indicates the site and immediate vicinity are mantled by Quaternary Ground Moraine deposits (Qgt). This deposit is described as thin glacial ablation till over lodgment till deposited by the Vashon Stade of the Frasier glaciation. The lodgment till is further described as a compact, unsorted mixture of sand, silt, clay, and gravel, commonly termed “hardpan”. The ablation till is described as being compositionally similar to lodgment till, but much less dense. Tertiary age Renton Formation deposits are mapped about ½ mile to the northeast of the site. The Renton formation is described as arkosic sandstone, mudstone, and shale with several beds of coal. The Renton Formation has an extensive history of surface and subsurface coal mining. Stratigraphic information presented in the referenced geologic map indicates that Renton Formation deposits underlie the surficial glacial deposits at an unspecified depth. Soil Conditions ZGA completed a subsurface exploration of the site as part of our geotechnical evaluation of the 2016 hazard tree removal plan for the subject site. Based on our 2021 reconnaissance observations, it is our opinion that the subsurface information collected by ZGA in 2016 is adequate and suitable for use in the current 2021 geotechnical evaluation. A summary of the 2016 subsurface exploration program is presented below. The 2016 subsurface exploration for the site included four hand auger borings (HA-1 through HA- 4) located near proposed tree removal areas and steep site slopes. The hand auger borings extended approximately 3½ to 6 feet below the existing ground surface. The approximate exploration locations are shown on the enclosed Site and Exploration Plan, Figure 2. 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 augers HA-1 and HA-2 were completed in the northern portion of the site. In general, the explorations encountered about 7 to 10 inches of loose, organic-rich silty sand interpreted as topsoil. Below the topsoil, loose to medium dense silty sand with some gravel was encountered to a depth DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 6 of about 2½ to 3 feet and was interpreted as weathered glacial till. Dense, silty sand with some gravel, interpreted as unweathered glacial till, was encountered below the weathered horizon and extended to the total depth explored of 3½ feet below existing grade. Hand augers HA-3 and HA-4 were completed in the west-central and central portions of the site, respectively. These explorations encountered about 12 inches of loose organic-rich silty sand topsoil over primarily granular deposits interpreted as glacial outwash. In hand auger HA-3, the outwash consisted of medium dense sand with some gravel and silt which extended to the total depth explored of 3½ feet below existing grade. In hand auger HA-4, the outwash consisted of medium dense fine to medium sand with some silt which extended to the total depth explored of 6 feet below existing grade. Groundwater Conditions Slight groundwater seepage was observed in hand auger HA-2. Groundwater seepage was not observed in the other hand auger borings. The groundwater observed in HA-2 was interpreted to represent water perched above lower permeability glacial till soils encountered at a depth of 3 feet. Given the relatively low permeability of the glacial till soils encountered in the northern portion of the site, perched groundwater may develop above the till or within sandy zones within the till during periods of extended wet weather. Perched groundwater may also develop above lower permeability layers within the glacial outwash deposits, which are often stratified. These observations represent groundwater conditions at the time of the subsurface 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. Historic Coal Mine Maps As part of our site evaluation, we completed a review of historic coal mine maps available from the Washington State Department of Natural Resources. Based on our review, it appears that the site is underlain by portions of the Sunbeam Mine and Renton Mine. A summary of our historic coal mine map review relative to these mines is presented below. Sunbeam Mine: The Sunbeam Mine was operated by the Renton Coal Company from 1922 to 1929. The Sunbeam portal (or mine entrance) is mapped about 2/3 of a mile north-northwest of the CHILD site and has a reported main “water level” gangway elevation of about 68 feet. The mine chutes appear to have extended up and to the south-southwest of the main gangway (away from the CHILD site) along a coal seam inclined or dipping to the north-northeast at an angle of about 65 degrees from horizontal. Based on the mapped data, it appears that the Sunbeam mine workings are primarily west of the CHILD site, with the eastern end of the mine underlying a portion of Benson Road South and the southwestern portion of the CHILD sit e. Given a site ground surface elevation of about 400 feet, it appears that those portions of the mine workings located below or near the CHILD site are on the order of 200 to 300 feet below the ground surface. The City of Renton has mapped the mine workings west-southwest of the site (west of Benson DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 7 Road South) as having a moderate to high risk, and the workings mapped below the CHILD site as having a moderate risk. Renton Mine: The Renton Mine was operated by the Seattle Electric Company in 1919 and the Renton Coal Company from 1920 to 1955. It appears that the Renton Mine 6th Gangway South and associated workings extended below the CHILD site. The historic maps do not include explicit information regarding the elevation of the workings in this area. However, the workings appear to be an extension of the generally north-northeast dipping coal seam worked in the Sunbeam Mine. Based on the elevation of the Sunbeam main water level gangway and the indicated strata dip angle and orientation, it appears that the Renton Mine workings in the vicinity of the site are likely greater than 300 feet below the ground surface. The City of Renton has mapped the Renton Mine workings at the project site and nearby vicinity as having a moderate risk. CONCLUSIONS AND RECOMMENDATIONS General Based on the results of our site reconnaissance, document review, subsurface exploration, and geotechnical analysis, it is our opinion that the proposed tree removal plan is feasible from a geotechnical perspective. Provided that the conditions and recommendations contained within this report and the provisions of the Renton Municipal Code (RMC) are satisfied, it is our opinion that the proposed 2021 Hazard Tree Removal Plan 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 Renton regulated ECAs and recommendations to reduce risks associated with the tree removal plan. Geologic Hazard ECA Considerations The site is mapped by the City of Renton as containing Coal Mine Hazard, Steep Slope Hazard, and Landslide Hazard Environmental Critical Areas (ECAs). We understand that the City has requested a geotechnical evaluation of the 2021 Hazard Tree Removal Plan to assess the risk to regulated Geologic Hazard ECAs and their associated buffers, as defined in Part 4-3-050-G-5 of the City of Renton Municipal Code (RMC) and provide recommendations to mitigate identified risks. Part 4-3-050-C-3 of the RMC indicates that the removal of dangerous trees which have been approved by the City and certified dangerous by a licensed landscape architect or certified arborist is exempt from RMC ECA restrictions and may be permitted by the City. The following sections present our evaluation of Coal Mine Hazard, Steep Slope Hazard, and Landslide Hazard ECA’s relative to the 2021 Hazard Tree Removal Plan. Coal Mine Hazard Areas: Coal Mine Hazard Areas include areas with known subsurface coal mine workings. The site is mapped within a Medium Coal Mine Hazard Risk Area and slightly northeast of a High Coal Mine Hazard Risk Area. Our review of historic coal mine maps for the site vicinity DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 8 (Renton Mine and Sunbeam Mine) indicate mine workings depths in excess of 200 feet. Based on our document review and site observations, it is our opinion that the site is consistent with the criteria for a Medium Coal Mine Hazard Risk Area. Steep Slope Hazard Areas: The City of Renton maps portions of the site as Steep Slope Hazard Areas. The City mapped hazard areas are primarily located along the eastern property line, within the northern, undeveloped, wooded portion of the site, and the slope area between the main school building and the playground area. The enclosed Tree Inventory Plan, Sheet L1.1 identifies Steep Slope Hazard Areas and an associated 15 foot wide buffer based on site topography. Based on our document review and site reconnaissance, it is our opinion that the critical areas and associated buffers presented on Sheet L1.1 meet the definition of RMC Steep Slope Hazard Area criteria. Landslide Hazard Areas: The City of Renton ECA maps generally identify the central and western portions of the northern, undeveloped, wooded area and the slope between the playground and the school facility as High Landslide Hazard Areas, although the City mapped hazard areas to not appear to correlate well to the site specific topography presented on Sheet L1.1. This may be due to the City’s use of a more regional topographic database to identify hazard area. The RMC defines a high landslide hazard area as an area with slopes greater than 40 percent, and areas with slopes between fifteen and forty percent and underlain by soils consisting largely of silt and clay. Based on our document review and subsurface explorations, it is our opinion that site slopes exceeding forty percent are consistent with the RMC criteria for Landslide Hazard Areas. These areas are generally identified on the Tree Inventory Plan, Sheet L1.1. Risk Evaluation Summary: Based on mapped mine working depths in excess of 200 feet below existing grade, it is our opinion that the 2021 Hazard Tree Removal Plan should have no measurable adverse impact on the Coal Mine Hazard and should not increase the risk of coal mine induced damage to the subject site or adjacent properties. Relative to Steep Slope and Landslide Hazard Areas, it is our opinion that the proposed 2021 Hazard Tree Removal Plan should 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 provided that the conditions and recommendations contained within this report and the provisions of the Renton Municipal Code (RMC) are satisfied. Tree Stump and Root System Retention 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. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 9 Based on our review of the Arboreal Report, it appears that the trees listed for removal consist of black cottonwood, big leaf maple, and red alder. Under healthy forest conditions, these tree species often survive cutting by generating new growth from the stump, often referred to as stump sprouting. Stump sprouts provide nourishment for maintenance of the root system while greatly reducing the negative impact of tree weight and wind driven soil loosening. Unfortunately, most of the trees listed for removal at the subject site are dead or declining due to limited access to water and/or fungal infection and the potential for reliable stump sprouting is considered low and decay of the tree root systems over time is likely. In our experience, it typically takes about 3 to 5 years to realize a significant reduction in the bio- reinforcement provided by a tree that dies naturally or does not survive cutting. As such, we recommend that the trees listed for removal within Steep Slope ECAs and their buffers be cut off near the ground surface or reduced to wildlife snags if safe to climb and that the root systems be left in place to provide ongoing erosion protection and reinforcement during the decay period. We anticipate that there may be some benefit to potential utilization of the site by removing stumps from the gently sloping bench areas located in the northern portion of the site. In our opinion, the removal of stumps within the bench areas is feasible from a geotechnical perspective without adversely affecting the site slopes. A 12-inch diameter big leaf maple (tree #2495) is listed for removal from a grass lawn area just north of the main building. This tree is located within 2 feet of a larger tree that will remain. In our opinion, the stump of tree #2495 may also be removed to facilitate lawn maintenance without adversely affecting the slope provided the stump can be removed in a manner that will not adversely affect the root system of the nearby tree to remain. Replacement Plantings As previously discussed, the trees listed for removal are dead or declining due to limited access to water and/or fungal infection and the potential for reliable stump sprouting is considered low and decay of the tree root systems over time is likely. In our experience, it typically takes about 3 to 5 years to realize a significant reduction in the bio-reinforcement provided by a tree that dies naturally or does not survive cutting. We therefore recommend that the 2021 Hazard Tree Removal Plan include replacement plantings near trees located within Landslide Hazard ECAs. We recommend that native woody shrubs and 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. New plantings should also be resistant to the fungal types adversely affecting the site. The Tree Replacement Summary shown of the Tree Inventory Plan, Sheet L1.1 appears consistent with our recommendations and is adequate for the site conditions, in our opinion. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 Children’s Institute for Learning Differences Project No. 1601.01 February 2, 2021 Page 10 Temporary Erosion Control Considerations We anticipate that the tree removal process may 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 any soils exposed within Steep Slope ECAs and their buffers by the tree removal process be covered with a temporary erosion control material, such as sterile straw or arborist mulch. Retention of Woody Material In our opinion, it appears geotechnically feasible to retain much of the woody debris generated from the hazard tree removal as habitat enhancement within wooded portions of the site. However, based on our evaluation, we recommend that woody material greater than 6-inches in diameter be removed from the following areas as the weight of the material could adversely affect steep slopes or the performance of existing drainage features. • The drainage features near Benson Road South and adjacent steep slopes identified on Figure 2. • The steep slope located between the playground and the school facility and the 15-foot wide buffer above this steep slope area. 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 process, and that some soil loss from steep slopes should be expected over time, particularly in response to episodic severe storm events. The conclusions and recommendations presented in this report are intended to allow for the removal of hazard trees 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 by ZGA at the site. 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 The Children’s Institute for Learning Differences, Integrated Site Design, 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. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 B E N S O N R O A D S O U T H S 26TH S T R E E T S 27TH STREET REFERENCE: GOOGLE EARTH 2016. NOT TO SCALE APPROXIMATE PROJECT AREA FIGURE Job No. Zipper Geo Associates, LLC 19019 36th Ave. W.,Suite E Lynnwood, WA SHT. of 11 VICINITY MAP 1601.01Date: February 2021 1 CHILDREN'S INSTITUTE FOR LEARNING DIFFERENCES 2640 Benson Road South Renton, Washington DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 GROUNDWATER SEEPAGENEAR FENCE LINESTANDING WATER INDRAINAGE DITCHSTANDING WATER INDRAINAGE DITCHGROUNDWATER DISCHARGEFROM ROCKERY DRAIN PIPEFILLHA-1HA-2HA-3HA-4FIGUREJob No.Zipper Geo Associates, LLC19019 36th Ave. W.,Suite ELynnwood, WASHT. of11SITE AND EXPLORATION PLAN1601.01Date: February 20212CHILDREN'S INSTITUTE FOR LEARNING DIFERENCES2640 Benson Road SouthRenton, WashingtonLEGENDHA-1HAND AUGER NUMBER ANDAPPROXIMATE LOCATIONAPPROXIMATE LOCATION OFGROUNDWATER SEEPAGEAPPROXIMATE LOCATION OFSURFACE WATER FLOWDocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 APPENDIX A SUBSURFACE EXPLORATION PROCEDURES AND LOGS DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 SUBSURFACE EXPLORATION PROCEDURES Subsurface Exploration Description Our subsurface exploration included four hand auger borings completed on March 16, 2016. The approximate boring locations are presented on the enclosed Site and Exploration Plan, Figure 2. Boring locations were determined in the field by measuring distances from existing site features with a fiberglass tape. Ground surface elevations at the explorations were interpolated from topographic lines presented on the enclosed Tree Inventory Plan. As such, the exploration locations and elevations 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. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 ZIPPER GEO ASSOCIATES, LLC 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 HAND AUGER HA-1 Location: See Site And Exploration Plan, Figure 2 Approximate Ground Elevation: 382 Feet Project: Children’s Institute Project No: 1601.01 Date Excavated: 03-16-2016 Depth (ft) Material Description Sample NC %M Testing Loose, moist to wet, dark brown, silty SAND, with roots and fine organic material (Topsoil) Loose to medium dense, moist grading to wet, mottled brown, silty SAND, some gravel, moderate iron oxide staining (Weathered Glacial Till) Dense, moist, gray to gray-brown, silty SAND, some gravel, slight iron oxide staining (Glacial Till) Hand Auger completed at 3.3 feet on 3/16/2016. No groundwater seepage observed at time of exploration. No caving observed. 1 2 S-1@1.5’ 3 4 5 6 7 8 9 10 11 12 Note: NC is the Dynamic Cone Penetrometer blow count per ASTM Special Publication #399. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 ZIPPER GEO ASSOCIATES, LLC 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 HAND AUGER HA-2 Location: See Site And Exploration Plan, Figure 2 Approximate Ground Elevation: 366 Feet Project: Children’s Institute Project No: 1601.01 Date Excavated: 03-16-2016 Depth (ft) Material Description Sample NC %M Testing Loose, moist to wet, dark brown, silty SAND, with roots and fine organic material (Topsoil) Loose to medium dense, moist to wet, mottled gray -brown, silty SAND, some gravel, moderate iron oxide staining (Weathered Glacial Till) Dense, moist, gray, silty SAND, some gravel (Glacial Till) Hand Auger completed at 3.5 feet on 3/16/2016. Very slight perched groundwater seepage observed at 3 feet at time of exploration. No caving observed. 1 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. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 ZIPPER GEO ASSOCIATES, LLC 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 HAND AUGER HA-3 Location: See Site And Exploration Plan, Figure 2 Approximate Ground Elevation: 374 Feet Project: Children’s Institute Project No: 1601.01 Date Excavated: 03-16-2016 Depth (ft) Material Description Sample NC %M Testing Loose, moist to wet, dark brown, silty SAND, with roots and fine organic material (Topsoil) Medium dense, moist, gray-brown SAND, some silt and gravel (Weathered Glacial Outwash) Hand Auger completed at 3.5 feet on 3/16/2016. No groundwater seepage observed at time of exploration. Slight caving observed below 12 inches. 1 2 S-1@1.5’ 3 4 5 6 7 8 9 10 11 12 Note: NC is the Dynamic Cone Penetrometer blow count per ASTM Special Publication #399. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760 ZIPPER GEO ASSOCIATES, LLC 19019 36th Avenue West, Suite E, Lynnwood, Washington 98036 HAND AUGER HA-4 Location: See Site And Exploration Plan, Figure 2 Approximate Ground Elevation: 398 Feet Project: Children’s Institute Project No: 1601.01 Date Excavated: 03-16-2016 Depth (ft) Material Description Sample NC %M Testing Loose, moist to wet, dark brown, silty SAND, with roots and fine organic material (Topsoil) Medium dense, moist, gray-brown, fine to medium SAND, some silt (Weathered Glacial Outwash) Hand Auger completed at 6 feet on 3/16/2016. No groundwater seepage observed at time of exploration. No caving observed. 1 2 S-1@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. DocuSign Envelope ID: 93E0402B-0AB8-4EC3-B01B-1DCE00FAB760