_2.4_CLASSIC_GeotechnicalReportFinalSealed.pdf

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March 25, 2025

Windsor Engineers 27300 NE 10th Ave Ridgefield, WA 98642

Report of Geotechnical Services Manzanita – Classic Street and Necarney City Road, Stormwater Improvements and Water Main Extension Project Manzanita, Oregon Project #074-24-015

1.0 INTRODUCTION

This report provides Pali Consulting Inc.’s (Pali Consulting’s) geotechnical evaluation and recommendations for the Manzanita – Classic St Road and Necarney City Road Stormwater Improvements and Water Main Extension Project (Project) in Manzanita, Oregon. The Project consists of geotechnical design recommendations for water system improvements to Classic Street and Necarney City Road. Improvements will include installation of a water main line, stormwater facilities as necessary, pavement widening, and installation of a shared use path along Classic Street. The Project area is shown on Figure 1.

Windsor Engineers requested that Pali Consulting provide geotechnical services for the Project. Our scope of work included a review of existing information, site reconnaissance, subsurface explorations, geotechnical analyses, and design recommendations for the Project. Our work was completed in general accordance with Task Order 09 of our master services agreement with Windsor Engineers, dated November 12, 2024, and subsequent modifications.

2.0 BACKGROUND REVIEW

2.1 PROJECT DESCRIPTION

The Project includes installation of a water mainline and appurtenant facilities in an undeveloped area known as the Highlands. The water system improvements will allow development of the area and will connect existing branches of the City of Manzanita’s water system.

The new water mainline will consist of 12-inch diameter HDPE pipe that will be installed with open trench and/or trenchless methods at depths of between 3 and 5 feet below ground surface (bgs) for much of the alignment. The current proposed alignment will follow existing roadways between the intersection of Classic Street and Laneda Avenue and the junction of Highlands Drive and Meadows Drive, then run overland to

March 25, 2024 Project No. 074-24-015 Page 2

Necarney City Road, where it will tie in with the existing water line near the junction of Necarney City Road and Clipper Court. The approximate alignment is shown on Figure 1.

2.2 BACKGROUND INFORMATION

2.2.1 TOPOGRAPHY AND GEOMORPHIC FEATURES

We reviewed U.S. Geological Survey (USGS) topographic maps, satellite imagery, as well as LiDAR data downloaded from the Oregon Department of Geology and Mineral Industries’ online mapping portal (DOGAMI, online mapping accessed November 2024), for analysis of topographic and geomorphic features in the Project area along Classic Street and the Highlands area.

Topography along the alignment is mostly flat to gently sloping where it traverses a dune complex which forms the Highlands development area. About 60 feet of total relief separates the highest and lowest points in the Project area. Although originally a dune complex, the topography has been highly altered by housing, road, and infrastructure development, and little of the pre-development topography remains. The low relief hills are relict sand dunes, of which the general shape still remains. Classic Street mostly occupies a cut bench across a former dune slope so is characterized by locally steeper slopes above and below the portion within the cut. A housing development lies above Classic Street through this segment and vacant land which is slated for development below. Other roadways generally follow gentler modified dune surfaces with little relief.

2.2.2 GEOLOGIC, LANDSLIDE, AND SOILS MAPPING

The geology within the Project area is mapped by DOGAMI. The geology consists of ancient sand dunes which were deposited in the last several hundred thousand years and have since stabilized from vegetation growth and development. These deposits are composed of eolian, or wind-deposited, fine sand.

Landslide mapping from DOGAMI’s SLIDO database maps one earth slide-rotational landslide about 450 feet west of Classic Street, outside of the Project area. It is about 140 feet wide and is mapped with low confidence. No other information regarding the landslide is available.

Faults are mapped by the United States Geologic Survey (USGS) in their online Quaternary Faults and Folds Database (https://www.usgs.gov/tools/interactive-us-fault-map, accessed November 2024). The nearest mapped fault to the Project area is the Tillamook Bay fault zone, 9-10 miles south near Garibaldi. Little is known about this fault zone, but its geomorphology suggests that it is active, though at a low slip rate less than 0.2 mm/yr. Other nearby faults include unnamed faults offshore that are related to the Cascadia Subduction Zone, but little is known about these faults other than that they are likely active in the last 12,000 years with slip rates of 2.0-5.0 mm/yr.

Geologic hazards were reviewed using DOGAMI’s Statewide Geohazards Viewer (HAZVU). Geologic hazards mapped along the alignment include landslides, earthquake shaking, liquefaction, coastal erosion, and tsunami inundation. Mapped landslide hazard is moderate to high along the length of Classic Street, where the steep sand cutbanks are susceptible to shallow landsliding. No hazard from deep-seated landsliding is mapped along Classic Street or in the Highlands area. DOGAMI assigns a 10-20% probability of damaging earthquake shaking in the next 50 years throughout Manzanita, including the Project area. A Cascadia Subduction Zone earthquake is expected to generate severe local shaking of 8 on the Modified Mercalli Scale, indicating widespread severe damage to structures. Earthquake shaking strong enough to be damaging would also produce liquefaction in areas of loose sediments saturated with water. The entirety of the Project area’s susceptibility to liquefaction is rated as High. About half of Classic Street is within the

March 25, 2024 Project No. 074-24-015 Page 3 evacuation zone for an expected Cascadia Subduction Zone tsunami, between approximately Dorcas Lane and Jackson Way.

The area’s soils are mapped by the Natural Resources Conservation Service (NRCS, Web Soil Survey accessed November 2024). The Project area is entirely underlain by the Netarts fine sandy loam, present on slopes of 5 to 30 percent. It is derived from eolian dune sands, and has a typical profile of slightly decomposed plant material from 0-2 inches depth, an A horizon of fine sandy loam from 2-5 inches, an E horizon of loamy fine sand from 5-9 inches, an AB horizon of loamy fine sand from 9-15 inches, a B horizon of fine sand from 15-54 inches, and a C horizon of fine sand from 54 to 67 inches depth. It is considered well-drained with infiltration rates of 1.98 – 5.95 inches/hr. The depth to both a restrictive feature and the water table is greater than 80 inches, according to NRCS.

2.2.3 LIDAR AND AERIAL IMAGERY REVIEW

LiDAR-generated bare earth hillshade mapping of the Project area was obtained from DOGAMI. Aerial photos from USGS Earth Explorer and Google Earth Pro for the years 1953, 1980, 1994, 2000, 2005, 2012 and 2021 were reviewed for evidence of instability or other changes. A discussion of the LiDAR imagery is provided below, and a summary of pertinent geomorphic and slope stability observations made from aerial imagery is given in Table 1.

Table 1 – Review of Aerial Imagery

Image

Number Date

Image

Source Notes

1 1953

USGS

Earth

Explorer

B/W photo shows that the area of Classic Street is forested and undeveloped, south of Dorcas Lane. No signs of slope instability are interpreted.

2 1986

USGS

Earth

Explorer

False-color photo shows that Classic Street is still undeveloped south of Dorcas

Lane. No signs of slope instability are interpreted.

3 1994 Google

Earth

B/W photo shows Classic Street present as a narrow unpaved road. Grading for housing developments is in progress to the southwest and southeast. Highlands

Drive is not present, but the Highlands area is clear of timber. No signs of slope instability are interpreted.

Dec.

Google

Earth

Color photo shows Classic Street apparently paved. Terracing for adjacent housing developments appears complete. Highlands Drive is not present. Large areas of bare sand are visible, likely due to recent earthmoving activity. No signs of slope instability are interpreted.

5 2021 Google

Earth

Color photo shows the project area much as it appears presently. All grading adjacent Classic Street is complete, and the Highlands development is partly graded as well, although Highlands Drive appears only about half constructed.

No signs of slope instability are interpreted.

6 2024 Google

Earth

Color photo shows project area as it appears presently. No signs of slope instability are interpreted.

Notes:

1. B/W = black and white

In summary, the air photo record shows stable dune slopes and the time history of existing development.

No indications of instability were interpreted in the air photos over the approximately 70-year record.

March 25, 2024 Project No. 074-24-015 Page 4

3.0 SITE CONDITIONS

3.1 SURFACE CONDITIONS

Classic Street is a narrow asphalt-paved road which is surrounded by development except for the central portion between Highlands Drive and Dorcas Lane where there are housing developments upslope (east) of the road and vacant land downslope (west) of the road. Where not covered by landscaping soil or vegetation, fine sand is visible throughout the Project area. Vegetation is limited to blackberries, some shrubs, occasional conifer trees, and grasses. The nature of the sandy subsoil means that there are no areas of persistent ponding or standing water.

The Project area also includes Highlands Drive, a residential connector extending northeast from Classic Street, and Necarney City Road, a paved county road which extends roughly east-west through Manzanita, as well as the undeveloped area between these two roads. The area is dominated by rolling hills of sand covered with primarily grassy vegetation.

Pavement cracking is prevalent along Classic Street between Dorcas Avenue and Highlands Drive, where the road is built on a bench cut into an old dune. Cracks are generally arcuate in shape, and areas of cracking are often noticeably subsided compared to surrounding pavement. Cracks range in width from hairline up to about ¾”, with slight vertical offset generally too small to be measured individually but adding up to about two inches across the damaged zone in some cases. Areas of cracking are mostly restricted to the westernmost (downhill) few feet of the roadway, although some areas extend to the approximate centerline of the roadway. The areas of cracking are variable in length, but up to about 70 feet long in the roadway direction.

3.2 SUBSURFACE CONDITIONS

We completed eleven machine-drilled borings and four hand auger explorations within the Project area.

Machine-drilled borings are designated B-1 through B-11 and were completed to depths between 11.5 feet and 51.5 feet bgs. Borings were completed using hollow-stem auger methods, except B-6 which used mud rotary methods. Hand auger explorations were completed to between 6 and 10 feet bgs and are designated HA-1 through HA-4. Additionally, we completed four drive probe soundings to evaluate subsurface conditions to a depth of 10 feet, designated DP-1 to DP-4. The approximate locations of our explorations are shown on Figure 2. Explorations were completed between November 12th and 15th, 2024. Descriptions and logs of our subsurface explorations are included in Appendix A.

Our site explorations generally encountered native eolian sands to 51.5 feet bgs, the maximum depth of exploration. The native sands were generally overlain by roadway gravels and variable depths of gravelly and sandy fill where in roadway areas, or by a thin layer of organic material where outside of roadway areas. We completed interpretive cross sections at the boring locations shown on Figure 3. Our interpretation of subsurface conditions at each cross section are provided in Figures 4A through 4G. The geologic units we encountered are described in more detail below.

3.2.1 ROADWAY ASPHALT AND FILL

We encountered well-graded roadway gravel and/or sand fill in all of our borings. The gravel fill extended from the ground surface to variable depths of up to 2.5 feet bgs, but generally ranged in thickness from 0.5 to 2 feet. In all borings except B-1, B-2, and B-7, the gravel was overlain by approximately 2 inches of asphaltic concrete (AC) pavement. The gravels generally contained clasts measuring ¾-inch to 2 inches in diameter with varying amounts of sand and silt. Two sieve gradation tests were completed near the base of the fill which determined the fill to consist of predominately fine sand, with 5 to 42 percent gravel and 11

March 25, 2024 Project No. 074-24-015 Page 5 to 7 percent fines (B-10 S1 and B-6 S1, respectively). In some borings, sand without gravel was interpreted as fill, but the similarity of the native and fill sand made distinguishing between the two uncertain.

No laboratory testing was completed on the Roadway Asphalt and Fill.

3.2.2 EOLIAN SANDS

Below the Roadway Asphalt and Gravels, we encountered orange to gray sands. These sands were poorly graded and fine-grained in size. They were generally uniform apart from color changes and occasional thin beds containing small amounts of organic material. The sand ranged from very loose to dense, with N-values of 2 to 65 bpf. Higher blow counts, those above about 25, were often, but not always at depths of about 30 feet bgs or greater.

Laboratory testing on samples of the eolian sands found moisture contents ranging from 1 to 25 percent.

Most sands encountered were dry to moist, but samples retrieved during mud rotary drilling (Boring B-6) and/or recovered from below the water table were moist to wet, with measured moisture contents of up to 25 percent. Fines content tests measured 2 to 3 percent fines in three samples from B2 and B7.

3.2.3 GROUNDWATER

We encountered groundwater in Boring B-5 at 38 feet bgs. We did not encounter groundwater in any of our other borings (depths of between 11.5 and 31.5 feet bgs). Groundwater could not be confirmed in Boring B-6, which extended to 51.5 feet bgs, due to the mud rotary drilling method used, but is presumed to occur at a similar depth as B-5.

Although groundwater was encountered at the depth and location noted above, groundwater conditions vary temporally due to seasons, precipitation, development and other factors. Perched (transient) groundwater could be encountered anywhere within the Project area during periods of heavy or prolonged precipitation, but the relatively clean sands throughout the Project area, suggest perched groundwater will not be common.

4.0 EVALUATION

Our background review and subsurface explorations found the primary geotechnical factors affecting the Project are the extensive loose dry eolian sand, the stability of site slopes along Classic Street, and seismic hazards overall. Based on our analysis, retaining walls as proposed will need to be designed to stabilize the roadway in general, currently failing areas in particular, and to support the widened roadway sections.

These key geotechnical factors affecting the Project and geotechnical design of retaining walls are further evaluated in the following sections.

4.1 SLOPE STABILITY

We completed numerical slope stability analyses (SSA) at representative sections of Classic Street where indications of road instability are visible (arcuate cracking as noted above) and significant grading is proposed. The locations of our SSA analyses are shown on Figure 2, as Sections A, C, and E. The SSA were completed using the two-dimensional commercial software SLIDE by RocScience. SLIDE uses two-dimensional limit equilibrium methods to analyze slope stability by determining a factory of safety (FS) against slope instability. The FS against slope instability can be generalized as the ratio of forces resisting slope movement (soil strength, soil mass, etc.) to forces driving slope movement (gravity, earth pressure, etc.). A FS equal to or less than 1.0 indicates a condition when the available soil shear resistance decreases below the shear stresses required to maintain stability of the slope and the slope will theoretically fail. FS above 1.0 indicates the slope is stable with increasing FS indicating increasing stability. The program also predicts the location and geometry of “critical slip surfaces.” Critical slip surfaces are the zones with the

March 25, 2024 Project No. 074-24-015 Page 6 lowest FS. Our SSA was completed using the Spencer and Morgenstern-Price Methods, which both satisfy moment and force equilibrium. The lowest calculated FS from these two search methods is reported.

4.1.1 CASES ANALYZED

The existing and proposed surface geometry of our models was developed from data provided by Windsor Engineers. As noted above, we analyzed three of our eleven boring locations: A-A’ (B-10), C-C’ (B-5), and E-E’ (B-11), which approximately correspond to Stations 19+00, 15+50, and 12+00, respectively. The locations of the cross sections are illustrated on Figure 3. Our subsurface interpretations were based on the findings of our borings and laboratory testing at the cross sections analyzed. We estimated soil properties under existing static conditions by back-analysis at the analyzed locations. Back-analyzed conditions were developed by iteratively varying soil properties until achieving a FS of approximately 1.0 with failure surface locations similar to those observed in the field (the extent of pavement cracking). This method provides soil shear strength (average) in their current conditions to be used in analyses. Using these properties, we then analyzed the following scenarios:

Existing static and seismic conditions.

Developed conditions with retaining walls at the locations proposed by Windsor Engineers, and o Designed to meet a static FS of 1.25, with the resulting seismic FS calculated (not meting ODOT requirements).

o Designed to meet both a static FS of 1.25 and seismic FS of 1.0 (meeting ODOT requirements).

A traffic surcharge was included within the roadway area as indicated by Windsor Engineers which was modeled as 250 pounds per square feet (psf) across the full traffic lanes.

Our seismic analyses utilized a horizontal acceleration of 0.254g, based on a peak ground acceleration of

0.5081 for the 975-year event, per Section 4.2.2.

Although not presented in this report, we evaluated both gravity and mechanically stabilized earth (MSE) walls. Our analyses found that the walls had different height and embedment requirements to meet the same required FS, but the differences were not large. Due to the additional excavation needed for placement of geogrid elements for the MSE walls and the resulting undermining of the slope (unless shored), we opined that the gravity wall was preferred for the Project and only present those results in our tables and in Appendix B. We also note that at some wall height/depth, a pile-wall may become preferable from a constructability and cost standpoint.

Other methods to stabilize the slopes were considered but not evaluated in detail. Buttresses require slopes that would not fit within the City ROW, groundwater is too low for drainage to be an option, and concrete cantilever walls would be more expensive and still require large excavations. So gravity walls were used in our analyses. Cantilever and/or tie-back pile walls are generally expected to be less cost effective, too, but at taller heights may become the preferred wall type as noted above.

4.1.2 DESIGN PARAMETERS

Based on the back analysis described above and on laboratory testing completed for this Project, we developed the soil properties for soil units used in the analyses, as summarized in Table 2. We note that the laboratory direct shear test completed for the Project (Appendix A) measured a significantly higher strength than the back-calculated values. Because the back-calculated values were consistent across the cross

March 25, 2024 Project No. 074-24-015 Page 7 sections and they estimate average values (not at a single location like a test sample), we used the back-calculated, not laboratory-derived, strength.

Table 2 – Stability Soil Properties For All Cross Sections

Soil Unit Description

Material

Color in

SLIDE

Total Unit

Weight

(pcf)

Friction Angle

(degrees)

Cohesion

(psf)

Native sand, loose 110 26 0

Road Fill 125 40 0

Concrete Blocks 150 Infinite Strength -

4.1.3 STABILITY ANALYSES RESULTS

Results from our SSA are summarized in Tables 3a through 3c. Graphical results are included in Appendix B.

For analysis of slope mitigation options, a global minimum FS of 1.25 was used under static loading conditions for roadway embankment sideslopes per ODOT GDM, Table 7.3. Table 7.3 recommends FS of

1.25 for roadway sideslopes not supporting structures and for landslide remediation, FS of 1.3 for slopes adjacent to but not supporting structures, and 1.5 for slopes supporting structures. Since mitigation structures will support the slopes (not vice-versa) and are designed to stabilize the slopes, it is our opinion that 1.25 is an appropriate FS for static design. If an FS higher than 1.25 is preferred, we can provide additional design information.

For seismic conditions, FS of 1.1 to as low as 1.0 are the minimum required by ODOT, depending on acceptable ground deformations at the site (ODOT GDM, Section 13.5.3.1). To design to ODOT seismic requirements is expected to increase mitigation costs substantially. Due to the cost and because the roadway is a City-owned road so not subject to ODOT jurisdiction, we understand that ODOT requirements will generally be considered but may not be implemented. We, therefore, provide the results for mitigation configurations to meet a seismic FS of 1.0, as well as the resulting seismic FS under the minimum static design requirements (1.25) for consideration by the City and design team. Under each scenario in Tables 3a through 3c, we note the wall heights needed to meet the design FS, for consideration in estimating the additional cost.

Based on the results in Tables 3a through 3c, we note the following:

- Stabilization of the roadway slopes utilizing gravity retaining walls meet the design FS (are stable) under static conditions (FS of 1.25 or greater) with the wall heights noted in the tables.

- Under seismic conditions, stabilization using the same wall heights do not achieve a minimum FS of 1.0.

- To achieve minimum FS of 1.0 for seismic conditions, the MSE walls would have to be increased in height by approximately 25 to 30 percent, as noted in the tables.

March 25, 2024 Project No. 074-24-015 Page 8

Table 3a – Cross Section A-A’ Stability Factor of Safety Summary

Figure Case Global

Minimum FS

Wall Height*

(feet)

B-1 A-A’ – Back Analysis (Existing Conditions) – Static 1.64 NA

B-2 A-A’ – Back Analysis (Existing Conditions) – Seismic 0.83 NA

B-3 A-A’ – Proposed with Retaining Wall – Static 1.49 10

B-4 A-A’ – Proposed with Retaining Wall – Seismic 0.92 10

B-5 A-A’ – Proposed with Retaining Wall – Seismic FS 1.0 1.16 12.5

*Wall heights include exposed and embedded portions of wall and are approximate

Table 3b – Cross Section C-C’ Stability Factor of Safety Summary

Figure Case Global

Minimum FS

Wall Height*

(feet)

B-6 C-C’ – Back Analysis (Existing Conditions) – Static 1.03 NA

B-7 C-C’ – Back Analysis (Existing Conditions) – Seismic 0.69 NA

B-8 C-C’ – Proposed with Retaining Wall – Static 1.25 22.5

B-9 C-C’ – Proposed with Retaining Wall – Seismic 0.82 22.5

B-10 C-C’ – Proposed with Retaining Wall – Seismic FS 1.0 1.09 30

*Wall heights include exposed and embedded portions of wall and are approximate

Table 3c – Cross Section E-E’ Stability Factor of Safety Summary

Figure Case Global

Minimum FS

Wall Height*

(feet)

B-11 E-E’ – Back Analysis (Existing Conditions) – Static 0.94 NA

B-12 E-E’ – Back Analysis (Existing Conditions) – Seismic 0.53 NA

B-13 E-E’ – Proposed with Retaining Wall – Static 1.27 30

B-14 E-E’ – Proposed with Retaining Wall – Seismic 0.82 30

B-15 E-E’ – Proposed with Retaining Wall – Seismic FS 1.0 1.18 40

*Wall heights include exposed and embedded portions of wall and are approximate

We also analyzed the stability of the slope above Classic Street under seismic conditions. Our analysis shows that the slope east of Classic Street is prone to failure if not mitigated. Table 4 summarizes the results.

Table 4 – Global Stability FS Summary

Figure Case Global

Minimum FS

B-16 Cross Section A-A’ – Cut Slope Seismic 0.70

B-17 Cross Section C-C’– Cut Slope Seismic 0.73

B-18 Cross Section E-E’– Cut Slope Seismic 0.67

March 25, 2024 Project No. 074-24-015 Page 9

As noted in Table 4, the cutslope above the roadway is not stable under seismic conditions. Under the design seismic event the cutslope would be expected to fail above the road, and would likely deposit significant debris onto the roadway surface.

4.2 SEISMIC HAZARDS

The Project site is in a seismically active area. In this section, we describe seismic sources at the site, identify the seismic site class, provide seismic response spectra, and outline our interpretation of other seismic hazards at the site.

4.2.1 SEISMIC SOURCES

The Project site is in a seismically active area. The seismicity of the region is controlled by the Cascadia Subduction Zone (CSZ). Plate tectonics cause the oceanic Juan de Fuca Plate to subduct beneath the continental North American Plate. Three types of earthquakes are associated with subduction zones:

interface, intraslab, and crustal earthquakes, as described below.

Interface Seismic Sources – Subduction zones are typically characterized by interactions between the oceanic Juan de Fuca Plate and the continental North American Plate. As the oceanic plate subducts beneath the continental plate, the two lock together. As they lock together, stresses build in the overlying continental plate. When the stresses become too large, the plate can rupture resulting in an interface earthquake. An example of an interface earthquake is the moment magnitude 9.0 (M9.0) event which occurred in 2011 in Tohoku, Japan. Interface earthquakes are some of the largest magnitude and most destructive earthquakes recorded across the globe.

Intraslab Seismic Sources – Intraslab earthquakes originate from a deeper zone of seismicity that is associated with bending and breaking of the subducting oceanic plate. Intraslab earthquakes occur at depths of 40 to 70 kilometers (km) and can produce earthquakes with magnitudes up to and greater than magnitude M7.0. An example of an intraslab earthquake is the 2001 M7.0 Nisqually earthquake which occurred in west-central Washington.

Crustal Sources – Shallow crustal faults are caused by cracking of the continental crust resulting from the stress that builds as the subduction zone plates remain locked together. Based on our review of available geologic maps (through DOGAMI HazVu), the closest mapped active fault to the site is approximately 10 miles to the south as described in section 2.2.2.

Details of these sources and their contributions to seismic hazard at the Project site are provided below.

4.2.2 SEISMIC SHAKING

We evaluated potential seismic shaking at the site in accordance with the ODOT GDM and AASHTO based on seismic shaking having a 7 percent probability of exceedance in 75 years (975-year return period); this is the standard AASHTO seismic design criteria (AASHTO, 2020).

We evaluated potential seismic shaking at the site using the updated ODOT Seismic Hazard Maps which are based on the USGS 2014 seismic shaking maps (ODOT 2016). The expected peak ground acceleration (PGA) at the site for the “Life Safety” criteria (975-year return period motion) is approximately 0.4369g based on the ODOT, 2016 maps. This value represents the peak acceleration on bedrock beneath the site and does not account for ground motion amplification due to site-specific effects. The site-adjusted PGA

March 25, 2024 Project No. 074-24-015 Page 10

(As) is determined by applying a site class factor to the PGA noted above and is presented in Section 4.2.3.

Refer to Section 4.2.4 below for a discussion of ground motion amplification.

Seismic sources contributing to the potential ground shaking above include shallow crustal faults, intraplate faults, and the CSZ megathrust interface fault. The data indicated that the “modal source” for shaking at the site under the 975-year design interval (Life Safety criteria) at all potential periods of interest (0.0 to

2.0 seconds) is a magnitude 9.1 earthquake epicentered at the CSZ approximately 32 km from the site. The modal source generally signifies the earthquake with the highest contribution to the site earthquake hazard, in this instance a rupture along the CSZ.

4.2.3 SEISMIC SITE CLASS

The “site class” is a classification used by the 2022 Oregon Structural Specialty Code (OSSC) and by ODOT to quantify ground motion amplification. The classification is based on the properties of the upper 100 feet of the soil and bedrock materials at a site.

The deepest exploration performed at the site was approximately 51.5 feet bgs. The SPT N-value obtained at the bottom of this exploration was extrapolated down to 100 feet in order to obtain a site class designation.

The weighted average N-values in the upper 100 feet of this boring were 19 blows per foot (bpf). As a result, we consider Site Class D to be an appropriate designation for the Project area.

However, we note this site class designation does not consider potential liquefaction of site soils, as discussed in Section 4.2.5.

4.2.4 DESIGN RESPONSE SPECTRUM

We obtained seismic design parameters for the 975-year AASHTO design event (AASHTO, 2020) at Latitude

45.715399 and Longitude -123.929722. The parameters provided in Table 1 were developed using the ODOT ARS Spreadsheet (ODOT, V.2014.16). The values provided in Table 5 are considered generally appropriate for AASHTO and ODOT code-based seismic design, except for liquefaction, as noted above.

Table 5 - Seismic Design Parameters for 975-year Event

Parameter Value

Site Class D

Mapped Spectral Response Acceleration (Short Period), Ss

0.9041

Mapped Spectral Response Acceleration (1-Second

Period), S1

0.3743

Peak Ground Acceleration Coefficient, Fpga 1.1631

Short Period Spectral Acceleration Coefficient, Fa 1.1383

Long Period Spectral Acceleration Coefficient, Fv 1.9257

As (Fpga x PGA) 0.5081

Spectral Response Acceleration (Short Period), SDS 1.0292

Spectral Response Acceleration (1-Second Period), SD1 0.7208

Peak Ground Acceleration (PGA) 0.4369

March 25, 2024 Project No. 074-24-015 Page 11

4.2.5 LIQUEFACTION HAZARDS

When cyclic loading occurs during an earthquake, the shaking can increase the pore pressure in some soils and cause liquefaction. The rapid increase in pore water pressure reduces the effective normal stress between individual soil particles, resulting in the sudden loss of shear strength in the soil. Granular soils (gravels and sands), which rely on interparticle friction for strength, are susceptible to liquefaction until the excess pore pressures can dissipate. Sand boils and flows observed at the ground surface after an earthquake are the result of excess pore pressures dissipating upwards, carrying soil particles with the draining water.

In general, loose, saturated sand soils with low silt and clay contents are the most susceptible to liquefaction.

Silty soils with low plasticity are moderately susceptible to liquefaction under relatively higher levels of ground shaking. For any soil type, the soil must be saturated for liquefaction to occur. Although the loose to medium dense sands at the site are subject to liquefaction where saturated, due to the depth of groundwater at the site (38 feet bgs, where encountered), the potential for soil liquefaction to affect the Project area is very low.

4.2.6 OTHER SEISMIC HAZARDS

4.2.6.1 Surface Fault Rupture

As noted previously, the nearest mapped active fault is approximately 10 miles south of the Project site.

Therefore, we consider the hazard from ground surface rupture on mapped active faults to be relatively low.

Unmapped or inactive faults may still exist that could increase the risk of ground fault rupture at the site.

4.2.6.2 Tsunami and Seiche

The proposed alignment is generally outside of the tsunami hazard area, but on its north end (from about the intersection of Classic St and Dorcas Ln to about the intersection of Classic St and Jackson Way) is within the local tsunami evacuation zone. As a result, tsunami hazards are likely to impact surface structures within the north end of the alignment if a tsunami occurs. The potential damage to buried structures, such as pipelines, is judged to remain low, however, as scour and erosion from tsunamis are not likely to reach them.

4.2.6.3 Seismic Subsidence or Uplift

Given the proximity of the site to the coastline, it is likely that the site will experience considerable coseismic subsidence associated with a rupture on the CSZ. Based on mapping by DOGAMI (Madin and Burns, 2013), between 3 and 4 feet of subsidence is anticipated following the design subduction zone earthquake. Generally, such subsidence is expected to be a widespread areal event which is not likely to have a significant effect on the alignment as differential displacement would be minimal.

4.2.6.4 Earthquake-Induced Landsliding

As described in Section 4.1.3 of this report, the steep slopes along Classic Sreet between Dorcas Lane and Highlands Drive will undergo earthquake-induced landsliding within this portion of the alignment. Outside of this area of steep slopes, the potential for earthquake-induced landsliding is low as slopes are generally flat.

4.2.6.5 Earthquake-Induced Settlement

It is well-known that seismically induced settlement of sand soils occur, even absent liquefaction (ODOT GDM, Section 13.5.4). We estimated sand settlement at the site and found that up to several inches of settlement is possible following a design earthquake.

March 25, 2024 Project No. 074-24-015 Page 12

4.3 RETAINING WALLS

We evaluated different methods to stabilize the slopes and to support the roadway along Classic Street, as discussed in Section 4.1.1. We determined that retaining walls are needed for this purpose.

We considered both gravity and MSE walls, but did not further consider concrete cantilever or pile walls for the reasons discussed in Section 4.1.1. For our slope stability analyses we determined the configurations (heights/embedments) needed to achieve global stability under static and seismic conditions as detailed in Section 4.1. We found that there was little difference in heights between the two wall types, however, MSE walls likely require more excavation and potential undermining of the roadway and eastern slope, compared to gravity walls. Therefore, we completed our slope stability analyses with gravity walls as described in Section 4.1. Wall heights/embedments necessary to meet design FS are noted in Tables 3a through 3c.

Final wall design will be completed by the Contractor and our recommendations for design of the walls are included in Section 6.2 of this report.

4.4 INFILTRATION TESTING

We completed two infiltration tests, IT-1 and IT-2, on each side of the intersection of Classic and Necarney City Road. The tests were completed on November 14, 2024, at the approximate locations shown on Figure

2. The tests were completed in general accordance with the encased falling head test in general accordance with the US Bureau of Reclamation (USBOR), as described in Attachment A of this report. We measured the following results in our infiltration tests:

Table 6. Field-Measured Infiltration Rates

Test # Unfactored Rate Max/Min (in/hr)

Notes

IT-1 52/39 West side of Classic St

IT-2 85/67 East side of Classic St

As indicated in Table 6, the measured field (unfactored) minimum infiltration rate varies from 67 to 39 inches per hour with the slowest of the two measurements in IT-1 and IT-2 averaging 53 inches per hour.

Given the depth of the water table in the area and consistent occurrence of sands within the area that are similar to those at the test locations, we anticipate an unfactored infiltration rate of 53 inches per hour is reasonable for the locations where testing was completed. Appropriate factors should be applied to this rate in design.

5.0 CONCLUSIONS

Based on our explorations, testing, and analyses, it is our opinion that the proposed Project is feasible from a geotechnical perspective, provided the recommendations in this report are included in design and construction. We offer the following general summary of our conclusions:

Soils at the site are loose sands within anticipated excavation depths and generally all depths of interest.

Groundwater is several tens of feet below ground surface and not anticipated to have an effect on the Project.

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Pavement cracking along the edge of Classic Street is interpreted as due to ongoing sliding and creep of loose sands beneath the roadway.

Gravity or MSE retaining walls are recommended to stabilize the downslope edge of Classic Street, with gravity walls preferred. Final design of the walls will be completed by the Contractor and should consider the information provided in this report, meet a static FS of 1.25, and meet a seismic FS as determined by the City.

The loose sand soils will not hold steep slope angles or have stable trench walls at significant excavation depths. They will also be prone to raveling. Temporary and permanent excavations should consider the loose nature of the soils and take appropriate measures to protect structures and avoid excessive overexcavation from slope raveling and collapse.

Excavations into steep slopes below the houses adjacent Classic Street should be avoided due to the potential for upslope raveling and damage to house foundations. If such excavations are planned, we should be contacted to provide recommendations and review grading plans.

Additional measures to protect upslopes homes from construction-related damage should be considered, including a pre-construction survey and the use of non-vibratory compaction for roadway subgrades and base rock.

The site is conducive to on-site stormwater infiltration per the recommendations in this report.

Pavement design, based on the traffic data provided by the City, should follow the recommendations in this report.

On-site soils are suitable for use as structural fill.

Subsurface conditions will make shallow trenchless methods difficult to complete, due to mud loss and heave at the surface. However, we understand that local contractors have been able to successfully advance utilities in the site soils with specific mud mixtures. Completion of trenchless utilities may require reliance on local contractors experienced in such soils.

Our geotechnical recommendations for the Project, which address the above, are provided in the following sections.

6.0 RECOMMENDATIONS

Our Earthworks and Retaining Wall Recommendations for the Project are provided in the following sections.

6.1 EARTHWORKS RECOMMENDATIONS

6.1.1 Site Preparation

Site preparation will depend on final selection of a pavement section. Where pavements remain in place, no significant site preparation is anticipated. However, where pavements are to be removed, they should be removed to the full depth they occur. The underlying base rock can generally be left in place, unless removal is necessary to reach site grades, or the rock is contaminated. Removed AC and base rock can be stockpiled and re-used later as structural fill as described later in this report.

Where retaining walls are to be constructed, site preparation should also include clearing of trees, grubbing stumps and other vegetation, and stripping any organics and duff within structural and work areas. We estimate that stripping will generally be less than 6 inches deep. Cleared, stripped, and grubbed materials should be hauled off-site and properly disposed of.

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Any utilities to be abandoned within the Project area should be fully removed or grouted full if left in place.

Areas disturbed by their removal should be repaired as recommended elsewhere in this report.

The exposed subgrade should be evaluated after site preparation activities are complete. Evaluation should be completed by proofrolling the subgrade with a fully-loaded dump truck or similar heavy rubber-tired construction equipment to identify remaining soft, loose or unsuitable areas. The proofroll should be conducted prior to placing any other fill. The proofrolling should be observed by a member of Pali Consulting’s staff who should evaluate the suitability of the subgrade and identify any areas of yielding that are indicative of soft or loose soil. If soft or loose zones are identified during proofrolling, these areas should be excavated to the extent indicated by the engineer and replaced with structural fill.

6.1.2 Wet Weather Construction

The sandy soils at the site are not very susceptible to wet conditions, except during periods of high precipitation or when saturated.

However, it is good practice to schedule earthwork for drier summer months, if possible. If earthwork is scheduled for the wet season or significant precipitation occurs during construction, the contractor should be prepared to employ wet weather measures to minimize disturbance to the subgrade from construction traffic. Such measures might include:

Constructing a temporary working pad of 12 to 24 inches or more of crushed rock over a geotextile fabric, Using tracked equipment and smooth-edge buckets to minimize subgrade disturbance, Covering soil stockpiles or subgrade areas with plastic to prevent erosion and saturation, Protecting footing subgrades with four or more inches of lightly compacted crushed rock.

Other measures as needed to protect structural areas of the site and structural materials.

Bearing soils that are disturbed during construction should be recompacted in place, if practical, or removed and replaced with structural fill.

6.1.3 Excavation

Site soils within expected excavation depths will generally consist of loose sand that is dry to moist. It is our opinion that conventional earthmoving equipment in proper working condition should be capable of making Project excavations in expected soil types. The earthwork contractor should be responsible to provide the equipment and procedures to excavate the site soils described in the exploration logs and text of this report.

6.1.4 Dewatering

Regional groundwater was encountered at over 30 feet deep, so is not expected to occur within anticipated excavation depths. During periods of high precipitation, perched groundwater may occur within planned excavation depths, but given the very uniform well-drained sandy soils at the site, such perched conditions are unlikely to be persistent for long periods of time. In addition to perched groundwater, surface water inflow to the excavations during the wet season could be problematic, especially adjacent to areas where AC pavements remain. Provisions for temporary ground and surface water control should be the responsibility of the contractor to select the means and methods best suited to the schedule and their equipment.

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6.1.5 Excavation Stability

Trench sidewalls throughout the Project will be prone to raveling and collapse at all depths. We recommend that all excavations be shored or laid back. All trench excavations should be made in accordance with applicable Occupational Safety and Health Administration (OSHA) and state regulations. Site soils are expected to be OSHA Type C throughout the Project area, but the soil type should be confirmed by a “competent person” under the direction of the contractor in the field based on actual conditions encountered.

Trenches should not be excavated adjacent the toe of any slope below a line projected from the toe of the slope at a 3H:1V gradient, unless evaluated by qualified personnel.

While this report describes certain approaches to excavation and shoring, the contractor should be responsible for selecting and designing the specific methods, monitoring the excavations for safety, and providing shoring required to protect personnel and adjacent structural elements.

6.1.6 Permanent Cut and Fill Slopes

Permanent cut slopes in the loose native aeolian sands should not exceed 3H:1V. Fill slopes can be constructed at maximum gradients of 2H:1V, if completed per Section 6.1.7 of this report. Slopes that will be maintained by mowing or adjacent to surface water should be 3H:1V or flatter. Footings, access roads and pavements should be located at least 5 feet horizontally from any slope face. If steeper slopes or closer setbacks are necessary, we should be contacted to provide additional recommendations, and additional explorations may be necessary.

Slopes should be planted with appropriate vegetation as soon as possible after grading to provide protection against erosion. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

6.1.7 Structural Fill and Backfill

All fill associated with roadways, retaining walls, and slopes over 5H:1V should be considered structural fill for this Project.

Structural fill soils should be free of debris, roots, organic matter, frozen soil, man-made contaminants, particles with greatest dimension exceeding 4 inches, and other deleterious materials. The suitability of soil for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines in the soil matrix increases, the soil becomes increasingly more sensitive to small changes in moisture content and achieving the required degree of compaction becomes more difficult or impossible.

Recommendations for suitable fill are provided in the following sections.

6.1.7.1 PIPE BEDDING AND PIPE ZONE MATERIAL

Utility trench backfill for pipe bedding and in the pipe zone should consist of well-graded granular material with a maximum particle size of 3/4-inch and less than 10 percent fines. The pipe bedding and pipe zone material should meet the pipe manufacturer’s recommendations, as well, including placement of the bedding and pipe zone material so that the pipe is evenly supported and backfilled.

6.1.7.2 TRENCH BACKFILL

Backfill above the pipe zone should consist of materials suitable for the overlying use of the area. Our recommendations for backfill within and outside of roadway areas follow, separately:

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6.1.7.2.1 BACKFILL IN ROADWAY AREAS

Within roadway areas we recommend that imported granular material be used as backfill. The material should be pit or quarry run rock, crushed rock, or crushed gravel and sand and should meet the specifications provided in the Oregon Department of Transportation (ODOT) Standard Specifications for Construction (SSC) 00330.14 – Selected Granular Backfill or SSC 00330.15 – Selected Stone Backfill. The imported granular material should also be angular, fairly-well graded between coarse and fine material, have less than 10 percent by dry weight passing the U.S. Standard No. 200 Sieve, and have at least two mechanically fractured faces. During dry weather, the fines content may be increased to a maximum of 20 percent.

6.1.7.2.2 BACKFILL IN NON-ROADWAY AREAS

Outside of roadway areas where no surcharge loads or traffic will occur, on-site granular material (sand) can be used provided the material meets the general requirements for structural fill. If the use of on-site soil as structural fill is problematic, imported granular material such as that specified for roadway areas or Imported Structural Fill can be used.

6.1.7.3 ROADWAY BASE ROCK

Imported granular material used as aggregate base (base rock) in roadway areas should be clean, crushed rock or crushed gravel and sand that is fairly-well graded between coarse and fine. The base aggregate should meet the specifications of SSC 00641 – Aggregate Subbase, Base, and Shoulder Base Aggregate, depending upon application, with the exception that the aggregate have less than 5 percent by dry weight passing a U.S. Standard No. 200 Sieve based on the minus 3/4-inch fraction and have at least two mechanically fractured faces. The aggregate base should have a maximum particle size of 1 to 1-1/2 inch, depending on future performance preference. Smaller aggregate material generally has more favorable drivability characteristics but shorter lifespan, while larger aggregates have the opposite characteristics where AC will not be placed over the base rock.

6.1.7.4 HAUL ROAD ROCK

If haul roads are constructed, rock to construct haul roads should consist of crushed rock that is well-graded between coarse and fine particle sizes, contains no unsuitable materials or particles larger than 4 inches, and has less than 5 percent by weight passing the U.S. Standard No. 200 sieve. It should be placed in a single lift, typically over a separation geotextile fabric, and compacted to a well-keyed state using a heavy non-vibratory roller.

6.1.7.5 IMPORTED SELECT STRUCTURAL FILL

Select imported granular material may be used as structural fill. The imported material should consist of pit or quarry run rock, crushed rock or crushed gravel and sand that is fairly well graded between coarse and fine sizes. It should have a maximum particle size of 4 inches and less than 5 percent passing the U.S.

No. 200 Sieve. During dry weather, the fines content can be increased to a maximum of 12 percent.

The material should be placed and compacted in lifts with maximum uncompacted thicknesses and relative densities as recommended in the tables that follow.

6.1.7.6 CRUSHED ROCK FILL

Crushed rock fill for aggregate base located under footings or other structures, should consist of imported clean, durable, crushed…