2-Final_Geotech_Report_-_Site_5B-2_W_Raye_Street_encrypted.pdf

FINAL GEOTECHNICAL REPORT

WATER MAIN REHAB PACKAGE 5B

SITE 5B-2: W RAYE 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 GEOTECHNICAL SITE CONDITIONS

3.2.1 Environmental Critical Areas

3.2.2 Mapped Geology

3.2.3 Subsurface Conditions

4.0 SEISMIC CONSIDERATIONS

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1 TEMPORARY EXCAVATIONS, DEWATERING, AND SHORING

5.1.1 Excavation Methods

5.1.2 Dewatering Considerations

5.1.3 Shoring Methods

5.1.4 Construction Vibration Considerations

5.1.5 Lateral Earth Pressures

5.1.6 Excavation Monitoring

5.2 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR ECAS

5.2.1 Steep Slope ECAs

5.3 THRUST BLOCK AND BURIED STRUCTURE DESIGN

5.3.1 Thrust Block Design

5.3.2 Buried Structure Design

5.4 OTHER CONSTRUCTION CONSIDERATIONS

5.4.1 Subgrade Preparation

5.4.2 Pipe Bedding

5.4.3 Backfill and Compaction

5.4.4 Wet Weather Earthwork

5.4.5 Construction Drainage and Erosion Control

6.0 LIMITATIONS AND ADDITIONAL SERVICES

7.0 REFERENCES

List of Tables (within text) Table 1 Groundwater Monitoring Results

List of Figures (following text) Figure 1 Site & Exploration Map Figure 2 Geology & ECA Map Figure 3 Lateral Earth Pressures for Non-Watertight Temporary

Shoring Design Figure 4 Lateral Earth Pressures for Watertight Temporary Shoring

Design Figure 5 Surcharge Loads Figure 6 Lateral Earth Pressures for Thrust Block Design Figure 7 Loads for Buried Structures

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-2: W RAYE STREET

SEATTLE, WASHINGTON

1.0 INTRODUCTION

This report presents the results of our geotechnical investigation and recommendations for design and construction of Site 5B-2 of the Seattle Public Utilities (SPU) Water Main Rehabilitation Package 5B project (project) located within the right-of-way (ROW) along W Raye Street between 3rd and 4th Avenue W in the North Queen Anne neighborhood of Seattle, as shown on Figure 1. The project includes installation of an 8-inch ductile iron pipe (DIP) watermain along W Raye Street between 4th Avenue W and just east of 3rd Avenue W and an 8-inch RJDIP water main within the intersection of 3rd Avenue W and W Raye Street. Based on our understanding of the 30% design plans, the majority of the watermain is to be located approximately 5 feet below ground surface (bgs). Excavations of up to 10 feet bgs may be required for the water main located within the intersection of W Raye Street and 3rd Avenue W.

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. Tables are included within the text. 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. Construction dewatering evaluation and recommendations are provided in Attachment 1.

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-2: W Raye 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. This section summarizes our understanding of the geotechnical conditions at the project site.

3.1 SITE SURFACE CONDITIONS

The project is located within the ROW along W Raye Street between 3rd Avenue W and 4th Avenue W. Within the project site, road surfacing along 3rd Avenue W and W Raye Street consists of asphalt pavement, and 4th Avenue W consists of concrete pavement.

The site topography along W Raye Street slopes moderately downward toward the east at approximately 22 percent from elevation 276 to 214 feet and gently downward towards the north at approximately 8 percent along 3rd Avenue W from elevation 218 to 212 feet.

Based on the 1894 USGS topographic map, a historical stream channel existed approximately 225 feet east of present-day 3rd Avenue W centerline. Elevations provided in this report are referenced to the North American Vertical Datum of 1988 (NAVD88) unless noted otherwise.

3.2 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.

3.2.1 Environmental Critical Areas

We reviewed SDCI GIS online map to determine if the site is within geotechnically relevant Environmentally Critical Areas (ECAs) or their buffers as defined in Chapter

25.09 of the Seattle Municipal Code (SMC). The geotechnical 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 W Raye Street between 2nd Avenue W and 4th Avenue W. Based on the mapped location of the steep slopes, portions of the proposed work will occur within the steep slope ECA buffer. Our recommendations for design or construction of the proposed improvements within ECAs or their buffers are provided in Section 5.4 of this report.

A wetland ECA is also mapped at the northeast corner of the intersection of W Raye Street and 3rd Avenue W. We recommend that the project team coordinate with a qualified wetland professional, if required, to review the proposed project details, complete a thorough evaluation of any wetlands within the mapped wetland ECA and/or project area, review proposed design elements to ensure the proposed improvements will not impact any mapped wetlands or their buffers, and identify any required permitting for work within wetlands or their buffers which may impact construction.

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. Search results show that there are no mapped contaminated sites or LUSTs within 500 feet of the site.

The site topography and the location of relevant ECAs in the vicinity of the project are shown on Figure 2.

3.2.2 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 mass wastage deposits and Vashon advance outwash deposits. During the Vashon stade of the Fraser glaciation, the advancing ice-sheet blocked northward lowland drainage resulting in the formation of pro-glacial lakes, the establishment of southerly drainage, and the deposition of laminated to massive silt and clayey silt deposits. Such deposits are mapped as Lawton Clay. As pro-glacial lakes drained to the south, 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.

The geology of the area has been further shaped by mass wastage deposits, which consist of loose to dense, intermixed fine and coarse-grained deposits of colluvium, soil, and landslide debris typically found along steep slopes.

In general, the soil encountered in the exploration that we completed for this project and in select historical exploration agree with the mapped geology at the site.

3.2.3 Subsurface Conditions

SPU Geotechnical Engineering completed one boring designated B-101 on February 12, 2024. The boring was completed to a depth of 26.5 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 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 W Raye Street prepared in 1905, W Raye Street required regrading to construct the present-day roadway alignment. The historical profile along W Raye Street indicates that the likely fill depth along W Raye Street from 4th Avenue W to the west ROW margin of 3rd Avenue W is between 0 to 5 feet and within the intersection of 3rd Avenue W and W Raye Street is between 5 to 10 feet (SeaDIR Record 522-12). We determined that nine historical explorations in the vicinity are geotechnically relevant.

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. Below is a summary of soil and groundwater conditions as documented in the historical exploration logs followed by descriptions of the soil deposits encountered in the exploration that we completed for the project. 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.

Historical Explorations

We reviewed seven historical explorations (BH-1, BH-2, BH-3, HH-1, TP-3, TP-4, and B-1) along the north and south sides of W Raye Street between 3rd Avenue W and 4th Avenue W. The explorations were advanced to depths ranging between 6.5 and 23.5 feet bgs. The exploration summary logs document loose to medium dense fill consisting of sand and silty sand from ground surface to depths ranging between 1 and 9 feet bgs.

Medium stiff to hard silt and clay deposits (BH-1, BH-3, and HH-1) and a combination of loose to medium dense silty sand and stiff to hard silt and clay deposits (BH-2, B-1, TP- 3, and TP-4) were documented below the fill and extended to the final exploration depth.

Historical explorations PG-1 and PG-2, which are located southeast of the 3rd Avenue W and W Raye Street intersection and were drilled to a depth of 16.5 feet bgs, document very loose to loose fill or colluvium deposits consisting of sand, silty sand, and silty clay from ground surface to depths ranging between 3 and 5 feet bgs. Below the fill or colluvium, the exploration logs document dense to very dense native soil deposits consisting of sand, silty sand, silt, and clay.

Exploration summary logs BH-3 and HH-1 document perched groundwater during drilling at a depth of 4 to 5 feet bgs (elevation 210 feet). Exploration PG-1 and PG-2 document groundwater at 12 feet bgs (elevation 224.5 feet) and 4 feet bgs (elevation 225 feet), respectively, at time of drilling. The summary log of exploration TP-3 documents groundwater during excavation at approximately 2 feet bgs.

Fill

Within boring B-101, we encountered soil that we interpret as fill from the bottom of the road pavement section to a depth of 16.5 feet bgs, which is generally consistent with the expected fill depths indicated on the historical roadway profile along W Raye Street for the area around B-101 (SeaDIR Record 522-12). The fill soil consists of very loose to loose, silty fine to medium sand with varying amounts of coarse sand and gravel (SM) 1 and occasional wood. We encountered a 2-inch layer of organic silt with reeds and wood at the contact between the fill and advance outwash deposit below. The natural water content values within the fill range from 10 to 22 percent with an average of 17 percent.

The N-values2 within the fill range from 2 to 9 with an average value of 5.

Advance outwash

Underlying the fill within boring B-101 and extending to the final boring depth of 26.5 feet bgs, we encountered soil consisting of medium dense sand with silt (SP-SM), which we interpret as advance outwash. The natural water content values within the fill range from 25 to 27 percent with an average of 26 percent. The N-values within the fill range from 18 to 20 with an average value of 19.

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)

Groundwater

At time of drilling boring B-101, we measured groundwater at 2.5 feet bgs (elevation

219.5 feet). To allow for more accurate and ongoing groundwater measurements, we installed a groundwater monitoring well within boring B-101 and developed the well by surging the well and pumping approximately 15 gallons of groundwater from the well.

An SPU geotechnical engineering representative measured groundwater within the well on four occasions between February 26 and June 24, 2024. Groundwater was measured at levels ranging between 0.15 feet bgs and -0.39 feet (above ground surface). In order to measure artesian groundwater, a 2-inch diameter, 4-foot-long riser pipe was affixed to the well casing. Table 1 presents the results of our groundwater monitoring.

Table 1: Groundwater Monitoring Results

Date Groundwater Level (feet)1 Approximate Groundwater Elevation2 (feet)

2/26/2024 0.15 221.9

3/23/2024 -0.39* 222.4

4/22/2024 -0.21* 222.2

6/24/2024 -0.06* 222.1 1Groundwater measurements are referenced to the top of the well monument (ground surface). * Negative values represent the height of groundwater above ground surface (artesian groundwater).

2Elevations are based off of 2-foot LiDAR contours.

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.

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-2 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-2 is not within the SFZ nor a SPU intense ground shaking region thus site-specific seismic analyses are not required for the proposed water main improvements. Site 5B-2 is also not within a Permanent Ground Displacement Area as defined in Table 5-13 of the SPU DSG.

Although site-specific seismic analyses are not required for the proposed water main improvements, we reviewed the geotechnical site conditions to determine the potential for permanent ground displacement due to liquefaction. The results of our review indicate that there is potential for permanent ground displacement due to liquefaction induced settlement of the fill soils within the intersection of 3rd Avenue W and W Raye Street.

Based on our understanding of the subsurface soil and groundwater conditions, significant fill is not expected west of the intersection of 3rd Avenue W and W Raye Street. In our opinion, given the density of the native Pre-Fraser deposits, the potential for liquefaction west of 3rd Avenue W is low.

We used the Idriss and Boulanger (2008) method based on SPT data from new and historical explorations to estimate liquefaction-induced ground surface settlement at the site following the DE with a magnitude of 7.11 and a MCER PGA of 0.63g. The results of our analysis indicate that more than 9 inches of liquefaction-induced settlement could occur within the intersection of 3rd Avenue W and W Raye Street following the DE.

When the underlying soil liquefies and loses shear strength, gradually sloped ground can move laterally towards a nearby slope face (lateral spread), or steeply sloped ground can lose stability (flow failure). The ground surface in the area susceptible to liquefaction, within the intersection of 3rd Avenue W and W Raye Street, slopes down to the north along 3rd Avenue W at approximately 7.8 percent towards the Lake Washington Ship Canal. In our opinion, given the site topography and potential for liquefaction within the intersection of 3rd Avenue W and W Raye Street, there is potential for liquefaction induced lateral spreading or flow failures which could result in excessive displacements.

Based on our review of geotechnical site conditions and our liquefaction analysis, we recommend that the water main at the site meet the requirements of Performance Specification 1.

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, DEWATERING, AND SHORING

We understand that construction of the project will require temporary excavations that extend up to 10 feet bgs. In general, we recommend that the temporary excavations, shoring, and dewatering are implemented in accordance with Section 2-04, 2-07 and 2-08 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 silty sand (fill) or medium stiff to hard silt and clay and medium dense silty sand (native 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 Dewatering Considerations

We anticipate that dewatering will be required to stabilize the base of excavation and/or maintain groundwater a minimum of 2 feet below the excavation depth during construction of the proposed water main improvements. 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 if work can be completed in the wet.

Based on our understanding of the site subsurface conditions, perched groundwater may be encountered atop the relatively impermeable glaciolacustrine and glacial till deposits expected west of the intersection of 3rd Avenue W and W Raye Street. In this area, excavations are expected to be less than 5 feet deep. Based on our understanding of the expected excavation depths and soil and groundwater conditions, sumps and pumps may be sufficient for excavations west of the intersection of 3rd Avenue W and W Raye Street.

Within the intersection of 3rd Avenue W and W Raye Street, groundwater has been observed to be artesian with a maximum height of 0.4 feet above existing ground surface.

Based on our understanding of the subsurface soil and groundwater conditions within the intersection of 3rd Avenue W and W Raye Street, the groundwater table may need to be lowered up to 14 feet to comply with provisions of the Standard Specifications and per the recommendations included in the dewatering evaluation. It is important to note that due to spatial constraints and utility conflicts, B-101 was advanced approximately 7 feet higher in elevation than the location of the water main replacement and that subsurface conditions may differ across the intersection. In our opinion, given the groundwater measurements from B-101 and the interpreted subsurface soil and groundwater conditions, sumps and pumps may not be sufficient for excavations within the intersection of 3rd Avenue W and W Raye Street and an active dewatering system may be required to stabilize the base of excavation and/or maintain groundwater a minimum of 2 feet below the excavation depth during construction of those improvements.

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 within the intersection of 3rd Avenue W and W Raye Street. Richard Martin’s recommendations are provided as Attachment 1 and are based on the assumption that non-watertight shoring is used for the excavation. If watertight shoring is used and dewatering is completed from within the shoring, the dewatering rates are expected to be lower than those provided in Attachment 1. Richard Martin also recommended additional explorations be completed prior to excavation activities to confirm the soil conditions.

We anticipate that potholing may be required to verify the location and depth of utilities within the intersection as part of ongoing design effort. If potholing is not planned as part of the design effort, we recommend that the contractor complete potholing prior to beginning excavation at the site to verify soil and groundwater conditions are consistent with those described in this report. We request that SPU Geotechnical Engineering be notified of the potholing schedule so that we may observe the subsurface conditions within the 3rd Avenue W and W Raye Street intersection during potholing.

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 SPU Geotechnical Engineering for review and approval.

As the dewatering volume increases, the potential for negative side effects also increases.

Potential negative side effects at the project site include dewatering related settlement and an increased cost for disposing of groundwater.

Dewatering Related Settlement

Lowering the groundwater table increases the effective stress in the soil which can lead to settlement at the ground surface. We analyzed the potential for dewatering related settlement within the intersection of 3rd Avenue W and W Raye Street based on the anticipated soil and groundwater conditions and our understanding of the excavation depth. For our analysis we assumed the base of excavation is 10 feet bgs, and that the groundwater is drawn down and maintained 2 feet below the base of excavation.

Dewatering drawdown related settlement is typically greatest at the point of dewatering, which we assume will be 2 feet from the excavation footprint in accordance with the recommendations included in Attachment 1 and decreases with horizontal distance from the point of dewatering. The results of our analysis indicate that up to 1 inch of dewatering settlement is anticipated at the ground surface adjacent to the excavation.

Surface supported structures can be expected to settle with the surrounding ground surface and the maximum allowable post-construction settlement for structures is typically 1 inch and the maximum post-construction differential settlement for structures is typically less than 0.5 inches. Our analyses indicate that the maximum settlement is expected to be less than 1 inch and the maximum differential settlement is less than 0.5 inches, therefore we expect that the risk that the estimated dewatering related settlement could result in damage to the nearby surface-supported structures is relatively low.

Buried utilities are also expected to settle with the surrounding ground, however, the magnitude of settlement at the embedded depth may be less than the estimated ground surface settlement. The maximum allowable settlement for buried utilities depends on the type of utility, age, current condition, and material that it is made of. Based on our review of City GIS data and review of information included on the Project Design Plan and Profiles dated 1/21/2022, a 30-inch by 27-inch rectangular telecommunications duct will cross the proposed water main alignment and several catch basins and drainage laterals are located in the intersection of 3rd Avenue W and W Raye Street. If the drainage utilities and telecommunications duct cannot withstand the estimated settlement, we should be contacted to provide additional dewatering requirements to limit settlement to acceptable values.

To further reduce the risk that nearby structures and infrastructure may be damaged by dewatering related settlement, we recommend that the magnitude and duration of groundwater table draw down is kept to the minimum necessary for the proposed work.

In addition, we recommend that a settlement monitoring program that includes monitoring points on the ground surface, surface-supported structures, and settlement sensitive utilities that are within 100 feet of the excavation is implemented during construction. Please refer to the Excavation Monitoring section of this memorandum for displacement limits recommended for a settlement monitoring program during construction.

5.1.3 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 and information on the Project Design Plan and Profiles dated 1/21/2022, the proposed water main will cross below a 30-inch by 27-inch telecommunications duct within the west half of the intersection of 3rd Avenue W and W Raye Street. We expect that the telecommunications duct will need to be temporarily supported during excavation for and installation of the new water main. Additionally, the very loose to loose fill soil expected within the intersection of 3rd Avenue W and W Raye Street may be prone to running when dry and flowing when wet. Running soils are unstable at slope angles greater than their angle of repose, typically 30 to 35 degrees.

When exposed at steeper slopes they run until the slope flattens to the angle of repose. In exposed slopes, saturated flowing soils can flow like a viscous fluid. Based on the groundwater and soil conditions expected and temporary support needs for the telecommunications duct, we recommend that project specifications require temporary shoring systems that restrict the movement of the sides of the trench within the intersection of 3rd Avenue W and W Raye Street. Typical shoring systems that restrict movement of the trench sides include slide rail, soldier pile and lagging, and sheet pile shoring. These shoring systems reduce settlement of the ground surface adjacent to the excavation; however, some settlement may occur.

In our opinion, given the relatively shallow excavation depths (less than 5 feet) and expected soil and groundwater conditions along W Raye Street west of the intersection with 3rd Avenue W, 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 or if settlement sensitive utilities are identified within the zone of influence, we recommend a trench support system be used for temporary excavation support.

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. In our opinion non-watertight shoring systems may be suitable for

Exhibit A - Zone of Influence for Temporary Excavations construction as long as a dewatering system is implemented to maintain groundwater below the base of the excavation and the dewatering-related risks discussed are not prohibitive. Watertight shoring systems (sheet piles) should be used in areas where the water table cannot be maintained below the base of excavation.

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.

5.1.4 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.

5.1.5 Lateral Earth Pressures

The temporary shoring must be designed to carry bending and shear stresses caused by soil that is retained by the shoring. Lateral earth pressures for the design of shoring depend on the type of wall selected and its ability to deform. Active earth pressures should be used for the design of walls that can yield at least 0.001 times the wall height, otherwise at-rest earth pressures should be used.

We provide recommended lateral earth pressures for design of cantilevered and braced shoring walls at each improvement area on Figures 3 and 4. Figure 3 should be used if the shoring will not be watertight and hydrostatic pressures do not need to be considered.

Figure 4 should be used if the shoring will be watertight. 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 Figures 3 and 4. Lateral loads due to surficial surcharge loading can be calculated using Figure 5.

For a shoring system with continuous vertical elements, such as sheet piles, earth pressures should be applied over the length of the wall. For a shoring system with discrete vertical elements, such as soldier piles and lagging, at-rest and active earth pressures should be applied over the length of the wall above the base of excavation and over the width of the vertical element below the base of excavation. The passive earth pressure can be assumed to act over three times the width of the discrete vertical element.

The width of the discrete vertical element is assumed to be the diameter of the drilled soldier pile if the drilled hole is filled with full strength concrete. The width of the discrete vertical element is assumed to be the width of the pile if the drilled hole is filled with controlled density fill or aggregate.

For a shoring system with discrete vertical elements, ground loss between the vertical elements is prevented by facing, which typically consists of timber lagging. We recommend that timber lagging is designed for 50 percent of the lateral earth and surcharge pressures indicated on Figures 3 through 5 to account for soil arching between the relatively stiff vertical elements. Drainage behind the lagging must be maintained, if not the lateral earth pressures should include hydrostatic water pressure.

5.1.6 Excavation Monitoring

The intent of a settlement monitoring program is to provide early warning of the potential need for remedial measures if the shoring does not perform as anticipated. We recommend that condition inspections are completed for structures that are within the zone of influence of the excavations. As indicated in Section 5.1.3 of this report, it appears that there may be structures and utilities in the zone of influence of the excavations within the intersection of 3rd Avenue W and W Raye Street. The condition inspections should be completed before and after construction. The pre-construction condition inspection will establish the baseline documentation of existing conditions, such as identifying the size and location of cracks and other damage.

We also recommend optical surveying of horizontal and vertical movement of nearby structures, utilities, the surface of adjacent streets and sidewalks, and the shoring system before, during, and after construction. We recommend the following displacement limits:

• If the total magnitude of displacement (resultant of X, Y, Z displacements) exceeds 0.5 inches, then the contractor should notify SPU, determine the cause, and develop remedial measures, if warranted.

• If the total magnitude of displacement exceeds 1 inch, or if unacceptable performance or other adverse impacts occur, then the contractor should notify SPU immediately to determine if contingency measures should be implemented.

Remedial measures may include more frequent monitoring and modifying construction practices to limit and/or correct detrimental displacements. Construction may need to cease until remedial measures are implemented.

5.2 DESIGN AND CONSTRUCTION CONSIDERATIONS FOR ECAS

5.2.1 Steep Slope ECAs

As discussed in Section 3.2.1 of this report, portions of the proposed water main improvements are expected to be within the 15-foot buffers above the top and below the toe of a steep slope to the north and south of W Raye Street between 4th Avenue W and 3rd Avenue W. Chapter 25.09 of the SMC includes provisions that restrict construction, site disturbance, or altering vegetation and trees within Steep Slope ECAs or 15-foot buffers. Because the proposed work consists of repairing a public utility, it is exempt from most restrictions of Chapter 25.09. However, regardless of the exemption, all portions of the work areas that are within Steep Slope ECAs or within the 15-foot buffers disturbed or affected by the work must be stabilized during and after construction.

In general, temporary construction loads from equipment or material stockpiles could affect stability of steep slopes. The potential for negatively affecting the stability of steep slopes can be reduced by limiting the magnitude of temporary construction loads or by maintaining sufficient setback distances between the loads and the top of slope. For work areas that are within Steep Slope ECAs, or within the 15-foot buffers at the top of the slope, construction equipment, building materials, or other surcharge loads such as excavated or imported soil should not be allowed within one third of the slope height from the top of the slope. For these sites, we also recommend that the work is completed with the smallest equipment that can feasibly complete the work.

For work areas that are within Steep Slope ECAs or their buffers, regardless of the type of shoring system that is selected for the site, we recommend that it is installed in such a way that the height of unsupported soil never exceeds 2 vertical feet, and that the excavation is only as wide as necessary to install the shoring system and any voids between the sides of the trench and the shoring system are backfilled with flowable aggregate immediately after the shoring system is installed to depth.

With time and the presence of seepage and/or precipitation, the stability of excavations 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.

Erosion Control and Stabilization

Disturbed and denuded slopes are susceptible to surficial erosion and instability during and after construction. We recommend the following best management practices (BMPs) to control erosion and stabilize areas that are within Steep Slope ECAs, or within the 15-foot buffers at the top and toe of the slope:

• Avoiding earthwork during the wet season (defined as October 31 to April 1),

• Using shallow ditches or low earthen berms to divert runoff from the site and surrounding steep slopes and using temporary sumps to collect runoff,

• Installing temporary erosion control measures such as plastic sheeting, erosion control netting/blankets or mulch to protect disturbed or denuded surfaces during or after construction until permanent erosion control measures are established, and

• Establishing permanent erosion control measures by planting disturbed or denuded surfaces with approved vegetation as quickly as possible after earthwork is complete.

5.3 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 10 feet bgs.

5.3.1 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 6. 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 6.

5.3.2 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 7. 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 7.

Bearing Capacity and Settlement

We estimated the allowable bearing capacity for buried structures based on the anticipated soil and groundwater conditions between 3 and 10 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 5.4.3. We recommend an allowable bearing capacity of 3,000 pounds per square foot (psf) for buried structures bearing on improved subgrade 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 7. 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.

5.4 OTHER CONSTRUCTION CONSIDERATIONS

5.4.1 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.

5.4.2 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 5.4.3.

5.4.3 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.