June 13, 2018 Project No E001. Issaquah School District 565 Northwest Holly Street Issaquah, Washington 98027

Transcription

1 June 13, 2018 Project No E001 Issaquah School District 565 Northwest Holly Street Issaquah, Washington Attention: Subject: Mr. Steve Crawford Subsurface Exploration and Landslide Hazard Evaluation Proposed Hilltop Elementary School NE Discovery Drive and 6 th Avenue NW Issaquah, Washington Dear Mr. Crawford: In accordance with your request, this letter presents the results of the landslide hazard evaluation recently completed for the subject project. This letter was prepared for the exclusive use of the Issaquah School District, and their authorized agents, for specific application to this project. No other warranty express or implied is made. SITE AND PROJECT DESCRIPTION The project site is located on the west side of 6 th Avenue NW, approximately 150 yards south of NE Discovery Drive in the Issaquah Highlands neighborhood of Issaquah (Figure 1). The site consists of a portion of a single, irregularly shaped parcel totaling approximately 20 acres (King County Parcel No ). The subject site is limited to a rectangular-shaped area that extends south from the northeast parcel corner a distance of approximately 900 feet, and west from the eastern site boundary a distance of approximately 335 feet. The topography across the majority of the site is relatively flat to moderately sloping, but slopes steeply downward to the west and southwest along its western and southwestern margins, respectively. Review of the Light Detection and Ranging (LIDAR) based topographic contours shown on Figure 2 indicates that slope inclinations in these areas generally range from approximately 40 to 65 percent over a total slope height ranging from approximately 160 to 275 feet. Areas of steep slope are also located along portions of the northern and eastern Kirkland Office 911 Fifth Avenue Kirkland, WA P F Everett Office 2911 ½ Hewitt Avenue, Suite 2 Everett, WA P F Tacoma Office 1552 Commerce Street, Suite 2 Tacoma, WA P F

2 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation property margins. Slope inclinations in these areas generally range from approximately 40 to 65 percent over heights of approximately to 25 feet. The majority of the site is naturally forested with mixed deciduous/coniferous trees and light to moderately thick underbrush. The forest within the subject site and upper portion of the adjoining steep slope to the west consists predominantly of coniferous trees. The forest on the lower portion of the steep slope west of the site consists predominantly of deciduous trees. A former King County Department of Transportation (DOT) maintenance yard is located in a relatively flat area beyond the toe of the steep slope in the western portion of the parcel. According to King County tax assessor records, the entire parcel is currently owned by the City of Issaquah. Our understanding of the project is based on review of a conceptual site layout provided by the School District. It is our understanding that project plans include the construction of a four-story school building in the northern portion of the site. Hard and soft-surface play areas will be located east and south of the building. A copy of the conceptual site layout is included in Appendix A. SUBSURFACE EXPLORATION Our field study included excavating 12 exploration pits and advancing three borings to gain subsurface information about the site. The various types of sediments, as well as the depths where characteristics of the sediments changed, are indicated on the exploration logs presented in Appendix B. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types in the field. Our explorations were approximately located in the field using a portable Global Positioning System (GPS) unit. The approximate locations of the explorations are shown on Figure 2. Exploration Pits The exploration pits were excavated using a track-mounted excavator. The pits permitted direct, visual observation of subsurface conditions. Materials encountered in the exploration pits were studied and classified in the field by an engineering geologist from our firm. All of the exploration pits were backfilled immediately after examination and logging. Samples collected from the exploration pits were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification and/or laboratory testing. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 2

3 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation Exploration Borings The exploration borings were completed using a track-mounted, sonic drilling rig. The exploration borings were continuously observed and logged by a geologist from our firm. The sonic drilling method produces a continuous core of the subsurface sediments by advancing a 7-inch outside-diameter, core barrel, and drilling inside of the 7-inch barrel with a 5-inch sample barrel. During the drilling process, the samples/cuttings are extracted so that a continuous lithologic sequence can be observed. Select portions of the sample are retained, typically where conditions change in the sequence. Samples were also generally obtained at depth intervals of approximately feet using the Standard Penetration Test (SPT) procedure in accordance with American Society for Testing and Materials (ASTM) D This test and sampling method consists of driving a standard, 2-inch outside-diameter, split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free-falling a distance of 30 inches. The number of blows for each 6-inch interval is recorded, and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance ( N ) or blow count. If a total of 50 is recorded within one 6-inch interval, the blow count is recorded as the number of blows for the corresponding number of inches of penetration. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are plotted on the exploration logs in Appendix B. SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site, and review of selected applicable geologic literature. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between and beyond the field explorations is necessary. It should be noted that subsurface conditions differing from those shown on the exploration logs in Appendix B may be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between and beyond the field explorations may not become fully evident until construction. Stratigraphy Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site, and review of applicable geologic literature. The natural sediments encountered in our explorations generally consisted of loose to medium dense, sand and gravel with variable silt content. These sediments were underlain at depth by dense to very dense, glacially consolidated sediments of variable textural composition. An area of fill soil was encountered near the northeastern portion of the site. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 3

4 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation The following section presents more detailed subsurface information organized from the shallowest (youngest) to the deepest (oldest) sediment types. Fill Fill soils, those not naturally placed, were encountered in the northeastern portion of the site at the location of exploration pit EP-1. The fill generally consisted of loose to medium dense, brown to gray, very gravelly, silty to very silty sand with scattered cobbles and trace quantities of plastic and woody debris. At the location of exploration pit EP-1, the fill extended beyond the maximum depth explored of approximately 16 feet. The existing fill is not suitable for foundation support. Forest Duff/Topsoil A surficial forest duff/topsoil horizon was encountered at all of the exploration locations with the exception of exploration pit EP-1. The forest duff/topsoil horizon was typically about 6-inches thick and is not considered suitable for foundation support or for use in a structural fill. Vashon Recessional Outwash Sediments encountered below the surficial forest duff/topsoil horizon in each of the exploration locations except exploration pit EP-1 generally consisted of loose to medium dense sand and gravel with minor to moderate silt content and areas of abundant cobbles. The upper portion of this unit was typically observed to contain high percentages of silt. At most of the exploration locations, the silty horizon extended to depths of approximately 2 to 3 feet, but silty sediments were observed to extend to depths of up to approximately 5 to 7 feet in a few areas. Localized strata composed of silty sediments were also encountered within this unit at greater depths in a few locations. We interpret these sediments to be representative of Vashon recessional outwash. The Vashon recessional outwash was deposited by meltwater streams that flowed off of the retreating glacial ice during the latter portion of the Vashon Stade of the Fraser Glaciation, approximately 12,500 to 15,000 years ago. At the locations of exploration pits EP-2 through EP-12, the recessional outwash sediments extended beyond the maximum depths explored of approximately 8 to 11 feet. At the locations of exploration borings EB-1 through EB-3, the recessional outwash sediments extended to depths of approximately 30 to 35.5 feet. Fraser to Pre-Fraser Undifferentiated Sediments Sediments encountered below the recessional outwash in borings EB-1 through EB-3 generally consisted of dense to very dense, sand with moderate to high silt content and variable gravel content with interbeds of hard silt, and scattered cobbles and boulders. A 4-foot-thick interbed of silty gravel was encountered within this unit in exploration boring EB-2 at a depth of approximately 49 feet. The high relative density of these sediments indicate that they have June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 4

5 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation been glacially consolidated, and therefore were deposited during or prior to the most recent glaciation of the Puget Lowland. The most recent glaciation of the Puget Lowland, known as the Fraser Glaciation, ended approximately 12,500 years ago. The Fraser to pre-fraser undifferentiated sediments extended beyond the maximum depths explored in borings EB-1 through EB-3 of approximately 61.5 feet. Geologic Mapping Review of the regional geologic map titled Geologic Map of the Issaquah 7.5 Quadrangle, King County, Washington, by Booth and Minard (1991) indicates that the area of the subject site is underlain by Vashon recessional outwash. Our interpretation of the sediments encountered in our explorations is in general agreement with the regional geologic map. Groundwater Groundwater seepage was not encountered in any of the explorations advanced for our study. It should be noted that the occurrence or level of groundwater seepage below the site may vary in response to changes in season, precipitation, and other factor LANDSLIDE HAZARDS Code Requirements Section of the Issaquah Municipal Code (IMC) defines Landslide Hazard Areas as any of the following: A. Slopes greater than 40 percent. B. Any area with a combination of: 1. Slopes greater than 15 percent. 2. Impermeable soils (typically silt and clay) frequently interbedded with granular soils (predominantly sand and gravel); and, 3. Springs. C. Any area which has shown movement during the Holocene epoch, or to which is underlain by mass wastage debris from that epoch. D. Any area potentially unstable as a result of rapid stream incision, stream bank erosion, or undercutting by wave action. E. Any area which shows evidence of, or is at risk from, snow avalanches. F. Any area located on an alluvial fan, presently subject to or potentially subject to inundation by debris flows or deposition of stream-transported sediments. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 5

6 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation The code also states that Steep Slope Areas (Item A, above) must be at least feet in height to classify as a critical area. Sections and of the IMC require that a minimum 50-foot native growth buffer be established around all sides of a Landslide Hazard Area, with an additional minimum 15-foot building setback beyond the buffer. The buffer may be reduced to a minimum of feet if a critical area study demonstrates that the reduction will not reduce the level of protection to the proposed development and the critical area as provided by the 50-foot buffer. Topographic Conditions The topography across the majority of the site is relatively flat to moderately sloping, but slopes steeply downward to the west and southwest along its western and southwestern margins, respectively. Review of the LIDAR based topographic contours shown on Figure 2 indicates that slope inclinations in these areas generally range from approximately 40 to 65 percent over a total slope height ranging from approximately 160 to 275 feet. Areas of steep slope are also located along portions of the northern and eastern property margins. Slope inclinations in these areas generally range from approximately 40 to 65 percent over heights of approximately to 25 feet. These steeply sloping areas classify as Landslide Hazard Areas under the IMC. Slope Reconnaissance We conducted a reconnaissance of the steep slope along the western margin of the site on March 20, The weather was dry and sunny at the time of our reconnaissance. Notable site features observed during our reconnaissance are summarized below. A landslide was observed on the steep slope below the northwestern portion of the site. The landslide extends over the full height of the slope beginning near the top of the slope just northwest of the northwest property corner. The location of the landslide is shown on Figure 2. Outside of the historic landslide area shown on Figure 2, the tree trunks on the steep slope generally appear fairly straight and plumb with some minor bowing observed. The bowing observed appeared consistent with that associated with normal soil creep. An overgrown cat road extends partway up the steep slope from the former DOT maintenance yard located at the toe of the slope. The approximate location of the road is shown on Figure 2. A polyvinyl chloride (PVC) monitoring well was observed on a narrow bench located on the steep slope below the southern portion of the site. The monitoring well consisted of machine slotted well screen with no monument or surface seal. Given the access June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 6

7 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation conditions in this area, the construction of the well, and the lack of any observable cuttings stockpiled in the area, it appears likely that the monitoring well was installed by hand. The monitoring well is partially overgrown by moss, and therefore appears to have been in place for an extended period of time. We did not have any well gauging equipment at the time of our reconnaissance, and therefore did not gauge the monitoring well to assess its total depth or presence of groundwater. We observed skunk cabbage growing on the bench adjacent to the well, which suggests that the soil in this area is poorly drained. The morphology of the narrow bench suggests that it might be a landslide feature. The location of the monitoring well/bench is shown on Figure 2. Soil observed during our reconnaissance on the face of the majority of the slope generally consisted of sandy gravel and gravelly sand, consistent with the recessional outwash observed in the explorations advanced at the subject site. Fine-grained sediments (silt) were observed on the lower portion of the slope, within approximately 20 feet of the slope toe. A concrete block wall, approximately 4 to 5 feet in maximum height, was observed at the toe of the slope, along the east side of the DOT maintenance yard. No emergent seepage was observed on the slope. The steep slope in the southwestern portion of the site exhibits a fairly uniform inclination over its full height, which suggests that this may be a man-made cut slope. The suspected cut slope is forested with moderately sized trees. The location of this slope is shown on Figure 2. LIDAR Mapping LIDAR is a remote sensing technology that can be used to generate a detailed expression of the topography of the ground surface even in densely vegetated areas. For this reason, LIDAR based topographic imagery can be helpful in distinguishing surface features (such as old landslide features) that may otherwise not be easily recognizable. A LIDAR based shaded relief map of the subject site and surrounding area is included in Appendix C (Figure C1). Geomorphic features evident on the shaded relief map include the following: The landslide observed on the steep slope below the northwestern corner of the site is clearly visible in Figure C1. The landslide extends from near the northwestern corner of the site to the toe of the steep slope. No other landslides are visible on or directly adjacent to the subject site. Another landslide is visible on Figure C1 approximately 200 yards south-southeast of the southeast corner of the property. Review of historical records indicates that a landslide occurred in this area in February 2017, which temporarily blocked the west-bound lanes of I-90. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 7

8 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation The suspected cut slope observed during our site reconnaissance is also visible in Figure C1. It should be noted that shallow, small-scale landslides may not be evident in LIDAR based imagery. Historical Aerial Photography Review of historical aerial photography shows that the suspected cut slope observed during our reconnaissance and visible on the LIDAR-shaded relief map (Figure C1) was cleared of vegetation in This is consistent with the origin of the topography in this area as a cut slope, possibly associated with former aggregate mining activities. A copy of the 1979 aerial photo is included in Appendix C (Figure C2). Slope Stability Analysis In order to evaluate suitable buffer/building setbacks for the project, an analysis of the global stability of the slope was conducted using the computer program Slope/W, version 7.23 by GeoSlope International. The program used the Morgenstern-Price method for evaluating a rotational failure. Input parameters for the analysis included slope geometry, geology, and soil strength parameters. The slope geometry was obtained from the LIDAR based topographic contour map depicted in Figure 2 along section line A-A. The geology of the slope was based on the subsurface conditions encountered in the explorations and soil conditions observed on the face of the steep slope during our reconnaissance. Soil strength parameters used for our analysis were assumed based on typical published values for similar materials and our prior experience. The values assumed for our analysis are shown on the Slope/W profiles included in Appendix D. For evaluation of slope stability under dynamic conditions, a horizontal ground acceleration of 0.305g was used for our analysis. This value is equivalent to ½ of the peak horizontal ground acceleration based on a seismic event with a 2 percent probability of exceedance in 50 years in accordance with the 2015 International Building Code (IBC). The stability of a slope can be expressed in terms of its factor of safety. The factor of safety of a slope is the ratio between the forces that resist sliding to the forces that drive sliding. For example, a factor of safety of 1.0 would indicate a slope where the driving forces and the resisting forces are exactly equal. Increasing factor of safety values greater than 1.0 indicate increased stability. Factors of safety below 1.0 indicate conditions where the driving forces exceed the resisting forces and landsliding is imminent. The IMC specifies a minimum factor of safety of 1.5 for static conditions, but does not specify a minimum value for dynamic conditions. As a typical standard of practice, a factor of safety of 1.1 is commonly considered to be a minimum acceptable value for dynamic conditions. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 8

9 Hilltop Elementary School Issaquah, Washington Subsurface Exploration and Landslide Hazard Evaluation Under static conditions, a minimum factor of safety of 1.36 was calculated for the steep slope below the western portion of the site. The minimum factor of safety was associated with a theoretical failure surface on the lower portion of the face of the slope. Larger failures associated with factors of safety between 1.36 and 1.50 were predicted in our analysis. The largest failure associated with a factor of safety less than 1.50 extended up to the top of the steep (>40 percent) slope. Under dynamic conditions, a minimum factor of safety of 0.75 was calculated for the slope. This value was associated with a relatively shallow, theoretical failure on the face of the steep slope. Larger failures associated with factors of safety between 0.75 and 1. were predicted in our analysis. The largest failure associated with a factor of safety less than 1. extended 236 feet beyond the top of the steep (>40 percent) slope. This failure extends completely through the proposed building area to within 50 feet of the eastern property boundary. Copies of the Slope/W profiles are included in Appendix D. CONCLUSIONS The steep slope in the western portion of the site has a history of landslide activity. Our analysis indicates that factors of safety associated with this slope remain low, particularly under dynamic conditions. Theoretical failures associated with low factors of safety extend through the proposed building area. Given site property constraints, there is not sufficient room on the site to completely mitigate landslide hazards by establishing buffers and building setbacks. Although alternative options to mitigate landslide hazards at the site, such as grading or construction of retaining walls, may be feasible, such efforts would be costly and may not be practical. June 13, 2018 ASSOCIATED EARTH SCIENCES, INC. TKP/ms E Projects\ \KE\WP Page 9

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11 ISSAQUAH SITE Grand Ridge Park 90 Hillside Park Tradition Plateau NRCA Document Path: G:\GIS_Projects\aaY2018\ Hilltop Elementary\mxd\180018E001 F1 VM_HilltopES.mxd Kitsap County Pierce County Squak Mountain State Park Snohomish County King County NE Discovery Dr DATA SOURCES / REFERENCES: USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/I-CUBED/NGS 2013 KING CO: STREETS, PARCELS, CITY LIMITS 1/18 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE 7th Ave NE "u ± FEET NOTE: BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION West Tiger Mountain NRCA Copyright: 2013 National Geographic Society, i-cubed VICINITY MAP HILLTOP ELEMENTARY SCHOOL BELLEVUE, WASHINGTON PROJ NO. DATE: FIGURE: E001 3/18 1

12 400 NE Discovery Dr LEGEND: SITE !( EXPLORATION BORING ") EXPLORATION PIT!2 OLD MONITORING WELL LOCATION NE Holl y S t HISTORIC LANDSLIDE 7th Ave NE CROSS SECTION LANDSLIDE SCARP ")!( EP-12 EB-2 ") EP-2 EP-1 ") CAT ROAD PROPOSED BUILDING PARCEL Document Path: G:\GIS_Projects\aaY2018\ Hilltop Elementary\mxd\180018E001 F2 SM_HilltopES.mxd 3r d Ave NE 1st Ave NE 90 A th St SE OLD MONITORING WELL!2 SUSPECTED CUT SLOPE FORMER BORROW AREA CAT ROAD KING COUNTY ") ") ")!( 400 EP-11!( 420 EB-1 EP- EP-9 EB ") ") ") EP-4 EP-6 EP EP-3 ") ") 420 EP-5 EP-7 ") 440 A DETENTION POND FILLED IN AND DEVELOPED RECENT LANDSLIDE ISSAQUAH NE Blakely Dr SWEDISH HOSPITAL DATA SOURCES / REFERENCES: PSLC 2016 KING CO. DELIVERY 3 FLOWN 3/2/16-3/29/16 GRID CELL SIZE IS 3. CONTOURS FROM LIDAR WA STATE PLANE NORTH (FIPS 4601), NAD83(HARN) NAVD88 GEOID03 (GEOID03), US SURVEY FEET KING CO: PARCELS, STREETS, 1/18 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE Kitsap County Pierce County CITY BOUNDARY CONTOUR 20 FT CONTOUR 5 FT NOTE: DETENTION POND TO EAST IS FILLED IN AND BUILT RESIDENTIAL AT TIME OF THIS REPORT Snohomish County King County FEET BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION SITE AND EXPLORATION PLAN LIDAR BASED TOPOGRAPHY HILLTOP ELEMENTARY SCHOOL KING COUNTY, WASHINGTON PROJ NO. DATE: FIGURE: E001 6/18 2

13 APPENDIX A Conceptual Building Layout

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15 APPENDIX B Exploration Logs

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17 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-1 1 of 2 Hilltop Elementary Ground Surface Elevation (ft) 425 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5/8/18,5/8/18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) S T Samples S-1 S-2 Graphic Symbol DESCRIPTION Forest Duff Moist, brown, silty, fine to medium SAND, trace coarse sand, trace fine gravel; abundant organic material (SM). Vashon Recessional Outwash Moist, brown, silty, fine to medium sandy, fine to coarse GRAVEL, trace coarse sand, trace rootlets; stratified (GM). Well Completion Water Level Blows/6" Blows/Foot Other Tests 5 S-3 S-4 Moist, brownish gray to dark gray, fine to medium sandy, fine to coarse GRAVEL, trace to some silt, trace coarse sand, trace cobbles; stratified (GW). Poor recovery 0 to feet due to hitting cobbles. S-5 Moist, brownish gray, fine to medium sandy, fine to coarse GRAVEL, trace to some silt, trace coarse sand, trace cobbles; stratified (GW). S-6 Moist, brownish gray, medium to coarse SAND, some silt, trace fine sand, trace fine to coarse gravel; stratified (SP-SM). 15 S-7 S-8 Moist, brownish gray, fine to medium sandy, fine to coarse GRAVEL, some silt, trace coarse sand; larger rock fragments broken during drilling; stratified (SW-SM). 20 S-9- Moist, brownish gray, medium to coarse SAND, some silt, trace fine sand, trace fine gravel; stratified (SP-SM) S-11 Moist, brownish gray, medium to coarse SAND, some silt, some fine to coarse gravel; stratified (SP-SM). 25 S-12 Very moist, brownish gray, medium to coarse SAND, trace fine sand, trace fine to coarse gravel; stratified (SP). S-13 Very moist, brownish gray, medium to coarse SAND, some silt, some fine to coarse gravel; stratified (SP-SM). 30 S-14 Fraser to Pre-Fraser Undifferentiated Sediments 50/3" 50/3" S-15 Moist, grayish brown, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML). AESIBOR GPJ May 31, S-16 S-17 S-18 Sampler Type (ST): Moist, grayish brown, silty, fine SAND, some fine to coarse gravel, trace medium to coarse sand; fining upward (SM). Moist, grayish brown, fine SAND, some silt, trace medium to coarse sand, trace fine gravel; fining upward (SP-SM). Moist, grayish brown, fine to medium SAND, some silt, some fine to coarse gravel, trace coarse sand; fining upward (SP-SM). Moist, brownish gray with mottled iron oxidation, very fine sandy, SILT, trace 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

18 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-1 2 of 2 Hilltop Elementary Ground Surface Elevation (ft) 425 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5/8/18,5/8/18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) S T Samples S S-21 Graphic Symbol DESCRIPTION fine to coarse and, trace fine gravel (ML). Moist, grayish brown, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel (SM). Moist, brownish gray, fine sandy, SILT, some medium to coarse sand, trace fine to coarse gravel (ML). Well Completion Water Level Blows/6" Blows/Foot Other Tests 45 S-22 S-23 Driller notes cobble / boulder at 55 feet. Moist, brownish gray, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML). Moist, brownish gray to reddish brown with a thin oxidized layer, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML). Moist, brownish gray, very fine SAND; fining upward (SP). 50 S S-26 S-27 Upper 4 inches: As above, grading to: Lower 11 inches: Moist, brownish gray with occasional mottled iron oxidation, very fine sandy, SILT, fining upward (ML). Moist, reddish brown with iron oxidation, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel (SM). Moist, brownish gray, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML) S-28 Driller notes cobble / boulder at 56 feet. S-29 Moist, brownish gray, fine sandy, SILT, trace to some fine to coarse gravel, trace medium to coarse sand (ML). 60 S /5" 50/5" Bottom of exploration boring at 61.5 feet Groundwater not encountered AESIBOR GPJ May 31, 2018 Sampler Type (ST): 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

19 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-2 1 of 2 Hilltop Elementary Ground Surface Elevation (ft) 430 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5/9/18,5/9/18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) 5 S T Samples S-1 S-2 S-3 Graphic Symbol DESCRIPTION Forest Duff Moist, dark brown, silty, fine to medium SAND, trace coarse sand, trace fine gravel; abundant organic material (SM). Vashon Recessional Outwash Moist, dark brown, silty, fine to coarse GRAVEL, trace fine to coarse sand; abundant organic material (GM). Moist, brownish gray, fine to coarse sandy, fine to coarse GRAVEL, trace silt, trace rootlets; stratified (GW). Well Completion Water Level Blows/6" Blows/Foot Other Tests S-4 Becomes more sandy. S-5-6 S-7 Becomes less sandy. Moist, brownish gray, fine to medium SAND, trace to some silt, trace fine to coarse gravel; stratified (SP) S-8 S-9 Moist, light reddish brown with iron oxidation, fine to coarse gravelly, fine to coarse SAND, trace to some silt, trace cobbles, trace rootlets; fragments of rock broken during drilling; stratified (SW). Moist, brownish gray, fine to coarse sandy, fine to coarse GRAVEL, trace silt, trace rootlets (GW). 20 S--11 S-12 Moist, brownish gray, fine to coarse gravelly, fine to coarse SAND, trace silt, trace rootlets (SW). Moist, brownish gray, silty, fine to coarse gravelly, fine to coarse SAND, trace cobbles (SM) S-13 S-14 Moist, brownish gray to reddish brown with iron oxidation, silty, fine to coarse gravelly, fine to coarse SAND, trace cobbles (SM). 30 S S-17 Fraser to Pre-Fraser Undifferentiated Sediments Moist, brownish gray, silty, fine to coarse gravelly, fine to coarse SAND, trace cobbles (SM) S-18 Silt coated gravel. AESIBOR GPJ May 31, 2018 S-19 Sampler Type (ST): 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

20 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-2 2 of 2 Hilltop Elementary Ground Surface Elevation (ft) 430 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5/9/18,5/9/18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) S T Samples Graphic Symbol DESCRIPTION Well Completion Water Level Blows/6" Blows/Foot Other Tests S S-22 Moist, brownish gray, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML) S-23 Moist, grayish brown, fine to medium sand, some silt, trace coarse sand, trace fine to coarse gravel (SP-SM). S-24 Moist, grayish brown, silty, fine SAND, trace medium to coarse sand, trace fine to coarse gravel (SM). 50 S-25 S-26 Moist, brownish gray, silty, fine to coarse GRAVEL, some fine to coarse sand (GM) S-27 Moist, brownish gray, silty, fine to coarse gravelly, fine to coarse SAND (SM). S-28 Moist, brownish gray, fine SAND, some silt, trace medium to coarse sand; stratified (SP-SM). 60 S /5" 50/5" Bottom of exploration boring at 61.5 feet Groundwater not encountered AESIBOR GPJ May 31, 2018 Sampler Type (ST): 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

21 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-3 1 of 2 Hilltop Elementary Ground Surface Elevation (ft) 4 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5//18,5//18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) S T Samples S-1 S-2 Graphic Symbol DESCRIPTION Forest Duff Moist, dark brown, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel; abundant organic material (SM). Vashon Recessional Outwash Moist, brown, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel, trace cobbles; abundant organic material; root fragments (SM). Well Completion Water Level Blows/6" Blows/Foot Other Tests 5 No recovery. No recovery. S-3-4 S-5 Moist, brownish gray, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel, trace rootlets (SM). Moist, brownish gray, silty, fine to coarse gravelly, fine SAND, trace medium to coarse sand, trace rootlets (SM). Moist, brownish gray, fine to coarse sandy, fine to coarse GRAVEL, some silt, trace rootlets (GW-GM) S-6 Moist, brownish gray, silty, fine to coarse sandy, fine to coarse GRAVEL (GM). S-7 Moist, brownish gray, silty, fine to coarse GRAVEL, some fine to coarse sand (GM). 20 S S- 25 S-11 S S S-15 S-16 As above. Moist, brownish gray, fine to coarse sandy, fine to coarse gravelly, SILT (ML). Moist, brown, fine sandy, SILT, trace medium to coarse sand, trace fine to coarse gravel (ML) AESIBOR GPJ May 31, S-17 S-18 Sampler Type (ST): Fraser to Pre-Fraser Undifferentiated Sediments Slightly moist, brownish gray, very fine, sandy, SILT, trace fine to coarse sand, trace fine to coarse gravel; harder than previous samples (ML). As above. 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

22 Project Name Location Driller/Equipment Hammer Weight/Drop Project Number Exploration Log Exploration Number E001 EB-3 2 of 2 Hilltop Elementary Ground Surface Elevation (ft) 4 Issaquah, WA Datum NAVD 88 Holt Services / Track-MountedSonic Rig Date Start/Finish 5//18,5//18 140# / 30" Hole Diameter (in) 7 inches Sheet Depth (ft) S T Samples S Graphic Symbol DESCRIPTION Moist, brownish gray, silty, very fine, SAND, trace fine to coarse sand, trace fine gravel (SM). Well Completion Water Level Blows/6" Blows/Foot Other Tests S S-22 S S Moist, brownish gray, silty, fine to medium SAND, trace coarse sand, trace fine to coarse gravel, no stratification apparent (SM). 50/6" 50/6" S-26 Moist, dark brown, silty, fine to coarse gravelly, fine to coarse sand (SM). Dry to slightly moist (baked by drilling process), very fine sandy, SILT, trace fine to coarse sand, trace fine to coarse gravel (ML). 55 S-27 S S Bottom of exploration boring at 61.5 feet Groundwater not encountered AESIBOR GPJ May 31, 2018 Sampler Type (ST): 2" OD Split Spoon Sampler (SPT) No Recovery M - Moisture Logged by: KRB 3" OD Split Spoon Sampler (D & M) Ring Sample Water Level () Approved by: CJK Grab Sample Shelby Tube Sample Water Level at time of drilling (ATD)

23 LOG OF EXPLORATION PIT NO. EP-1 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION Fill 1 2 Loose to medium dense, moist, brown, very gravelly, silty, SAND; contains scattered cobbles, scattered pieces of plastic, twigs, and roots (SM) Becomes very moist, gray, and very silty from 8 to 9 feet. Becomes brown and silty below 9 feet Bottom of exploration pit at depth 16 feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

24 LOG OF EXPLORATION PIT NO. EP-2 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION Topsoil Vashon Recessional Outwash Loose to medium dense, moist, tan, fine to medium SAND, some silt, trace gravel; contains abundant roots from 0 to 2.5 feet (SP/SM) Medium dense, moist, tan to gray, very sandy, fine GRAVEL; scattered cobbles (GP) Bottom of exploration pit at depth 11 feet No seepage. Minor caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

25 LOG OF EXPLORATION PIT NO. EP-3 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Forest Duff Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, silty, sandy, GRAVEL; abundant roots (GM). 2 3 Medium dense, moist, grayish tan, very sandy, GRAVEL, trace silt; abundant cobbles (GW) Becomes tannish gray below 6 feet Bottom of exploration pit at depth feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

26 LOG OF EXPLORATION PIT NO. EP-4 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Forest Duff Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, silty, sandy, GRAVEL; abundant roots (GM). 2 3 Medium dense, moist, grayish, tan, very sandy, GRAVEL, trace silt; abundant cobbles (GW) Becomes tannish gray below 6 feet Bottom of exploration pit at depth feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

27 LOG OF EXPLORATION PIT NO. EP-5 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, silty, fine to medium SAND; abundant roots (SM) Loose to medium dense, moist, grayish tan, fine to medium SAND, trace to some silt, trace fine gravel (SP/SM). Becomes medium dense and gravelly below 4 feet. 5 6 Medium dense, moist, tannish gray, very sandy, GRAVEL, trace silt; abundant cobbles (GW) Bottom of exploration pit at depth 11 feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

28 LOG OF EXPLORATION PIT NO. EP-6 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose, moist, reddish brown, silty, fine SAND; abundant roots (SM) Medium dense, tannish gray, very sandy, GRAVEL, trace silt; abundant cobbles (GW) Bottom of exploration pit at depth feet No seepage. Moderately severe caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

29 LOG OF EXPLORATION PIT NO. EP-7 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 2 Topsoil Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, silty, fine to medium SAND, trace gravel; abundant roots (SM). 3 4 Loose to medium dense, moist, tannish gray, medium to coarse SAND, trace to some fine gravel, trace silt (SP) Becomes gravelly below 9 feet. 11 Bottom of exploration pit at depth feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

30 LOG OF EXPLORATION PIT NO. EP-8 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, gravelly, silty, SAND; abundant roots (SM) Medium dense, moist, tannish gray, very sandy, GRAVEL, trace silt; abundant cobbles; scattered small boulders (GW). 6 7 Becomes moist to very moist below 7 feet Bottom of exploration pit at depth feet No seepage. No caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

31 LOG OF EXPLORATION PIT NO. EP-9 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, gravelly, silty SAND; abundant roots (SM) Medium dense, moist to very moist, tan, very sandy, silty, GRAVEL (GM) Medium dense, moist, grayish tan, very sandy, GRAVEL, trace silt; abundant cobbles (GW). Increased moisture below ~8 feet Bottom of exploration pit at depth 11 feet No seepage. Moderate caving below 7 feet KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

32 LOG OF EXPLORATION PIT NO. EP- Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose to medium dense, moist, reddish brown, silty, gravelly, SAND; abundant roots (SM) Loose to medium dense, moist, grayish tan, very gravelly, fine to medium SAND, trace to some silt (SP/SM) Medium dense, most, gray to tannish gray, gravelly, medium to coarse SAND, trace silt; contains lenses (<~1 foot thick) of sandy, GRAVEL; contains scattered small boulders (SP) Bottom of exploration pit at depth 11 feet No seepage. Minor caving below 6 feet KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

33 LOG OF EXPLORATION PIT NO. EP-11 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose, moist, reddish brown, silty, gravelly, SAND; abundant roots (SM). 2 3 Loose to medium dense, moist, gray, very gravelly, medium to coarse SAND, trace silt; abundant cobbles (SP) Bottom of exploration pit at depth 9 feet No seepage. Severe caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

34 LOG OF EXPLORATION PIT NO. EP-12 Depth (ft) This log is part of the report prepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be read together with that report for complete interpretation. This summary applies only to the location of this trench at the time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are a simplfication of actual conditions encountered. DESCRIPTION 1 Topsoil Vashon Recessional Outwash Loose, moist, reddish brown, silty, gravelly, SAND; abundant roots (SM). 2 3 Loose to medium dense, moist, tannish gray, very sandy, GRAVEL, trace silt; abundant cobbles; scattered small boulders (GW) Bottom of exploration pit at depth 8 feet No seepage. Severe caving KCTP GPJ April 6, 2018 Logged by: TJP Hilltop Elementary Issaquah, WA Project No E001 Approved by: CJK 3/12/18

35 APPENDIX C LIDAR Shaded Relief Map and Historical Aerial Photo

36 7th Ave NE NE Disc ove ry D r HISTORIC LANDSLIDE NE Blak ely Dr ISSAQUA H ve ta 74th S t 1s DETENTION POND FILLED IN AND DEVELOPED E NE S SUSPECTED CUT SLOPE FORMER BORROW AREA KING COUN TY E sd rn nd RECENT LANDSLIDE hla King County Hig Document Path: G:\GIS_Projects\aaY2018\ Hilltop Elementary\mxd\180018E001 C1_SlideHilltopES.mxd 90 NE Bir ch S t LEGEND: ± SITE CITY BOUNDARY PARCEL 0 DATA SOURCES / REFERENCES: PSLC: KING COUNTY 2016, GRID CELL SIZE IS 3'. DELIVERY 3 FLOWN 3/2/16-3/29/16 WA STATE PLANE NORTH (FIPS 4601), NAD83(HARN) NAVD88 GEOID03 (GEOID03), US SURVEY FEET. KING CO: STREETS, PARCELS 1/18 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE LIDAR BASED TOPOGRAPHY FEET NOTE: BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION HILLTOP ELEMENTARY SCHOOL KING COUNTY, WASHINGTON PROJ NO. DATE: E001 FIGURE: 6/18 C1

37 Site Cleared Area (Suspected Borrow Area) Figure C Aerial Photo