FS31_Interim_Geotechnical_Report_200807.pdf

GEOTECHNICAL REPORT

FS 31 Interim Station N Northgate Way and Interlake Avenue N

Seattle, Washington

Project No. T-8326

Prepared for:

Seattle Department of Finance and Administrative Services Seattle, Washington

August 7, 2020

8-7-2020

TABLE OF CONTENTS

Page No.

1.0 Project Description

2.0 Scope of Work

3.0 Site Conditions

3.1 Surface

3.2 Soils

3.3 Groundwater

3.4 Geologic Hazards

3.4.1 Seismic Considerations

3.4.2 Landslide Prone and Erosion Hazard Areas

3.4.3 Volcanic Hazard Areas

3.4.4 Peat Settlement Prone Areas

3.5 Seismic Design Parameters

4.0 Discussion and Recommendations

4.1 General

4.2 Site Preparation and Grading

4.3 Excavations

4.4 Foundation Support

4.5 Floor Slab-on-Grade

4.6 Infiltration Feasibility

4.7 Drainage

4.8 Utilities

4.9 Pavement

5.0 Additional Services

6.0 Limitations

Figures

Vicinity Map ......................................................................................................................... Figure 1 Exploration Location Plan .................................................................................................... Figure 2

Appendix

Field Exploration and Laboratory Testing ........................................................................ Appendix A

Geotechnical Report FS 31 Interim Station

N Northgate Way and Interlake Avenue N Seattle, Washington

1.0 PROJECT DESCRIPTION

The project consists of redeveloping the single tax parcel totaling approximately 0.44 acres with an Interim Fire Station and associated access and utilities. Based on the provided site plan, prepared by TCA, dated March 2, 2020, the living quarters and office space will be constructed in the western portion of the site, the fitness and gear storage buildings will be in the southern portion of the site, and the apparatus tents will be located in the northeast and eastern portion of the site. Parking will be in the northwest corner of the site. Based on the existing topography, we expect grading will be minimal.

We expect that the building will be a single-story, steel-framed temporary structures with a weatherproof fabric exterior, constructed at grade. Structural loading should be relatively light, with isolated columns carrying maximum loads of 1 to 2 kips.

The recommendations in the following sections of this report are based on our understanding of the preceding design features. We should review design drawings as they become available to verify that our recommendations have been properly interpreted and to supplement them, if required.

2.0 SCOPE OF WORK

Our work was completed in accordance with our authorized proposal, dated March 11, 2020. Accordingly, on March 18, 2020, we observed soil conditions at 3 test borings advanced to depths of 16.5 feet below existing site grades. Using the information obtained from the subsurface exploration and laboratory testing, we performed analyses to develop geotechnical recommendations for project design and construction. Specifically, this report addresses the following:

Soil and groundwater conditions.

Seismic design parameters per the current Seattle Building Code (SBC).

Geologic Hazards per the Seattle Municipal Code (SMC).

Site preparation and grading.

Excavation

Foundations

Slab-on-grade floors.

Infiltration feasibility.

Utilities

Drainage

Pavements

Page No. 2

It should be noted that recommendations outlined in this report regarding drainage are associated with soil strength, design earth pressures, erosion, and stability. Design and performance issues with respect to moisture as it relates to the structure environment are beyond Terra Associates, Inc.’s purview. A building envelope specialist or contractor should be consulted to address these issues, as needed.

3.0 SITE CONDITIONS

3.1 Surface

The project site is located at the northwest corner of North Northgate Way and Interlake Avenue North in Seattle, Washington. The site is bordered by a church to the west, residences to the north, and the roadways to the south and east. A modular block wall, with a maximum height of about four feet was observed along the southern portion of the site. The approximate site location is shown on Figure 1.

The site is currently developed as a playground with a small shack and a playground equipment with associated access and landscaping. Site topography is generally flat with no obvious signs of sloping.

3.2 Soils

In general, the soil conditions at the site consisted of approximately six inches of topsoil overlying approximately three to five and one half feet of medium dense, inorganic fill material overlying dense to very dense silty sand and silty sand with gravel (glacial till) to the termination of the test borings. The exception to this was observed in Test Boring B-2 where the native soils were observed at the surface. The fill material consisted of various layers of silty sand and gravel.

The Geologic Map of Northeastern Seattle (Part of the Seattle North 7.5’ x 15’ Quadrangle), King County, by Derek B. Booth, Kathy G. Troost, and Scott A. Shimel (2009) maps the southern half of the site as glacial till (Qvt) and the northern half with advance outwash (Qva). The native soils we observed in the borings are more consistent with the glacial till mapping.

The preceding discussion is intended to be a general review of the soil conditions encountered. For more detailed descriptions, please refer to the Test Boring Logs in Appendix A. The approximate locations of the test borings are shown on Figure 2.

3.3 Groundwater

No groundwater seepage was observed within the test borings. We observed trace amounts of mottling within some of the native glacial till soils between about five to ten feet below existing grades. Mottling is typically an indication that shallow groundwater seepage develops during the normally wet winter and spring seasons within this mottled zone. This is due to precipitation infiltrating through the upper more permeable soils and becoming perched on the underlying denser glacial till soils. The glacial till soils have a relatively low permeability which impedes the continued downward migration of the infiltrated surface water. As a result, the water becomes perched and will flow laterally along the contact. Groundwater seepage levels are expected to be at their highest during the normally wet winter and spring months (November through May).

Page No. 3

3.4 Geologic Hazards

Section 25.09.012.A.1 of the Seattle Municipal Code (SMC) defines “Geologic Hazard and Steep Slope Erosion Hazard Areas” as:

“Geologic hazard areas are liquefaction-prone areas, landslide-prone areas, peat settlement-prone areas, seismic hazards areas, and volcanic hazard areas described in subsections 25.09.012.A.2, 25.09.012.A.3, 25.09.012.A.5, 25.09.012.A.6, and 25.09.012.A.7. Landslide-prone areas include steep slope erosion hazard areas. Steep slope erosion hazard areas that are regulated for additional erosion hazards are described in Subsection 25.09.012.A.4.”

3.4.1 Seismic Considerations

Section 25.09.012.A.6 of the SMC defines Seismic Hazard Areas as:

“a. Areas of the City subject to ground shaking from seismic hazards that are addressed by the Building Code (SMC Title 22).

b. The Seattle Fault zone as delineated in Troost et al., 2005, The geologic map of Seattle, a progress report, U.S. Geological Survey, Open-file report 2005-1252 or as the Director determines is more accurately mapped by the U.S Geological Survey, as set out in a Director's Rule.

c. Tsunamis

1) The waterbody of Lake Washington and for tsunamis and tsunami inundation, the water body and land area as shown in Walsh, et al., 2003, Tsunami hazard map of the Elliott Bay area, Seattle, Washington:

Modeled tsunami inundation from a Seattle Fault earthquake, Washington State Department of Natural Resources and National Oceanic and Atmospheric Administration, Washington Division of Geology and Earth Resources Open File Report 2003-14, or as the Director determines are more accurately mapped by the National Oceanic and Atmospheric Administration, the U.S. Geological Survey or the Washington State Department of Natural Resources, as set out in a Director's Rule.

2) The shoreline and upland areas surrounding Lake Washington are classified as an unknown risk from tsunamis under WAC 365-190-120(4)(c).

d. Seiches

1) The waterbodies of Elliott Bay, Lake Union, and Lake Washington.

2) The shoreline and upland areas surrounding the waterbodies in this Subsection 25.09.012.A.6.e are classified as an unknown risk from seiches under WAC 365-190-120(4)(c).”

The site is not mapped within the Seattle Fault Zone. Therefore, the site would not be classified as a seismic hazard area as defined by the SMC.

Section 25.09.012.A.2 of the SMC defines Liquefaction Prone Areas as “areas typically underlain by cohesionless soils of low density, usually in association with a shallow groundwater table, that lose substantial strength during earthquakes.”

Page No. 4

Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength due to an increase in water pressure induced by vibrations. Liquefaction mainly affects geologically recent deposits of fine-grained sand that is below the groundwater table. Soils of this nature derive their strength from intergranular friction. The generated water pressure or pore pressure essentially separates the soil grains and eliminates this intergranular friction; thus, eliminating the soil’s strength.

Based on soil and groundwater conditions observed in the test borings, in our opinion, the risk for soil liquefaction to occur within the native soils at the site is negligible. Therefore, the site is not a liquefaction prone area as defined by the SMC.

3.4.2 Landslide Prone and Erosion Hazard Areas

Section 25.09.012.A.3 of the SMC defines landslide prone areas as:

“a. Known landslide areas identified by documented history, or areas that have shown significant movement during the last 10,000 years or are underlain by mass wastage debris deposited during this period.

b. Potential landslide areas:

1) Those areas that are described as potential slide areas in "Seattle Landslide Study" (Shannon & Wilson, 2000 and 2003).

2) Areas with indications of past landslide activity, such as landslide headscarps and sidescarps, hummocky terrain, areas with geologic conditions that can promote earth movement, and areas with signs of potential landsliding, such as springs, groundwater seepage, and bowed or backtilted trees.

3) Areas with topographic expression of runout zones, such as fans and colluvial deposition at the toes of hillsides.

4) Setbacks at the top of very steep slopes or bluffs, depending on soil conditions.

5) Slopes with an incline of 40 percent or more within a vertical elevation change of at least 10 feet.

For the purpose of this definition, a slope is measured by establishing its toe and top and averaging the inclination over at least 10 feet of elevation difference. Also, for the purpose of this definition:

a) The "toe" of a slope means a distinct topographic break in slope that separates slopes inclined at less than 40 percent from slopes inclined at 40 percent or more. Where no distinct break exists, the "toe" of a slope is the lowermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.

b) The "top" of a slope is a distinct topographic break in slope that separates slopes inclined at less than 40 percent from slopes inclined at 40 percent or more. Where no distinct break exists, the "top" of a slope is the uppermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.

6) Areas that would be regulated under one of Subsections 25.09.012.A.3.b.2 through 25.09.012.A.3.b.5, but where the topography has been previously modified through the provision of retaining walls or non-engineered cut and fill operations.

7) Any slope area potentially unstable as a result of rapid stream incision or stream bank erosion.”

Page No. 5

None of the above conditions were observed at the site. Therefore, the site would not be considered a landslide prone area as defined by the SMC.

3.4.3 Volcanic Hazard Areas

Section 25.09.012.A.7 of the SMC defines landslide prone areas as:

“Volcanic hazard areas are areas subject to inundation by lahars or related flooding resulting from volcanic activity on Mount Rainier, as delineated by the U.S. Geological Survey in Hoblitt, et al., 1998, Volcano Hazards from Mount Rainier, Washington, Revised 1998: U.S. Geological Survey Open-File Report 98-428, or as the Director determines are more accurately mapped by the U.S. Geological Survey, as set out in a Director's Rule.”

The site is not mapped within a lahar or related flooding resulting from volcanic activity. Therefore, the site would not be considered a volcanic hazard area as defined by the SMC.

3.4.4 Peat Settlement-Prone Areas

Section 25.09.012.A.5 of the SMC defines peat settlement-prone areas as:

“Peat settlement-prone areas. Peat settlement-prone areas consist of Category I and Category II peat settlement-prone areas that are delineated on Maps A1 through A26, Peat Settlement-prone Area Boundaries Maps, codified at the end of this Chapter (SMC 25.09). This parcel-specific delineation is based on the location of the relevant bog or bogs identified in City of Seattle Identified Bogs (Troost 2007) plus a buffer of 50 feet for Category I peat settlement-prone areas or a buffer of 25 feet for Category II peat settlement-prone areas. On parcels larger than 50,000 square feet, the Director may consider a parcel-specific delineation, provided by the applicant, of the peat settlement-prone area boundary on a parcel. Where a parcel-specific delineation conflicts with the Peat Settlement-prone Area Boundaries Maps, the parcel-specific delineation shall apply.”

The site is not mapped as a peat settlement area and no peat was observed in our test borings. Therefore, it is our opinion that the project site is not a peat settlement-prone area as defined by the SMC.

3.5 Seismic Design Parameters

Based on soil conditions noted in the test borings and our knowledge of the area geology, per 2015 Seattle Building Code (SBC), site class “D” should be used in structural design.

4.0 DISCUSSION AND RECOMMENDATIONS

4.1 General

Based on our study, in our opinion, there are no geotechnical considerations that would preclude development of the site, as currently planned. The structures can be supported by conventional spread footing foundations bearing on competent existing fill or native soils. If required, the use of screw-down anchors bearing within the competent native soils below the fill soils can be used to assist in resisting uploading. Building floors and pavements can be similarly supported.

Page No. 6

The existing fill and native soils observed at the site contain a sufficient to significant amount of soil fines and will be difficult to compact as structural fill when too wet or too dry. The ability to use soils from site excavations as structural fill will depend on its moisture content and the prevailing weather conditions at the time of construction.

If grading activities will take place during winter, the owner should be prepared to import clean granular material for use as structural fill prior to incorporating pavements.

The following sections provide detailed recommendations regarding the preceding issues and other geotechnical design considerations. These recommendations should be incorporated into the final design drawings and construction specifications.

4.2 Site Preparation and Grading

To prepare the site for construction, all vegetation, organic surface soils, and other deleterious material should be stripped and removed from the site upon completion of the demolition of existing structures. Surface stripping depths of approximately six inches should be expected to remove the organic surface soils. Demolition of existing structures should include abandonment of underground septic systems and other buried utilities. Abandoned utility pipes that fall outside of new building areas can be left in place, provided they are sealed to prevent intrusion of groundwater seepage and soil. Organic topsoil will not be suitable for use as structural fill but may be used for limited depths in nonstructural areas.

Once demolition and stripping operations are complete, cut and fill operations can be initiated to establish desired grades. Prior to placing fill, all exposed bearing surfaces should be observed by a representative of Terra Associates, Inc. to verify soil conditions are as expected and suitable for support of new fill. Our representative may request a proofroll using heavy rubber-tired equipment to determine if any isolated soft and yielding areas are present. If excessively yielding areas are observed, and they cannot be stabilized in place by compaction, the affected soils should be excavated and removed to firm bearing and grade restored with new structural fill. Beneath the roadway subgrade, if the depth of excavation to remove unstable soils is excessive, the use of geotextile fabrics, such as Mirafi 500X, or an equivalent fabric, can be used in conjunction with clean granular structural fill. Our experience has shown that, in general, a minimum of 18 inches of a clean, granular structural fill placed and compacted over the geotextile fabric should establish a stable bearing surface.

The existing fill and native soils encountered at the site contain a sufficient amount of soil fines that will make them difficult to compact as structural fill when too wet or too dry. The ability to use these soils from site excavations as structural fill will depend on its moisture content and the prevailing weather conditions at the time of construction.

If wet soils are encountered, the contractor will need to dry the soils by aeration during dry weather conditions.

Alternatively, the use of an additive such as Portland cement, cement kiln dust (CKD), or lime to stabilize the soil moisture can be considered. If the soil is amended, additional Best Management Practices (BMPs) addressing the potential for elevated pH levels will need to be included in the Storm Water Pollution Prevention Program (SWPPP) prepared with the Temporary Erosion and Sedimentation Control (TESC) plan.

Page No. 7

If grading activities are planned during the wet winter months, or if they are initiated during the summer and extend into fall and winter, the owner should be prepared to import wet weather structural fill. For this purpose, we recommend importing a granular soil that meets the following grading requirements:

U.S. Sieve Size Percent Passing

6 inches 100 No. 4 75 maximum

No. 200 5 maximum*

* Based on the ¾-inch fraction.

Prior to use, Terra Associates, Inc. should examine and test all materials imported to the site for use as structural fill.

Structural fill should be placed in uniform loose layers not exceeding 12 inches and compacted to a minimum of 95 percent of the soil’s maximum dry density, as determined by American Society for Testing and Materials (ASTM) Test Designation D-1557(Modified Proctor). The moisture content of the soil at the time of compaction should be within two percent of its optimum, as determined by this ASTM standard. In nonstructural areas, the degree of compaction can be reduced to 90 percent.

4.3 Excavations

All excavations at the site associated with confined spaces, such as utility trenches, must be completed in accordance with local, state, and federal requirements. Based on regulations outlined in the Washington Industrial Safety and Health Act (WISHA), the upper existing fill soils would be classified as Type C. The underlying native till soils would be classified as Type B.

Accordingly, temporary excavations in Type C soils should have their slopes laid back at an inclination of 1.5:1 (Horizontal:Vertical) or flatter, from the toe to the crest of the slope. Type B soils should have slopes laid back at an inclination of 1:1. All exposed temporary slope faces that will remain open for an extended period of time should be covered with a durable reinforced plastic membrane during construction to prevent slope raveling and rutting during periods of precipitation.

The above information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Terra Associates, Inc. assumes responsibility for job site safety. It is understood that job site safety is the sole responsibility of the project contractor.

4.4 Foundation Support

The buildings may be supported on conventional isolated or continuous footing foundations bearing on competent native or existing fill soils. Perimeter foundations exposed to the weather should be at a minimum depth of 18 inches below final exterior grades. Interior foundations can be constructed at any convenient depth below the floor slab.

Page No. 8

We recommend designing foundations supported on competent existing fill or native soils for a net allowable bearing capacity of 2,500 pounds per square foot. For short-term loads, such as wind and seismic, a one-third increase in this allowable capacity can be used. With the anticipated building loads and this bearing stress applied to the soil, we estimate total foundation settlement on the native soils would not exceed one-half inch.

For designing foundations to resist lateral loads, an ultimate base shearing resistance equivalent to 35 percent of the vertical static dead-load can be used. Passive earth pressure acting on the sides of the footings can also be considered. We recommend calculating the passive lateral resistance using an equivalent fluid weight of 300 pounds per cubic foot (pcf). This value assumes the footings will be constructed neat against competent native soils or backfilled with structural fill. The friction and passive values are allowable and include a safety factor of 1.5.

For the screw down anchors, the uplift capacity would need to be determined based on the type of anchor selected.

Each manufacture has different capacities based on the type of anchor and the ground conditions. For anchors that extend through the upper three to five feet of fill material, the designer can use a dense silty sand soil for selection of the capacity. We can assist in determining the capacity once the anchor type has been selected.

4.5 Floor Slab-on-Grade

Slab-on-grade floor subgrades should be prepared as described in Section 4.2 of this report. Immediately below the floor slab, we recommend placing a four-inch thick capillary break layer composed of clean, coarse sand or fine gravel that has less than three percent passing the No. 200 sieve. This material will reduce the potential for upward capillary movement of water through the underlying soil and subsequent wetting of the floor slab.

The capillary break layer will not prevent moisture intrusion through the slab caused by water vapor transmission.

Where moisture by vapor transmission is undesirable, such as covered floor areas, a common practice is to place a durable plastic membrane on the capillary break layer and then cover the membrane with a layer of clean sand or fine gravel to protect it from damage during construction, and to aid in uniform curing of the concrete slab. It should be noted that if the sand or gravel layer overlying the membrane is saturated prior to pouring the slab, it will not be effective in assisting uniform curing of the slab and can actually serve as a water supply for moisture bleeding through the slab, potentially affecting floor coverings. Therefore, in our opinion, covering the membrane with a layer of sand or gravel should be avoided if floor slab construction occurs during the wet winter months and the layer cannot be effectively drained. We recommend floor designers and contractors refer to the current American Concrete Institute (ACI) Manual of Concrete Practice for further information regarding vapor barrier installation below slab-on-grade floors.

4.6 Infiltration Feasibility

The infiltration feasibility has been evaluated under a separate report that includes the required testing in accordance with the City of Seattle requirements.

Page No. 9

4.7 Drainage

Surface

Final exterior grades should promote free and positive drainage away from the structures at all times. Water must not be allowed to pond or collect adjacent to foundations or within the immediate building areas. We recommend providing a positive drainage gradient away from the building perimeters. If this gradient cannot be provided, surface water should be collected adjacent to the structures and disposed to appropriate storm facilities.

Subsurface

We recommend installing perimeter foundation drains adjacent to shallow foundations. The drains can be laid to grade at an invert elevation equivalent to the bottom of footing grade. The drains can consist of four-inch diameter perforated PVC pipe that is enveloped in washed pea gravel-sized drainage aggregate. The aggregate should extend six inches above and to the sides of the pipe. Roof and foundation drains should be tightlined separately to the storm drains. All drains should be provided with cleanouts at easily accessible locations.

4.8 Utilities

Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) or the City of Seattle specifications. As a minimum, trench backfill should be placed and compacted as structural fill, as described in Section 4.2 of this report. As noted, depending on the soil moisture when excavated, most existing fill and native soils on the site should be suitable for use as backfill material during dry weather conditions. The contractor should be prepared to aerate soils to reduce moisture and facilitate proper compaction. However, if utility construction takes place during the wet winter months, it will likely be necessary to import suitable wet weather fill for utility trench backfilling.

4.9 Pavement

Pavements should be constructed on subgrades prepared as recommended in Section 4.2 of this report. Regardless of the degree of relative compaction achieved, the subgrade must be firm and relatively unyielding before paving.

Proofrolling the subgrade with heavy construction equipment should be completed to verify this condition.

The pavement design section is dependent upon the supporting capability of the subgrade soils and the traffic conditions to which it will be subjected. We expect traffic at the facility will consist of cars and light trucks, along with heavy traffic in the form of fire engines and ambulance vehicles. For design considerations, we have assumed traffic in parking and in car/light truck access pavement areas can be represented by an 18-kip Equivalent Single Axle Loading (ESAL) of 50,000. For heavy traffic pavement areas, we have assumed an ESAL of 300,000 would be representative of the expected loading. This ESAL accounts for outrigger loading from the fire trucks. With a stable subgrade prepared as recommended for the design ESAL values, we recommend the following pavement sections:

Light Traffic/Car Access:

Two inches of hot mix asphalt (HMA) over four inches of crushed rock

Full depth HMA – 3.5 inches

Page No. 10

Heavy Traffic/Truck Access:

Three inches of HMA over eight inches of crushed rock

Full depth HMA – 5 inches

The paving materials used should conform to the Washington State Department of Transportation (WSDOT) specifications for ½-inch class HMA and CRB.

Long-term pavement performance will depend on surface drainage. A poorly-drained pavement section will be subject to premature failure resulting from surface water infiltrating the subgrade soils and reducing their supporting capability. For optimum performance, we recommend surface drainage gradients of at least two percent. Some degree of longitudinal and transverse cracking of the pavement surface should be expected over time. Regular maintenance should be planned to seal cracks as they occur.

5.0 ADDITIONAL SERVICES

Terra Associates, Inc. should review the final design drawings and specifications in order to verify that earthwork and foundation recommendations have been properly interpreted and implemented in project design. We should also provide geotechnical service during construction to observe compliance with our design concepts, specifications, and recommendations. This will allow for design changes if subsurface conditions differ from those anticipated prior to the start of construction.

6.0 LIMITATIONS

We prepared this report in accordance with generally accepted geotechnical engineering practices. No other warranty, expressed or implied, is made. This report is the copyrighted property of Terra Associates, Inc. and is intended for specific application to FS 31 Interim Station project in Seattle, Washington. This report is for the exclusive use of Seattle Department of Finance and Administrative Services and their authorized representatives.

The analyses and recommendations present in this report are based on data obtained from the subsurface exploration completed on-site. Variations in soil conditions can occur, the nature and extent of which may not become evident until construction. If variations appear evident, Terra Associates, Inc. should be requested to reevaluate the recommendations in this report prior to proceeding with construction.

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APPENDIX A

FIELD EXPLORATION AND LABORATORY TESTING

FS 31 Interim Station Seattle, Washington

On March 18, 2020, we completed our site exploration by observing soil conditions at 3 test borings drilled to a maximum depth of 16.5 feet below existing site grades. The test borings were advanced using a tracked drill rig equipped with hollow stem augers. Test boring locations were determined in the field by measuring from existing site features. The approximate locations of the test borings are shown on the attached Exploration Location Plan, Figure 2. Test Boring Logs are attached as Figures A-2 through A-4.

A geotechnical engineer from our office conducted the field exploration. Our representative classified the soil conditions encountered, maintained a log of each test boring, obtained representative soil samples, and recorded water levels observed during drilling. During drilling, soil samples were obtained in general accordance with ASTM Test Designation D-1586. Using this procedure, a 2-inch (outside diameter) split barrel sampler is driven into the ground 18 inches using a 140-pound hammer free falling a height of 30 inches. The number of blows required to drive the sampler 12 inches after an initial 6-inch set is referred to as the Standard Penetration Resistance value or N value. This is an index related to the consistency of cohesive soils and relative density of cohesionless materials. N values obtained for each sampling interval are recorded on the Test Boring Logs, Figures A-2 through A-4. All soil samples were visually classified in accordance with the Unified Soil Classification System (USCS) described on Figure A-1.

Representative soil samples obtained from the test borings were placed in closed containers and taken to our laboratory for further examination and testing. The moisture content of each sample was measured and is reported on the individual Test Boring Logs. Grain size analyses were performed on selected samples. The results of the grain size analyses are shown on Figure A-5.

DECKER

Figure No.

Project: Project No:

Logged By:Driller:

Location: Approx. Elev:

Client:

Relative Density

Consistency/ Soil Description

SPT (N)

Blows/foot

10 30 50 M oi st ur e C on te nt

Date Drilled:

Depth to Groundwater:

S am pl e

In te rv al

D ep th ft) pertains only to this boring location and should not be interpeted as being indicative of NOTE: This borehole log has been prepared for geotechnical purposes. This information other areas of the site

A-2LOG OF BORING NO. B-1

FS 31 Interim Station T-8326 3/18/2020

SLKBoretecCity of Seattle Dept. of Finance & Admin. S.

Seattle, Washington N/A N/A

Medium Dense

Dense

Very Dense

Dense

Very Dense

(GRASS at surface)

FILL: Dark brown silty SAND, fine to medium sand, moist, some gravel, trace organics. (SM)

FILL: Gray/brown GRAVEL, coarse gravel, moist, trace silt and sand.

(GP)

*Small layer of buried topsoil observed above mottled soils at bottom of sampler.

Red/orange silty SAND with gravel, fine to medium sand, fine gravel, moist. (SM)

Gray/brown silty SAND with gravel, fine to medium sand, fine gravel, moist, trace mottling. (SM)

*Coarse gravel observed.

Gray silty SAND, fine to medium sand, moist. (SM)

*Fine to coarse gravel observed.

Boring terminated at 16.5 feet.

No groundwater.

16.9 2.2

11.6

8.7

9.2

11.2

5.0

Logged By:Driller:

Location: Approx. Elev:

Client:

Relative Density

Consistency/ Soil Description

SPT (N)

Blows/foot

10 30 50 M oi st ur e C on te nt

Date Drilled:

Depth to Groundwater:

S am pl e

In te rv al

D ep th ft) pertains only to this boring location and should not be interpeted as being indicative of NOTE: This borehole log has been prepared for geotechnical purposes. This information other areas of the site

A-3LOG OF BORING NO. B-2

FS 31 Interim Station T-8326 3/18/2020

SLKBoretecCity of Seattle Dept. of Finance & Admin. S.

Seattle, Washington N/A N/A

Very Dense

Dense

Very Dense

(SAND and GRASS at surface)

(No sample recovery at 2.5 feet)

Gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, dry. (SM)

Gray brown-white silty SAND with gravel, fine to sand, fine to coarse gravel, moist. (SM)

Gray/brown silty SAND, fine to medium sand, moist, trace gravel.

(SM)

*Increase in silt and gravel content.

Boring terminated at 16.5 feet.

No groundwater.

5.4

7.8

8.7

10.3

15.6

50/6"

50/4"

77/9"

Logged By:Driller:

Location: Approx. Elev:

Client:

Relative Density

Consistency/ Soil Description

SPT (N)

Blows/foot

10 30 50 M oi st ur e C on te nt

Date Drilled:

Depth to Groundwater:

S am pl e

In te rv al

D ep th ft) pertains only to this boring location and should not be interpeted as being indicative of NOTE: This borehole log has been prepared for geotechnical purposes. This information other areas of the site

A-4LOG OF BORING NO. B-3

FS 31 Interim Station T-8326 3/18/2020

SLKBoretecCity of Seattle Dept. of Finance & Admin. S.

Seattle, Washington N/A N/A

Medium Dense

Dense

Very Dense

Dense

Very Dense

(GRAVEL and WOOD CHIPS at surface)

FILL: Dark brown silty SAND, fine sand, moist, trace organics, trace gravel. (SM)

Red/orange silty SAND, fine to medium sand, moist, trace gravel.

(SM)

Gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM)

Gray silty SAND, fine to medium sand, moist, trace gravel. (SM)

Boring terminated at 16.5 feet.

No groundwater.

17.7 15.0 6.7

10.5

7.3

8.6

10.6

9.0

11.3

Tested By: FQ

LL PL D85 D60 D50 D30 D15 D10 Cc Cu

Material Description USCS AASHTO

Project No. Client: Remarks:

Project:

Location: Test Boring B-1 Depth: -7.5 feet Sample Number: 3

Location: Test Boring B-2 Depth: -15 feet Sample Number: 6 Location: Test Boring B-3 Depth: -12.5 feet Sample Number: 5

Terra Associates, Inc.

Kirkland, WA Figure

8.8983 0.6006 0.2790

0.5298 0.2413 0.2099 0.1591

0.7133 0.2500 0.2164 0.1750 0.1092

Silty SAND with gravel SM Silty SAND SM Silty SAND SM

T-8326 City of Seattle Department of Finance and Administration

A-5

P E

R C

E N

T F

IN

E

R

GRAIN SIZE - mm.

0.0010.010.1110100

% +3" Coarse

% Gravel Fine Coarse Medium

% Sand Fine Silt

% Fines Clay

0.0 0.0 22.9 8.1 12.7 25.6 30.7

0.0 2.9 6.6 0.8 7.8 66.8 15.1

0.0 0.0 0.8 1.5 33.9 51.4 12.4 in in in

1½ in in

¾ in

½ in

3/ in

#4 #1

#2

#3

#4

#6

#1

#1

#2

Particle Size Distribution Report

FS 31 Interim Station

Seattle, Washington

Tested on March 23, 2020

Tested on March 23, 2020

Tested on March 23, 2020