Final Geotech Report - Spruce B_09072019.docx

IMPORTANT INFORMATION

SUBMITTED TO:

AECOM

1111 Third Avenue, Suite 1600 Seattle, Washington 98101

BY:

Shannon & Wilson, Inc.

3990 Collins Way, Suite 100 Lake Oswego, OR 97035

503-210-4750 www.shannonwilson.com

Geotechnical Report GR 12-18 Spruce Railroad Trail Segment B

WA NPS OLYM 2017(1)

Clallam County, Washington

September 2019 Shannon & Wilson No: 24-1-04087-002

September 2019 A-2

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Attn: Mr. Tom GutBryan Huschka, PE

Shannon & Wilson prepared this report and participated in this project as a subconsultant to AECOM. Our scope of services was specified in Subcontract No. 93420, Modification No. 2 with AECOM signed April 10, 2019. If you have questions concerning this report, or we may be of further service, please contact us.

We appreciate the opportunity to be of service to you on this project. If you have questions concerning this report, or we may be of further service, please contact us.

Sincerely, SHANNON & WILSON, INC.

Rachel K. Acosta, EIT Staff Engineer

AAJH:RKA:RPP/las

CONTENTS

Exhibits

Table FB-MultiPier Input Parameters Cut and Fill Slope Summary

Figures Vicinity Map Site Plan (7 sheets) Interpretive Subsurface Cross Sections (10 sheets) Global Stability Analysis (5 sheets) Factored Bearing Resistance Versus MSEW Reinforcement Length Tunnel Map Stereonet Tunnel Support Tunnel Drainage Tunnel Support Guidelines Tunnel Support Guidelines Part 2 Unwedge Tunnel Support View Station 67+50 to 68+31 Unwedge Tunnel Support View Station 66+88 to 67+50 Appendices Appendix A: Site Photographs Appendix B: AECOM Final Design Plans and Cross Sections Appendix C: Subsurface Field Explorations Appendix D: Laboratory Test Results Appendix E: MJA Geotechnical Recommendations Report Appendix F: Unstable Slopes Hazard Assessment Important Information

CONTENTS

September 2019 iv Project Understanding Project Location and Purpose This report presents our geotechnical evaluation for Segment B of the Spruce Railroad Trail in Olympic National Park, Washington. The location of Segment B is shown on the Vicinity Map, Figure 1. We understand that Western Federal Lands Highway Division (WFLHD) of the Federal Highway Administration (FHWA), the National Park Service (NPS), and Clallam County (the County) are planning improvements to Segment B of the Spruce Railroad Trail, the scope of which is discussed further in the Project Description section of this report. The purpose of this report is to provide the Project design team, AECOM and WFLHD, with the results of our geotechnical evaluation to support the current design phase. The content of this report includes our observations of the site and our understanding of the project, results and interpretation from our subsurface explorations, our design recommendations for the retaining walls, bridge foundation elements, the Short Tunnel rehabilitation, and construction considerations.

Site Description The Segment B project site is located generally along the existing Spruce Railroad Trail through Olympic National Park, approximately 30 miles west of Port Angeles in Clallam County, Washington, as shown on Figure 1, Vicinity Map. Spruce Railroad Trail is part of the greater Olympic Discovery Trail. The approximate layout of Segment B, shown on Figure 2, Site and Explorations Plan, is approximately 1.8 miles long, extending from Project Station 54+00 to Station 152+20. Segment B is located along the northern side of Lake Crescent, and photos of the site during our reconnaissance and exploration efforts are presented in Appendix A.

The proposed bridge and retaining wall locations along the Project site generally exhibit some erosional sloughing and oversteepening downslope of the trail down to the lake water level, in addition to some partially failed log crib and dry-stacked stone walls previously constructed for the earlier Spruce Railroad alignment.

The Segment B alignment includes one approximately 143-foot-long tunnel (the Short Tunnel), which extends from Station 66+88 to Station 68+31. Like the rest of the Spruce Railroad alignment, the tunnel was constructed around 1918. We understand that the eastern portal area was blasted in the early 1950s, when the railroad was abandoned, and that the original length of the tunnel may have been closer to 170 feet.

The remaining 143foot-long tunnel is currently unsupported and unlined, with variable amounts of rockfall and debris accumulated along the invert. Photographs of typical conditions in and around the tunnel are presented in Appendix A. Addition observations of the Project site conditions are discussed in Site Reconnaissance section of this report.

Project Description We understand that the Project includes trail improvements to Segment B of the Spruce Railroad Trail to generally create a 12 to 13footwide cleared trail area that will provide a 3footwide gravel surface and an 8footwide paved, multiuse path. The trail improvements include retaining walls, a bridge, and rehabilitation to the Short Tunnel. All stations for the design elements presented in our report were provided by AECOM via the final design plans for the Project, which are provided in Appendix B. Cross sections spaced every 20 feet perpendicular to the trail alignment showing ground surface and proposed design trail configuration elevations were provided on May 14, 2019. Additionally, we received estimates of water levels for Lake Crescent from AECOM on October 5, 2018.

Our previous draft of this geotechnical report, dated November 15, 2018, presented design elements based on our understanding of the Project 30 percent design plans. During the development of the final design plans, several aspects of the Project design have changed, including trail realignment and narrowing and elimination of several retaining walls.

We understand that the Project will follow FHWA Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FHWA FP-14) and that the proposed bridge superstructure will be designed in accordance with LRFD Guide Specifications for the Design of Pedestrian Bridges and will be designed by the contractor.

In this report, we have provided information about the Project site from our reconnaissance efforts and from our subsurface field explorations and laboratory testing, summarized in Appendix B and Appendix C, respectively. We have also provided our results and recommendations for the external design of the Project retaining walls, the foundation design for the bridge, and the design of the Short Tunnel rehabilitation. We performed the analyses presented herein under static conditions; we understand seismic design is not required for the Project.

Some design aspects (i.e., internal design) of the retaining walls will be performed by the contractor, and WFLHD will design the rockfall hazard mitigation aspects of the Project outside of the Short Tunnel. We understand that rockfall work will be completed before retaining wall and bridge construction, and we expect that construction for the retaining walls and the bridge will be able to be performed along the trail as widening is achieved, in order to avoid barge construction.

The Project will rehabilitate the Short Tunnel to accommodate the proposed multiuse trail. Rehabilitation will include removal of rockfall and debris from the invert; reducing the rockfall hazard; stability improvements; and construction of the trail, tunnel drainage conveyance, and timber façades at the portals. We understand that WFLHD is designing rockfall mitigation measures outside the tunnel portals and that these will interface with the tunnel support behind the timber façades.

The Short Tunnel and adjacent portal areas were previously mapped and evaluated by McMillen Jacobs Associates (MJA), as described in their Spruce Railroad Tunnel Evaluation - Geotechnical Recommendations Report, dated February 2016. WFLHD provided this report to Shannon & Wilson for review. A copy is included in Appendix E for reference.

Shannon & Wilson performed a preliminary reconnaissance of the Short Tunnel on February 14, 2017. Based on our observations from that reconnaissance and review of the 2016 MJA report, we provided 30 percent design recommendations for tunnel rehabilitation to AECOM on March 22, 2018. Under our current scope of services, we performed a more detailed reconnaissance of the tunnel on September 28, 2018. The purpose of this additional reconnaissance was to provide more information to further refine design of the tunnel rehabilitation.

Scope of Services Our scope of services was completed in accordance with WFLHD Task Order No. DTFH7017F00046, Modification No. 2, signed April 10, 2019, which was issued under WFLHD IDIQ Contract No. DTFH7015D00001. Our scope of services included the following tasks:

Site reconnaissance, including use of the Unstable Slopes Management Program (USMP) for Federal Land Management Agencies (FLMA) to evaluate unstable soil and rock slopes along the alignment;

Preparing a Draft Investigation Work Plan (provided under a separate cover, dated February 14, 2018);

Preparing of a memorandum summarizing the USMP data (provided under a separate cover, dated October 6, 2017);

Providing input on design plans and specifications to the final design level for the trail and Short Tunnel;

Field explorations, including seven geotechnical borings and five geophysical profiles;

Engineering evaluations to support final design of five retaining walls, one bridge foundation, and rehabilitation of the Short Tunnel; and Preparing this Geotechnical Report.

General Geologic Setting The Spruce Railroad Trail Segment B lies along the north side of Lake Crescent, in the north-central portion of the Olympic Mountains Physiographic Province of northwestern Washington. Bedrock along the Project alignment has been mapped by Schasse (2003) as early to middle Eocene (53- to 45-million-year-old) marine basaltic rocks of the Crescent Formation.

The Crescent Formation generally includes pillow basalts and basalt breccias, volcaniclastic sandstones and conglomerates, minor basalt flows, and minor gabbroic dikes and sills. The formation is generally interpreted to represent a marginal rift-basin sequence (marine volcanic and sedimentary rocks) that was scraped off the ocean floor and onto the North American continent during subduction of the oceanic crust. Convergence between the North American tectonic plate and adjacent subducting oceanic plates has led to uplift, contortion, and extensive faulting of the accreted rocks, producing the rugged peaks of Olympic National Park.

The modern-day valleys at and near the project site were shaped partly by glaciers during the last ice age. The most recent glacial period occurred from about 110,000 to 10,000 years ago, and the last major glaciation of that period, within the vicinity of the project area, is known as the Fraser Glaciation.

The Fraser Glaciation occurred between about 25,000 and 10,000 years ago. The Vashon ice sheet of the Fraser Glaciation advanced from Canada and into the project area around 18,000 to 17,500 years ago, reaching its maximum extent about 17,000 years ago (Porter and Swanson, 1998). The Vashon ice sheet bifurcated as it advanced southward, with part of it following the Puget Lowland and part of it following the Strait of Juan de Fuca.

The Crescent Valley, which originated along the parallel Crescent Thrust Fault, was further carved out by ice associated with the Juan de Fuca Lobe of the Vashon ice sheet. The valley drained through Indian Creek until about 10,000 to 8,000 years ago, when a massive landslide created a natural dam on the modern-day east side of the lake (Logan and Schuster, 1991; Judd, 2014). Today, the lake drains toward the north into the Lyre River, over the Lyre River Falls.

Variable thicknesses of talus and colluvial sediments (slope deposits) overlie the bedrock in the project area. Several landslides are mapped by Schasse (2003) in the Crescent Formation around Lake Crescent, including one that dammed the Indian Creek Valley to create the lake. In more recent times, cuts and fills were made along the project alignment during construction of the Spruce Railroad.

Field Explorations Site Reconnaissance Shannon & Wilson observed conditions along the Segment B alignment over the course of multiple site visits. Apart from an initial scoping visit made on November 3, 2016, our first visit was made on February 14, 2017. The primary focus of the February 14, 2017, visit was Segment A, but we also performed a brief initial reconnaissance of the Short Tunnel. Our second site visit was made from August 14 to August 16, 2017. This visit included an onsite meeting with AECOM, WFLHD, and the County, as well as reconnaissance for proposed explorations and identification of unstable rock and soil slopes, which were evaluated using the USMP for FLMA. Additional reconnaissance of the bridge and wall locations was performed during the geotechnical drilling, which occurred between September 10 and 20, 2018. Also, an additional reconnaissance of the Short Tunnel was performed on September 28, 2018.

The following sections present a summary of our reconnaissance observations regarding the proposed bridge, proposed walls, and Short Tunnel. Key photographs taken during the various reconnaissance visits are presented in Appendix A, Site Photographs. Also provided at the end of Appendix A are additional photographs provided from WFLHD taken from the lake towards the trail. Stationing, referenced below, was estimated from the 30 percent Project design plans, which were available at the time of our reconnaissance (attached in our draft report dated November 15, 2018). The wall locations described below were as depicted in the 30 percent plans and cross sections, and some walls have since been shortened or eliminated with changes to the proposed trail alignment; we have only provided our reconnaissance observations below for wall sites that are remaining in the design plans. Site observations are also noted on Figure 3, Interpretive Subsurface Cross Sections. Reconnaissance observations related to the USMP mapping efforts were presented in our Unstable Slopes Hazard Assessment letter dated October 7, 2017, a copy of which is included in Appendix F for reference.

Wall and Bridge Reconnaissance Observations A total of 14 walls and one bridge were planned and designed during the preliminary design phase. We performed site reconnaissance for all wall locations. During the final design phase, nine walls were eliminated. Five walls are designed in the final design phase, including Walls, 4, 9, 10, 12, and 14. The following paragraphs describe our site reconnaissance and observations for these five walls and the bridge locations.

Wall 4 (Sta. 64+30 to 65+60): The slope above the trail consists of a steep rock face sitting behind a wedge of talus and colluvium. Rockfall has left some large boulders along the trail. The slope below the trail contains colluvial soil and wood debris material from a previously installed log crib wall.

Narrow trail area between rock and lake (Sta. 71+00 to 71+40): We understand this is a proposed rockfall mitigation area, and material from the rockfall work will likely be used as fill to obtain the desired trail alignment.

Wall 9 (Sta. 103+10 to 104+30): The slope above the trail contains variable exposures of colluvium and rock. There are trees upslope and downslope of trail, some of which have fallen.

Wall 10 (Sta. 105+70 to 106+80): The slope above the trail consists of colluvium. Trees cover the slopes above and below the trail, some of which have fallen.

Wall 12 (Sta. 113+60 to 115+00): The slope above the trail contains variable exposures of colluvium and possible rock. The slope below the trail contains wood debris material from a previously installed log crib wall.

Wall 14 (Sta. 118+20 to 120+20): There is a steep rock face upslope from trail. There is a partially-failed, dry-stacked stone wall downslope of the trail. Photographs taken from the lake (provided by WFLHD) show a rock shelf exposed near water level, below the dry-stacked stone material.

Bridge area (Sta. 69+25 to Sta. 70+20): The slope above the trail consists of a steep rock face sitting behind a wedge of talus and colluvium. Exposed rock was observed along the slope below the trail approximately 1 to 2 feet above the water level. The stations of the bridge abutment locations have been altered from that shown in the 30 percent plans in the design presented herein to improve the feasibility of the bridge foundation construction.

Tunnel Reconnaissance Observations Two Shannon & Wilson licensed engineering geologists visited the Short Tunnel on September 28, 2018. During the visit, we observed conditions in the tunnel including tunnel shape, rock mass characteristics, common orientations and characteristics of discontinuities, existing rockfall and debris accumulated along the invert, and groundwater conditions. Photographs of typical conditions in and around the tunnel are presented in Appendix A.

The Short Tunnel is approximately 143 feet long and extends from Project Station 66+88 to Station 68+31. The azimuth of the tunnel axis is approximately 049° and the plunge of the axis is near-horizontal. The distance from crown to invert ranges from about 25 to 31 feet (excluding debris piles) and the width from sidewall to sidewall ranges from about 19 to 25 feet. While the tunnel is currently unsupported and unlined, wood debris observed in the invert suggests that it may have originally been supported by timber sets.

From approximately Station 66+88 to Station 67+56, the tunnel has a fairly regular arch-shape in the crown and quarter-arches and the invert contains minor rockfall and wood debris. Beyond Station 67+56, the shape becomes more irregular due to overbreak along a shear zone, and the invert contains piles of rock debris that increase in height toward the eastern portal, where the debris pile extends to within a few feet of the tunnel crown. One debris pile, centered around Station 67+70, appears to have originated from a failure in the crown and quarter-arch along the shear zone. A much larger debris pile, centered just outside the eastern portal, appears to have resulted from a partial tunnel collapse following the intentional abandonment blast in the 1950s. The sidewalls of the tunnel near the eastern portal are obscured by this debris pile. During our September 28, 2018 visit, we noted that no major rockfall events appeared to have occurred within the tunnel since our last visit on February 14, 2017, a span of over 19 months.

During the reconnaissance, we mapped the rock exposed in the tunnel based on the Rock Mass Rating (RMR) System (after Bieniawski, 1989) and Q System (after Barton and others, 1974). In both systems, numerical values are assigned to various rock mass parameters and used to calculate an overall score. RMR is calculated as the sum of parameters that include intact rock strength; Rock Quality Designation (RQD); and discontinuity spacing, surface condition (roughness), water, and orientation relative to the tunnel. Q is calculated as the product of three parameter ratios and involves RQD; the number of discontinuity sets; discontinuity surface roughness, surface alteration, and water; and a stress reduction factor. Both RMR and Q can be used to estimate tunnel support requirements based on published relationships between their values and tunnel support systems that have proven successful in databases of case histories (Bieniawski, 1989; Grimstad and Barton, 1993).

Our mapped values of RMR and Q throughout the tunnel are shown graphically in the Tunnel Map, Figure 6. The map also includes the approximate location of major structural features that we observed, including the shear zone and highly persistent cross-cutting discontinuities.

During our reconnaissance, we also measured joint orientations in the tunnel. These are plotted on the Stereonet, Figure 7. The four typical joint orientations used by MJA in their evaluation of the Short Tunnel, described in their 2016 report, are also shown on Figure 7 for reference. Structures that make up the shear zone between Stations 67+56 and 68+31 generally have dips that range from about 59° to 71° and dip directions that range from about 346° to 014°. The shear zone is up to 10 feet wide and includes a layer of gouge that is up to about 1 foot thick.

The tunnel was dry during our September 28, 2018 reconnaissance. However, we noted dripping conditions during our February 14, 2017 site visit, particularly along and on the northeast side of the shear zone. The gouge layer observed within the shear zone may act as an aquitard and collect water from the rock mass upslope, particularly during periods of heavy precipitation. We understand that WFLHD personnel have observed flows of several gallons per minute or more along the shear zone during the winter months. From these observations, we conclude that seepage varies in the tunnel from one location to another and seasonally, with conditions ranging from dry in the summer months to localized flows potentially in excess of several gallons per minute in the winter months.

Geotechnical Borings The subsurface field exploration program performed by Shannon & Wilson included eight geotechnical borings designated B-1, B-4, B-5, B-6, B-7, B-8, B-9A, and B-9B. Proposed borings B-2 and B3 were excluded from the exploration program. Proposed boring locations were measured in the field using a hand-held GPS unit during reconnaissance and were measured during drilling in reference to site features. Approximate boring locations are shown on Figure 2, Site and Explorations Plan.

The borings were drilled between September 10 and 19, 2018, and ranged in depth from 9.6 to 30 feet below the existing ground surface. The borings were drilled using an SS Viper modular drill rig that was provided and operated by Salisbury and Associates, Inc., of Chewelah, Washington. The borings were advanced using casing advancer and HQ3 wireline coring techniques.

A Shannon & Wilson representative was on site during drilling to locate the borings, observe drilling, collect samples, and maintain logs of the materials encountered. Details of the drilling program, including techniques used to advance and sample the borings, logs of the materials encountered, and photographs of the rock cores are presented in Appendix C, Subsurface Field Explorations.

Geophysical Explorations The subsurface exploration program also included five seismic refraction profiles. The purpose of these geophysical profiles was to help characterize the seismic velocity of the soil and rock units and to identify bedrock depth. Approximate locations of the profiles are shown on Figure 2, Site and Exploration Plan. Generally, the geophysical profiles were taken along the lakeside edge of the existing trail. Collection and reduction of all geophysical data was performed by Earth Dynamics, LLC, of Portland, Oregon. A summary report for the geophysical profiles, prepared by Earth Dynamics, is included at the end of Appendix C.

As discussed in the referenced summary report, the material observed in the borings typically correlated well with the material inferred from the seismic refraction profiles. However, material encountered throughout the full depth of boring B-8 did not appear to represent intact basalt whereas the seismic refraction model in the vicinity of boring B-8 resulted in a seismic velocity greater than 5,000 feet per second (ft/s) within about 10 feet below ground surface. A seismic velocity greater than 5,000 ft/s typically correlated with intact basalt encountered at the other Project borings where geophysical exploration was performed.

Although there is inherent uncertainty in the interpretation of seismic refraction profiles, correlation to materials encountered in the borings located along respective geophysical explorations help provide an indication of the subsurface profiles at sites. The range of seismic velocities for various material types is provided in more detail in the summary report for the geophysical profiles in Appendix C. Exhibit 3-1 presents a summary of type, approximate locations, and purpose of our site explorations.

Exhibit 31: Site Exploration Summary

NOTES:

The purpose of our explorations was based on our understanding of 30 percent design plans; changes to the alignment since these explorations are noted in parenthesis.

N/A = not applicable.

Laboratory Testing The samples we obtained during our field explorations were transported to our laboratory for further examination. During our review, we selected sample intervals in the rock core for uniaxial compressive strength and point load testing. FHWA performed uniaxial compressive strength tests in accordance with ASTM D7012-07 and point load tests in accordance with ASTM D 5731-07. Results of the laboratory tests, provided by FHWA, are included in Appendix D, Laboratory Test Results.

Subsurface Conditions Geotechnical Units We grouped the materials encountered in the field explorations into four geotechnical units. Our interpretation of the subsurface conditions is based on the explorations and regional geologic information from published sources. The geotechnical units are as follows:

Fill: very loose to medium dense, Poorly Graded Gravel (GP) to Poorly Graded Gravel with Silt and Sand (GP-GM) and Silty Gravel with variable amounts of Sand (GM); commonly contains cobbles and boulders;

Colluvium: very loose to very dense, Poorly Graded Gravel (GP) to Poorly Graded Gravel with Silt and Sand (GP-GM) and Silty Gravel with Sand (GM); commonly contains cobbles and boulders;

Residual Soil: medium dense to very dense, Silty Sand with Gravel (SM), Clayey Sand with Gravel (SC); Silty Gravel with Sand (GM); and Clayey Gravel with Sand (GC); commonly contains cobbles and boulders; and Crescent Formation: medium strong to strong (R3-R4) Basalt and strong to very strong (R4-R5), Basalt Breccia.

These geotechnical units were grouped based on their engineering properties, geologic origins, and distribution in the subsurface. Our interpretation of the unit distributions within the subsurface near the proposed retaining walls and bridge abutments is presented on Figure 3, Interpretive Subsurface Cross Sections. The locations of the respective cross sections are referenced to Project Stationing.

Although all of the retaining wall cross sections shown on Figure 3 are sections perpendicular to the proposed trail alignment which is generally perpendicular to the surrounding slopes, the bridge abutment cross sections are cut perpendicular to the slope below the existing trail to show a more representative ground surface for the subsurface condition profiles. Contacts between the units may be more gradational than shown in the boring logs, seismic refraction profiles, and cross sections, and subsurface conditions are expected to vary.

Standard Penetration Test (SPT) N-values presented on the boring logs in Appendix B and on Figure 3 are in blows per foot (bpf) as counted in the field (i.e. no corrections have been applied).

Depth to bedrock was estimated conservatively for global stability analyses where no data from borings or geophysical explorations were available. It is anticipated that there may be shallower bedrock at some sites, which may be encountered during construction. Further discussion of the possible occurrence of shallow bedrock encountered during construction is provided in the Engineering Conclusions and Recommendations section of this report.

Groundwater The borings were drilled using casing advancer and HQ3-wireline coring techniques, both of which introduce water into boreholes during drilling. This makes it difficult to discern the depth to groundwater, if it is encountered.

Based on the materials observed in the borings, we assume that the subsurface beneath the alignment is hydrologically connected to Crescent Lake, with groundwater levels being at or near the lake elevation. For slope stability analyses, we assumed average groundwater elevations of 584 feet and average maximum groundwater elevations of 587 feet, based on average lake level and maximum average lake levels provided by AECOM.

The groundwater elevation adjacent to the alignment likely increases with the local topography as it rises away from the lake. Cuts into the slope adjacent to or beneath the trail may encounter groundwater seepage above the lake level.

Within the Short Tunnel, groundwater seepage varies from one location to another and seasonally, with conditions ranging from dry in the summer months to localized flows potentially in excess of several gallons per minute in the winter months. We understand that the heaviest flows occur along the shear zone between Stations 67+56 and 68+31.

Engineering Conclusions and Recommendations We have prepared our design conclusions and recommendations based on information obtained from our site reconnaissance efforts and field explorations. We have also considered input from the design team provided during the design workshop held at the WFLHD office in Vancouver, Washington on October 12, 2018, as well as during several phone conferences. Our design recommendations for Project elements including five retaining walls, the bridge foundations, and the tunnel rehabilitation are presented in the following sections.

Retaining Walls The proposed wall sites initially presented in the 30 percent plans were evaluated along the proposed trail alignment. We understand that retaining walls are desired in locations where the vertical and horizontal position of the proposed trail alignment may not be able to be achieved with a cut and fill approach. In total, fourteen retaining wall sites were initially under consideration for the Project. During the development of the 70 percent design, WFL determined that two walls (Wall 2 and 3) be designed and constructed prior to the construction of Segment B. Also, the design team revised the proposed trail alignment. Consequently, a total of seven walls are eliminated. Therefore, a total of five walls including Walls 4, 9, 10, 12, and 14 are designed during the final design phase. In the following sections, we present our design recommendations for the five remaining retaining walls. The lengths of the proposed retaining walls range from approximately 450 to 160 feet.

Design Alternatives We considered three design alternatives at each wall location to evaluate the feasibility of different types of retaining structures at each site. The types of design alternatives considered for the Project included mechanically stabilized earth walls (MSEWs), reinforced soil slopes (RSSs), and riprap revetments. These design alternatives were evaluated due to their anticipated construction feasibility and probable relative cost, as opposed to alternatives such as a soldier pile wall, which is a robust alternative that we expect could also be implemented at most retaining wall sites at a higher cost.

We presented the selected suite of conceptual retaining wall design alternatives to WFLHD and AECOM at the aforementioned design workshop and aimed to gain a consensus from the design team of the preferred design alternative at each wall location. During the design workshop, we learned which deciding factors were most influential to the design team. For instance, it is important that the preferred design alternative for the retaining walls limits that amount of material, such as riprap, placed below the water level of the lake. Also, to limit disturbance to land downslope of the proposed retaining walls, it was preferred that riprap revetments not be applied to slopes that appeared stable with established vegetation, unless required to obtain a wall configuration that satisfied performance criteria.

An additional factor that was important in selecting a preferred design alternative at each wall location was the history of each specific site; if it was observed that a previous wall, such as a log crib or dry stacked stone wall, had been constructed at a site, the design team anticipated that permitting requirements may be less stringent due to the nature of work required to replace the preexisting wall to provide a stable slope.

Following the design workshop, we understood that MSEWs were the preferred design alternatives for the majority of the proposed retaining walls, largely due to site configuration constraints, and RSSs also remained as a potential design alternative if the configuration was appropriate for specific wall sites. It was discussed that several locations along the proposed trail alignment may be adjusted to reduce the extents of, or eliminate, a potential retaining wall.

As stated previously, a total of five walls are required for the final design. We have selected preferred design alternatives for the five remaining retaining walls based on feedback from the design team and our further evaluation of the alternatives and results from our analyses.

Exhibit 61 presents the preferred design alternative selected for each retaining wall and the key reasoning leading to the decision. Generally, MSEWs reduced the footprint of the retaining wall and reduced or eliminated material from encroaching into the lake. Several wall site configurations appeared to potentially be suitable for an RSS; however, upon further evaluation of global stability, an RSS generally required greater wall height and longer reinforcements to achieve acceptable stability and the toe of the proposed slopes was observed to encroach towards the lake more than the MSEW alternative.

Although RSSs are generally considered to be more economical and potentially easier to construct than MSEWs due to lower quality reinforced fill being accepted and a less steep slope face, we believe that MSEWs are a more appropriate design alternative given the site conditions for all of the retaining wall sites evaluated along the proposed trail alignment of Segment B. Additionally, the higher quality reinforced fill required for MSEWs is beneficial when considering wall durability, drainage, constructability, and better soil-reinforcement interaction; higher quality fill material can be more easily compacted and provides higher internal strength of the reinforced zone than a lower quality fill with greater fines content (material passing the No. 200 sieve).

Exhibit 61: Preferred Design Alternatives for Retaining Walls

1. Approximate extents of wall stationing estimated from our understanding of final design plans.

MSEW Design General We have provided our MSEW design recommendations in accordance with the standard design procedures outlined in the FHWA manual (NHI-10-024), Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes (Berg and others, 2009). We have evaluated global stability, bearing capacity, settlement, and construction considerations for the vertical MSEWs, which we present in the following sections. We understand that the contractor will perform the internal design for the MSEWs, including sliding and overturning, and selection of reinforcement and facing materials. The internal design should also conform to the requirements in NHI-10-024.

For facing of the MSEWs, we recommend galvanized welded wire baskets. We understand this facing option is acceptable to WFLHD and the aesthetics conform to the Project requirements. Currently the face is assumed to be vertical. However, the wall face can be battered if the contractor satisfies the required width of the top of the walls. The welded wire baskets should have support struts connected at sufficiently close spacing to limit bulging at the face. Material composed of 3- to 6inch rock (FHWA FP14, Section 705.01(c)2) should be placed as fill in the welded wire facing unit in front of the geogrid wrapped reinforced fill. A nonwoven geotextile (separation geotextile; FHWA FP14, Section 714.01, Class 1 or 2 Nonwoven, Type A) should be placed behind the geogrid reinforcement at the face of the geogrid wrapped reinforced fill to retain the material. Exhibit 62 shows an example of a welded wire basket and a section of the MSEW facing described, which are shown in the design plans attached in Appendix B.

Exhibit 62: Example Welded Wire Basket Sections for MSEW Facing

MSEW Design Parameters We recommend material conforming to the requirements in the NHI10024 for MSEW select granular reinforced fill, with a maximum particle size of 3/4inchdiameter, less than 15 percent fines, and a maximum plasticity index of 6 for material passing the No. 40 sieve. In our opinion, a material meeting FHWA FP-14, Section 703.05(b), D (Base) Aggregate Material meets these gradation requirements for the reinforced fill of the MSEWs, and the material should also conform to the additional requirements in FHWA FP14, Section 704.08, Select Granular Backfill. We assume the same material for retained backfill material for the MSEWs. The reinforced fill and retained backfill should be placed and compacted in accordance with FHWA FP-14, Section 255.05, within 2 percent of optimum moisture content to 95 percent of maximum dry density per AASHTO T-99.

We consider the foundation soil to be generally Colluvium with a range of representative soil parameters, depending on specific materials encountered in our subsurface explorations. For retaining wall sites without subsurface explorations performed, we estimated subsurface materials and design parameters based on our site reconnaissance observations and the design parameters of similar materials encountered at the Project sites. Exact design parameters for the materials considered in our analyses are shown on Figure 4. Exhibit 63 presents the estimated design parameters of the reinforced fill, retained backfill, and the generalized foundation soil used herein for MSEW design.

Exhibit 63: MSEW Geotechnical Design Parameters

pcf = pounds per cubic foot psf = pounds per square foot Lateral pressures on the MSEWs should be calculated using the design parameters of the retained backfill and should include active earth pressure and pressures from applied surcharge loading. In our analyses, we have considered a surcharge of 125 pounds per square foot (psf) over the finished trail width.

Global Stability We performed global stability analyses at each wall site with the aid of the computer program SLOPE/W, Version 9.1 (GeoStudio 2018 R2) to satisfy both force and moment limit equilibrium considering rotational and irregular surface failure mechanisms (Spencer analyses method). Global stability is influenced by various factors including 1) the geometry of the soil mass and subsurface materials, 2) the weight of soil materials overlying the failure surface, 3) the shear strength of soils along the failure surface, and 4) the hydrostatic pressure (groundwater levels) present within the analyzed mass and along the failure surface. Comment by Morehouse, Diann (FHWA): Please provide information to the reader about the lateral load that is shown in the analysis.in the figures. It is not clear the purpose of the lateral load. The magnitude of the lateral load is not shown on the figures. Comment by Rachel Acosta: Graphics showing reinforcement loading were only for illustrative purposes to show approximate reinforcement layers within the MSE block. We have revised figures to clarity.

Global stability is expressed in terms of factor of safety (FS), which is defined as the ratio of resisting forces to driving forces. At equilibrium, the FS is equal to 1.0, and the driving forces are balanced by the resisting forces. Failure occurs when the driving forces exceed the resisting forces, i.e., FS less than 1.0. An increase in the FS above 1.0, whether by increasing the resisting forces or decreasing the driving forces, reflects a corresponding increase in the stability of the mass.

The actual FS may differ from the calculated FS due to variations in soil strengths, subsurface geometry, failure surface location and orientation, groundwater levels, and other factors that are not completely known or understood. In this regard, we have used information developed from our field explorations, geologic reconnaissance and mapping, and our experience with similar materials to develop the stability analysis models in SLOPE/W. Our analyses are based upon the assumption that subsurface conditions everywhere within each retaining wall site are not significantly different from those encountered by the field explorations, where present, or those assumed from site observations. We evaluated both static and rapid drawdown conditions.

For MSEWs where it is determined a riprap revetment is required extending down the slope in front of the wall, a 3foot thickness of riprap normal to the slope is assumed in our global stability models. The riprap material is considered as FHWA FP14, Section 705.02, Class 3 material which is informed by a hydraulic engineer. The recommended riprap zones are shown on Figure 4, Static – MSEW Global Stability Analysis, as well as in the MSEW toe embedment details in the design plans provided in Appendix B. We anticipate riprap material may encroach into the lake at Wall 12, and potentially at Walls 4 and 14 depending on the extents of existing log crib wall and dry stacked stone wall material, respectively. For Walls 12 and 14, the riprap may encroach into the lake. Therefore, the we expect in-water work permits will be required for construction. For Walls 12 and 14, the assumed riprap slope thickness is provided in the stability models above the average lake water level, elevation 584 feet, and additional material is assumed to be placed below the water level as necessary to provide sufficient stability of the walls and a stable configuration of the revetment above the water level. Comment by Morehouse, Diann (FHWA): It was my understanding is riprap is not required at wall 4 during the Aug 20th telecon. How is this information dealt with in the plans? Comment by Rachel Acosta: Riprap is not anticipated, but if the log crib wall material extends deeper than the required extents of the wall footprint excavation, the slope will need to be rebuilt, and may require riprap. This detail is included in the plans under Footnote 1 of Sheet H.3 We evaluated each wall site and selected a cross section for global stability analysis that we believe to be representative, considering subsurface conditions interpreted from our site observations and boring and geophysical explorations and geometry of the existing and proposed trail configurations. We expect actual conditions along the extents of each wall site will vary, and the riprap configurations at the toe of MSEWs, as they are shown in the analysis results, may vary as well. Consequently, riprap may encroach into the lake. An in-water work permit will be required for construction.

Previously constructed wall material is present at Walls 4, 12, and 14. As discussed in the Site Reconnaissance Section, we observed previous log crib wall material at Walls 4 and Wall 12. In our global stability model, we have considered a configuration of Wall 4 where log crib wall material does not extend below the bottom of the MSEW and the wall is founded on Colluvium. The material to embed the toe is assumed to embankment fill material be (compacted 6-inch minus native material with 5 percent or less fines content). If log crib wall material is excavated below the MSEW at Wall 4, the slope will need to be rebuilt with compacted select granular backfill (same material as MSEW backfill) and a riprap slope of 1V:1.25H:1V (vertical:horizontal), with the same riprap thickness and configuration as described previously. At Wall 14, we observed previous dry stacked stone wall material. We have considered riprap to be placed directly in front of the toe of Wall 14 and the existing dry stacked stone wall material to be left in place where stable in front of the MSEW. Since we do not know the stability of the remaining wall material, the slope should be reconstructed with a riprap slope below the MSEW, as shown in our global stability model.

We have designed the MSEWs for an FS against global stability failure of 1.3, which we believe is appropriate for the application of the retaining walls for the multiuse trail, and we understand this FS is acceptable to WFLHD. For our static analyses, we considered the average lake level and a surcharge loading of 125 psf is assumed for pedestrian traffic and maintenance access. We evaluated the sensitivity of wall stability for rapid drawdown conditions and observed sufficient resistance at this condition as well. We have also implemented additional material weight applied to the top of the MSEWs, shown as the asphalt and base course layer in the analysis models.

We considered the minimum reinforcement length to be eight feet or 0.7H, whichever is greater, in accordance with NHI10024, where H is the height of the MSEW from the top of the leveling course pad at the bottom level of reinforcement to the bottom of base material required for paved trail surfacing. We have assumed the bottom of the leveling pad to be at least one foot above the high-water level of lake (estimated as elevation 587 feet), which, along with proper MSEW toe embedment and wall drainage details, will further protect against material loss of the MSEW system from lake wave action.

The minimum reinforcement length was then evaluated and increased if necessary in our global stability analyses to provide an allowable factor of safety. As stated previously, we assumed a conservative rock depth for global stability analyses in order to not overestimate stability of MSEWs regarding the uncertainties of subsurface conditions along the extents of the walls.

The global stability models were developed from the design cross sections provided by AECOM. To perform our global stability analyses, we estimated the ground surface and bathymetry below the surveyed elevation from field observations of the slope toe near the water level and a roughly linear slope below the water level down to elevation 570 feet.