1-Final_Geotech_Report_-_Site_5B-1_NW_55th_Street_encrypted.pdf

FINAL GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 5B

SITE 5B-1: NW 55TH STREET

SEATTLE, WASHINGTON

Project Number C601097 April 2025

707 South Plummer Street Seattle, Washington 98134

Page

1.0 INTRODUCTION

2.0 PURPOSE AND SCOPE OF WORK

3.0 SITE CONDITIONS

3.1 SITE SURFACE CONDITIONS

3.2 ENVIRONMENTAL SITE CONDITIONS

3.3 GEOTECHNICAL SITE CONDITIONS

3.3.1 Environmental Critical Areas

3.3.2 Mapped Geology

3.3.3 Subsurface Conditions

4.0 SEISMIC CONSIDERATIONS

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1 TEMPORARY EXCAVATIONS AND SHORING

5.1.1 Excavation Methods

5.1.2 Shoring Methods

5.1.3 Construction Vibration Considerations

5.2 TEMPORARY CONSTRUCTION DEWATERING

5.3 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR ECAS

5.3.1 Potential Slide Area ECAs

5.4 THRUST BLOCK AND BURIED STRUCTURE DESIGN

5.4.1 Thrust Block Design

5.4.2 Buried Structure Design

5.5 OTHER CONSTRUCTION CONSIDERATIONS

5.5.1 Subgrade Preparation

5.5.2 Pipe Bedding

5.5.3 Backfill and Compaction

5.5.4 Wet Weather Earthwork

5.5.5 Construction Drainage and Erosion Control

5.5.6 Environmental Media Handling and Disposal

6.0 LIMITATIONS AND ADDITIONAL SERVICES

7.0 REFERENCES

List of Figures (following text)

Figure 1 Site & Exploration Map Figure 2 Geology & ECA Map Figure 3 Thrust Block Design Figure 4 Buried Structure Design

Appendix A: Field Exploration Program Figure A-1 Key to Symbols and Terms Used on Boring Log (2 sheets) Figures A-2 B-101 Summary Exploration Log

Appendix B: Geotechnical Laboratory Testing

Appendix C: Historical Explorations

Attachment 1: Construction Dewatering Evaluation and Recommendations

FINAL GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 5B

SITE 5B-1: NW 55TH STREET

SEATTLE, WASHINGTON

1.0 INTRODUCTION

This report presents the results of our geotechnical investigation and recommendations for design and construction of Site 5B-1: NW 55th Street of the Seattle Public Utilities (SPU) Water Main Rehabilitation Package 5B project (project) located within the right-of-way (ROW) along NW 55th Street between 3rd Avenue NW and Baker Avenue NW in the Phinney Ridge neighborhood of Seattle, as shown on Figure 1. The project includes installation of an 8-inch restrained joint ductile iron pipe (RJDIP) water main along NW 55th Street and both 8-inch and 12-inch RJDIP within the intersection of NW 55th Street and 3rd Avenue NW. Based on our understanding of the 30% design plans, the majority of the water main is to be located approximately 5 feet below ground surface (bgs) with a maximum depth of 7 feet just east of the intersection with 3rd Avenue NW.

We have organized this report into five sections. The first two sections describe our understanding of the project and the purpose and scope of our work. The third and fourth sections describe the surface and subsurface conditions and seismic considerations for the project. The fifth section includes our geotechnical recommendations for design and construction of the project. Figures illustrating project features and design recommendations are provided at the end of the text. Field data are provided in Appendix A, geotechnical laboratory test results are provided in Appendix B and relevant historical explorations are provided in Appendix C.

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 geotechnical engineering and environmental recommendations to support design and construction of the project.

Our scope of work for this project included:

Reviewing readily available geologic, geotechnical, and environmental information for the site.

Completing a geotechnical exploration program consisting of one boring exploration and geotechnical laboratory testing.

Preparing this geotechnical engineering report that summarizes our investigation and recommendations.

Water Main Rehab Package 5B Site 5B-1: NW 55th Street Final Geotechnical Report

3.0 SITE CONDITIONS

We reviewed existing surface and subsurface information and completed one geotechnical exploration to develop an understanding of the geotechnical conditions that could affect design and construction of the water main improvements at the project. This section summarizes our understanding of the geotechnical conditions at the project site.

3.1 SITE SURFACE CONDITIONS

The project is generally located within the ROW along NW 55th Street between 3rd Avenue NW and Baker Avenue NW. Between these blocks, NW 55th Street is a concrete paved non-arterial street with single family residential dwellings situated to north and south. 3rd Avenue NW is a concrete paved arterial street bordered by single and multi-family dwellings. Between Baker Avenue NW and 3rd Avenue NW, site topography slopes moderately downward toward the west from 200 to 156 feet elevation. All elevations in this report are referenced to the NAVD88 vertical datum, unless noted otherwise.

3.2 ENVIRONMENTAL SITE CONDITIONS

We performed a cursory review on the Washington Department of Ecology (Ecology) What’s in My Neighborhood online mapping tool and Confirmed and Suspected Contaminated Sites List (CSCSL) database to identify potential sources of contaminated soil and groundwater near the site. According to Ecology’s database, a cleanup site (cleanup site ID [CSID] 12786) is listed in the CSCSL database approximately 300 feet west of the project at 404 NW Market Street.

As shown on Figure 2, CSID 12786 is located at an elevation approximately 30 feet lower than the project elevation. According to the 2015 initial investigation field report (Ecology 2015) available on Ecology’s website for CSID 12786, a complainant reported seeing paint being emptied into a hole in soil at an active construction site located on the north side of NW 55th between 4th Avenue NW and 5th Avenue NW. Ecology identified this construction site to be located at 404 NW Market Street. No soil samples were collected from the suspected location. Ecology listed the site as a suspected contaminated site due to the solvents in the photographs provided by the complainant and the unknown nature and extent of the dumping.

Based on the distance of the project to CSID 12786, the limited extent of the dumping shown in the photographs (hole in soil with a limited depth) and the difference of elevation between the project and CSID 12786, it is unlikely that contamination associated with CSID 12786 will be encountered at the project site and therefore no environmental investigations were conducted.

No other known cleanup sites, leaking underground storage tanks or environmental covenants are mapped within 500 feet of the project site.

3.3 GEOTECHNICAL SITE CONDITIONS

Our understanding of the geotechnical conditions at the project site is based on conversations with the project team, our review of existing information from sources such as the Seattle Department of Construction and Inspections (SDCI) GIS online mapping database, the geologic map of Seattle published by the United States Geological Survey (USGS) in 2005, the Washington State Department of Natural Resources historical subsurface exploration database, and SPU Geotechnical Engineering’s internal database of historical subsurface explorations. We also completed one new subsurface exploration and laboratory testing to supplement the existing geotechnical information.

A description of the existing information that we reviewed and our geotechnical subsurface exploration program are provided below.

3.3.1 Environmental Critical Areas

We reviewed the Seattle Department of Construction & Inspections (SDCI) GIS online map to determine if the project is within geotechnically relevant Environmentally Critical Areas (ECAs) or their buffers as defined in Chapter 25.09 of the Seattle Municipal Code (SMC). The ECAs that we consider relevant for design and/or construction of the project improvements include steep slope, potential slide area, abandoned landfill, known slide, and peat settlement prone areas.

A steep slope ECA buffer is defined as the area within 15-feet of the top and toe of the steep slope. According to the SDCI GIS online map, steep slope ECAs are mapped on the north and south side of NW 55th Street between Baker Avenue NW and 3rd Avenue NW.

Based on the mapped location of the steep slopes, the proposed work will not occur within any of the mapped steep slope ECAs or their buffers. A known slide area ECA is mapped approximately 300 feet south of the project and is associated with a mapped 1997 landslide that was the result of failing roadway fill for Baker Avenue NW near the intersection of 53rd Avenue NW. A potential slide area ECA is mapped on the slope to the south of the project site and is mapped within the westernmost end of the proposed improvements. Our recommendations for design or construction of the water main improvements within the potential slide area ECA is provided Section 5.3 of this report.

The location of geotechnically relevant ECAs in the vicinity of the project are shown on Figure 2.

3.3.2 Mapped Geology

The general geologic condition of the Puget Sound area is a result of glacial and non-glacial activity that occurred over the course of millions of years. The most recent glacial activity was the Vashon stade of the Fraser glaciation that ended about 10,000 years ago.

Following the Fraser glaciation, non-glacial erosion and deposition shaped the region.

Based on our review of the geologic map of Seattle (Troost, et al, 2005), the project is mapped within Vashon advance outwash deposits. During the Vashon stade of the Fraser glaciation, meltwater channels flowing from the toe of the glacier transported and deposited well sorted sands and gravels ahead of the advancing glacier. These advance outwash deposits were subsequently overridden and consolidated under the advancing ice-sheet.

In general, the soil encountered in the new and historical explorations agree with the mapped geology at the site.

3.3.3 Subsurface Conditions

SPU Geotechnical Engineering completed one boring designated B-101 on November 22, 2023. The boring was completed to a depth of 30 feet bgs. We completed geotechnical laboratory tests on select soil samples collected from the boring. We reviewed historical subsurface exploration logs in the site vicinity and determined that two historical explorations are geotechnically relevant to the project. Additionally, we reviewed historical roadway profiles in the Seattle Digital Infrastructure Records (SeaDIR) online database to gain an understanding of the project site history. Based on our review of a historical roadway profile along NW 55th Street prepared in 1907, NW 55th Street required regrading to construct the present-day roadway alignment. The historical profile along NW 55th Street indicates that between 3 and 8 feet of soil was removed during regrading of the roadway (SeaDIR Record 561-12).

Details of our subsurface exploration program and summary boring log are provided in Appendix A, geotechnical laboratory test results are provided in Appendix B. Logs of historical explorations are provided in Appendix C. The location of new and historical explorations in the vicinity of the site are shown on Figures 1 and 2.

Based on our exploration program and review of existing subsurface data, we have characterized the subsurface conditions as described in the following text. Soil deposits are listed in the order of stratigraphic sequence, with the youngest unit described first, followed by a description of groundwater conditions.

The conclusions and analyses that we provide in this report are based on subsurface soil and groundwater conditions interpreted from the new and historical explorations. The nature and extent of variations between the explorations and current 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.

Fill

We encountered, within boring B-101, soil that we interpret as fill from the ground surface to a depth of 9 feet bgs. The fill consists of very loose to loose, sand, sand with silt, and silty sand with varying amounts of gravel (SP, SP-SM, SM) 1. The natural water content within the fill ranged from 8 to 31 percent. A N-value2 of 5 was recorded for each of the three samples driven in the fill. The exploration summary logs for historical borings B-1 and B-2 document fill to a depth of 5 feet bgs with soil textures consistent with those described above.

Advance outwash

We encountered soil interpreted as advance outwash underlying the fill and extending to the final boring depth of 30 feet bgs. The advance outwash consists of medium dense fine sand (SP) over dense to very dense silty fine sand (SM). Silt was encountered at approximately 29 feet bgs. The natural water content within the advance outwash ranged from 25 to 32 percent with an average of 29 percent. The N-values within the advance outwash range from 23 to 56 with an average value of 36. The exploration summary logs for historical borings B-1 and B-2 document advance outwash below the fill and extending to a final boring depth of 26 and 22 feet bgs, respectively, with soil textures consistent with those described above.

Groundwater

At time of drilling boring B-101, we measured groundwater at 7.5 feet bgs (approximate elevation 190 feet). To allow for more accurate and ongoing groundwater measurements, we installed a groundwater monitoring well and developed the well using pump and surge methods.

It should be noted that monitoring well B-101 is near the upslope end of the project alignment and horizontal extrapolation of the subsurface conditions may not be valid.

Additionally, we understand that seepage has been observed in the past through joints and cracks in the pavement along NW 55th Street within the project area. It is unclear whether

1 Soil classification in accordance with the Unified Soil Classification System (USCS).

2 N-values are defined as the number of blows required to drive a 2.0-inch outside diameter sampler one foot, using a 140-pound hammer falling a distance of 30 inches. Refer to ASTM D-1586 (ASTM, 2007) seepage may be due to groundwater or other sources but may indicate groundwater may be present at or near ground surface within the project area.

Based on 6 groundwater level measurements taken in B-101 between November 2023 and April 2024, the depth to groundwater has ranged between 8.7 and 9.2 feet bgs. Based on the groundwater observations in B-101 and the presence of seepage through the pavement of NW 55th Street east of 3rd Avenue NW, we recommend that groundwater be expected at or near ground surface within the western half of the project area.

In general, we expect that groundwater will fluctuate throughout the year and be at its highest during the late winter and spring seasons and its lowest during the late summer and early fall seasons.

4.0 SEISMIC CONSIDERATIONS

The site is located in a seismically active area. We anticipate that seismic design of the water main improvements will be completed in accordance with Section 5.10.1 of the 2024 SPU Design Standards and Guidelines (DSG), which specifies the level of analysis and performance requirements for water main design and construction based on the water main class/criticality and the earthquake hazards that the water main is exposed to. The SPU DSG indicates that the earthquake hazards should be assessed for an earthquake with 2 percent probability of exceedance in 50 years, which corresponds to an average return period of 2,475 years. In this report we refer to this seismic event as the Design Event (DE).

We understand that the water main at Site 5B-1 is classified as an ordinary main. In accordance with the SPU DSG, site-specific seismic analysis is not required for an ordinary main, instead one of three performance requirements is selected based on the location of the main relative to mapped seismic hazards. Ordinary mains that are within areas with the potential for seismic or non-seismic permanent ground displacement must meet the requirements of Performance Specification 1. Ordinary mains that are within the Seattle Fault Zone (SFZ) or an SPU intense ground shaking region, defined as areas where the peak ground acceleration (PGA) for the DE is greater than or equal to 0.6g, must meet the requirements of Performance Specification 2. All other ordinary water mains have no seismic design requirements. Site 5B-1 is not within the SFZ nor within a SPU intense ground shaking region thus site-specific seismic analyses are not required for the proposed water main improvements. Portions of Site 5B-1 are within a Permanent Ground Displacement Area (potential slide area ECA) as defined in Table 5-13 of the SPU DSG. Design and construction considerations for ECAs are discussed in Section 5.3 of this report.

Although site-specific seismic analyses are not required for the proposed water main improvements, we reviewed the geotechnical site conditions for the site to determine the potential for permanent ground displacement due to liquefaction. The results of our review indicate that the potential for permanent ground displacement due to liquefaction induced settlement at Site 5B-1 is very low.

5.0 CONCLUSIONS AND RECOMMENDATIONS

This section presents our geotechnical engineering recommendations for design and construction 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;

Thrust Block and Buried Structure Design; and

Other Construction Considerations.

We developed these recommendations based on our understanding of the proposed project elements, as described in Section 1.0 and this section, and on subsurface conditions we interpreted from new and historical subsurface explorations. If there appear to be differences between our understanding of the project and the actual design, or if the nature or locations of the project elements change after completion of this study, we should be notified so we can confirm or change the recommendations.

5.1 TEMPORARY EXCAVATIONS AND SHORING

We understand that construction of the project will require temporary excavations that extend up to 7 feet bgs. In general, we recommend that the temporary excavations and shoring are implemented in accordance with Section 2-04 and 2-07 of the City of Seattle Standard Specifications for Road, Bridge and Municipal Construction (Standard Specifications). In accordance with Section 2-07.3(1), sloping or benching as a means of stabilizing excavations is not permitted within paved roadways, sidewalks, or other improved areas. Because the expected excavations are within a paved roadway, sloping and benching is not permitted and shoring methods should be used to stabilize the excavation.

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 recommendations for feasible excavation and shoring methods to aid in planning and design.

5.1.1 Excavation Methods

Based on our interpretation of subsurface conditions at the site, and the proposed maximum excavation depth, it appears that onsite excavations will be made in variable density sand to silty sand (fill) or medium dense to dense sand to sandy silt (advance outwash deposits). In general, we do not anticipate that specialty equipment will be required to excavate the site soil. 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. Debris should be anticipated within the fill.

5.1.2 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 protection 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 Exhibit A.

Based on our review of City GIS data, there are no known structures or utilities anticipated in the zone of influence of the expected excavations. In our opinion, given the relatively shallow excavation depths (less than 7 feet), trench safety systems may be suitable for trench support provided the groundwater can be maintained below the base of the excavation and no sensitive utilities are identified within the zone of influence of the trench. If groundwater cannot be maintained below the base of the excavation if

Exhibit A - Zone of Influence for Temporary Excavations settlement sensitive utilities are identified within the zone of influence, we recommend a trench support system be used for temporary excavation support. 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 the project site.

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.

5.1.3 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 if there are sensitive structures or utilities in close proximity to the project site.

5.2 TEMPORARY CONSTRUCTION DEWATERING

We anticipate that dewatering will be required to maintain groundwater a minimum of 2 feet below the excavation depth during construction of the proposed improvements. The excavation should be dewatered in accordance with Section 2-08 of the Standard Specifications.

Richard Martin Groundwater LLC (Richard Martin) completed hydrogeologic testing and dewatering analysis to provide estimated hydraulic conductivities for the site soil, anticipated dewatering rates, and recommendations for feasible dewatering methods.

Richard Martin’s recommendations are provided as Attachment 1 and are based on the assumption that non-watertight shoring is used for the excavation.

The anticipated dewatering volumes are affected by construction means and methods including the length of time the water is drawn down at any given location and the type of shoring that is used. The contractor should choose a dewatering method based on the available information and their selected construction techniques. If required, dewatering wells, vacuum well points, engineered sumps, and other external dewatering systems should be designed by a qualified professional engineer or geologist, and the dewatering plan should be submitted to us for review and approval.

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 no more than necessary for the proposed work to avoid damage to the adjacent utilities and structures. Based on the soil and groundwater conditions encountered in B-101 and indicated on the historical exploration logs, we anticipate that the potential for dewatering related settlement at the site is low.

5.3 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR ECAS

5.3.1 Potential Slide Area ECAs

As discussed above, a portion of the proposed water main improvements are located within a potential slide area ECA. Within this area, the topography is relatively flat, and the roadway grades on the east and west side of the intersection slope moderately down to the west. Based on our understanding of the location of the proposed water main improvements within the potential slide area ECA, site subsurface conditions, and site topography in the area of the proposed improvements, special design and construction considerations for the potential slide area ECA are not warranted and the proposed water main within the potential slide area ECA does not need to meet the requirements of Performance Specification 1.

5.4 THRUST BLOCK AND BURIED STRUCTURE DESIGN

We understand that the proposed water main improvements will include thrust blocks and other buried structures. When developing design recommendations for these features, we assumed that the invert of water mains and base of thrust blocks and buried structures will be between 3 and 7 feet bgs.

5.4.1 Thrust Block Design

Thrust blocks resist lateral loads through a combination of base friction and passive lateral soil resistance. Our recommendations for the allowable coefficient of base friction to resist sliding for a thrust block bearing directly on the anticipated soil at the site is provided on Figure 3. The passive lateral soil resistance can be calculated using equivalent fluid densities for the soil surrounding the thrust block. The resulting earth pressure has a triangular distribution, that increases linearly with depth over the height of the thrust block. Equivalent fluid densities for thrust block design are provided on Figure 3.

5.4.2 Buried Structure Design

Lateral Earth Pressures

Buried structures 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 site are provided on Figure 4. 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 4.

Bearing Capacity and Settlement

We estimated the allowable bearing capacity for buried structures based on the anticipated soil and groundwater conditions between 3 and 7 feet bgs at the site. We anticipate that soft or loose soil that is generally unsuitable for subgrade support for buried structures will be encountered at the site. We recommend that the subgrade is improved by over excavating one foot and replacing the excavated soil with compacted structural fill meeting the gradation requirements of Type 17 material in Section 9-03.14, Mineral Aggregate Chart in the Standard Specifications if the subgrade is not saturated or quarry spalls if the subgrade is saturated. The structural fill should be placed and compacted in accordance with the recommendations provided in Section 5.5.3. We recommend an allowable bearing capacity of 3,000 pounds per square foot (psf) for buried structures bearing on improved at the site. If the subgrade soil encountered during construction is saturated or is softer or less dense than the anticipated soil described in Section 3.3.3, 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.

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 4. Conservatively we recommend the groundwater table is assumed to be at the ground surface. We recommend neglecting the frictional resistance between the structure walls and the adjacent soil. We recommend minimum factors of safety against static and seismic uplift of 1.2 and 1.0, respectively.

5.5 OTHER CONSTRUCTION CONSIDERATIONS

5.5.1 Subgrade Preparation

Subgrade preparation for areas where water mains, maintenance structures, valve chambers, and pavements will be built 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. We should be contacted to observe the subgrade, to evaluate the adequacy of the bearing stratum and to confirm that subsurface conditions are suitable for the recommended allowable bearing capacities.

For large areas, this evaluation should be performed by proof-rolling the exposed subgrade with a fully loaded dump truck. For smaller areas where access is restricted, the subgrade should be evaluated by probing the soil with a steel rod. A layer of mineral aggregate may be used to protect the subgrade once it is exposed.

Soft/loose soils identified during subgrade preparation should be compacted to a firm and unyielding condition or stabilized. Typical subgrade stabilization measures include over excavation 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 fabric can also be used to minimize the required thickness of imported fill. We should be contacted to evaluate the subgrade and provide additional recommendations for subgrade improvement.

5.5.2 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 5.5.3.

5.5.3 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 work 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 we anticipate will be encountered at the site generally has more than 5 percent fines content. Therefore, in our opinion, it is unlikely that the excavated material at the site 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 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. The Contractor is responsible for protecting stockpiled material from becoming unsuitable while it is on site.

Fill placed in all structural areas and for five feet around such areas, should be compacted to a minimum of 95 percent of the maximum dry density (MDD) as determined by test method ASTM D1557. Within a lateral distance of three feet of any retaining wall or subgrade wall, smaller, possibly hand-held equipment should be used in conjunction with thinner soil lifts to achieve the required compaction and limit compaction forces on the wall.

The procedure to achieve the specified minimum relative compaction depends on the size and type of compacting equipment, the number of passes, thickness of the layer being compacted, and certain soil properties. When the size of the excavation restricts the use of heavy equipment, smaller equipment can be used, but the soil must be placed in thin enough lifts to achieve the required compaction. A sufficient number of in-place density tests should be performed as the fill is placed to verify the required relative compaction is being achieved.

5.5.4 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;

Immediately place and compact a suitable thickness of clean structural fill 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.

5.5.5 Construction Drainage and Erosion Control

Surface runoff and erosion at the work 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.

5.5.6 Environmental Media Handling and Disposal

Based on environmental information publicly available on Ecology’s database, it is our opinion that soil and groundwater in the project area is unlikely to be contaminated by nearby known cleanup sites. No special handling requirements pertaining to exposure to contaminants are anticipated. However, should evidence of contamination be encountered during excavation activities, including visual staining, odors, or sheen, the contractor retained to perform the work should adhere to Section 1-07.29 Discoveries of Contaminated Material(s), Dangerous Waste(s) and Toxic Substances Control Act (TSCA) Waste(s) and any other relevant sections of the Standard Specifications.

6.0 LIMITATIONS AND ADDITIONAL SERVICES

This geotechnical report is intended to provide information and recommendations to support preliminary engineering activities for this project. 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, 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.

The conclusions and recommendations in this report are based on geotechnical and environmental information obtained from subsurface explorations completed by SPU Geotechnical Engineering and publicly available documents. SPU Geotechnical Engineering is not responsible for the accuracy or completeness of publicly available documents. The conclusions and interpretations presented in this report should not be construed as a warranty of the subsurface conditions.

We recommend that a Geotechnical Engineer and an Environmental Professional from SPU Geotechnical Engineering review the Project Manual to verify that our recommendations have been interpreted and implemented as intended. Sufficient monitoring, testing, and consultation should be provided during construction to confirm that the conditions encountered are consistent with those indicated in this report and to verify that the geotechnical and environmental aspects of construction comply with the Project Manual. Recommendations for design changes will be provided should conditions revealed during construction differ from those anticipated.

We appreciate the opportunity to be of service.

Sincerely, SPU GEOTECHNICAL ENGINEERING

Aaron Clark, L.G. Kori Wilson, P.E.

Senior Geologist Senior Geotechnical Engineer

Ophélie Encelle Senior Environmental Specialist

04/10/25

04/10/2025 https://seattlegov.na1.adobesign.com/verifier?tx=CBJCHBCAABAADERxOCqnDv7KlvHrwW5B2RvKe_x2jd3e https://seattlegov.na1.adobesign.com/verifier?tx=CBJCHBCAABAADERxOCqnDv7KlvHrwW5B2RvKe_x2jd3e https://seattlegov.na1.adobesign.com/verifier?tx=CBJCHBCAABAADERxOCqnDv7KlvHrwW5B2RvKe_x2jd3e

7.0 REFERENCES

American Society of Civil Engineers, ASCE 7 Hazard Tool, online application. Accessed March 2023. https://asce7hazardtool.online/

ASTM International, 2021. American Society of Testing Materials Annual Book of Standards, Vol. 4.08, West Conshohocken, PA.

City of Seattle, 2023, Standard Plans for Municipal Construction 2023 Edition.

City of Seattle, 2023, Standard Specifications for Road, Bridge and Municipal Construction 2023 Edition.

Seattle Department of Construction & Inspections GIS. City of Seattle, Online Geodatabase: http://seattlecitygis.maps.arcgis.com. Accessed May 2024.

Seattle Public Utilities, 2024. SPU Design Standards and Guidelines.

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.

United States Geological Survey. Unified Hazard Tool, online application. Accessed May 2024. https://earthquake.usgs.gov/hazards/interactive/.

Washington State Department of Ecology, Cleanup Site Details Cleanup Site ID: 12786 Accessed May 2024.

https://apps.ecology.wa.gov/cleanupsearch/reports/cleanup/sitedetails/12786

Washington State Department of Ecology, 2015, NW Market St Paint Dump - Initial Investigation Field Report. February 3.

Washington State Department of Ecology, What’s In My Neighborhood, online application. Accessed May 2024. https://fortress.wa.gov/ecy/neighborhood/.

Washington Department of Natural Resources, Subsurface Geology Information System, online geodatabase. Accessed May 2024. https://geologyportal.dnr.wa.gov/.

Geotechnical Engineering

B-101

NW

TH

PL

3R D A

VE

N

W

BAKER AVE NW

NW 55TH ST

B-2 (2016)

B-1 (2016)

224 220

304 234

5411B 5415A5415

5413 5415B

5415C 5417 5419

307 5421

5423215

3035419

D oc um en t P at h:

P

:\G eo te ch

\G E

O

TE

C H

\G eo te ch ni ca l P ro je ct s\ S

P U

\W at er m ai n\

(C

) W at er m ai n

R eh ab P ac ka ge

B -1

,2 ,3

(K W

)\P re lim E ng \F ie ld

L ab W or k\

S ite

M ap s\ W

M P

5_ Si te M ap _A

JC

.m xd

Water Main Rehab Package 5B Site 5B-1: NW 55th Street

Seattle, Washington

SITE AND EXPLORATION MAP

FIGURE 1

May 2024

Project No. C601097

25 0 25 5012.5

Feet

NAVD88-

Project Area

!< Boring with Piezometer

!( Historical Explorations

Arterial Streets

Residential Streets

Pavement Edge

Distribution Line

Distribution Main

Topography - 10 Foot

Topography - 2 Foot

F Addresses

Parcels

NW Market St Paint Dump CSID 12786)

B-101

NW 53RD ST

NW 54TH ST

4T H A

VE

N

W

3R D A

VE

N

W

2N D A

VE

N

W

BAKER

AV

E N W

NW

TH

PL

NW 55TH ST

NW MARKET ST

Qva B-2 (2016)

B-1 (2016)

D oc um en t P at h:

P

:\G eo te ch

\G E

O

TE

C H

\G eo te ch ni ca l P ro je ct s\ S

P U

\W at er m ai n\

(C

) W at er m ai n

R eh ab P ac ka ge

B -1

,2 ,3

(K W

)\P re lim E ng \F ie ld

L ab W or k\

S ite

M ap s\ W

M P

5_ S ite M ap _A

JC

.m xd

Water Main Rehab Package 5B Site 5B-1: NW 55th Street

Seattle, Washington

GEOLOGY AND ECA MAP

FIGURE 2

May 2024

Project No. C601097

50 0 50 10025

Feet

NAVD88-

Legend

Project Area

!< Boring with Piezometer

!( Historical Explorations

! Confirmed and Suspected Contaminated Sites

Arterial Streets

Residential Streets

Pavement Edge

Distribution Line

Distribution Main

Topography - 10 Foot

Parcels

((\ Known Slide (Initiation Point)

Known Slide (Affected Property)

Potential Slide Area

Steep Slope (40%)

Qva Vashon advance outwash deposits

In se rt Fi le P at h

FIGURE 3

April 2025

Project No. C601097

Water Main Rehab Package 5B Site 5B-1: NW 55th Street

Seattle, Washington

Lateral Earth Pressures for Thrust Block Design

Definition of Variables:

d1 = depth below ground surface to top of thrust block dw = depth below ground surface to groundwater d = depth below ground surface to base of thrust block A1 = active equivalent fluid densities above groundwater table A2 = active equivalent fluid densities below groundwater table P1 = passive equivalent fluid densities above groundwater table P2 = passive equivalent fluid densities below groundwater table

Notes:

1. All dimensions are in feet.

2. Active equivalent fluid densitites provided in the table below are ultimate values.

3. Passive equivalent fluid densities provided in the table below are allowable values and includea Factor of Safety of 1.5.

4. Use design groundwater levels in the main body of the report to determine dw for each design zone.

Recommended Equivalent Fluid Densities in PCF

Passive

P1 (above GWT)

P2 (below GWT)

A2 (below GWT)

A1 (above GWT)

Active Coefficient of Base Friction

Site 5B-1 40 18 221 101 0.37 d (FT)

Ground Surface

Adf

Adw + B(d - dw)

Burried Structure dw (FT) df (FT)df (FT)

62.4(d - dw) 62.4(d - dw)

(Uplift Pressure)

Adw

D

Cdf

At -Rest Earth Pressure

Hydrostatic Pressure

Surcharge Pressure

In se rt Fi le P at h

FIGURE 4

June 2024

Project No. C601097

Water Main Rehab Package 5B Site 5B-1: NW 55th Street

Seattle, Washington

Loads For Buried Structure Design

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 pressurse are provided

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

4. Surcharge pressure includes a 250 PSF vehiclular and construction surcharge. Additional surficial surcharge loads should be considered on a case by case basis.

5. All dimensions are in feet.

Recommended Loads

A

(PSF)

In-Situ Soil Properties

Unit Weight (PCF) Friction Angle

At Rest Earth Pressure

Coefficient (Ko) B

(PSF)

C

(PSF)

D

(PSF)

Site 5B-1 115 29 0.52 59 27 120 130

APPENDIX A

FIELD EXPLORATION PROGRAM

SPU GEOTECHNICAL ENGINEERING A-1

APPENDIX A

FIELD EXPLORATION PROGRAM

GEOTECHNICAL SOIL BORINGS

We explored subsurface conditions at the project by completing one boring using hollow stem auger drilling techniques. The exploration, designated B-101, was completed to a depth of 30 feet on November 22, 2024. The approximate location of the exploration is illustrated in Figures 1 and 2 in the main body of the text. The exploration was located relative to existing site features. The approximate ground surface elevation at the exploration location was determined based on City GIS and is referenced to the NAVD88 datum.

The boring was drilled by Holocene Drilling, Inc. of Puyallup, Washington, using Diedrich D-50 low-tower drill rig with 4-1/4-inch inner diameter (ID) auger flights. Split spoon soil samples were obtained from the boring at 2.5-foot and 5-foot depth intervals using the Standard Penetration Test (SPT, ASTM D-1586). The 2.0-inch outside diameter (OD) SPT sampler was driven into the soil a distance of 18 inches using a 140-pound drive automatic trip hammer falling a distance of 30 inches.

Recorded blows for each 6-inches of sampler penetration (blow counts) are shown on the summary log in this appendix. The blow counts provide a qualitative measure of the relative density of cohesionless soil, or the relative consistency of fine-grained soils. Representative portions of all recovered samples were placed in sealed containers and transported to our laboratory for further observation and testing.

We installed a 2-inch diameter Schedule 40 PVC monitoring well in B-101 to facilitate more accurate measurements of the groundwater, and for hydrogeologic testing, if needed. The monitoring well was installed with a 10-foot-long 0.010-slotted screen, 10/20 or equivalent silica sand filter pack, and bentonite seal. A traffic-rated flush-mount monument was installed at the ground surface.

The monitoring well should be properly decommissioned in accordance with Washington Administrative Codes (WACs) at the conclusion of the project.

The results of the exploration are provided on the individual summary boring log, which is included here as Figure A-2. A key to the symbols and terms used on the summary log is included as Figure A-1.

P:\MatLab\Secure\GEOTECH\FORMS\Office\Boring Log Key

SOIL CLASSIFICATION AND

EXPLORATION LOG KEY

FIGURE A-1

(Sheet 1 of 2)Geotechnical Engineering

NOTES:

UNIFIED SOIL CLASSIFICATION SYSTEM - ASTM D2488

LETTER

SYMBOL GROUP NAMEGROUP

SYMBOLMAJOR DIVISION

GRAVEL WITH

BETWEEN 5%

AND 15% FINES

GW-GM

GP-GC

HIGHLY ORGANIC SOILS PT

COARSE

GRAINED

SOILS

CONTAINS

LESS THAN

50% FINES

SAND AND

SANDY SOILS

MORE THAN

50% OF

COARSE

FRACTION

PASSING ON

NO. 4 SIEVE

GRAVEL AND

GRAVELLY

SOILS

MORE THAN

50% OF

COARSE

FRACTION

RETAINED ON

NO. 4 SIEVE

FINE

GRAINED

SOILS

CONTAINS

MORE THAN

50% FINES

SILT

AND

CLAY

TPTOPSOIL

GRAVEL WITH

< 5% FINES

GRAVEL WITH

> 15% FINES

SAND WITH

< 5% FINES

SAND WITH

BETWEEN 5%

AND 15% FINES

SAND WITH

> 15% FINES

LIQUID LIMIT

GREATER

THAN 50

LIQUID LIMIT

LESS THAN 50

GW

MH

SW

SP

SW

GW

CH

OH

GP

GP

GW-GC

GP-GM

GM

GC

SC

CL

OL

SP

SW-SM

SW-SC

SP-SM

SM

SP-SC

ML

CL

ML

Well-graded GRAVEL

Well-graded GRAVEL WITH SAND

Poorly graded GRAVEL

Inorganic SILT, low plasticity

Inorganic SILT WITH SAND, low plasticity

Elastic inorganic SILT, moderate to high plasticity

PEAT soils with high organic contents

TOPSOIL

ORGANIC SILT, low plasticity

Well-graded GRAVEL WITH SILT

Well-graded GRAVEL WITH CLAY

Poorly graded GRAVEL WITH SILT

Well-graded SAND

Well-graded SAND WITH GRAVEL

Poorly graded SAND

Poorly graded SAND…