4-Geotech_Report_WMRHP5B_Site_5B-4_Beacon_Site_V2.pdf

GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 5B

BEACON SITE 5B-4

SEATTLE, WASHINGTON

Project Number C601097 February 2025

707 South Plummer Street Seattle, Washington 98134

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

2.0 PURPOSE AND SCOPE OF WORK

3.0 METHODOLOGY

3.1 FIELD INVESTIGATION

3.2 PREVIOUS STUDIES

3.2 LABORATORY TESTING

4.0 SITE CONDITIONS

4.1 SITE SURFACE CONDITIONS

4.2 ENVIRONMENTAL CRITICAL AREAS AND KNOWN CONTAMINATED SITES

4.3 SITE SUBSURFACE CONDITIONS

4.2.1 Fill

4.2.2 Blakely Formation

4.2.3 Groundwater Conditions

5.0 SEISMIC CONSIDERATIONS

5.1 SEISMIC SETTING

5.2 SEISMIC BASIS OF DESIGN

5.3 SEISMIC DESIGN PARAMETERS

5.4 SEISMICALLY INDUCED GEOTECHNICAL HAZARDS

6.0 CONCLUTIONS AND RECOMMENDATIONS

6.1 GENERAL

6.2 UNDERGROUND UTILITIES

6.2.1 Subgrades

6.2.2 Bedding

6.3 VAULT STRUCTURE BEARING CAPACITY

6.4 RETAINING SYSTEMS AND VAULT WALLS

6.4.1 Lateral Earth Pressures

6.5 PAVEMENTS

6.6 CONSTRUCTION CONSIDERATIONS

6.6.1 Excavation

6.6.2 Dewatering

6.6.3 Temporary Slopes and Shoring

6.6.4 Shoring Related Settlement

6.6.5 Subgrade Preparation

6.6.6 Backfill and Compaction

6.6.7 Construction Drainage and Erosion Control

7.0 LIMITATIONS AND ADDITIONAL SERVICES

8.0 REFERENCES

SPU GEOTECHNICAL ENGINEERING

ii

List of Tables (within text)

Table 1 Seismic Design Parameters Table 2 Lateral Earth Pressures Table 3 Seismic Surcharge Loads

List of Figures (following text)

Figure 1 Site & Exploration Map Figure 2 Geology & ECA Map

Appendix A: Historical Explorations

Appendix B: Field Exploration Logs Figure B-1 Key to Symbols and Terms Used on Exploration Logs (2 sheets) Figures B-2 through B-4 Summary Hand Boring Logs

GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 5B

BEACON SITE 5B-4

SEATTLE, WASHINGTON

1.0 INTRODUCTION

Seattle Public Utilities (SPU) Geotechnical Engineering was retained to complete a geotechnical engineering study for the SPU Water Main Rehabilitation Package 5B (WMRP5B) - Beacon Site Project (project) in the Beacon Hill neighborhood of Seattle, Washington. The project location is shown on Figure 1, Site & Exploration Map. We understand that the improvements include the installation of a pressure regulating station within the public right of way (ROW) to alleviate the diminished service with the out of service Albro feed under the Cleveland High School sports field. The proposed pre-cast concrete pressure regulating vault will have a footprint of approximately 7 feet by 11 feet and will be installed to a depth of approximately 8 feet below the roadway surface. The vault will connect to the existing 8-inch ductile iron (DI) water main.

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 work site. The fifth section includes our geotechnical recommendations for design and construction of the water main improvements. Tables throughout the report provide data that is 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 and field exploration data are presented in Appendix B.

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 recommendations to support design and construction of the project.

Our scope of work included:

Reviewing readily available geotechnical/geologic and environmental information for the project Site and vicinity;

Conducting a geotechnical exploration program including the completion of three hand auger explorations; and, Preparing this report, which summarizes our investigations, geotechnical assessments, analysis, conclusions, and recommendations.

WMRHP5B Project –Beacon Site 5B-4 Geotechnical Report

3.0 METHODOLOGY

3.1 FIELD INVESTIGATION

SPU Geotechnical Engineering personnel conducted subsurface explorations on October 24th, 2023, by completing three soil hand auger borings (HA-101 to HA-103) with dynamic cone penetration (DCP) tests. The hand auger borings for this project provide subsurface information for design recommendations of the proposed vault, pavement, and construction considerations.

We used a 2¾-inch inner diameter (ID) hand auger to complete our explorations. Due to shallow refusal, the soil borings ranged from 2 to 3 feet deep. Hand auger samples were collected continuously to depth. Representative portions of the recovered samples were placed in sealed containers and transported to our laboratory for further observation and testing. Soils were classified in general accordance with ASTM D-2488 Standard Practice for Description and Identification of Soils (Visual-Manual Procedure). Summary exploration logs are provided in Appendix A.

A Sowers Dynamic Cone Penetrometer (DCP) was used to provide a qualitative measure of the relative density of cohesionless soil and the relative consistency of fine-grained soil. The Sowers DCP uses a 15-pound steel mass falling 20 inches to drive a 1.5-inch diameter 45-degree cone into the bottom of a hand augered hole. The cone is driven a total distance of between 1.75 and 4.25-inches into the soil. Recorded blows for each 1.75 inches of cone penetration are shown on the summary logs. The Sowers n-value shown on the summary logs is the average of the recorded blows for each 1.75-inch interval.

The current hand boring locations are shown on Figures 1 and 2. The summary logs of the subsurface explorations are included as Figures B-2 through B-4. Figure B-1 is a key to the terms and symbols used on the logs.

3.2 PREVIOUS STUDIES

As part of our scope of services we performed a search of readily available geotechnical reports and explorations in the vicinity of the Site. Our search found 5 historical explorations conducted within 450 feet of the proposed vault. The boring logs are presented in Appendix A and shown on Figure 2.

3.2 LABORATORY TESTING

We determined the natural water content of select samples in general accordance with ASTM D2216, Standard Test Method for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. The natural water content within the fill ranged from 9 to 12 percent. Test results are graphically indicated at the appropriate sample depth on the summary logs in Appendix A.

4.0 SITE CONDITIONS

4.1 SITE SURFACE CONDITIONS

The project site is located within the paved S Graham St minor arterial westbound travel lane at an approximate elevation of 138 feet. The two-way S Graham St roadway at the project site is classified as 12% slope (Approx. 21% grade). The proposed vault will be located approximately 70 feet west of hydrant (1930 S Graham St - E13) and 175 feet east of the intersection of S Graham St and Swift Ave S.

All elevations in this report are referenced to the NAVD88 vertical datum, unless noted otherwise. We assume that the elevations reported within the historical logs are the Old (Obsolete) City of Seattle Datum. Per City of Seattle Standard Plan 001, the Seattle Datum varies between 9.1 and 9.9 feet below NAVD88 depending on the location in Seattle.

The road surface is asphalt concrete (AC) overlying Portland Cement Concrete (PCC).

4.2 ENVIRONMENTAL CRITICAL AREAS AND KNOWN CONTAMINATED SITES

The project site is located adjacent to a Steep Slope Environmentally Critical Areas (ECAs) as mapped by the Seattle Department of Construction and Inspections (SDCI).

The ECAs are shown on Figure 2. We reviewed the Washington State Department of Ecology (Ecology) What’s in My Neighborhood online mapping tool and database to identify potential sources of contaminated soil or groundwater near the site. Our review did not identify any cleanup sites within 500 feet of the site.

4.3 SITE SUBSURFACE CONDITIONS

The general geologic condition of the Puget Sound region is a result of glacial and non-glacial activity that occurred over the course of millions of years. Review of a geologic map (Troost, et al, 2005) indicates that the project alignment is underlain by the Blakely formation deposits. The geologic units within the project area are shown on Figure 2. The Blakely formation consists of medium to coarse-grained sandstone and minor siltstone and can be fresh to highly weathered. The mapped geologic overprints across the site consist of mass wastage and landslide deposits.

Based on our exploration program and review of historical exploration logs, we have characterized the subsurface conditions as described in the following text. Below are descriptions of the soil deposits encountered in the historical and current explorations in the order of stratigraphic sequence, with the youngest unit described first, followed by a description of groundwater conditions.

4.2.1 Fill

Fill material consisting of very loose to dense brown to gray silty sand with varying amounts of gravel (SP, SM, SW) was observed in historical explorations TP-1 (approx.

elevation 124 feet, reported elevation 110 feet), TP-2 (approx. elevation 126 feet, reported elevation 117 feet), TP-3 (approx. elevation 158 feet, reported elevation 147 feet), and D-69 (approx. elevation 118 feet) and all current explorations. Fill consisting of soft to stiff sandy silt (ML) was encountered within historical exploration B-1 (approx.

elevation 122 feet, reported elevation 110 feet). Numerous organics and construction debris (wire, asphalt fragments) was encountered within the current explorations. Large gravels and scattered cobbles (3-inches) were observed in all current hand augers at termination depth. The fill extended to the depth augured in HA-101 (3 feet) and HA-102 (2 feet), and HA-103 (2 feet). Occasional to numerous weathered sandstone and siltstone (Blakely Formation) fragments were encountered within the fill material at depths of about 1.5 to 3 feet depth below ground surface (bgs). We interpret the fill to potentially be reworked fill material that included Blakely Formation deposits and was placed within the shoulder near the project site during grading or utility trench excavation. N-values1 for the fill within historical exploration B-1 unit, ranged from 5 to 50 blows per 3-inches of driven sample.

4.2.2 Blakely Formation

Weathered Blakely Formation which consisted of brown sandstone was observed within historical exploration D-69 from the surface to the bottom of the exploration at 5.6 feet bgs. Weathered brown siltstone was observed below the fill material in historical borings TP-2 and TP-3 at depths of 8 feet and B-1 at a depth of 57.5 feet bgs. The siltstone extended to the termination depths of each exploration. The N-value for this unit, within the historical exploration B-1 was 46. The number of blows of the described DCP equipment, ranged from 33 to 38 per 1.75 inches of penetration of the cone.

4.2.3 Groundwater Conditions

Groundwater was not observed in the hand augers during the field investigation performed on October 24, 2023.

Groundwater was not encountered in historical explorations TP-1 to TP-3, or D-69 but was within B-1 at a depth of approximately 7.5 feet bgs. Groundwater depths are anticipated to fluctuate at the project site based on time of year and location.

1 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)

5.0 SEISMIC CONSIDERATIONS

The site is in a seismically active area. In this section we discuss the seismic setting at the project site, provide seismic design parameters, and discuss earthquake-induced geotechnical hazards at the site including fault rupture, liquefaction, post-liquefaction vertical settlement, and lateral spreading.

5.1 SEISMIC SETTING

The seismicity of Puget Sound is dominated by the Cascadia Subduction Zone (CSZ) in which the offshore Juan de Fuca plate subducts beneath the continental North American plate. Three main types of earthquakes are typically associated with subduction zone environments: crustal, intraplate, and interplate.

Seismic records in the Puget Sound area clearly indicate the existence of a distinct shallow zone of crustal seismicity that may have surficial expressions and can extend to depths of 25 to 30 kilometers (km). Several minor earthquakes occur in the area each year, most of which are not even felt. However, some of the shallow faults are capable of producing significant, damaging earthquakes. Perhaps the most notable of these faults are the Seattle Fault Zone (SFZ) and the South Whidbey Island Fault Zone (SWIFZ).

Research indicates that both the SFZ and the SWIFZ are capable of producing an earthquake with a magnitude 7.0 or higher which, given the shallow depth and proximity to the Seattle urban area, could produce intense shaking at the project site. The site is located within the SFZ and approximately 18 km south of the SWIFZ.

A deeper zone of seismicity is associated with the subducting Juan de Fuca plate and produces intraplate earthquakes at depths of 40 to 70 km beneath the Puget Sound region (e.g., the 1949 Western Washington magnitude 7.1, 1964 Olympia magnitude 6.7, and 2001 Nisqually magnitude 6.8 earthquakes), and interplate earthquakes, at depths that can range from 60 km to over 500 km, near the Washington coast (e.g., the 1700 Cascadia earthquake with an approximate magnitude of 9.0).

5.2 SEISMIC BASIS OF DESIGN

We expect that the proposed vault will be designed in accordance with the 2018 Seattle Building Code (SBC). The SBC requires that structures be designed for inertial forces induced by earthquake motions in accordance with ASCE 7-16 (ASCE 7). The basis of design for ASCE 7 is two-thirds of the hazard associated with the Risk-Targeted Maximum Considered Earthquake (MCER), with 2 percent probability of exceedance in 50 years, which corresponds to an average return period of 2,475 years. The ASCE 7 event is referred to as the Design Event (DE).

5.3 SEISMIC DESIGN PARAMETERS

We developed seismic design parameters at the ground surface using code-based methods provided in ASCE 7. Characterization of the soil profile type is required in ASCE 7 to determine the site class definition. Based on standard penetration test (SPT) data collected from new and historical exploration logs, we determined the soil profile corresponds to Site Class D. In accordance with ASCE 7, site class determinations are based on a weighted average of the SPT blow counts observed to a depth of 100 feet bgs.

Because the explorations for the project terminated at less than 100 feet bgs, we assumed the material density below the deepest sample is the same as that of the deepest sample for our determination of the site class.

We obtained mapped spectral accelerations for peak ground acceleration, short-period, and 1-second period (PGA, Ss, and S1), for the project using a web interface developed by the California Office of Statewide Health Planning and Development, and the Structural Engineers Association of California which retrieves data from the 2014 National Seismic Hazard Maps published by the United States Geological Survey. We then determined appropriate site coefficients (FPGA, Fa, and Fv) using ASCE 7 Tables 11.8-1, 11.4-1, and 11.4-2.

The site modified MCER response accelerations (PGAM, SMS, SM1) were determined by applying the site coefficients to the mapped spectral accelerations for the site. In accordance with requirements provided in ASCE 7 Supplement 3 for structures on Site Class D soil profiles, we increased the SM1 value by fifty percent, instead of completing a site-specific hazard analysis.

Finally, we determined the DE spectral response accelerations (PGAD, SDS, SD1) by reducing the MCER response accelerations by a factor of 2/3. The seismic design parameters for the project site are provided in Table 1.

Table 1—Seismic Design Parameters

Site Class

Mapped Spectral Accelerations, (g)

Site Coefficients MCER Response

Accelerations, (g) Design Response Accelerations, (g)

PGA SS S1 FPGA Fa Fv PGAM SMS SM1 PGAD SDS SD1

D 0.65 1.52 0.53 1.1 1.0 1.77 0.72 1.52 0.93 0.48 1.01 0.93

In accordance with Section 11.4.8 of ASCE 7, based on the site class determination and mapped spectral accelerations, a site-specific hazard analysis would need to be completed to determine seismic design parameters for the proposed buried vault walls because the structure is on a soil profile corresponding to Site Class D with a mapped spectral acceleration for 1-second period greater than 0.2g. However, there are three potential exceptions to this requirement for structures that are not seismically isolated and do not include a damping system. We anticipate that the second exception is applicable to the proposed vault. The second exception indicates that for a structure with a fundamental period less than 1.5 times the short period transition period (Ts) for the site, a site-specific analysis is not required, as long as the value of the seismic response coefficient (Cs) used for seismic design of the structure is calculated using equation 12.8-2 of ASCE 7. If any of our assumptions are incorrect, or if the Design Team does not intend to comply with the requirements of this exception, the design parameters provided for the proposed precast vault in Table 1 may not be valid and we should be notified so that we can determine appropriate seismic design parameters.

5.4 SEISMICALLY INDUCED GEOTECHNICAL HAZARDS

Potential seismically induced geotechnical hazards may include surface fault rupture, liquefaction, lateral spreading, and landslides. Our review of these hazards is based on the soils indicated on the historical exploration logs, regional experience, and our knowledge of local seismicity.

The project site is located within the SFZ. Although there is some potential for surface rupture at the site, the risk is difficult to quantify, and in our opinion, special design considerations for surface fault rupture are not warranted for this project.

As part of our seismic evaluation, we analyzed the potential of the soil within the project site to liquefy. Liquefaction is a momentary loss of some portion of soil shear strength during a seismic event. The loss of shear strength can potentially cause settlement or lateral spreading.

Groundwater was not encountered in our current explorations or within the closest historical explorations TP-1 or TP-2. Based on our understanding of the subsurface materials and conditions at the site, we anticipate that the soils below assumed groundwater table elevations are too dense or hard to be susceptible to liquefaction. As a result, the risk of liquefaction is considered low and does not need to be considered for design. Based on the low risk of liquefaction, it is our opinion that the risk of post-seismic settlement, and lateral spreading at the site is also low and does not need to be considered for design.

6.0 CONCLUTIONS AND RECOMMENDATIONS

6.1 GENERAL

The primary geotechnical concerns for the project site are subgrade suitability, lateral earth pressures and foundation support for the proposed vault structure.

Based on the results of our investigations and analyses, it is our opinion that the proposed project is feasible from a geotechnical perspective, within the limitations presented below, provided the recommendations of this report are incorporated into the design and construction.

6.2 UNDERGROUND UTILITIES

We understand that SPU plans to install a precast concrete pressure regulating vault with connections to the existing 8-inch DI pipe within the S Graham St ROW using a traditional open cut excavation method.

6.2.1 Subgrades

While our current hand auger explorations were terminated at shallow depths of 3-feet and less due to refusal, based on geologic mapping, logs of historical explorations, and the relatively light loading of the proposed vault, we expect that the majority of the existing fill soil should be suitable for installation of the vault and associated underground utilities, provided subgrades remain in an undisturbed condition, and the pipes and structures are bedded as described in the following section.

Determining whether the refusal that was met, within all three of the SPU Geotechnical Engineering hand auger explorations, resulted from a hard layer (upper zone of the weathered Blakely Formation siltstone/sandstone) or from individual fragments of weathered rock within the zone of existing fill, is not possible. We anticipate that it is likely that the weathered Blakely Formation bedrock is present at relatively shallow depths within and near the site (and possibly within the upper 8 feet – this would indicate that the vault would likely bear on weathered siltstone/sandstone). Review of the nearest explorations and corresponding elevations indicate that encountering the weathered bedrock, at relatively shallow depths, appears likely.

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

If unsuitable subgrade conditions are encountered at the time of construction, the subgrade should be evaluated and the course of action determined by SPU Geotechnical Engineering. Typical courses of action may include overexcavation and replacement with structural fill, stabilization with quarry spalls, or judicious use of geosynthetics.

6.2.2 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 2023 Standard Specifications for Road, Bridge and Municipal (City of Seattle 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 6.6.5: Backfill and Compaction.

6.3 VAULT STRUCTURE BEARING CAPACITY

We expect that the maximum excavation depth for the base of the proposed vault will be approximately 9 to 10 feet bgs. We expect that the Weathered Blakely Formation siltstone/sandstone or the existing fill, comprised reworked weathered Blakely Formation deposits should generally provide suitable support for the anticipated structural loads.

We recommend a maximum allowable bearing pressure of 3,600 pounds per square foot (psf) for structures bearing on the weathered Blakely Formation deposits. For structures bearing on existing fill we recommend an allowable bearing pressure of 2,500 psf. The foundation subgrade should be observed and evaluated, at the time of construction, by a representative of SPU Geotechnical Engineering. Re-compaction of the existing fill may be recommended to verify that the subgrade is suitable for the recommended allowable bearing pressure.

The recommended maximum allowable bearing pressures may be increased by 1/3 for short term transient conditions such as wind and seismic loading. Assuming construction is accomplished as recommended herein, and for the foundation loads anticipated, we estimate total settlement of less than about 1 inch. It is anticipated that the majority of the estimated settlement will occur during construction, as loads are applied.

The subgrade should be observed by the SPU Geotechnical Engineering to evaluate the adequacy of the bearing stratum and to confirm that subsurface conditions are suitable for the recommended design bearing pressure values.

6.4 RETAINING SYSTEMS AND VAULT WALLS

The concept project summary indicates that regulator vault will measure 11 by 7 feet and will be installed at about 8 feet, bgs. The below grade vault walls will be designed as a retaining wall (likely designed for the restrained/unyielding condition).

Recommendations for permanent retaining wall design are presented in the subsequent sub-sections.

6.4.1 Lateral Earth Pressures

Permanent retaining walls must be designed to carry the bending and shear stresses caused by the soil retained by the retaining system. Lateral earth pressures for the design of retaining systems depend on the type structure selected and its ability to deform. We provide active and at-rest earth pressures for cantilevered retaining walls. Active earth pressures should be used for the design of cantilevered walls that are allowed to yield at least 0.001 times the wall height. At-rest earth pressures can be used for the design of cantilevered walls to limit lateral deflections and resulting ground surface settlement.

Lateral loads due to surficial surcharge loading (adjacent footings, fill, traffic, etc.)

should be considered by the designer on a case-by-case basis.

Below-grade walls, including vault or maintenance hole walls and other retaining structures should be designed to resist the lateral loads from the retained soil and other related surcharge loads. The wedge of soil influencing the pressure on the wall is illustrated in Figure A.

Figure A –Soil Pressure Wedge

The size of the wedge depends on the engineering properties of the soil. Table 2 presents information for soils above the groundwater table. The table lists the angle, , for each of the soil units that could be within this active wedge. Table 2 also lists the equivalent fluid densities to use for calculation of the active or at-rest pressures on the walls. An equivalent fluid density for a particular soil unit should be used if the wedge is comprised solely of that particular material. The greater of the respective equivalent fluid densities should be used if the wedge contains multiple soil units.

Lateral loads due to surficial surcharge loading (adjacent footings, areal fill, traffic, etc.)

should be considered. These lateral loads can be determined by multiplying the vertical pressures by the factors listed in Table 2.

Wall

Table 2 – Lateral Earth Pressures

Soil Unit Phi

Φ3

Theta

Equivalent Fluid Densities

Factor for Transfer of Vertical Loads

Active Case1

At-Rest Case1

Active Case1

At-Rest Case1

Existing On-Site Fill 30 30 34 pcf 55 pcf 0.31 0.50

Properly Compacted Backfill2 36 27 33 pcf 56 pcf 0.24 0.41

Existing Weathered Blakely Formation Siltstone/Sandstone

36° 27° 30 pcf 52 pcf 0.24 0.41

Notes: 1) These values assume drained conditions behind the wall in pounds per cubic feet (pcf).

2) Backfill meets the requirements of Section 6.6.5 of this report.

3) Angle of internal friction of material

Wind, earthquakes, and unbalanced earth loads may subject the proposed structures to lateral forces. Lateral forces on these structures will be resisted by a combination of sliding resistance of the bases of the structures on the underlying soil, and passive resistance against the walls. For use in design, a coefficient of friction of 0.55 may be assumed along the interface between the base of the precast concrete regulator vault and existing fill soils (on-site material containing a significant percentage of weathered siltstone/sandstone fragments). The coefficient of friction value is an ultimate value, and thus does not incorporate a factor of safety. A passive earth pressure of 450 pounds per cubic foot (pcf) may be assumed for existing fill soils adjacent to the vault walls. The upper 1-foot of passive resistance should be neglected in design. The passive pressure is an ultimate value, thus it does not include a safety factor. The passive earth pressure was determined using log spiral methods.

To reduce the potential for build-up of hydrostatic pressure, behind retaining walls, installation of a drainage zone, is recommended. The drainage zone should include a minimum 1-foot width of free draining mineral aggregate, along the full height of the wall. We recommend that the free draining mineral aggregate meet specifications for a City of Seattle Standard Mineral Aggregate, from Pages 9-23 and 9-24 (Section 9-03.14) of the City of Seattle Standard Specifications. The drainage material could consist of Type 17 or a similar aggregate, with less than 5 percent fines (material passing the U.S.

Standard No. 200 Sieve). We recommend that a minimum 4-inch diameter perforated or slotted PVC pipe be installed along the rear side of the wall, just above the wall footing, in the lower portion of the drainage zone.

6.4.1.1 Seismic Lateral Earth Pressure Increases

During seismic shaking, inertial effects impose increased lateral earth pressures on subsurface walls. The total active or at-rest earth pressure should be analyzed for seismic loading conditions using the average dynamic load increments presented in Table 3. The restrained condition should be used when wall movements are limited to less than 0.001H. The seismic surcharge values are based on one-half of the value of design peak ground acceleration of 0.72g (determined in accordance with the 2018 SBC, based on ASCE 7). The resultant of the seismic surcharge loads should be assumed to act a distance of 0.6H above the base of the wall, resulting in an inverted trapezoidal pressure distribution (see Figure B). The load increment can be multiplied by 1.6 and 0.4 to calculate the pressures at the top and bottom of the wall, respectively.

Table 3 below presents the recommended lateral pressure surcharges from seismic loading.

Table 3–Seismic Surcharge Loads

Wall Condition Average Seismic Surcharge Load

Unrestrained/Yielding 9H

Restrained/Unyielding 10H

Note: Values in pounds per square foot (psf) where H is the height of the retained soil in feet. H is measured from the top of the adjacent exterior grade to the top of the lateral restraint at the base of the wall. Restrained/Unyielding calculated using Zhang et al (1998) method.

Figure B – Seismic Earth Pressure Distribution

6.5 PAVEMENTS

We understand that, after backfilling the utility trenches, the disturbed roadway areas will be paved. We assume right-of-ways will be paved with asphalt concrete. In our opinion, the flexible pavement may be designed assuming an effective subgrade resilient modulus, MR, of 9,000 pounds per square inch (psi) when placed on compacted backfill. This recommendation is based on our interpretation of surficial ground conditions at the site

0.6H

1.6 x Average

0.4 x Average

H and the guidelines of the American Association of State Highway and Transportation Officials Guide for the Design of Pavement Structures (AASHTO, 1993). The effective modulus takes into account the regional climate of Seattle.

6.6 CONSTRUCTION CONSIDERATIONS

6.6.1 Excavation

Based on our interpretation of subsurface conditions at the site, and the proposed grading, it appears that onsite excavations will be made in fill and weathered or unweathered Blakely Formation. The Contractor should be responsible for determining equipment suitability for the excavation of the varying soils noted in this report, as well as for other soil strata, which may be encountered. Excavation in the unweathered Blakely Formation siltstone/sandstone may be difficult. The earthwork Contractor should be prepared with equipment available that is capable of excavating in the bedrock material (considered relatively hard for excavation, however relatively soft as a sedimentary rock layer).

6.6.2 Dewatering

Based on available information, we anticipate that construction dewatering of the subsurface, during construction, should not be required assuming all excavations will be no greater than 10 feet deep.

If it is anticipated that some degree of dewatering the excavations will be required, the excavations should be dewatered 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 only a portion of the water will have to be removed.

The contractor should choose a method based on the available information and their selected construction techniques. To avoid damage to adjacent utilities and structures due to settlement, the groundwater table should be drawn down no more than necessary for the proposed work. 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.

6.6.3 Temporary Slopes and Shoring

Temporary excavations and protective systems (i.e. shoring or sloping) will be required for this project, if temporary slopes cannot be sloped back at suitable angles. These measures should be implemented in accordance with Section 2-04 of the City of Seattle Standard Specifications. All specific design parameters for temporary slopes, trench shields, or other shoring methods should be developed by the Contractor’s competent person. For planning purposes, a 1.5H:1V (horizontal to vertical) inclination can be assumed for temporary sloped excavations in the existing fill and 1H:1V inclination in the weathered Blakely Formation siltstone/sandstone. The contractor is responsible for the sloping of temporary excavations in accordance with the WAC Chapter 296-155. The means and methods of sloping or shoring temporary excavations for the protection of workers are the contractor’s responsibility. Shoring to protect workers typically consists of passive shoring systems (i.e. trench boxes) that allow the sides of the trenches to slough and still protect the workers. This type of shoring is acceptable where settlement sensitive structures and/or utilities are not present within the zone of influence of the trench. The zone of influence is defined graphically in Figure C.

Figure C - Zone of Influence for Excavations

Where settlement sensitive facilities are within the zone of influence, additional analysis should be done to determine both the amount of movement that is expected at the location of the facility, as well as the amount of movement that is acceptable. If the movement anticipated at the location of the facility is unacceptable, the project plans and specifications should require support systems that restrict the movement of the sides of the trenches. Support systems should be designed for the specific situation by a registered professional engineer as defined in Title 18 of the Revised Code of Washington (RCW).

With time and the presence of seepage and/or precipitation, the stability of temporary cuts can be significantly reduced. Therefore, construction should proceed as rapidly as feasible to limit the time the excavations are left open, and runoff water should be prevented from entering excavations. Heavy construction equipment, building materials, and surcharge loads such as excavated or imported soil should not be allowed within one third of the slope height from the top of any excavation.

Trench 1 to 1.5H:1V Zone of

Influence

Structure

Utility

6.6.4 Shoring Related Settlement

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.

Clough and O’Rourke (1990) developed empirical relationships between the depth of braced excavations and settlement of the ground surface adjacent to the excavation. Their research indicates that the maximum settlement is typically less than 0.3 percent of the excavation depth. The ground settlement profile for excavations is generally linear from the point of maximum settlement at the edge of excavation to less than 0.1 percent of the excavation depth at a horizontal distance from the excavation equal to 2 times the excavation depth. As noted above these empirical relationships were developed for braced excavations and as a result, are not applicable for excavations supported by trench boxes and are marginally applicable for excavations supported by slide rail shoring systems.

Using the Clough and O’Rourke empirical relationships for the anticipated 8 to 10-foot-deep excavation at the site, we calculated approximately 0.4 inches of settlement at the edge of the excavation tapering to approximately 0.1 inch of settlement at a horizontal distance of approximately 20 feet from the excavation. These estimates assume standard construction practices are employed. Lateral deflection of the shoring system and resulting ground surface settlement can increase 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 excavations.

6.6.5 Subgrade Preparation

Subgrade preparation for areas where new fills, pavements and utilities will be built should begin with the removal (stripping) of all deleterious matter, asphalt, and concrete.

The exposed subgrade soils should be evaluated by SPU Geotechnical Engineering. 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 probe.

Soft/loose soils identified during subgrade preparation should be compacted to a firm and unyielding condition or over-excavated and replaced with structural fill, as described below. The depth of overexcavation, if required, should be determined by SPU Geotechnical Engineering at the time of construction.

6.6.6 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 project 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 Project site 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 fill soils we anticipate will be encountered generally will have high variability and will be difficult to control and verify compaction. Therefore, in or opinion, it is unlikely that the excavated material will meet the requirements for Selected Material unless site conditions encountered during construction indicate otherwise. If reuse is not feasible, we recommend that the excavations is backfilled with Borrow Material meeting the gradation requirements of Type 17 material in Section 9-03.14, Mineral Aggregate Chart in the Standard Specifications.

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 D1557-12. 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.

Earthwork should be accomplished in small sections to minimize exposure to wet weather. Excavations or the removal of unsuitable soil should be followed immediately by the placement and compaction of a suitable thickness of clean structural fill, as described below. The size of construction equipment used may have to be limited to prevent soil disturbance.

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:

Material used as trench backfill should consist of clean, granular soil, of which not more than 5 percent by dry weight passes the U.S. Standard No.

200 sieve, based on wet sieving the fraction passing the ¾ inch sieve. The fines should be non-plastic;

The ground surface in the construction area should be sloped and sealed with a smooth drum roller to promote rapid runoff of precipitation, to prevent surface water from flowing into excavations, and to prevent ponding of water;

No soil should be left uncompacted so it can absorb water. Soils that become too wet for compaction should be removed and replaced with clean granular materials; and

Excavation and placement of fill should be observed on a full time basis by a person experienced in wet weather earthwork to verify that all unsuitable materials are removed and suitable compaction and site drainage is achieved.

6.6.7 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.

Permanent erosion control measures should be implemented to reduce the potential for future erosion events. Denuded areas should be mulched and/or planted with approved vegetation.

7.0 LIMITATIONS AND ADDITIONAL SERVICES

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.

The scope of our work did not include environmental assessments or evaluations regarding the presence or absence of wetlands. However, a limited review of publicly and readily available documents related to environmental liabilities in the vicinity of the site was performed by an Environmental Professional as defined in Title 40 Part 312 of the Code of Federal Regulations. We did not encounter apparent indications of contamination in our explorations.

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 recommend that SPU Geotechnical Engineering be retained to review the plans and specifications and verify that our recommendations have been interpreted and implemented as intended. Sufficient geotechnical monitoring, testing, and consultation should be provided during construction to confirm that the conditions encountered are consistent with those indicated by explorations and to verify that the geotechnical aspects of construction comply with the contract plans and specifications. Recommendations for design changes will be provided should conditions revealed during construction differ from those anticipated.

WMRHP5B Project -Beacon Site 5B-4

Geotechnical Report

February 2025

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We appreciate the opportunity to be of service.

Sincerely, SPU GEOTECHNICAL ENGINEERING

1�'-1<� Hilja K. Hartt Sean L. Caraway, P .E.

Associate Civil Engineering Specialist Senior Geotechnical Engineer

8.0 REFERENCES

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, Washington, D.C.

ASTM (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.

Dennis Joule, P.E., 1994, Geotechnical Investigation - Swift Avenue Lots, Seattle, WA, February 1994.

Seattle Building Code, 2018. Seattle Department of Construction & Inspections.

Seattle Engineering Department, 1987, South Graham Street Project, Seattle, WA.

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.

USGS (2009). National Seismic Hazard Map, 2008 Data. April 2009, Denver, Colorado.

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

Accessed January 2024. https://fortress.wa.gov/ecy/neighborhood/.

Washington State Department of Highways, 1962, 15th Avenue S and S Albro Place Roadway Project, Seattle, WA.

Washington Geologic Information Portal. Washington Department of Natural Resources, Online Geodatabase: https://geologyportal.dnr.wa.gov/. Accessed 2023.

Geotechnical Engineering

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Project No. C601097

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APPENDIX A

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GEOTECHNICAL INVESTIGATION

SWIFT AVENUE LOTS

S110RTSunmv1s10N850J86J LoTSA, B, b, E, F

Seattle, Washington Febntary 18, 1994

Prepared For

Swift Development Corporation 1402 Third Ave., Suite 621 Seattle, Washington 98101

Prepared By

Dennis Joule, P.E.

32729 S.E. 44th Street

Fall City, Washington 98024

(206) 222-4661

GEOTECIINICAI. ENGINEERING GROUND AND SURPACE WAIBR HYDRAULICS

Scale: 1"=40' Gr©Jhm

TP-1 I Indicates Backhoe Test Pits Logged 11-8-93

08-l Indicates Test Boring By Seattle Engr. Dept.

Logged 1-16-87

Recent 2: l Fill Slope, Supporting New Alignment of So. Grahm St.

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