Final_Geotechnical_Report_WMP6_North_Sept_2023.pdf

FINAL GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 6 - NORTH

SEATTLE, WASHINGTON

Work Authorization No.: C600739 September 2023

707 South Plummer Street Seattle, Washington 98134

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

2.0 PURPOSE AND SCOPE OF WORK

3.0 SITE DESCRIPTION

3.1 NW 201ST STREET

3.2 24TH AVENUE NW

3.3 WOODLAWN AVENUE N

3.4 8TH AVENUE NE

4.0 SUBSURFACE SOIL AND GROUNDWATER CONDITIONS

4.1 NW 201ST STREET

4.2 24TH AVENUE NW

4.3 WOODLAWN AVENUE N

4.4 8TH AVENUE NE

5.0 SEISMIC CONSIDERATIONS

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 TEMPORARY EXCAVATIONS AND SHORING

6.1.1 Excavation Methods

6.1.2 Shoring Methods

6.1.3 Construction Vibration Considerations

6.2 TEMPORARY CONSTRUCTION DEWATERING

6.2.1 Dewatering Related Settlement

6.3 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR STEEP SLOPE ECAS

6.4 BURIED STRUCTURE DESIGN

6.4.1 Lateral Earth Pressure

6.4.2 Bearing Capacity and Settlement

6.4.3 Uplift Resistance

6.5 CONSTRUCTION CONSIDERATIONS

6.5.1 Subgrade Preparation

6.5.2 Pipe Bedding

6.5.3 Backfill and Compaction

6.5.4 Wet Weather Earthwork

6.5.5 Construction Drainage and Erosion Control

6.5.6 Landfill Gas Mitigation

7.0 LIMITATIONS

8.0 REFERENCES

ii

List of Tables (within text) Table 1 Project Site Summary Table

List of Figures (following text) Figure 1 Site Overview Map Figure 2 Site and Existing Information Map – NW 201st Street Site Figure 3 Site and Existing Information Map – 24th Avenue NW Site Figure 4 Site and Existing Information Map – Woodlawn Avenue N Site Figure 5 Site and Existing Information Map – 8th Avenue NE Site Figure 6 Loads for Buried Structure Design (Vault) – NW 201st Street Site

Appendix A: Historical Explorations

Attachment 1: Environmental Recommendations Memorandum – WMRP6 North:

NE 55th St & Roosevelt Way NE

FINAL GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 6 – NORTH

SEATTLE, WASHINGTON

1.0 INTRODUCTION

This report presents the results of our geotechnical investigation and our recommendations for design and construction of new water main lines in association with the Water Main Rehab Package 6 – North project, one of three sub-packages of Water Main Rehab Package 6. Separate reports are being prepared for the Central and South sub-packages of the Water Main Rehab Package 6 project. We understand that Water Main Rehab Package 6 – North project includes water main improvements at four sites, two of which are located in the City of Shoreline and two are located in the City of Seattle. Our current understanding is that the proposed water main improvements will be installed at a maximum depth of 8 feet below ground surface (bgs) using open trench excavation methods. The site designation, site location and approximate water main replacement length for each site are summarized in Table 1 below.

Table 1: Project Site Summary Table

Neighborhood Street and Site Designation Cross Streets

Approximate Replacement Length (ft)

Shoreline NW 201st Street 18th Avenue NW PRV Shoreline 24th Avenue NW NW 196th Street – 197th Street 310

Green Lake

Woodlawn Avenue N

Meridian Avenue N

N 63rd Street – N 65th Street

N 62nd Street – Woodlawn Avenue N

University District 8th Avenue NE NE 55th Street – NE 60th Street 1,335

A site overview map is shown on Figure 1 and individual site maps are shown on Figures 2 through 5.

We have organized this report into seven primary sections. The first three sections describe the purpose and scope of our work and our understanding of existing site conditions. The remaining sections present the site subsurface conditions, seismic considerations, and our geotechnical engineering findings and recommendations.

Tables throughout the report provide data that are described in the text. Figures illustrating project features are presented at the end of the text. Logs of relevant historical explorations are provided in Appendix A.

Water Main Rehab Package 6 – North FINAL Geotechnical Report

SPU GEOTECHNICAL ENGINEERING

2.0 PURPOSE AND SCOPE OF WORK

The purpose of our work is to provide the SPU Project Team with subsurface information and interpretation, and preliminary geotechnical engineering recommendations to support their design of the project.

Our scope of work for this project included:

• Assessing subsurface conditions using historical geotechnical reports and explorations;

• Providing results of a limited field investigation for the 8th Avenue NE site (NE 55th St & Roosevelt Way NE intersection) and limited environmental assessment;

• Providing limited geotechnical recommendations; and,

• Preparing this geotechnical engineering report.

3.0 SITE DESCRIPTION

In this section, we discuss the proposed improvements and mapped surface features at each of the four water main sites. Elevations provided in this report are referenced to the North American Vertical Datum of 1988 (NAVD88) unless noted otherwise. Our review of mapped surface features for each site included the following:

• City of Seattle Environmentally Critical Areas (ECAs),

• City of Shoreline Online Interactive Mapping website,

• Washington State Department of Ecology (Ecology) contaminated site data,

• City of Seattle Slide Event Inventory data, and

• An overview of the site topography.

3.1 NW 201ST STREET

The NW 201st Street site is located within the City of Shoreline right-of-way (ROW) at the northeast corner of the intersection of NW 201st Street and 18th Avenue NW and includes installation of one new pressure relief valve (PRV) and associated pipe and appurtenances. The water main at this site is an existing 12-inch cast iron (CI) slip joint (SJ) pipe. The location of the site improvements is shown on Figure 2. The site topography is relatively flat at approximate elevation 280 feet. The topography surrounding the site slopes gently down towards the south.

According to the City of Shoreline Online Interactive Mapping website, the NW 201st Street site is not located in any mapped ECAs. We also compared the location of the site to the Washington Department of Ecology (Ecology) Confirmed and Suspected

Contaminated Sites List (CSCSL) and did not identify any potential sources of contaminated soil and/or groundwater near the NW 201st Street site.

3.2 24TH AVENUE NW

The 24th Avenue NW site is located within the City of Shoreline 24th Avenue NW ROW between NW 196th Street and NW 197th Street. The site improvements are as follows:

• Plug and abandon 280 lineal feet (LF) of 4-inch Kalamein and 30 LF of 6-inch CI pipe;

• Install a new 310 LF, 8-inch restrained joint (RJ) ductile iron (DI) water main;

• Install two 8-inch gate valves (GVs); and

• Install a new hydrant in NW 197th Street.

The location of the site improvements is shown on Figure 3. The site topography along 24th Avenue NW slopes gently down from northeast to southwest from approximate elevation 125 feet at the northeast corner of the intersection with NW 197th Street to elevation 105 feet at the northwest corner of the intersection with NW 196th Street. The topography surrounding the site slopes down from northeast to southwest towards Puget Sound.

According to the City of Shoreline Online Interactive Mapping website, the NW 201st Street site is not located in any mapped ECAs. We also compared the location of the site to the Ecology CSCSL and did not identify any potential sources of contaminated soil and/or groundwater near the 24th Avenue NW site.

3.3 WOODLAWN AVENUE N

The Woodlawn Avenue N site is located in the Green Lake neighborhood of Seattle and includes improvements in the Woodlawn Avenue N ROW between N 63rd Street and N 65th Street and in the Meridian Avenue N ROW between N 62nd Street and Woodlawn Avenue N. The site improvements are as follows:

• Plug and abandon approximately 715 LF of 2-inch Galvanized Iron (GI) water main in Woodlawn Avenue N;

• Install approximately 750 LF of 8-inch restrained joint ductile iron pipe (RJDIP) in Woodlawn Avenue N and intersections;

• Install thirteen 8-inch GVs, three at N 63rd Avenue, five at N 64th Street, and five at N 65th Street;

• Install two fire hydrants, one at N 64th Street and one at N 65th Street; and

• Replace approximately 150 LF of 2-inch GI water main in Meridian Avenue N ROW with approximately 100 LF of 2-inch copper water main.

The location of the site improvements is shown on Figure 4. The site topography along Woodlawn Avenue N slopes down from the northeast to southwest from approximate elevation 214 feet at the intersection of N 65th Street to approximate elevation 190 feet at the intersection of N 63rd Street. The site topography along the Meridian Avenue N ROW slopes down gently to the north from approximate elevation 298 feet at the intersection of N 62nd Street to approximate elevation 190 feet at the north end of the intersection with Woodlawn Avenue N. The overall topography surrounding the site slopes down to the west towards Green Lake.

According to the City of Seattle GIS maps, the site is not located within any ECAs;

however multiple steep slope ECAs are located along the boundary of the ROW and private properties along Woodlawn Avenue N between N 64th Street and N 65th Street and along the north and south sides of the N 64th Street ROW east of Woodlawn Avenue N. Based on our review of Google Street View® imagery, the mapped areas correspond to rockeries and retaining wall structures along the ROW boundaries. We also compared the location of the site to the Ecology CSCSL and did not identify any potential sources of contaminated soil and/or groundwater near the Woodlawn Avenue N site.

Portions of the project along Woodlawn Avenue N north of its intersection with N 65th Street are within 1,000 feet of an abandoned landfill (Green Lake landfill). Although the City of Seattle GIS maps do not include a 1,000-foot buffer for the Green Lake landfill and label it as a non-methane landfill, a previous study conducted by SPU Geotechnical Engineering’s environmental consultant on behalf of Seattle Parks and Recreation (SPR) has documented the presence of methane in the vicinity of the Green Lake Community Center (Herrera, 2021). Recommendations for construction adjacent to methane producing landfills are provided in Section 6.5.6 of this report.

3.4 8TH AVENUE NE

The 8th Avenue NE site is located in the University District neighborhood of Seattle and includes improvements in the 8th Avenue NE ROW between NE 55th Street and NE 60th Street and in the intersection of NE 55th Street and Roosevelt Way NE. The site improvements are as follows:

• Plug and abandon approximately 1255 LF of 2-inch GI water main in 8th Avenue

NE;

• Install approximately 1385 LF of 12-inch RJDIP water main in 8th Avenue NE;

• Install eighteen 12-inch GVs within the six intersections of 8th Avenue NE between NE 55th Street and NE 60th Street;

• Install/replace 6 fire hydrants in 8th Avenue NE;

• Establish connection to an existing 12-inch CI water main in NE 55th Street;

• Install approximately 100 LF of new 12-inch water main in the intersection of Roosevelt Way NE and NE 55th Street; and

• Install four 12-inch GVs, one in Roosevelt Way NE and three in NE 55th Street.

The location of the site improvements is shown on Figure 5. The site topography along 8th Avenue NE slopes down to the north from approximate elevation 200 feet at the intersection of NE 55th Street to approximate elevation 184 feet at the intersection of NE 60th Street. The site topography at the intersection of NE 55th Street and Roosevelt Way NE is relatively flat at approximate elevation 202 feet. The overall topography surrounding the site slopes down to the north – northeast towards NE Ravenna Boulevard.

According to the City of Seattle GIS maps, the site is not located within any ECAs;

however, one steep slope ECA is mapped on private property bordering the west side of the 8th Avenue E ROW just north of the intersection with NE 58th Street. Based on our current understanding the existing water main features and proposed improvements, all work is expected to be at least 30 feet east of the base of the steep slope and is not within the buffer to the steep slope ECA.

We compared the location of the site to the Ecology CSCSL and identified a potential source of contaminated soil and/or groundwater near the 8th Avenue NE site, specifically within the vicinity of the intersection of NE 55th Street and Roosevelt Avenue NE. As a result, we completed environmental explorations and laboratory testing to inform soil disposal and dewatering considerations for construction of the water main improvements in the vicinity of the intersection. The results of our environmental investigation and recommendations for construction are summarized in an environmental memorandum included as Attachment 1.

4.0 SUBSURFACE SOIL AND GROUNDWATER CONDITIONS

We based our interpretation of subsurface conditions at each of the water main sites on published geologic maps, and information obtained from historical subsurface explorations. The locations of relevant historical explorations are shown on Figures 2 through 5. Logs of relevant historical explorations are provided in Appendix C. The conclusions and analyses provided in this report are based on our interpretation of the subsurface conditions at each site.

It is important to note that many of the historical explorations are more than 100 feet away from the proposed improvements and are sparsely distributed along the improvement alignments. As a result, our understanding of subsurface conditions is limited and may not accurately represent localized anomalies. If the SPU Project Team requires a greater level of accuracy in the subsurface conditions at the water main sites, we recommend that additional explorations are completed. If additional explorations are not completed, it should be anticipated that subsurface conditions encountered during construction may vary from what is indicated in this report, and that adjustments to construction methods may be required. The nature and extent of variations between our understanding of subsurface conditions and the actual conditions may not become evident until additional explorations are completed, or construction begins. If variations are encountered, it will be necessary to reevaluate the conclusions and recommendations in this report.

Based on our understanding that the excavation depths required to complete the water main improvements will be 8 feet or less, the subsurface conditions discussed in Sections

4.1 through 4.4 focus on soil and groundwater conditions documented in the upper 15 feet of the historical exploration summary logs.

Note that measured groundwater levels at all sites are representative for the time of year indicated on the exploration logs. We would expect the groundwater level to fluctuate throughout the year and be at its highest during the late winter and spring seasons and at its lowest during the late summer and early fall seasons.

4.1 NW 201ST STREET

Review of the Geologic Map of Surficial Deposits in the Seattle 30’x60’ Quadrangle, Washington (Yount, et. al.) and Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington (Minard) indicates that the NW 201st Street site is generally underlain by Vashon glacial till. Glacial till, a mix of poorly sorted silt, sand, and sub-rounded to well-rounded gravels and cobbles, is transported by the glacier and deposited under the ice resulting in a very dense to over consolidated deposit. Vashon advance outwash deposits are mapped to the northeast and recessional outwash deposits are mapped to the west and northwest. The historical explorations reviewed are generally consistent with the mapped geology.

We reviewed five explorations in the general vicinity of the NW 201st Street Site (Figure 2). Historical explorations were advanced to depths ranging from 8 to 380 feet bgs. E- 404, TP-1, TP-7, and 88099 were advanced through 0 to 5 feet of fill over dense glacial till within the upper 15 feet bgs. E-103 was advanced through advance outwash deposits.

Based on our interpretation of the historical explorations in the site vicinity and surficial geologic maps, we anticipate that the site is underlain by fill, dense glacial till or advance outwash deposits.

No groundwater or seepage was encountered in any of the historical explorations in the site vicinity; however, wet soil was described in the loose to medium dense deposits overlying the dense to very dense till in TP-1 and TP-7. Perched groundwater may be present at the contact between the dense, glacially consolidated deposits, seasonally.

4.2 24TH AVENUE NW

Review of the Geologic Map of Surficial Deposits in the Seattle 30’x60’ Quadrangle, Washington (Yount, et. al.) and Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington (Minard) indicates that the 24th Avenue NW site is generally underlain by Vashon glacial till. The historical explorations reviewed are generally consistent with the mapped geology.

We reviewed two explorations in the general vicinity of the 24th Avenue NW site (Figure 3). Historical explorations were advanced to depths ranging from approximately 3 to 13 feet bgs. TB-13 was advanced through approximately 6 feet of fill comprised of loose, brown, silty sand with gravel overlying dense, gray silty fine to medium sand with some gravel (glacial till) to the base of the exploration. 1+75 encountered gray-brown slightly silty sand with scattered gravel which is interpreted as either fill or glacial till deposits.

Based on our interpretation of the historical explorations in the site vicinity and surficial geologic maps, we anticipate that the site is underlain by near surface fill and dense glacial deposits.

Wet soil was described in TB-13 at approximately 4 feet bgs near the contact between the loose fill and dense glacial till deposits. Perched groundwater may be present at the contact between the dense, glacially consolidated deposits, seasonally.

4.3 WOODLAWN AVENUE N

Review of the Geologic Map of Seattle (Troost, et. al.) indicates that the Woodlawn Avenue N site is generally underlain by Vashon glacial till. The historical explorations reviewed are generally consistent with the mapped geology.

Historical explorations completed near the Woodlawn Avenue N site generally encountered 6 feet of fill soils overlying medium dense to very dense glacial till deposits.

Groundwater was not encountered in the historical exploration or soil cut observations described in the historical geotechnical reports. Perched groundwater may be present at the contact between the dense, glacially consolidated deposits, seasonally.

4.4 8TH AVENUE NE

Review of the Geologic Map of Seattle (Troost, et. al.) indicates that the 8th Avenue NE site is generally underlain by Vashon glacial till. The historical explorations reviewed are generally consistent with the mapped geology.

Historical explorations completed near the 8th Avenue NE site generally encountered up to 10 feet of loose fill overlying glacially consolidated deposits including glacial till and ice contact deposits, which are mapped to the southwest of the 8th Avenue NE site.

Explorations completed at the intersection of NE 55th Street and Roosevelt Way NE, B- 101 and B-102, encountered 8 to 10 feet of silty sand and silt fill overlying glacial till deposits. The fill was generally comprised of very loose to loose silty sand with variable gravel. The ice contact deposits encountered in PG-1 were comprised of medium stiff to very stiff sandy silt. Loose to medium dense silt with sand was encountered below the fill in BH-1-21 and BH-2-21 which we interpret to be ice contact deposits. Glacial till was encountered below the fill in PG-2 and in the test hole completed at the 5518 Roosevelt Way NE site. Based on our review of existing subsurface information, we anticipate that the water main site is underlain by fill, glacial till, and ice contact deposits. Recessional outwash deposits may also be encountered in the northern portion of the site along 8th Avenue NE.

Wet conditions, rust mottling, and perched groundwater was described in all new and historical explorations at the contact between the loose coarse-grained fill and the underlying ice contact and glacial till deposits. We interpret the described groundwater conditions as a perched groundwater layer. The rust mottling described in the new historical explorations may indicate seasonal variability of the perched groundwater. We would expect the groundwater level to fluctuate throughout the year and be at its highest during the late winter and spring seasons and at its lowest during the late summer and early fall seasons.

5.0 SEISMIC CONSIDERATIONS

Based on conversations with the SPU Project Team, we understand that all water mains and improvements included in this package are considered ordinary and do not need to be designed based on seismic criteria. As a result, we have not included considerations for seismic design and earthquake-induced geotechnical hazards.

6.0 CONCLUSIONS AND RECOMMENDATIONS

This section presents geotechnical engineering recommendations for the installation of the proposed water main improvements. This includes recommendations for the geotechnical engineering aspects of:

• Temporary Excavations and Shoring;

• Temporary Construction Dewatering;

• Design and Construction Considerations for ECAs; and

• Other Construction Considerations.

6.1 TEMPORARY EXCAVATIONS AND SHORING

We understand that the proposed water main improvements will require temporary excavations to depths of up to 8 feet at all sites. The contractor should be responsible for determining equipment suitability for excavation of the varying soils noted in this report, as well as for other soil strata, which may be encountered. Temporary excavations and shoring should be implemented in accordance with Section 2-04 and 2-07 of the 2020 City of Seattle Standard Specifications for Road Bridge and Municipal Construction (Standard Specifications). In accordance with the Standard Specifications, the design parameters and the means and methods of shoring temporary excavations are the contractor’s sole responsibility. However, we provide our recommendations for feasible shoring methods to aid in planning and design.

6.1.1 Excavation Methods

Based on our interpretation of subsurface conditions at the sites, and the proposed grading, it appears that onsite excavations will predominantly be made in fill, glacial till, and ice contact deposits. We anticipate that the site soils for all of the water main sites can generally be excavated using standard excavation equipment. Variability of subsurface soil conditions should be expected; however, the nearest exploration logs and geologic mapping do not indicate a high probability for subsurface obstructions.

6.1.2 Shoring Methods

In accordance with the Standard Specifications, shoring systems are classified as either trench safety systems, which allow the sides of the trench to slough while providing protections for workers in the trench, or trench support systems, which restrict movement of the sides of the trench. Because trench safety systems do not provide positive support for the sides of the trench, structures that are within the zone of influence of the trench can experience excessive settlement when the sides of the trench move. As a result, trench safety systems should only be used in areas where settlement sensitive structures are not present within the zone of influence of the trench and groundwater can be controlled using sumps and pumps. The zone of influence is defined graphically in Figure A.

Once the location and dimensions of excavations have been determined for each site, we recommend that the SPU Design Team specifies trench support systems for all sites

Figure A - Zone of Influence for Temporary Excavations where settlement sensitive structures or utilities are within the zone of influence.

Commonly used trench support systems include slide rail, soldier pile and lagging, and sheet piles.

In general, non-watertight shoring systems (slide rail and soldier pile and lagging), can only be used in areas where groundwater can be maintained below the base of excavation. Watertight shoring systems (sheet piles) should be used in areas where the water table cannot be maintained below the base of excavation. In our opinion, based on our understanding of groundwater conditions, watertight shoring should not be required at any of the sites.

The lateral deflection of shoring walls can lead to settlement of the ground surface adjacent to the shored excavation. The magnitude of ground settlement is dependent upon soil and groundwater conditions, the contractor’s shoring installation methods, and the stiffness of the shoring system. Lateral deflection of the shoring system and resulting ground surface settlement are greater if poor shoring installation techniques are used.

Construction practices that can lead to increased settlement include improper control of groundwater, delayed placement of lagging and supports/bracing, and excavation below designed support/bracing elevations.

6.1.3 Construction Vibration Considerations

Ground vibrations could be generated during installation and removal of shoring systems.

Installation and removal of sheet piles, using a vibratory hammer would be likely to generate continuous or steady state vibrations, whereas, soldier pile installation and removal would be likely to generate transient vibrations. The vibration levels and potential damage to nearby structures generated by installation and removal of each shoring system are difficult to quantify; however, driven sheet piles and drilled soldier piles would likely cause the most and least significant vibration and associated damage, respectively. Trench box placement and removal would likely generate relatively few vibrations, which would be transient. It should be noted that vibration levels that can be perceived by humans are well below levels that typically cause damage to structures.

Regardless of the shoring method selected at each site, we recommend that a pre-construction survey is completed for all structures within the zone of influence of the excavation, and for any other sensitive structures located in close proximity, to provide a baseline for possible damage claims. We also recommend a vibration expert be retained to provide vibration thresholds and recommendations for a monitoring program.

6.2 TEMPORARY CONSTRUCTION DEWATERING

The excavations should be dewatered, as needed, in accordance with Section 2-08 of the Standard Specifications. Typically, sumps and pumps are sufficient for dewatering when excavations are less than 3 feet beneath the water table, excavation faces are supported or sloped appropriately, or if work can be completed in the wet. If conditions and construction methods do not allow for open sumps, a deep well or well point system may be a viable alternative.

The majority of the historical explorations encountered perched groundwater seepage, wet soil, or rust mottling at the time of exploration, at depths ranging between 2 and 6 feet bgs across all of the sites. The perched groundwater seepage, wet soil, and rust mottling was predominately concentrated within the loose to medium dense fill deposits overlying dense glacial till at the NW 201st Street, 24th Avenue NW, and Woodlawn Avenue N sites and within the loose to medium dense fill deposits overlying the medium stiff to stiff silt ice contact deposits or glacial till deposits at the 8th Avenue NE site.

Based on our understanding of the proposed improvements and expected excavation depths, perched groundwater should be expected during excavation at all sites.

Long term groundwater monitoring data in the areas of the subject water main sites was not available for our review, therefore determining the highest groundwater levels with certainty is not feasible at the project sites. Based on our review of historical exploration logs, we anticipate that the project sites included in this package will maintain bottom of excavation depths at generally less than 3 feet below the groundwater level. This would indicate a greater probability that dewatering, in such sites, could be achieved with sumps and pumps, particularly if construction takes place during the dry season. Groundwater would be expected to be highest during winter and early spring and lowest in the summer and early fall.

The contractor should choose a dewatering method based on the available information and their selected construction techniques. Well points and other external dewatering systems should be designed by a qualified professional engineer or geologist. The dewatering plan should be submitted to SPU Geotechnical Engineering for approval.

6.2.1 Dewatering Related Settlement

Lowering the groundwater table increases the effective stress in the soil which can lead to settlement at the ground surface, particularly in compressible soils. As a result, the groundwater table should be drawn down (if applicable) no more than necessary for the proposed work, to avoid damage to adjacent utilities and structures. Based on the soil and groundwater conditions indicated on the new and historical exploration logs, we anticipate that the potential for dewatering related settlement at the water main sites is low.

6.3 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR STEEP SLOPE ECAS

As discussed in Section 3.3, the Woodlawn Avenue NW site improvements are located in close proximity to several mapped steep slope ECAs along the boundary of the ROW and private properties along Woodlawn Avenue N between N 64th Street and N 65th Street and along the north and south sides of the N 64th Street ROW east of Woodlawn Avenue

N. Based on our review of Google Street View® imagery, the mapped areas correspond to rockeries and other retaining wall structures along the ROW boundaries. We anticipate that the proposed improvements will be located in close proximity to the existing water main and therefore we expect that the excavation limits will be beyond the 15-foot buffer to the mapped steep slope ECAs and special considerations for design and construction are not warranted.

As discussed in Section 3.4, the 8th Avenue NW site improvements are located approximately 30 feet east of a steep slope bordering the west side of the 8th Avenue E ROW just north of the intersection with NE 58th Street. Based on our current understanding the existing water main features and proposed improvements, the proposed work is not within the steep slope ECA nor within the buffer to the steep slope ECA and special considerations for design and construction are not warranted.

If the aforementioned assumptions differ from the site conditions during design or construction, we should be notified so that we can provide updated recommendations for work adjacent to or within steep slope ECAs.

6.4 BURIED STRUCTURE DESIGN

We understand that the proposed water main improvements at the NW 201st Street site will include installation of a new vault. When developing design recommendations for these features, we assumed that the base of vault will be between 3 and 8 feet bgs.

6.4.1 Lateral Earth Pressure

The proposed vault should be designed to resist lateral and vertical loads from the soil surrounding the structure and other related surcharge loads. Recommended loads for design of buried structures at the NW 201st Street site is provided on Figure 6. We provide at-rest earth pressures because we anticipate that the walls of the relatively rigid structures will not be free to rotate. Lateral loads due to surficial surcharge loading (adjacent footings, fill, traffic, etc.) should be considered by the designer on a case-by-case basis and should be added to the lateral earth pressures provided on Figure 6.

6.4.2 Bearing Capacity and Settlement

We estimated the allowable bearing capacity for buried structures based on the anticipated soil and groundwater conditions between 3 and 8 feet bgs at the NW 201st Street site. We anticipate that the native glacial till at the NW 201st Street site is generally suitable for subgrade support for the vault. We recommend an allowable bearing capacity of 3,000 pounds per square foot (psf) for buried structures bearing on native glacial till. If the subgrade soil encountered during construction is saturated or is softer or less dense than the anticipated soil described in Section 4.1, we should be contacted to evaluate the subgrade and provide additional recommendations for subgrade improvement.

Assuming construction is accomplished as recommended herein, and for the anticipated loads, we estimate total static settlement of less than 1 inch. It is anticipated that the majority of the estimated static settlement will occur during construction, as loads are applied.

6.4.3 Uplift Resistance

Buried structures that are watertight and that are at least partially below the groundwater table are subjected to permanent hydrostatic uplift pressures. The buoyant force on the bottom of these structures is resisted primarily by the weight of the structure and any soil above the structure. The uplift pressure can be calculated using the formula provided on Figure 6. We recommend a minimum factor of safety against static uplift of 1.2. Based on our understanding of the depth of the proposed structure and the anticipated soil and groundwater conditions, we do not anticipate that the structure will be subjected to significant static uplift pressures.

6.5 CONSTRUCTION CONSIDERATIONS

6.5.1 Subgrade Preparation

Subgrade preparation for areas where water main sections and vault will be installed should begin with the removal (stripping) of all deleterious matter, asphalt, and concrete.

A smooth-bladed excavator bucket should be used to excavate to the subgrade elevation and foot traffic on the subgrade should be minimized to reduce the amount of disturbance to the subgrade. A layer of mineral aggregate or structural concrete may be used to protect the subgrade once it is exposed.

The exposed subgrade, within the water main trenches and vault excavation, should be observed by the Geotechnical Engineer-of-Record to evaluate the adequacy of the bearing stratum, for pipe support, prior to initial placement of bedding material. For the anticipated smaller areas where access is restricted, the subgrade should be evaluated by probing the soil with a steel rod.

Soft/loose soils identified during subgrade preparation should be compacted to a firm and unyielding condition or stabilized. Typical subgrade stabilization measures include:

overexcavation and replacement with up to 12 inches of structural fill if the subgrade is not saturated, or 6 to 18 inches of quarry spalls if the subgrade is saturated. Geosynthetic materials can also be considered for reduction of the required thickness of imported fill or quarry spalls. The depth of overexcavation, if required, should be determined by the Geotechnical Engineer-of-Record at the time of construction.

6.5.2 Pipe Bedding

Bedding is material placed at the bottom of the trench to provide uniform support along the bottom of a buried utility. Bedding material and placement procedures should meet the appropriate requirements and criteria of the Standard Specifications, depending on the utility in question. In areas where a trench box is used, the bedding material should be placed before the trench box is advanced. Bedding material disturbed by movement of trench boxes should be recompacted prior to final backfilling. Care should be taken not to disturb the utility as the trench box is advanced.

Trench backfill will be placed on top of the bedding. Refer to the backfill recommendations in Section 6.5.3.

6.5.3 Backfill and Compaction

In general, we recommend that excavations in areas that support pavements, structures or utilities are backfilled and compacted in accordance with Sections 2-10 and 2-11 of the Standard Specifications. Section 2-10 of the Standard Specifications requires that soil excavated from the site that is considered suitable for fill applications (Selected Material) must be used as backfill before new material is imported (Borrow Material). In accordance with the Standard Specifications, the Engineer is responsible for determining the suitability of onsite material for fill applications at the time of construction. However, to aid in project planning and design, we provide our opinion on the probability that the soil at the sites will meet the requirements for Selected Material.

The suitability of excavated soil for reuse as fill depends upon the gradation and moisture content of the soil when it is placed. As the percent of fines (that portion passing the Number 200 sieve) increases, the soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult to achieve. Soil containing more than 5 percent fines, by weight, cannot be consistently compacted to a dense non-yielding condition when the water content is greater than 3 percent above or below optimum. In accordance with the Standard Specifications the soil must also contain less than 5 percent, by volume, of organic and other unsuitable material.

The soil that we anticipate will be encountered within 8 feet of the ground surface at the four water main sites generally has a high fines content. Therefore, in our opinion, it is unlikely that the excavated material will meet the requirements for Selected Material unless it can be moisture conditioned to within 3 percent of the optimum moisture content. Depending on the project schedule, area available for moisture conditioning, and the time of year when construction is planned, moisture conditioning may be very difficult or not feasible. If moisture conditioning is not feasible, we recommend that the excavations at the four water main sites are backfilled with Borrow Material meeting the gradation requirements of Type 17 material in Section 9-03.14, Mineral Aggregate Chart in the Standard Specifications.

6.5.4 Wet Weather Earthwork

If earthwork is to be performed or fill is to be placed in wet weather or under wet conditions when control of soil moisture content is not possible, the following recommendations should apply:

• Complete earthwork in small sections to minimize exposure to wet weather;

• Place and compact a suitable thickness of clean structural fill immediately following excavations or the removal of unsuitable soil;

• Limit the size of construction equipment if needed to prevent soil disturbance;

• Use clean, granular soil for trench backfill with not more than 5 percent by dry weight passing the U.S. Standard No. 200 Sieve. The fines should be non-plastic;

• Slope and seal the ground surface in the construction area with a smooth drum roller to promote rapid runoff of precipitation, prevent surface water from flowing into excavations, and to prevent ponding of water;

• Protect uncompacted soil so it does not absorb water. Soils that become too wet for compaction should be removed and replaced with clean granular materials; and

• Excavate and place fill under the full-time observation of a person experienced in wet weather earthwork to verify that all unsuitable materials are removed, and suitable compaction and site drainage are achieved.

6.5.5 Construction Drainage and Erosion Control

Surface runoff and erosion at the site can be controlled during construction by careful grading practices and observance of best management practices (BMPs). Such practices typically include the construction of shallow, upgrade perimeter ditches or low earthen berms, and the use of temporary sumps to collect runoff. Erosion during construction can be minimized by judicious use of erosion control devices. If used, these devices should be in place and remain in place throughout construction.

Erosion and sedimentation of exposed soils can also be minimized by quickly re-vegetating exposed areas of soil, and by staging construction such that large areas of the project site are not denuded and exposed at the same time. Areas of exposed soil requiring immediate and/or temporary protection against exposure should be covered with either mulch or erosion control netting/blankets.

6.5.6 Landfill Gas Mitigation

As discussed in Section 3.3, portions of the project along Woodlawn Avenue N north of its intersection with N 65th Street are within 1,000 feet of the Green Lake landfill. The Title 10 King County Board of Health Solid Waste Regulation governs construction standards and methane controls on abandoned landfills. Specifically, construction restriction applicability is described in Section 10.09.060 (A) for all construction activities within 1,000 feet of an active, closed, or abandoned landfill with methane gas concentrations at the lower explosive limit (LEL, 5 percent by volume) or greater levels.

A previous study performed by SPU Geotechnical Engineering’s consultant on behalf of SPR has confirmed concentrations of methane at or above the methane LEL within the vicinity of the Green Lake Community Center that lies within the footprint of the historical Green Lake landfill. Although it is unlikely that contaminated soil or groundwater associated with the landfill will be encountered during construction of the Woodlawn Avenue N improvements, the proposed work must adhere to all federal, state, and local regulations that are meant to protect the public from the risks of migrating landfill gases.

7.0 LIMITATIONS

This report was prepared in accordance with generally accepted professional principles and practices in the field of geotechnical engineering at the time the report was prepared.

All environmental work products, analyses, recommendations, and statements herein have been prepared or performed by an Environmental Professional as defined in Title 40 Part 312 of the Code of Federal Regulations. The scope of our work did not include environmental assessments or evaluations regarding the presence or absence of wetlands.

This geotechnical report is intended to provide information and recommendations to support preliminary engineering activities for this project. The conclusions and interpretations presented in this report should not be construed as a warranty of the subsurface conditions.

We appreciate the opportunity to be of service.

Sincerely, Mark Jusayan, L.G. Kori Wilson, P.E.

Senior Environmental Geologist Senior Geotechnical Engineer

09/07/23 09/07/2023 https://seattlegov.na1.adobesign.com/verifier?tx=CBJCHBCAABAASlQeJTeRx81jcidsZ26y4UR1i_-dm6Iz https://seattlegov.na1.adobesign.com/verifier?tx=CBJCHBCAABAASlQeJTeRx81jcidsZ26y4UR1i_-dm6Iz

8.0 REFERENCES

City of Seattle, 2020, Standard Plans for Municipal Construction 2020 Edition.

City of Seattle, 2020, Standard Specifications for Road Bridge and Municipal Construction 2020 Edition.

Seattle Public Utilities, 2020. SPU Design Standards and Guidelines.

Minard, J.P., 1983, Geologic map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington – U.S. Geological Survey Miscellaneous Field Studies Map MF-1541, Scale 1:24,000.

Troost, K.G., Booth, D.B., Wisher, A.P., and Shimel, S.A.,2005. The geologic map of Seattle - U. S. Geological Survey Open file report 2005-1252, scale 1:24,000.

Washington Department of Ecology, What’s In My Neighborhood, online application.

Accessed October 2022. https://fortress.wa.gov/ecy/neighborhood/.

Washington Department of Natural Resources, Subsurface Geology Information System, online geodatabase. Accessed October 2022.

https://fortress.wa.gov/dnr/geology/?Theme=subsurf.

Yount, J.C., Minard, J.P., Dembroff, G.R., 1993. The Geologic Map of Surficial Deposits in the Seattle 30’x60’ Quadrangle, Washington – U.S. Geological Survey Open file report 93-233, scale 1:100,000.

Geotechnical Engineering

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SITE OVERVIEW MAP

Water Main Rehab Package 6 - North Seattle, Washington

FIGURE 1

December 2022

Project No. C600739

NAVD88-

8th Avenue E Site

Woodlawn Avenue N

Site

NW 201st Street Site

24th Avenue NW Site

8th Avenue E Site

3,000 0 3,000 6,0001,500

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Pleistocene continental glacial drift

Water Main Rehab Package 6 - North NW 201st Street Site Seattle, Washington

SITE & EXISTING INFORMATION

FIGURE 2

December 2022

Project No. C600739

100 0 100 20050

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25th Ave NW

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W

NW 194th Pl

NW 196th St

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SITE & EXISTING INFORMATION MAP

FIGURE 3

December 2022

Project No. C600739

50 0 50 10025 water

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SITE & EXISTING INFORMATION

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Seattle, Washington

FIGURE 4

December 2022

Project No. C600739

75 0 75 15037.5

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!( SPU Boring

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Feeder Main Supply Main Water Services Historic Stream Topography - 10 Foot Topography - 2 Foot Parcels

Confirmed and Suspected Contaminated Sites

No Further Action Clean-up Sites

Regulated Underground Storage Tanks

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SITE & EXISTING INFORMATION

Wate Main Rehab Package 6 - North 8th Avenue NE Site Seattle, Washington

FIGURE 5

December 2022

Project No. C600739

125 0 125 25062.5

In se rt Fi le P at h

FIGURE 6

September 2023

Project No. C600739

Water Main Rehab Package 6 - North NW 201st Street Site Seattle, Washington

Loads For Buried Structure Design (Vault)

Notes:

1. Earth pressures and other loads presented here are ultimate values.

2. It is assumed that buried structures will be sufficiently stiff to limit wall deflection, as a result at-rest earth pressures are provided.

3. Design groundwater levels are provided in the report, and shall be used to determine depth to groundwater (dw).

4. Surcharge pressure includes a 250 PSF vehicular and construction surcharge, which is equivalent to a 2-foot-thick layer of soil with a unit weight of 125 PCF placed at the ground surface. Additional surficial surcharge loads should be considered on a case by case basis.

5. All dimensions are in feet, and pressures are in pounds per square foot.

120df + 250 60df 60dw 60dw + 29(d-dw)

APPENDIX A

HISTORICAL EXPLORATIONS

A-1

APPENDIX A

HISTORICAL EXPLORATIONS

We reviewed historical explorations to gain an understanding of the subsurface conditions along the project alignments. Figures 2 through 5 show the approximate locations of the historical explorations in the vicinity of the proposed project improvements.

SPU Geotechnical Engineering is not responsible for the accuracy or completeness of these exploration logs that were developed by others.

NW 201ST STREET SITE

Project: King County WTD I Brightwater Conveyance System

Project Location: King & Snohomish Counties, Washington

Contract Number: E23007E

Date(s) Drilled 4/8/03 - 4/15/03 J Geotechnical

Consultant Camp Dresser & McKee lnc.J ~~gged

Drilling Method/Rig TypeWireline/ Porta-drill Drilling Contractor…