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HomeMy WebLinkAbout1601.01_Geotechnical Report_CHILD_02-02-202119019 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 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) 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 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. 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 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. 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. 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 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 site. 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 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 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. 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. 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. 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 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 FLOW APPENDIX A SUBSURFACE EXPLORATION PROCEDURES AND LOGS 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. 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. 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. 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. 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.