D236_Geotechnical_Report_-_2026-MPW-174.pdf

HALEY & ALDRICH, INC.

3131 Elliott Ave Ste 600 Seattle, WA 98121 206.324.9530 www.haleyaldrich.com

June 18, 2026 File No. 0213211-000

Pierce County Planning and Public Works 2702 S 42nd Street, Suite 109 Tacoma, Washington 98409

Attention: David Chain, P.E.

Project Manager

Subject: Geotechnical Engineering - Soil Nail Wall Recommendations Parkview Point Soil Nail Wall Gig Harbor, Washington

Dear David Chain:

Haley & Aldrich, Inc. (Haley & Aldrich) is pleased to present this letter to Pierce County Planning and Public Works (County) summarizing our geotechnical engineering design and construction recommendations for the proposed Parkview Point soil nail wall at Gig Harbor, Washington. Our work was performed in general accordance with SC-11274 Work Assignment No. 5, dated September 8, 2025.

Project Background

Landau Associates, Inc. (Landau) previously provided geotechnical design services for the “Point Evans on the Narrows Bluff Stabilization” project, summarized in memoranda dated April 24, 2020, and May 16, 2023 (Attachments 1 and 2). Pierce County requested that Haley & Aldrich, Inc. continue geotechnical services for the project because Landau is no longer providing geotechnical services.

Based on the previous reports, Haley & Aldrich understands that a portion of the steep bluff overlooking The Narrows located behind a single-family residence at 906 32nd Street Court NW and a condominium at 802 30th Street NW experienced erosion after a stormwater line broke. The eroded bluff is mostly located within a stormwater easement owned by Pierce County, though also extends onto the adjacent private properties. The erosion resulted in approximately 12 feet of bluff retreat.

The Point Evans on the Narrows Homeowners’ Association (HOA) submitted a claim to Pierce County requesting repair of the bluff erosion. Pierce County considered options to reduce or limit continued bluff retreat and selected a soil nail wall stabilization concept. The bluff in this area is subject to natural erosion processes. The intent of the soil nail wall repair is to address the portion of bluff retreat attributed to the broken stormwater line and to regain time lost due to anthropogenic erosion, rather than to stop natural bluff retreat outside the repair area.

Pierce County Planning and Public Works June 18, 2026

Soil and Groundwater Conditions

The soil and groundwater conditions summarized below are based on Landau’s memoranda dated April 24, 2020, and May 16, 2023. Haley & Aldrich did not perform additional subsurface explorations for this phase of work.

Based on Landau’s explorations, the soils beneath the existing ground surface, which generally consisted of grass-covered areas, were grouped into three general geologic soil units:

Fill: generally consisted of sand with variable silt and gravel content in a medium dense, moist condition, extended from 1 to 5 feet below ground surface (bgs).

Advance outwash: beneath the fill and typically consisted of sand with variable silt and gravel content in a moist, medium dense to very dense condition. The advance outwash extended to approximately 54 feet bgs.

Glacial till: beneath the advance outwash and typically consisted of very dense to hard silt with variable sand content.

During Landau’s investigation, groundwater was observed at approximately 55 feet bgs, perched atop the glacial till. The groundwater conditions summarized in this letter are based on the specific locations and dates reported by Landau and may not represent conditions at other locations or times.

Groundwater conditions may vary with subsurface conditions, seasonal precipitation, storm events, site drainage, tidal influence, and other factors.

Haley & Aldrich staff visited the site and reviewed aerial photography collected by Haley & Aldrich (as discussed below) of the exposed scarp, and consider the subsurface characterization of the site soils completed by Landau to be representative of soils that we would typically expect to be found in the project vicinity. We utilized their subsurface information to complete our updated analyses of the proposed soil nail wall system.

Wet Season Monitoring

In the Settlement Agreement between the County and HOA, it was required that the County monitor the eroded bluff area where the precipitation reached 1 inch or more during a 24-hour period. Haley & Aldrich employed an Unmanned Aerial System (UAS; also known as a drone) equipped with a high-density, multi-return light detection and ranging (LiDAR) sensor capable of penetrating moderate vegetation to capture both ground and canopy surfaces. Simultaneously, the UAS captured high-resolution imagery to generate an orthomosaic and orthoplex of the site.

An initial baseline flight was performed on October 16, 2025. Subsequent wet season monitoring flights were performed on October 30 and December 29, 2025, and February 11 and May 4, 2026. Visual observations were also performed on October 8, December 9, and December 12, 2025, and March 18, 2026. In general, we observed no signs of regression of the crest of the bluff failure or significant changes to the bluff face or body of the failure area. However, due to thick vegetation, detailed change detection within the body of the failure area was not feasible.

Updated Geotechnical Design

As part of our work for the County, Haley & Aldrich updated the design of the soil nail wall, including performing an analysis of seismic stability, which had not previously been completed by Landau. A brief discussion of our updated analyses is provided in the following sections.

SEISMIC DESIGN PARAMETERS

We evaluated potential seismic shaking at the site in accordance with the 2024 International Building Code (IBC; International Code Council, 2024). The maximum considered earthquake considered by the IBC has a 2 percent probability of exceedance in 50 years, corresponding to a 2,475-year return period.

In accordance with American Society of Civil Engineers (ASCE) 7-16, the “Site Class” is a designation to quantify ground motion amplification. The classification is based on the stiffness in the upper 100 feet of soil and bedrock materials at a site, as evaluated using standard penetration test or shear wave velocity data. We classified the on-site material using Landau’s boring data and determined a Site Class D classification for this site. We recommend the seismic design parameters provided in Table 1.

Table 1. ASCE Seismic Design Parameters Parameters Values Latitude 47.2875 Longitude –122.5509 Site Class D Spectral Response Acceleration at Short Periods, SS 1.5 Spectral Response Acceleration at 1-second Periods, S1 0.49

Site Class adjusted PGA (PGAm) 0.62g

Site Coefficient for Short Period Spectral Response Acceleration SS, Fa 1.2 Site Coefficient for 1-second Period Spectral Response Acceleration, Fv N/A Site Coefficient for mapped PGA, FPGA 1.2 Notes:

Values in this table are for use in design based on the ASCE Web Tool.

N/A = not applicable PGA = peak ground acceleration

SLOPE STABILITY ANALYSIS

General

Haley & Aldrich performed slope stability analyses for the bluff using Slide2 by Rocscience. Slide2 performs two-dimensional limit equilibrium analyses to evaluate slope stability and calculate a factor of safety (FS) against slope movement. For our analyses, we used the Morgenstern-Price and Spencer methods.

The FS represents the ratio between resisting forces, such as soil shear strength, and driving forces, such as gravity, earth pressure, and seismic loading. A calculated FS of 1.0 or less indicates that the shear stresses required for equilibrium equal or exceed the available shear resistance for the modeled conditions.

Landau developed the original slope stability models for the project. We reviewed and updated those models to evaluate the slope conditions and proposed stabilization approach described in this letter.

The analyses used the subsurface conditions interpreted from Landau’s explorations, available topographic information, groundwater conditions reported by Landau, and design assumptions summarized in the following sections.

Scenarios Considered

The soil nail wall section analyzed was oriented generally perpendicular to the bluff crest, running east to west. The section was analyzed for three general conditions: the assumed original pre-failure ground surface, the existing post-failure ground surface based on LiDAR data, and the proposed ground surface with soil nails installed.

Landau’s previous design did not include seismic slope stability conditions. Haley & Aldrich therefore evaluated seismic conditions as part of the updated analyses. The material properties and subsurface layering used in Haley & Aldrich’s analyses are generally consistent with those used in Landau’s design and analyses and are summarized in Table 2.

Table 2. Summary of Soil Layers and Engineering Properties for Slope Stability Analysis

Soil Layers Description Friction Angle (degree) Cohesion (psf) Unit Weight (pcf)

1 Medium Dense Poorly Graded Sand with Silt [Fill] 34 0 125

2 Dense Poorly Graded Sand 38 0 125

3 Dense Poorly Graded Sand with Silt 38 0 130

4 Dense Silty Sand 38 0 125

5 Hard Silt (Glacially Consolidated) 34 200 125

Notes:

pcf = pounds per cubic foot psf = pounds per square foot

Pre-Slide Slope

Because LiDAR or survey data are not available for the slope before the erosion failure, we estimated the original ground surface used in the Slide2 model based on a projected slope profile and geometry.

We modeled the assumed original ground surface to compare the calculated failure surfaces and FS for the proposed soil nail wall with the interpreted pre-slide condition. This comparison was used to evaluate whether the proposed stabilization approach would meet or improve the modeled stability of the slope before the stormwater line-related erosion occurred.

Existing Slope

The existing slope profile used in the Slide2 model is based on LiDAR scans completed at the site using drone flights during the wet-season monitoring period. The drone flights were completed to observe slope conditions, document surficial changes, and supplement the available topographic and visual information used to evaluate the existing slope condition.

Proposed Soil Nail Wall

We analyzed several soil nail wall design configurations. The analyses included the design proposed by Landau and design variations with longer soil nails, a shallower slope inclination, or both.

Seismic Slope Stability

The original Landau design did not explicitly include seismic loading in the slope stability analyses.

Landau stated that, based on its evaluation, the proposed soil nails would increase the seismic stability of the bluff compared with the unimproved slope.

Because the proposed soil nail wall will protect existing improvements near the bluff crest, Haley & Aldrich evaluated seismic stability using a target FS of 1.1 per standard geotechnical engineering practice and as outlined in the Washington State Department of Transportation (WSDOT) Geotechnical Design Manual (GDM; 2022)1 Section 7.4. Pseudo-static slope stability analyses typically apply a horizontal seismic coefficient equal to one-half of the PGA (Table 1). For this site, that approach would result in a seismic coefficient of 0.31g. However, we also applied a reduction to this seismic coefficient per the guidance of National Cooperative Highway Research Program Report 611 that accounts for dispersion of seismic shaking due to tall slopes. This reduction assumes that 1 to 2 inches of seismic deformation is considered acceptable, which is within the range of standard engineering allowances. Applying that approach to the site results in a reduced seismic coefficient of 0.22g. We used the reduced seismic coefficient of 0.22g in the pseudo-static slope stability analyses.

Design Approach

Haley & Aldrich understands that the soil nail wall is intended to stabilize the immediate area around the slope failure. The wall is not intended to stabilize the entirety of the approximately 300-foot-tall bluff.

Slide2 allows the user to define search limits that focus on the evaluation of potential failure surfaces.

These limits can control where a modeled failure surface starts near the top of slope and where it exits

1 WSDOT, 2022. Geotechnical Design Manual. M46-03.16, February 10.

on or near the lower slope. To evaluate how the proposed soil nail wall affects slope stability, we assessed two search-limit conditions: focused limits and full-slope limits.

The primary difference between the focused limits and full-slope limits is where the modeled failure surfaces were allowed to exit the slope. The focused limits allowed failure surfaces to exit only as far downslope as the soil nail wall extends, approximately 40 vertical feet below the top of slope, at the toe of the bluff failure that occured. The full-slope search did not limit the exit location; therefore, modeled failure surfaces could exit the slope at any elevation.

We used the focused limits to evaluate the stability of the area proposed for repair and to develop a soil nail wall configuration for the failed area. We used the full-slope limits to evaluate whether the soil nail wall improves the modeled stability of the bluff compared with the existing and assumed pre-slide conditions. However, the soil nail wall is not intended to achieve target FS for the full approximately 300-foot-tall bluff. For comparison, we developed a design that targets improved FS for relative to a full slope search. The results as described below in Table 3 required increasing the lengths of soil nail bars 20 to 40 percent of the focused limit.

As part of our long-term slope stability analyses, we included a uniform surcharge of 250 psf for the nearby condominium building.

Slope Stability Results

As shown in Table 3, the proposed soil nail wall improves the calculated FS compared with the assumed pre-slide condition for the modeled scenarios. The proposed Haley & Aldrich soil nail wall design (case 3) also meets the target FS when the analysis uses focused search limits to evaluate the immediate area of the slope failure. The slope stability results are provided in Attachment 3.

Table 3. Summary of Factors of Safety for Slope Stability Analysis

Model Limits Scenario Pre-Slide

Slope (Case 1)

Existing Slope (Case 2)

Focused Stability (Case 3)1

Full Slope Stability (Case 4)2

Target

Seismic Full Slope A 0.47 0.58 0.97 1.06

1.1 Focused B 0.77 0.59 1.34 1.52

Static Full Slope C 0.74 0.9 1.37 1.5

1.5 Focused D 1.19 0.91 1.74 2.0

Notes:

1. Nail lengths for Case 3 are 25 feet for the upper eight rows and 30 feet for the bottom two rows.

2. Nail lengths for Case 4 are 30 feet for the upper eight rows and 40 feet for the bottom two rows.

SNAIL STABILITY ANALYSES AND RESULTS

We analyzed the internal stability of the proposed soil nail wall using California Department of Transportation SNAIL software. The internal stability was analyzed under both static and pseudostatic loading conditions for a minimum FS of 1.3 and 1.1, respectively, per the WSDOT GDM and the Federal Highway Administration design manual. The general soil nail details are summarized in Table 4.

The SNAIL models included a temporary construction surcharge of 1000 psf and a uniform condominium building surcharge of 250 psf. The soil nail lengths for each section and row are listed in Table 5.

Table 4. Soil Nail Details Parameter Value

Number of rows 9 Maximum Vertical Spacing 4 feet

Maximum Horizontal Spacing 4 feet Minimum Bar Size #9

Minimum Bar Grade Grade 75 Nail Inclination 15 degrees from horizontal

Minimum Nail Hole Diameter 6 inches

Table 5. SNAIL Analysis Soil Nail Lengths Soil Nail Row Numbera Soil Nail Length (feet)

1 to 7 25 8 to 9 30

Note:

a. Soil nail rows are listed from top down.

Permanent Wall Stability

We evaluated the stability of the permanent, full-height wall at the completion of construction under both static and seismic conditions. The results of our SNAIL analyses are summarized in Table 6. The results of the analysis and illustrations of the wall are included in Attachment 4.

Table 6. SNAIL Analysis Stability Results – Permanent Wall Model Global Stability FS Nail Head Service Load (To in kips) Static 1.8 19.1

Seismic 1.28 26.3

Temporary Construction Stability

Construction staging cases were also evaluated under static conditions using SNAIL for all design sections. The staging cases evaluated the temporary stability of the wall geometry as each lift is excavated to install a row of soil nails, but before the row of nails is installed, to ensure no intermediate failure is likely to occur during construction. These analyses assumed a maximum depth of 2.5 feet of over-excavation beneath the new row of nails to be installed.

The minimum FS for static and temporary conditions of soil nail walls is 1.2, per the WSDOT GDM Section 15-7.5. A summary of the FS results for the static, temporary case of each section is shown below in Table 7. The corresponding nail head service load is included for each case.

Table 7. Temporary Construction Stability Results

Construction Stage SNAIL Static FS Nail Head Service Load (kips)

Lift 1a 2.03 20.7 Lift 2b 2.42 14.1 Lift 3c 2.23 11.9 Lift 4d 2.0 12.2 Lift 5e 1.84 13.4 Lift 6f 1.59 12.3 Lift 7g 1.48 11.1 Lift 8h 1.39 17.5

Notes:

a. Excavation of second cut, one row of soil nails installed.

b. Excavation of third cut, two rows of soil nails installed.

c. Excavation of fourth cut, three rows of soil nails installed.

d. Excavation of fifth cut, four rows of soil nails installed.

e. Excavation of sixth cut, five rows of soil nails installed.

f. Excavation of seventh cut, six rows of soil nails installed.

g. Excavation of eighth cut, seven rows of soil nails installed.

h. Excavation of ninth cut, eight rows of soil nails installed.

Temporary minimum allowable facing resistance is 28.3 kips.

Construction Recommendations

Construction should conform to the WSDOT 2026 Standard Specifications, the 2015 Federal Highway Administration Reference Manual, and the project plans and specifications. The County should review and approve deviations from these requirements before the contractor implements them.

Use of this Report

We completed this work in general accordance with our work assignment dated September 8, 2025. This report is for the exclusive use of Pierce County Planning and Public Works and their authorized agents, for specific application to this project and site. We completed this work in accordance with generally accepted geotechnical practices for the nature and conditions of the work completed in the same or similar localities, at the time the work was performed. We make no other warranty, express or implied.

Sincerely yours, HALEY & ALDRICH, INC.

Michael Liu, P.E.

Project Geotechnical Engineer

Daniel Trisler, P.E.

Principal Geotechnical Engineer

Attachments:

1. Landau Associates Inc, 2020, Summary of Geotechnical Engineering Services Point Evans on the

Narrows Bluff Erosion Repair

2. Landau Associates Inc, 2023, Summary of Geotechnical Design Services Point Evans on the

Narrows Bluff Erosion Repair and Soil Nail Wall

3. Haley & Aldrich SLIDE Results

4. Haley & Aldrich SNAIL Results https://haleyaldrich.sharepoint.com/sites/PierceCounty/Shared Documents/0213211.Parkview Point Soil Nail Wall/Deliverables/Soil Nail Wall Design Letter/Final/2026_0618_HAI_ParkviewPoint_SoilNailWallLetter_F.docx

ATTACHMENT 1

Landau Associates Inc, 2020

Summary of Geotechnical Engineering Services Point Evans on the Narrows Bluff Erosion Repair

Technical Memorandum

955 Malin Lane SW, Suite B • Tumwater, Washington 98501 • (360) 791-3178

TO: Mr. Colby Richards, Claims and Safety Technician, Pierce County Risk Management

FROM: Calvin McCaughan, PE, and Daniel Simpson, PE

DATE: April 24, 2020

RE: Summary of Geotechnical Engineering Services Point Evans on the Narrows Bluff Erosion Repair Gig Harbor, Washington Project No. 0309070.010.011

Introduction

This memorandum summarizes the results of geotechnical engineering services provided by Landau

Associates, Inc. (LAI) to support repair of an eroded bluff at the Point Evans on the Narrows condominiums in Gig Harbor, Washington (Parcel No. 7589000011; site; Figure 1). Services were provided in accordance with the scope outlined in LAI’s proposal, dated April 13, 2020.

Project Understanding

A portion of steep bluff overlooking Puget Sound has experienced soil erosion as the result of a broken stormwater line. The bluff is located within a stormwater easement owned by Pierce County

(County). The County plans to decommission the easement. Erosion has resulted in approximately 12 feet (ft) of bluff loss, and has encroached on the site, undermining the stability of a fence. The Point

Evans on the Narrows homeowners’ association has filed a claim with the County, requesting that the bluff erosion be repaired. Erosion has also encroached on adjacent Parcel No. 40001006260. The approximate parcel layout and extent of soil erosion are shown on Figure 2.

LAI has been retained by the County to identify feasible erosion repair alternatives and associated costs.

Geology and Bluff Retreat Processes

The steep marine bluffs located near the site are common to the Puget Sound region. Approximately

13,500 years ago, continental glaciers covered much of the region (Galster and Laprade 1991). The weight of the glaciers caused underlying soil deposits to partially consolidate into a material with apparent cohesion, allowing for the formation of steep coastal bluffs. Nature and anthropogenic factors weather the glaciated soils and degrade the apparent cohesion, resulting in slope failures near the exposed edges of the bluff and depositing colluvium near the base of the bluff (Tubbs 1974).

Wave action or tidal flows then remove the colluvium at the base of the bluff, preventing the slope from stabilizing. These beach erosion processes also contribute to bluff retreat.

Typically, retreat occurs episodically as weathering affects near-surface material until a chunk of soil dislodges, exposing fresh material and repeating the process. The time between calving events and

Landau Associates

Summary of Geotechnical Engineering Services Point Evans on the Narrows Bluff Erosion Repair 2 April 24, 2020 the magnitude of the events (i.e., the amount of bluff lost) are non-uniform (Hapke 2004). In the

Puget Sound region and similar geologic settings, researchers have attempted to quantify the rate of retreat. Hapke (2004) notes the episodic nature of bluff retreat (this may be why some researchers report retreat rates on time bases, such as centuries).

Local to the Puget Sound region, Galster and Laprade (1991) and Nashem et al. (2000) reported a bluff retreat rate of 10 to 75 ft per century when bathymetry data were analyzed over an approximately

5,000-year period. Prior to construction of the nearby Parkview Point development, AESI (1987) applied photogrammetric techniques to aerial images from 1966, 1971, and 1985, and estimated a bluff retreat rate of 0 to 1.5 ft per year for the 19-year study period. Hapke (2004) reports bluff retreat rates on the order of 20 centimeters per year (0.66 ft per year) for coastal bluffs in northern

Monterey Bay, California. Bluffs in this region consist of poorly indurated sandstone and siltstone, a material geotechnically similar to the partially consolidated glacial sediments found in the Puget

Sound region. LAI reviewed aerial images of the portions of bluff adjacent to the condominiums. The photographs were taken in 1990 and 2006 (Figure 3); the results of LAI’s qualitative review indicate that bluff retreat is ongoing. Similar comparisons can be made for other portions of bluff adjacent to the site.

Site Conditions

The portion of bluff between the condominiums and the shore of Puget Sound is approximately 300 ft tall. The upper 50 to 100 ft of bluff is inclined between 60 degrees and vertical, while the lower portion is inclined between 30 and 50 degrees. The upper bluff primarily consists of glacial till and advance outwash (AESI 1987; DNR 2020), both glacially overridden soils.

On October 31, 2019, LAI completed a reconnaissance of the lower bluff. LAI also reviewed drone footage of the upper bluff face (Pierce County 2019a). Based on LAI’s findings, widespread groundwater seepage is not present on the eroded bluff face. A glacial till cap (AESI 1987) likely contributes to the lack of groundwater in the underlying advance outwash. LAI interprets the lower bluff to consist of colluvium, though the soil beneath (behind) the colluvium is likely earlier

Pleistocene glacial soil.

The upper bluff erosion that is subject of this memorandum is shown on Figure 4. Additional erosion channels extend farther downslope, but are not likely to affect the condominiums. Figure 4 also includes a juxtaposition of the current site and the site as it appeared in the early 1990s, when a retaining wall was constructed on the condominiums’ property.

Basis of Design

As shown on Figure 2, approximately 11 ft of bluff retreat has occurred along an alignment that passes through the point nearest the condominium structure (demonstrated by concentric circles centered

Point Evans on the Narrows Bluff Erosion Repair 3 April 24, 2020 on the building corner and tangent to the pre- and post-erosion top of bluff line). To develop Figure 2, LAI reviewed satellite (Pierce County 2020) and drone imagery (Pierce County 2019a) as well as measurements taken on site. Based on the geologic review and site conditions noted in previous sections, 11 ft of bluff retreat can be equated to time using a bluff retreat rate, i.e., the condominiums lost 22 years of usable building setback. The estimated time lost is based on a bluff retreat rate of 0.5 ft per year, the approximate average of the values previously reported. The lost building setback time can be used to establish a minimum design life for the erosion repair.

The minimum horizontal extents of erosion repairs are depicted on the following diagrams. Horizontal extents were established to limit building setback loss and property loss during the design life of the repair. LAI used limit-equilibrium slope stability calculations, performed with SLIDE Version 2018, to estimate size and strength requirements for the erosion repair alternatives. To develop the slope stability model, LAI estimated stratigraphy using drone imagery (Pierce County 2019a), geologic information provided by AESI (1987), and soil engineering properties reported by Koloski et al. (1989).

Erosion Repair Alternatives

Based on the results of LAI’s engineering analyses, five erosion repair alternatives were considered.

LAI completed an alternatives analysis for the three repair options found to be most feasible.

Rock Infill Buttress (Infeasible)

LAI considered mitigating soil erosion by filling the eroded portions of the slope with quarry spalls or other large, angular rock. The rock would protect areas of exposed soil from erosion, and increase the stability of the upper bluff. Conceptually, the rock buttress would be supported on level ground in the plunge pool. Site geometry (steep slope conditions) makes construction of a rock infill buttress infeasible.

Plastic Sheeting/Revegetation (Infeasible)

LAI considered mitigating soil erosion by covering the slope face with plastic sheeting during the wet season, and revegetating during dry, warm months. Because the upper 30 ft of bluff is inclined at a near-vertical angle, vegetation is unlikely to establish unless the slope is flattened. Conceptually, the slope would need to be cut back 20 to 30 ft to allow for long-term revegetation. To accommodate the cut back, the condominium would require a remaining setback 5 ft from the slope, making revegetation an infeasible repair alternative.

Conventional Retaining Wall

LAI considered mitigating soil erosion by constructing a conventional retaining wall at the top of the slope. As shown on Photograph 1, a crib wall was constructed at the site in, or around, 1991, and remains in place today.

Point Evans on the Narrows Bluff Erosion Repair 4 April 24, 2020

Diagram 1. Conceptual conventional wall layout.

Photograph 1. 1991 (approximate date) wall construction (Pierce County 2019b).

A conventional retaining wall, such as a mechanically stabilized earth wall, would slow erosion at the top of the bluff, while maintaining a level and usable yard. This wall type would not directly slow erosion below the wall.

LAI evaluated the possibility of installing a conventional wall near the top of the slope. An 11-ft setback would be required between the base of the wall and the top of the slope below the wall. The setback would protect the wall from bluff erosion. A conceptual layout for a 6-ft-tall conventional wall is shown on Diagram 1.

Reference fence post

Point Evans on the Narrows Bluff Erosion Repair 5 April 24, 2020

LAI concludes that construction of a conventional retaining wall is feasible. An 11-ft slope setback would achieve the estimated 22-year design life of the wall, assuming an average bluff retreat rate.

However, the condominiums and adjacent Parcel No. 4000100260 would be forced to yield yard space to accommodate the slope setback. LAI estimates that, including the 11-ft setback and any required wall batter, the new top of slope would be located 15 to 18 ft from the condominium.

Reticulated Micropiles

LAI considered mitigating soil erosion using reticulated micropiles to construct a subsurface, or

“hidden” wall, as shown on Diagram 2. (Note: The micropile wall depicted on Diagram 2 was installed for a deep excavation project; however, a similar layout could be used to mitigate bluff erosion.)

Micropiles (small-diameter bored piles filled with steel reinforcement and grout) would be installed with a track-mounted drill rig that could access the top of the bluff. The wall face would eventually be exposed by soil erosion, but the design would limit erosion beyond the wall face. Landscaping could be used to cover the grade beam at the top of the wall so that it is essentially invisible.

Diagram 2. Typical reticulated micropile wall (FHWA 2000).

Based on LAI’s preliminary calculations, approximately 200 micropiles, each 50 ft long, would be required to construct the wall. A reinforced concrete grade beam would be used to connect the tops of the piles. The wall would be installed several feet from the current top of bluff. The grade beam could be buried, but would likely extend 8 to 10 ft onto adjacent parcels. A conceptual wall layout is shown on Diagram 3.

Point Evans on the Narrows Bluff Erosion Repair 6 April 24, 2020

Diagram 3. Conceptual micropile wall alignment.

Soil Nail Wall

LAI considered mitigating soil erosion by constructing a soil nail wall, as shown on Diagram 4. (Note:

The soil nail wall depicted on Diagram 4 was installed for a roadway-widening project; however, a similar layout could be used to mitigate bluff erosion.) This wall type typically consists of soil nails that reinforce retained soil and shotcrete facing that provides erosion protection. Soil nails typically consist of threaded steel bars that are installed in 4- to 6-inch-diameter holes filled with Portland cement grout. Wall facing typically includes one to two layers of shotcrete reinforced with welded wire mesh.

Bolts and bearing plates are used to transfer loads between the wall facing and soil nails.

Diagram 4. Typical soil nail wall (FHWA 2015).

Point Evans on the Narrows Bluff Erosion Repair 7 April 24, 2020

Based on LAI’s calculations, 24-ft-long soil nails, consisting of No. 8 threaded bars, would be required to construct the wall. Five to six feet of vertical and horizontal spacing should be included between nails. A conceptual wall layout is shown on Diagram 5. The wall would be installed along the current bluff face, and would extend, vertically, from the top of the erosion scarp to the base of the plunge pool, peaking at a height of approximately 40 ft. Prior to construction, loose material would be removed from the bluff face, resulting in approximately 1 to 3 ft of additional bluff retreat. Erosion protection would be installed at the toe of the wall.

Diagram 5. Conceptual soil nail wall layout.

Alternatives Analysis

A comparison of the three feasible repair alternatives is provided in the following table. Detailed cost estimates are provided in Attachment 1.

Alternative Estimated

Cost Nominal

Design Life

Property Value/Use

Impacts

Easements Required

Construction Impacts Other Comments

Conventional Wall

$140k 22 years High A, B

High – requires major regrading and revegetation of Parcels A and B, as shown on

Photograph 1.

High property value/use impact to condominiums (11 to 16 ft of yard loss required).

Soil Nail Wall $460k 50 years Low A, B, C

Requires accessing Parcel B with a crane and making associated landscaping repairs.

Little to no impact to

Parcel A.

Estimated 1.5-month construction period.

Only option likely to extend onto Parcel C and require coordination with parcel owner(s).

Point Evans on the Narrows Bluff Erosion Repair 8 April 24, 2020

Alternative Estimated

Cost Nominal

Design Life

Property Value/Use

Impacts

Easements Required

Construction Impacts Other Comments

Reticulated Micropiles

$1.17M 50 years Low A, B

Likely to require 2 months of construction (with impacts to Parcels A and B).

Note: A = Condominiums; B = Parcel No. 4000100260; C = Parcel No.0221214016. Costs rounded to nearest $5k. See cost estimates in Attachment 1. Property loss not included in estimated costs.

The three erosion repair alternatives will have varying impacts on the properties adjacent to the bluff.

Temporary construction easements will be required; permanent easements could also be required. A site survey should be completed to support final design and confirm easement requirements for the selected erosion repair.

Other types of drilled, screwed, or launched anchor walls, similar to a soil nail wall, are feasible and cost comparable. If a soil nail wall is selected, more than one anchor wall type (with anchors on the order of 24 ft long) should be considered during final design.

Alternative Selection

Though it is the least expensive repair alternative, the conventional wall would have a significant impact (loss of useable yard space) on the condominiums.

In LAI’s opinion, a soil nail wall will provide suitable erosion protection at less than half the cost of reticulated micropiles. Additionally, the soil nail wall will not result in the yard loss seen with a conventional wall. A soil nail wall is the recommended repair alternative.

Use of This Technical Memorandum

Landau Associates, Inc. (LAI) has prepared this technical memorandum for the exclusive use of Pierce

County Risk Management for specific application to the Point Evans on the Narrows Bluff Erosion

Repair project in Gig Harbor, Washington. No other party is entitled to rely on the information, conclusions, and recommendations included in this document without the express written consent of

LAI. Reuse of the information, conclusions, and recommendations provided herein for extensions of the project or for any other project, without review and authorization by LAI, shall be at the user’s sole risk. LAI warrants that, within the limitations of scope, schedule, and budget, its services have been provided in a manner consistent with that level of skill and care ordinarily exercised by members of the profession currently practicing in the same locality under similar conditions as this project. LAI makes no other warranty, either express or implied.

Point Evans on the Narrows Bluff Erosion Repair 9 April 24, 2020

Closing

We appreciate the opportunity to assist you with this project. If you have questions or comments, please contact the undersigned at (360) 791-3178.

LANDAU ASSOCIATES, INC.

Daniel Simpson, PE Associate Engineer

Calvin McCaughan, PE Principal Engineer

DCS/CAM/mcs [Y:\0309\070.010\R\POINT EVANS ON THE NARROWS BLUFF EROSION REPAIR TECHNICAL MEMORANDUM 4.24.2020.DOCX]

Attachments: Figure 1. Vicinity Map Figure 2. Site Features Figure 3. 1990 to 2006 Bluff Retreat Figure 4. Eroded Area Comparison (1991 to 2019) Attachment 1. Detailed Cost Estimates for Feasible Repair Alternatives

Point Evans on the Narrows Bluff Erosion Repair 10 April 24, 2020

References

AESI. 1987. Subsurface Exploration and Geological Hazards Evaluation, Pierce County, Washington.

Associated Earth Sciences, Inc.

DNR. Geologic Information Portal: topoView portal, 24k Scale. Washington State Department of

Natural Resources. Accessed April 15, 2019. Available online at:

https://ngmdb.usgs.gov/topoview/.

FHWA. 2000. Micropile Design and Construction Guidelines: Implementation Manual. U.S. Publication

No. FHWA-SA-97-070. Department of Transportation Federal Highway Administration. June.

FHWA. 2015. Soil Nail Walls Reference Manual. Publication No. FHWA-NHI-14-007. U.S. Department of

Transportation Federal Highway Administration.

Galster, R.W., and W.T. Laprade. 1991. Geology of Seattle, Washington, United States of America.

Bulletin of the Association of Engineering Geologists. Vol. XXVIII(3):253–302.

Hapke, C. 2004. “The Measurement and Interpretation of Coastal Cliff and Bluff Retreat.” U.S.

Geological Survey Professional Paper No. 1693.

Koloski, J.W., S.D. Schwarz, and D.W. Tubbs. 1989. Volume I. Geotechnical Properties of Geologic

Materials. In: Engineering Geology in Washington. Washington Division of Geology and Earth

Resources.

Nashem, W.D., W.T. Laprade, C.A. Robertson, and T.E. Kirkland. 2000. Seattle Landslide Study.

Shannon and Wilson. Report No. W-7992-07.

Pierce County. 2020. PublicGIS. Accessed April 21, 2020. Available online at:

https://matterhornwab.co.pierce.wa.us/publicgis/.

Pierce County. 2019a. “Re: Parkview Pt. 1995 [sic] Fix.” From Ingo Kutcha, Pierce County Stormwater and Flooding, to Calvin McCaughan, Landau Associates, Inc. December 30.

Pierce County. 2019b. “Re: Ohop.” From Rachel Corradi, Pierce County, to Calvin McCaughan, Landau

Associates, Inc. October 23.

Tubbs, D. 1974. Landslides in Seattle. State of Washington Department of Natural Resources, Division of Geology and Earth Resources. Information Circular No. 52.

Point Defiance

Park

TacomaTacoma NarrowsNarrows AirportAirport

Puget Sound

The Narrows

WollochetWollochet

RustonRuston

Shore AcresShore Acres

N 2 6Th S t

West ga te Bl vd

24T h S t N W

Poin t F osd i ck D rNW

O l ymp ic Dr NW

N Pe ar l S t

S to n e D r NW

Sou ndv iew D r

N Va ssa ult

St N Park Way

Hol l ycroft St

Re idDrNW

Th

A ve

N W

N Nar r ow s D r

N 17T h S t

Th

A ve

N W

N a r ro ws Dr

ST163

ST16

Data Source: Esri 2012 Point Evans on the Narrows

Bluff Erosion Repair Gig Harbor, Washington

Vicinity Map Figure1

0 0.5 1

Miles

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Project Location

W a s h i n g t o nW a s h i n g t o n Olympia

Tacoma Spokane

Everett Seattle

Project Location

Building wall line

Reference Fence Post Photo 1 view

Parcel No.

4000100260

Parcel No.

0221214016

Parcel No.

7589000011

Approximate top of slope pre erosion

Photo 2 view

Approximate top of slope post erosion

County easement

Approximate top of slope pre erosion

Parcel No.

4000100250

Distance to top of slope post erosion 31'

Distance to top of slope pre erosion 42'

0 60 120

Scale in Feet

Notes

1. Black and white reproduction of this color original may reduce its effectiveness and lead to incorrect interpretation.

2. The map features are approximate and are intended only to provide an indication of said feature. Additional areas that have not been mapped may be present. This is not a survey. Orthophotos and other data may not align. Landau Associates assumes no liability for variations ascertained by actual survey. ALL DATA IS EXPRESSLY PROVIDED ‘AS IS’ AND ‘WITH ALL FAULTS’. Landau Associates makes no warranty of fitness for a particular purpose.

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Point Evans on the Narrows Bluff Erosion Repairs

Gig Harbor, Washington Site Features

Figure

Photo 1 view

Photo 2 view

Figure

1990 to 2006 Bluff Retreat

Point Evans on the Narrows Bluff Erosion Repairs

Gig Harbor, Washington

4/24/2020 Y:\0309\070.010\R\Figure 3 1990_2006 bluff.docx

Reference fence post Reference fence post

1990 2006

Bluff retreat, demarcated by unvegetated area that appeared between 1990 and 2006

Google Earth 2020 Google Earth 2020

Crib wall

Crib wall

Eroded Area Comparison

(1991 to 2019)

Point Evans on the Narrows Bluff Erosion Repairs

Gig Harbor, Washington

4/24/2020 C:\Users\dsimpson\Desktop\9th ave\Point Evans Comparison.docx

Reference fence post

Reference fence post

Reference fence post

Estimated date of photograph is 1991.

Photograph and inset from 2019 drone footage.

Plunge pool

Detailed Cost Estimates for Feasible Repair Alternatives

Pierce County – Point Evans on the Narrows Conventional Wall Alternative

Landau Associates, Inc. 4/24/2020

Item Quantity Units Unit Cost Cost

1 L.S. $10,000 $10,000

1 L.S. $2,000 $2,000

75 L.F. $20 $1,500

25 L.F. $15 $375

248 C.Y. $90 $22,320

72 C.Y. $400 $28,800

12 C.Y. $125 $1,500

43 C.Y. $125 $5,375

1 L.S. $12,000 $12,000

1 L.S. $10,000 $10,000

1 L.S. $10,000 $10,000

Mobilization

Clearing and grubbing

High-visibility silt fence Removal of damaged fence Structure

Excavation including haul (limited -access equipment, includes 11-ft setback bench) mechanically stabilized earth wall (65 ft by 6 ft tall)

Foundation Material Class A (limited-access equipment)

Gravel Backfill (limited-access equipment)

Construction Items

Abandon (cut, plug, remove) 70-ft high-density polyethylene pipe on steep slope Landscape restoration (above wall) Landscape restoration (below wall) Construct new fence 25 L.F. $100 $2,500

$106,370

Engineering (30% of construction ‐ Wall Design, Geotechnical Report, Survey, Inspection, etc.) $31,911

$138,281

Construction Subtotal

Construction Total

Soil Nail Wall Alternative

Landau Associates, Inc. 4/24/2020

Item Quantity Units Unit Cost Cost

1 L.S. $15,000 $15,000

75 L.F. $20 $1,500

25 L.F. $15 $375

1,600 S.F. $175 $280,000

6 WKS $7,400 $44,400

1 L.S. $12,000 $12,000

5 EA $300 $1,500

1 L.S. $1,500 $1,500

25 L.F. $100 $2,500

1 L.S. $10,000 $10,000

Preparation

Mobilization (includes crane and manlift)

High-visibility silt fence Removal of damaged fence Structure

Soil nail wall ( 40' by 40' ‐ slow construction operating from crane basket) Construction Items

Crane rental/Operator (6 weeks) Abandon (cut, plug, remove) 70-ft high-density polyethylene pipe on steep slope Planting (tree/shrub) Hydroseeding

Construct new fence Landscaping restoration Armor rock/Grading in plunge pool

40 CY $300 $12,000

$380,775

Engineering (20% of construction ‐ Structural Wall Design, Geotechnial Report, Survey, Inspection, etc.) $76,155

$456,930

Reticulated Micropile Wall Alternative

Landau Associates, Inc. 4/24/2020

Item Quantity Units Unit Cost Cost

1 L.S. $15,000 $15,000

1 L.S. $750 $750

75 L.F. $20 $1,500

15 L.F. $15 $225

200 EA $5,000 $1,000,000

50 L.F. $500 $25,000

1 L.S. $8,000 $8,000

4 L.S. $300 $1,200

Preparation

Mobilization

Clearing and grubbing

High-visibility silt fence Removal of damaged fence Structure

Micropiles Pile cap Construction Items

Landscape restoration Planting (tree/shrub) Abandon (cut, plug, remove) 70-ft high-density polyethylene pipe on steep slope

1 L.S. $12,000 $12,000

$1,063,675

Engineering (10% of construction ‐ Structural, Survey, Inspection, Materials, etc.) $106,368

$1,170,043

ATTACHMENT 2

Landau Associates Inc, 2023

Summary of Geotechnical Design Services Point Evans on the Narrows Bluff Erosion Repair and Soil Nail Wall

Technical Memorandum

2107 S C St • Tacoma, WA 98402 • 253.926.2493

TO: Mr. Ingo Kuchta, Pierce County Department of Planning and Public Works

FROM: Amy Power, PE, and Daniel Simpson, PE

DATE: May 16, 2023

RE: Summary of Geotechnical Design Services Point Evans on the Narrows Bluff Erosion Repair and Soil Nail Wall Gig Harbor, Washington Project No. 0309070.020.021

1.0 Introduction

This memorandum summarizes the results of geotechnical engineering services provided by Landau

Associates, Inc. (Landau) in support of the Point Evans on the Narrows Bluff Erosion Repair Soil Nail

Wall project, located near 917 30th Street Northwest in Gig Harbor, Washington (site; Figure 1).

Services were provided in accordance with the scope outlined in Contract No. SC-106846, Work

Assignment No. 6.

This memorandum has been prepared with information provided by Pierce County Planning and

Public Works (County, project owner) and with data collected during Landau’s geotechnical field exploration and laboratory testing programs.

1.1 Project Understanding

In April 2020, Landau completed a geotechnical alternatives analysis to support repair of a bluff eroded by a damaged stormwater pipe located within a County-owned stormwater easement. Of the options considered for reducing/limiting bluff retreat, the County chose a soil nail wall option. The bluffs in the area retreat by natural erosion. The intent of the soil nail wall erosion repair is to regain time lost due to the anthropogenic erosion.

The County has retained Landau to provide geotechnical design services for the soil nail wall. Landau understands that the County intends to distribute final design drawings and specifications during project bidding. This memorandum should be made a part of the bid documents.

2.0 Site Conditions

The following sections describe the subsurface conditions observed during Landau’s February 2022 field investigation. Interpretations of site conditions are based on a review of available geologic information and on the results of the site reconnaissance, subsurface explorations, and geotechnical laboratory testing.

Point Evans on the Narrows Bluff Erosion Repair Landau Associates

Point Evans on the Narrows 2 March 9, 2023

2.1 Subsurface Explorations

Near-surface soil conditions consist of glacial till and advance outwash, as described in Landau’s April

24, 2020 technical memorandum.

On February 8, 2022, Landau’s drilling subcontractor advanced one hollow-stem auger boring (B-1)-approximately 61.5 feet (ft) below ground surface (bgs). The approximate location of the exploration is shown on Figure 2.

Landau personnel coordinated and monitored the exploration, obtained representative soil samples, and maintained a detailed record of the subsurface soil and groundwater conditions observed.

Subsurface conditions were described using the soil classification system shown on Figure 3, in general accordance with ASTM International (ASTM) standard D2488, Standard Practice for

Description and Identification of Soils (Visual-Manual Procedures). A summary log of the subsurface soil and groundwater conditions is presented on Figure 4.

The standard penetration test (SPT) procedure was used to determine the in situ geotechnical properties of subsurface soils. Soil samples were obtained at regular intervals using a 1.5-inch-inside-diameter split-spoon sampler. A 140-pound automatic hammer, falling approximately 30 inches, was used to drive the sampler 18 inches (or a portion thereof) into the undisturbed soil. The number of blows required to drive the sampler for the final 12 inches (or a portion thereof) of soil penetration is noted on the summary log, adjacent to the appropriate sample notation.

Samples collected from the exploration were transported to Landau’s geotechnical laboratory for further examination and testing. Grain size analyses were performed on select samples in accordance with ASTM standard test method D422, Standard Test Method for Particle-Size Analysis of Soils.

Samples selected for grain size analysis are designated with a “GS” in the “Test Data” column on

Figure 4. The results of the grain size analyses are presented on Figure 5. Natural moisture content determinations were performed on select soil samples in general accordance with ASTM standard test method D2216, Standard Test Methods for Laboratory Determination of Water (Moisture) Content of

Soil and Rock by Mass. The natural moisture content is shown as W = xx (i.e., percent of dry weight) in the “Test Data” column on Figure 4. To evaluate the composition of fine-grained site soils, select samples were tested for index properties in accordance with ASTM standard test method D4318, Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. Samples selected for Atterberg limits tests are designated with an “AL” in the “Test Data” column on

Figure 4. The results of the Atterberg limits test is presented on Figure 6.

Soil corrosivity testing was completed by Benchmark Geotechnical Labs, LLC. Test results are presented in Attachment 1.

Point Evans on the Narrows 3 March 9, 2023

2.1.1 Soil Conditions

The soils observed underlying existing surface conditions (i.e., grass) were categorized into three general units:

• Fill: Fill generally consisted of sand with variable silt and gravel content in a medium dense, slightly damp condition. The fill extended from 1 to 5 ft bgs.

• Advance outwash: Advance outwash was observed beneath the fill and typically consisted of sand with variable silt and gravel content in a damp, medium dense to very dense condition.

The advance outwash extended to approximately 54 ft bgs.

• Glacial till: Glacial till was observed beneath the advance outwash and typically consisted of very dense to hard silt with variable sand content. Boring B-1 was terminated in this unit.

2.1.2 Groundwater Conditions

During Landau’s February 2022 field investigation, groundwater was observed at approximately 55.0 ft bgs in boring B-1. The groundwater conditions reported herein are for the specific location and date indicated and may not be representative of other locations and/or times. Groundwater conditions will vary depending on local subsurface conditions, weather conditions, and other factors.

3.0 Soil Nail Wall Erosion Protection System Design Parameters

Based on the results of Landau’s geotechnical field investigation, site soil and groundwater conditions are suitable for the proposed improvements.

3.1 Design Soil Strength Parameters

The soil parameters in Table 1 are based on the results of Landau’s geotechnical field exploration and laboratory testing programs. Soil unit weights, friction angles, and shear strength parameters were estimated using correlations based on SPT values, in general accordance with the Federal Highway

Administration’s (FHWA) 2015 Soil Nail Walls Reference Manual…