GM43-21 HurricaneRidge Obstruction Point Roads Final Rec_1-13-2022.pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: January 13, 2021

TO: John Davis Project Manager

FROM: Diann Morehouse Geotechnical Engineer

SUBJECT: Geotechnical Memorandum GM43-21 Final Geotechnical Recommendations Repair Slumps on Hurricane Ridge and Obstruction Point Roads

WA NPS OLYM 12(7)

Clallam County, Olympic National Park

INTRODUCTION

This memorandum provides the final geotechnical recommendations for the Repair Slumps on Hurricane Ridge and Obstruction Point Roads project. The project is within Olympic National Park (NPS). Diann Morehouse, WFL Geotechnical Engineer, conducted the initial site visit on September 25, 2018 with Skip Willoughby, NPS Roads Supervisor. She returned to the site to gather more information on September 20, 2019. On June 9 and 10, 2021, Diann visited the site with Doug Anderson, WFL Geotechnical Team Functional Manager, Skip Willoughby, and Lisa Turecek, NPS Chief of Facility Management.

PROJECT DESCRIPTION

Hurricane Ridge Road begins in Port Angeles, Washington, and heads in a southerly direction for approximately 17 miles to Hurricane Ridge Visitor Center and is the only vehicle access to the visitor center and to Obstruction Point Road. Obstruction Point Road is an 8-mile, single lane gravel road and generally heads in a southeasterly direction from Hurricane Ridge Visitor Center. The road provides remote vehicle access into the Olympic Mountain Range and several trailheads. See Project Location and Project Vicinity Maps, Figures B-1 and B-2, respectively.

The concerns for this project include:

• Hurricane Ridge Road roadway subsidence and lateral movement affecting approximately 35 feet of the outside lane and 80 feet of the paved pullout.

• Erosion of exposed bedrock and embankment fill along several locations along Obstruction Point Road is reducing the roadway width and has become an emergent safety issue with the public accessing the road.

TOPOGRAPHY AND REGIONAL GEOLOGY

Topography

Elevation of the project is roughly 5200 feet on Hurricane Ridge Road with a grade of about 5 percent. From the intersection of the two roads, Obstruction Point Road steadily decreases elevation at about 11 percent grade. The project is located on a southern to western facing slope of Hurricane Ridge with a topographic relief ranging from approximately 5400 to1900 feet. The Lillian River runs along the valley floor south of the project and is a tributary of the Elwha River, located about 3 miles southwest.

Regional Geology

The uplift of the Olympic Mountains is the result of the Juan de Fuca plate descending beneath the North American plate along the Cascadia subduction zone. As the Juan de Fuca plate slides under the North American Plate, marine sediments are scraped off and pushed upwards causing overlying material to bulge and uplift. Figure B-3 shows an excerpt from Geologic Map of the Mount Olympus 1:100,000 Quadrangle, Washington (Gerstel, 2003).

The project is located within the central portion of the Olympic Mountains along Hurricane Ridge. The bedrock is mapped as an Oligocene-Eocene-age (23 to 33.9 million years ago (mya)) marine rhythmites and other thin-bedded sedimentary rocks (φEmr) consisting of laminated and or thin-bedded (1-20 cm), lithofeldspathic and feldspatholithic, micaceous sandstone, siltstone, slate, and argillite; rare thick-bedded sandstone, granule conglomerate, and thick-layer (>60 cm) of semi-schist, all containing platy-slate clasts grading to platy-clast breccia.

Interbedded with the marine rhythmites sedimentary rock (φEmr) is Oligocene-Eocene-age (23 to 33.9 mya) basalt (φEvb) consisting of dark green-gray basalt, basaltic tuffs, breccia, sandstone, and greenstone with rare pillow or amygdaloidal basalt.

The Juan de Fuca Lobe of the Cordilleran ice sheet, which advanced from Canada during the late Wisconsinan-age (approximately 17,000 years ago). The large continental ice sheet directly affected the Elwha Valley in many ways, which impacted the present-day geologic character of the site. The southern limit of the Juan de Fuca glacial lobe of the Cordilleran Ice Sheet is mapped on the east side of Hurricane Ridge and crossing the Elwha Valley, about six miles northwest of the project. The solid blue line shown in Figure B-4 indicates the approximate location of the southern limit of the Juan de Fuca glaciation with the ice residing east and north of this line from the 1:250,000 scale map. The blue line with hatched bars is the greatest extent of the glacier’s limits from a 1:100,000 scale map with the bars pointing in the direction of the occupation of the glacial ice. The Juan de Fuca Continental glaciation ice lobe advanced to about the 3800-foot contour and left behind a 100-foot high terminal moraine deposit. The ice front would have also dammed the north flowing Elwha River causing a glacial lake to form towards the south (in the vicinity of the previous Lake Mills).

Using geomorphic markers, such as hanging delta fans on the upper valley walls, the lake level may have been as high as the present day 1,200-foot elevation. Soils left after the recession of the Juan de Fuca lobe to the south of the glacial limits are interpreted as late Wisconsinan-aged glacio-lacustrine sediments (Qgl). The recession of the Juan de Fuca lobe is generally assumed to have occurred about 15,000 to 14,000 years ago; however, the recession of the glacial lobe may have occurred as late as 8,000 years ago. Evidence of river terraces comprised of late-Pleistocene-aged alluvium deposits are found along the 200-foot contour of the modern valley floor (Polenz, 2004). Late-Wisconsinan continental till deposits (Qgt) including poorly sorted cobbles, gravel, and boulders in a clay and silt matrix are identified in localized areas. Other localized deposits include Holocene-aged alluvium sediments (Qaf), landslide deposits (Qls and Qlsf), and late Pleistocene-aged landslide deposits (Qolsf).

As noted above, bedrock within the Lillian River valley is mapped as Miocene-Eocene-aged (5.3 to 33.7 mya) marine rhythmites and other thin-bedded marine sedimentary rocks (MEmr).

Rapid draw down due to ice dam failure or overtopping resulted in landslides within the saturated glacio-lacustrine sediments and marine rhythmite rocks.

Regional faults are part of a series of east-dipping, young westward, thrust faults. Faults shown in Figure B-3 may be considered older than Quaternary-age (<2.58 mya). Figure B-5 shows an excerpt from USGS Quaternary Faults Interactive Map. The Lake Creek-Boundary Creek fault is mapped as late Quaternary-age (>15,000 years), with a left lateral slip sense, and is well-constrained. The Unnamed fault along Barnes Creek is mapped as undifferentiated and of Quaternary-age (<1.6 mya) with a thrust slip sense that is well-constrained. The Calawah fault is late Quaternary-age, left lateral slip sense, and they are moderately constrained. All the faults located in the Strait of Juan de Fuca are unnamed and identified as a group of faults of various age with a normal slip sense and are moderately confined. Note for clarity that all the nearby faults illustrated in Figure B-5 are not identified.

All faults are identified with a slip rate of less than 0.2 mm/year.

Site Seismicity

Figure B-5 shows an excerpt from the USGS Quaternary Faults and Folds Database web-based mapping program. This program shows an absence of Quaternary-aged (<2.58 mya) faults within 6 miles of the project.

The American Association of State Highway and Transportation Officials (AASHTO) criteria for seismic design allows for some structural damage to occur when the structure is subjected to 1000-year strong ground motion. For Extreme Event I Limit State Design, an earthquake load and resistance factor of 1.00 should be used during the LRFD design of structures.

The procedures outlined in Section 3.10 of the 9th Edition of the American Association of State Highway and Transportation Officials LRFD Bridge Design Specifications Manual (2020) (AASHTO, BDS) were followed to characterize the seismic hazard during the design activity.

These procedures use a design earthquake of seven percent probability of exceedance (PE) in 75 years, which equates to a return period of approximately 1000 years. Based on the AASHTO BDS procedures, and the standard penetration resistance in the top 100 feet of the soil profile at the project site, the AASHTO Seismic Site Class for this project is Site Class C.

The USGS Seismic Design Map Web Services was used following 2011 AASHTO Guide Specifications to determine the peak ground acceleration and spectral accelerations at the project site based on Site Class B, rock. The spectral accelerations were then scaled to Site Class C using the site factors recommended by AASHTO (2020) to represent the seismic accelerations based on actual ground conditions.

The scaled peak ground acceleration, As, for an earthquake with a return period of approximately 1,000 years was determined to be 0.388g using the USGS Seismic Design Maps website. Seismic design for retaining walls or reinforced slopes is typically required by AASHTO (2020) where the As is over 0.4g. Embankment and cut slopes are generally not designed for earthquake loading for roadways except in close proximity to bridge abutments or adjacent critical facilities. We do not meet these requirements for seismic design consideration.

It should be noted that landslides and displacements of retaining structures could occur during strong ground shaking from an earthquake, and that movement of the existing prehistoric landslide, or smaller portions of it, could also affect the stability of the roadway.

SITE RECONNAISSANCE OBSERVATIONS

Site observations discussed in this section have been developed based on the conditions present during the geotechnical site visits performed in 2018, 2019, and 2021. Both roads have embankments and cuts in soil and rock within the general project area. This subsection also includes global site observations using satellite and Light Detection and Ranging (LiDAR) imagery provided in Figures B-2 and B-6, respectively.

Hurricane Ridge Road

A rock cut, up to 80 feet high, is located along about 1100 feet of Hurricane Ridge Road with a slope ratio between 2V:1H to nearly vertical. The rock, mapped as marine sedimentary rock, is fissile and slaty and is actively eroding. See Photograph A-1 of the marine rhythmites sedimentary rock. The ditch line has a varying amount of rock and soil debris that has fallen from the cut slope with an average size of approximately 9 inches, with some boulders up to 2 feet in dimension observed. The existing fill embankment slope ratios range between 1V:1.2H and 1V:1.4H. The steeper slopes are unvegetated while the shallower slopes are somewhat vegetated with evidence of low creeping plants and trees.

During site visits in September 2018 and 2019, pavement distress was only observed within the paved pullout. During the 2018 site visit, a tension crack, up to three fourths of an inch wide was measured and extending from the edge of pavement to about 3 feet from the fog line in the pullout, terminating abruptly at the west end with vertical displacement up to two inches. In 2019, the area was patched with asphalt. The patching was uneven and had a small separation along the location of the previous tension crack. Additional tension cracking along the edge of the pavement (from the east end of the 2018 observed tension crack) in the paved pullout extending in an easterly direction, approximately 60 feet with an open width of about an inch. See Photographs A-2, A-3, A-4.

Observations during the June 2021 field visit noted an extension of the tension cracks, starting near Station 7+90, in the middle of the outside travel lane for about eight feet. The tension crack then changes direction, about 30 degrees from centerline, toward the outside edge of pavement at about Station 9+10, see Photographs A-5, A-6, and Figure B-7. From this point the tension crack was observed following the edge of pavement until about Station 9+50. The west end of the tension crack observed during this site visit is very close to the same location as observed in 2018, except it had retrogressed to about 4 feet from centerline rather than being contained in the pull out area.

Obstruction Point Road

Obstruction Point Road is a gravel surfaced, single lane road with an occasional wide area to allow for two vehicles to pass by each other. The first 1900 feet of the road is situated on a steep slope that is exhibiting active slope erosion above and below the road. Exposed rock, mapped as marine sedimentary rock, is fissile and slaty and is actively eroding. Fill slopes above and below the road are described above in the previous subsection. The slope below Obstruction Point road extends approximately 200 feet below the road before a shallower sloped, vegetated bench is reached before continuing to the valley floor. Existing cut slope ratios are about 2.5V:1H.

The ditch line along the road is relatively nonexistent and grading by Park Maintenance usually wind rows soil along the outside edge of the roadway to direct surface water to established, less actively eroding drainage areas. Due to the narrow width of the road and frequent, small rockfall and debris originating from the cut slope, maintaining a ditch line is not practical. In addition, cross drainage is problematic due to the highly erodible soils (and rock) present down slope of the road. Concentrated flows from cross culverts exacerbate embankment slope erosion by causing deep rilling and gullying, which eventually works its way back up to the road, causing problems and reducing the safe width of the roadway.

Hurricane Ridge Road has a 24-inch cross culvert with an outlet on the embankment slope near Station 3+60. This outlet is located above Obstruction Point Road about 600 feet from its intersection with Hurricane Ridge Road. Photograph A-7 and Figure B-2 shows the location of the culvert outlet about 13 feet below Hurricane Ridge Road and 30 feet above Obstruction Point Road. On the embankment slope below Obstruction Point Road, there are open screen (breeze) concrete blocks that measure 12-inches by 12-inches by 4-inches and extend about 10 to 15 feet down the slope in an attempt to armor the fill slope. It appears this is the where the Hurricane Ridge cross culvert water flows down the embankment slope from above, crosses Obstruction Point Road and then drains down the embankment slope. The wind row of soil along the outside edge of the eroding embankment section of the road also directs surface water to this location. Our site observations suggest that the concrete screen blocks have been effective in protecting the road fill slope from excessive erosion and gullying, and are a cheap and effective method for maintenance to help minimize erosion on these embankment slopes.

Site 2 consists of two sites shown in Photograph A-8 and the width of the existing road is about 10.5 feet. Site 2A is located at Station 51+10; about 430 feet east of its intersection with Hurricane Ridge Road, see Figure B-8. This section of the road passes through a rock cut on the inboard side of the road and was constructed nearly full bench on a rock outcrop that can be seen on the outboard side of the road in Photograph A-9. The outcrop surface expression on the outboard side of the road consists of two humps of rock near road elevation with an area between the two humps that allows surface water to flow through and erode the weaker rock. The cyclical wetting and drying sequences are making this weaker rock area erode faster than the other portions of this outcrop.

It is theorized that Park Maintenance originally placed a concrete beam across the two humps in the rock outcrop to keep vehicles to the inside of the road. They also constructed an earthen berm up against the beam to direct surface drainage away from this highly erodible area between the two humps. As noted above, the surface drainage is directed down the road prior to it draining off the side of the embankment at one of the breeze block armored embankment slopes. The concrete beam measuring about 9 inches by 12 inches by 9 feet long is sitting atop of the two humps of the rock outcrop. Photographs A-10, A-11, and A-12 shows the concrete beam in 2018, 2019, and 2021, respectively. In 2021, the size of the void between the concrete beam face and edge of road measured horizontally about 2.4 feet and vertically about 2.5 feet. Comparing Photographs, A-10 and A-11, the fourth set of bolts (counting from left to right) appears to have more clearance in 2019. In Photographs A-11 and A-12, it shows a similar widening of the void in 2021 compared to 2019; although it is difficult to compare with the earthen berm in front of the concrete beam in Photograph A-12.

Photograph A-13 shows the eastern end (right upper corner) of the beam sitting on the rock.

It was also noted in 2018 and 2019, water seepage was coming out of the roadway cut slope in rock about 4 feet above the road. This seepage was not noted in 2021.

Site 2B, shown in Photographs A-8 and A-14, is approximately 15 feet east of Site 2A and located at Station 51+25. This site also has eroded from cyclic freezing and drying and from surface water running over the edge of the embankment; although, it appears this erosion is occurring in soil and not in rock. The erosion has caused about a one-foot loss of the roadway width and is about 1.8 feet deep at the worst point. The length of the erosional area along the road is about 6.5 feet.

During the June 2021 site visit, Skip Willoughby, NPS Roads Supervisor, mentioned Obstruction Point Road is typically graded with a cross slope back towards the inboard cut slope to discourage surface water from sheeting across the road surface and causing erosion of the embankment slopes.

Global Observations

About 550 feet west of Site 1 and about 400 feet east of the beginning of Hurricane Ridge parking area, the road exhibits differential settlement with a skew orientation to the road centerline. From Google Earth imagery, dated July 28, 2016, it shows an outline of an asphalt patch across the road. In 2018, a sawcut in the pavement surfacing, approximately 40 degrees from parallel with the road, was observed at Station 1+80 and is shown in Photographs A-15, A-16, and A-17. In 2019 the same area had a patch covering the sawcut.

According to Park Maintenance, asphalt was used in lifts to reduce the vertical sag in the roadway that was occurring. During the 2021 site visit, small discontinuous cracks were observed in the patch along the trend of the sawcut. After further observations, it was noted that the orientation of the pavement sawcut and the differential settlement area in Hurricane Ridge Road was in line with the lateral scarp of a large, prehistoric landslide feature noted in the LiDAR, see Figure B-6. Photograph A-18 and Figure B-2 also shows an interpreted delineation of the prehistoric landslide feature. The eastern edge of the landslide feature is not as clearly defined in the satellite and LiDAR imagery, which is assumed to be due to roadway building construction of embankments and cut slopes over the top of it.

At Site 2 on Obstruction Point Road and along the 2018 saw cut 2019 asphalt patch on Hurricane Ridge Road (directly above Site 2), it appears this is the location of the western lateral scarp of the prehistoric landslide. After a landslide occurs, it is not uncommon for ground water to find the path of least resistance along the lateral (scarps) extents of the landslide. Based on this and our field observations, it appears that ground water is piping soil and rock particles through these lateral scarp zones, resulting in subsidence of the roadway section. No lateral displacement was observed on Hurricane Ridge Road.

At the Hurricane Ridge Road pull out site, located along the apparent eastern margin of the prehistoric landslide, a tension crack in the middle of the outside lane has a separation of about 1-inch. The amount of vertical or horizontal displacement under the patching within the pullout is not known, however above the most recent patching, the vertical displacement was measured to be about 2 inches in June 2021.

It appears from our observations and interviews with Maintenance personnel that the following process occurs. As embankment material from the Hurricane Ridge pull out creeps and moves along the eastern margin of the larger prehistoric landslide feature noted in the LiDAR, it delivers material to Obstruction Point Road. This requires Maintenance to reestablish the width of Obstruction Point Road, which removes the toe to the Hurricane Ridge embankment slope creating a cyclical creep response. Snow melt and ditch water runoff along Hurricane Ridge road is likely delivering surface water to the prehistoric landslide eastern lateral scarp where the pullout failure is occurring. Subsequent groundwater running along this lateral limit of the prehistoric landslide also further exacerbates this process. It was originally thought this cyclical process and the ditch water crossing under the road was causing the pavement deformation. It is now our opinion that slope creep along with water migrating under the roadway along the lateral margin of the prehistoric landslide is the cause of the pavement subsidence and deformation.

It should be noted that subsurface investigation and further study of this larger landslide is beyond the scope of this project. Such a study would require additional subsurface investigation, instrumentation and monitoring, and preliminary analysis and conceptual design option development. This plan can be developed upon request.

SUBSURFACE INVESTIGATION AND PROFILE

Drilling

One subsurface investigation boring was drilled on September 15 and 16, 2021 in the pullout on Hurricane Ridge Road to a depth of 71.5 feet. Using a recreational-grade GPS unit, the boring is located at 47.97005°N, 123.49055°W and is shown on Figure B-2. Drilling operations and field inspection for the subsurface investigation were contracted with Budinger & Associates, Inc. located in Spokane Valley, Washington and completed using a Geoprobe 7822DT truck-mounted drill rig. The boring was advanced with 8.25-inch O.D., hollow-stem auger until refusal after which the boring was advanced using HQ3 (3.8-inch O.D.) coring methods.

Standard Penetration Tests (SPTs) were performed by an automatic hammer. The SPTs were performed in the soil at 2.5-foot and 5-foot intervals, using 140-pound hammer dropped from 30 inches. A standard, 2-inch O.D., as well as a 3-inch O.D., split barrel samplers were used to recover the disturbed SPT soil samples. The results of penetration testing are shown on the boring log presented in Attachment C. Rock core photographs are presented in Attachment D.

Laboratory Testing

Representative soil samples collected during the subsurface explorations were submitted to the WFLHD Materials Laboratory for soil index testing. Laboratory testing was conducted in accordance with appropriate AASHTO specifications to obtain engineering properties necessary for design. The results of the soil index tests were used to classify the samples according to the Unified Soil Classification System (USCS). The USCS classification of individual laboratory tested soil samples is reported on the boring logs. A summary of the laboratory tests performed and the test results are shown in Table 1 and Table 2, respectively.

Laboratory test reports are contained in Attachment E.

Table 1: Laboratory Test Descriptions

Laboratory Test Testing Standard Particle Size Analysis AASHTO T 88 Liquid Limit of Soils AASHTO T 89 Plastic Limit and Plasticity Index of Soils AASHTO T 90 Apparent Specific Gravity of Soils AASHTO T 100 Soil Classification of Soils AASHTO M 145 Natural Moisture Content of Soils AASHTO T 265

Table 2: Laboratory Test Results Summary

Sample No.

Sample Depth (Ft.)

USCS Water Content (Percent)

Liquid Limit

(Percent)

Plastic Limit

(Percent)

Plasticity Index

(Percent)

Passing No. 200 Sieve

(Percent)

S-6 & -7 17.5 – 21.0 GP-GC 2.6 23 17 6 7.6

S-8 22.0 – 23.5 SC 10.3 27 17 10 26.3

S-10 26.5 – 28.0 SC 5.5 24 16 8 18.1

Subsurface Profile

Boring BH21-01 was advanced to a depth of 71.5 feet below ground surface (bgs). The boring encountered loose to dense silty gravel with sand to a depth of 17 feet, which is interpreted to be fill. Between the depths of 17 and 21.5 feet, medium dense poorly graded gravel with silty, clay, and sand was encountered, and it is interpreted to be colluvium. From

21.5 to 23.75 feet and from 26 to 28.5 feet, medium dense clayey sand with gravel was encountered and it is interpreted to be highly weathered siltstone or a shear zone of a prehistoric landslide. Between the depths of 23.75 and 26 feet, a very dense poorly graded gravel with sand, as classified through laboratory testing, was encountered that is interpreted to really be a cobble. SPT blow counts for this layer is suspected to be overstated. Starting at

28.5 feet to termination of the boring at 71.5 feet, the boring encountered very fine grained, dark gray to black, medium strong (R3) to strong (R4) siltstone.

ANALYSIS

Slope stability analyses showing the FOS for existing slope conditions and proposed construction conditions are presented in Attachment F. Slope stability, was evaluated utilizing the software program SLIDE 2 version 9.003 by Rocscience Inc. for Site 1 on Hurricane Ridge Road at about Station 8+00 and for Site 2 on Obstruction Point Road at about Station 51+20.

SLIDE 2 is a two-dimensional limit equilibrium slope stability program. The models were developed from information gathered during the subsurface drilling boring log, laboratory testing results, projecting subsurface materials based on field observations and measurements, and additional information from the survey and LiDAR information to produce a generalized critical cross-section model. Two conditions were considered for the back analyses. One condition analyzed the site with a groundwater level that represents high groundwater or “spring thaw” and the second condition representing a drier groundwater condition. Both circular and non-circular failure modes were analyzed for both sites along with engineering judgment to determine appropriate shear strength parameters for the represented material layers. Soil and rock parameters for each layer are shown in Table 3.

Table 3: Material Properties used in SLIDE Analysis

Soil Layers Unit Weight, γm

(pcf)

Internal Friction Angle, θ (deg.)

Cohesion, C

(psf)

Generalized Hoek-Brown

UCS

(ksf)

GSI mi D

Fill – Obstruction Point Road 125 38 30 - - - - Colluvium/Fill – Hurricane Ridge Road 125 40 30 - - - - Poorly-graded Gravel with silty clay & sand 125 38 50 - - - - Clayey Sand with gravel 115 35 40 - - - - Marine Rhythmites-Siltstone 135 - - 60 20 7 0

For Site 1, Hurricane Ridge Road, both conditions were evaluated because past sites visit observations and interviewing Maintenance personnel suggests movement does not appear to occur during the drier seasons. The outboard slope limit in the analysis was placed about 8 feet toward centerline from the embankment hinge to model the site conditions observed in 2018, Figures F-1 and F-2 non-circular, and F-4 and F-5 circular. For the initial dry conditions, a Factor of Safety (FOS) of 1.059 and 1.075 was tabulated. A FOS of 0.885 and 0.976 was tabulated for the wet, “spring thaw” conditions. As would be expected, the analyses showed a slightly higher FOS when the head scarp retrogressed into the outboard travel lane, matching the 2021 field observations and measurements.

For Site 2 on Obstruction Point Road, a back-analysis in dry conditions to a FOS of 1.545 and 1.529, and a FOS of 1.469 and 1.474 was tabulated for the wet, “spring thaw” conditions.

These higher FOS values are not surprising since this is primarily an erosion issue and there is no evidence of recent lateral movement upon Hurricane Ridge Road, directly above Site 2.

DESIGN RECOMMENDATIONS AND DISCUSSION

Hurricane Ridge Road; Site 1

Slope creep along the eastern margin of the prehistoric landslide with an over-steepened embankment slope overlying steeply dipping weak bedrock with the likely concentration of groundwater piping material from under the road at this location is resulting in cracking and subsidence of the pavement of Hurricane Ridge Road. A reinforced soil slope (RSS) was analyzed for improving the slope stability of the embankment below Hurricane Ridge Road.

Figures F-3 and F-6 show “spring thaw” condition models for non-circular and circular failure surfaces with a FOS of 1.217 and 1.264, respectively. The models show a lowered groundwater level due to an underdrain system within the inboard foreslope. The RSS provides tensile strength within upper portion of the roadway embankment and the underdrain intercepts shallow groundwater from flowing into to the prehistoric landslide lateral scarp. An RSS is recommended to reduce the risk of future instability at this location on Hurricane Ridge Road. An RSS is very effective for stabilizing over steepened slopes and where surficial slope instabilities occur. However, if the instability is due to movement within the prehistoric landslide, this risk reduction measure should be considered an interim solution.

Construct an RSS on Hurricane Ridge Road from station 6+90 to 9+50. Between stations 7+10 and 9+00 the full-depth RSS should be constructed to an excavation depth of 6.25 feet, measured vertically below the top of subgrade elevation. In the tapering transitions, between the RSS beginning and end stations and full-depth RSS, the depth of the RSS should be stepped up uniformly. The full-depth RSS should consist of eight layers of Type IV reinforcement geogrid at 9-inch vertical spacing. The reinforcement geogrid should be lapped back 4 feet at the slope face except for the upper most lap, which will extend the full width of the excavation. Between stations 6+90 and 9+00 construct the RSS with the reinforcements extending 50 feet from the new embankment RSS face or to the inboard edge of the road.

Between stations 9+00 to 9+50 the RSS reinforcements should extend 10 feet from the new embankment RSS face. A thin layer of gravel should be placed between the lap and the overlying layer to prevent direct contact between adjacent geogrid layers. Construct RSS face with 4-inch by 4-inch W5xW5 hot dipped galvanized welded wire form extending 6-inches from the RSS geogrid and galvanized hardware cloth lining face and bottom, immediately adjacent to the welded wire form. The Park will provide direction to the materials to be packed between the geogrid and the hardware cloth. Place a non-woven, class 1, type A geotextile filter between the reinforcement geogrid and the select granular backfill, extending 2 feet behind the geogrid face. Also place the non-woven, class 1, type A geotextile and along the bottom and back slope of the excavation prior to placing the bottom most reinforcement. Construct the RSS slope face to match adjacent existing slope face at approximately 1V:1.25H. RSS typical and detail drawings are provided in Figures B-9 and B-10.

Installation of an underdrain system is required at the back side of the RSS. The underdrain system should consist of an 8-inch diameter perforated collector pipe in a 12-inch wide by 12-inch deep trench backfilled with granular backfill and lined with Class 1 Type A geotextile filter running the length of the RSS. Non-perforated outlet pipes should be installed to drain across the road and daylight on the slope about every 100 feet along the face of the RSS and at all low points in the RSS profile.

In addition to the RSS, a 4-foot-deep underdrain system in the existing Hurricane Ridge Road inboard ditch line is recommended between Station 6+40 and 9+50. This site appears to exhibit a greater magnitude of movement during periods of high groundwater levels, an underdrain system will lower the groundwater table to improve landslide stability. Generally, drainage reduces porewater pressure by lowering the groundwater table within a landslide mass and may result in slowing, or halting progression of landslide movement. Due to the moderate roadway grade ground water may be infiltrating and flowing under the road west of the site until it reaches the eastern lateral extent, increasing the porewater pressure within adding the shear plane. Therefore, it is recommended to begin the underdrain system at station 6+40. The trench drain will consist of an 18-inch wide (minimum) trench lined with class 1, type A filter geotextile at the bottom of an 8-inch diameter perforated drainpipe installed 2-inches above the bottom of the trench. The total depth of the trench is 5 feet from the existing groundline; which the lower 4 feet is filled with granular backfill and the top 1-foot is backfilled with impermeable backfill material. The impermeable backfill inhibits surface water to enter the underdrain system and to avoid damage from typical ditch maintenance activities. At Station 9+50, the trench underdrain will end and transition to a solid outlet pipe, continuing eastward in the current ditch line for about 500 feet before crossing under the road and outletting on to the embankment slope where vegetation is present, at approximately GPS coordinates 47.9693°N, 123.4877°W. The outlet pipe should be at least 18 inches below the flow line in the ditch. The proposed trench underdrain detail is provided in Figure B-11.

Obstruction Point Road

Site 2A and 2B

Roadway excavation into the inboard slope is recommended to shift the road over to avoid the continued erosion and retrogression of the outboard slope. It is anticipated that the erosional channels will continue to enlarge over time; however, if the road is cross sloped toward the inboard side on Obstruction Point road and is maintained, it should reduce the rate of the progression of the erosion. This risk reduction measure should be considered an interim solution with a design life of approximately 5 to 10 years.

Roadway excavation of approximately 6 feet into the existing cut is recommended between Station 50+80 and 51+40, no steeper than a slope ratio of 1V:1.25H. The maximum slope ratio is to be constructed only where the distance will be between 3 and 4 feet, as measured from the top of the cut slope hinge point to the edge of Hurricane Ridge Road. If the distance is greater than 4 feet, construct the cut at a shallower angle. Cut slopes shallower than about 1V:1.5H are vegetated based on field observations of the surrounding subalpine slopes.

Taper the cut on either side to match existing slopes within 30 feet of these station limits. The proposed cut slope will result in the loss of the small gravel pullout on Hurricane Ridge Road, directly above this site.

Before starting the roadway excavation, remove the vegetation in mats, with soil intact to protect the roots. The mats should be covered during construction and kept moist and hydrated. After the cut is completed, the mats should be carefully place back on the newly exposed slopes with a 1V:1.5H slope ratio or shallower. Slopes steeper than 1V:1.5H should be covered, with a FP-14, 713.18, Type 5A, permanent rolled erosion control product.

Based on discussions with Park Maintenance and our field observations, There is not adequate room for maintaining a ditch line due to frequent slope raveling events in this highly fractured rock material, so we recommend cross sloping (insloping) the road to the inboard side of the road to discourage surface water from continuing to direct water to the eroding outboard slope. A disadvantage to cross sloping the runoff on the road is an erosional channel along the toe of the cut slope can develop when not in rock. Allowing the water to cross the road, either as a concentrated flow or sheeting across the road will result in similar erosional channels on the embankment slopes that we observed. With this in mind, we strongly recommend identifying areas below the road that currently have vegetation growing for crossing and concentrating flow, like where the concrete screen blocks are located. For example, reconstructing the concrete screen blocks channel, enlarging it from one block to two blocks wide, may provide additional protection to the embankment and natural slopes below Obstruction Point Road.

To direct the surface water from the inboard side of the road to outboard side of the road several options were considered. We recommend a drainage blanket crossing consisting of a rock-filled trench orientated about 110 degrees from centerline filled with riprap to allow water to pass through and encased with separation-stabilization geotextile, presented in Figure B- 12.

We recommend using the rock-filled trench crossing to direct water from the inboard ditch to the outboard side of the road at the location of the concrete screen blocks. This recommendation will require welded wire forms placed at the face of the embankment to contain the riprap. Increased sediment load from the newly cut slope should be expected in the surface water following construction. This option is not maintenance free and may require periodic cleaning where the sediment may clog the drainage blanket in the future.

As observed, where present along Obstruction Point Road, the construction of concrete screen block lined channels on the embankment slope of the road at strategic outfall locations may be useful to reduce heavy erosion from developing on the embankment slope.

Obstruction Point Road - Long-term Outlook

As discussed in the Site Observation section of this memorandum, the slopes above and below the first 1900 feet of Obstruction Point Road exhibit poorly- vegetated, over-steepen soil slopes, and highly erodible, exposed bedrock. In addition, this portion of the road crosses a larger prehistoric landslide possibly showing signs of movement along its margins. It is anticipated that this section of the road will require considerable maintenance effort to maintain this section of Obstruction Point road over time. For instance, as the bedrock outcrop continues to erode at Site 2, more expensive mitigation structures, such as anchored walls should be anticipated. This section of Obstruction Point road is at risk of failure and potential loss of the roadway in this upper section. With this in mind, we have conducted a brief field reconnaissance to look at where there may be a more resilient access point to Obstruction Point Road from Hurricane Ridge Road (see Figure B-2 yellow shaded area). It appears a viable alternative may be approximately 2200 feet east of the current intersection.

The road would be outside the current prehistoric landslide feature at the upper portion of Obstruction Point Road and bedrock appears close to the surface as it runs along the top of the ridge. This may be a long-term option the Park would like to investigate to reduce long-term roadway maintenance expenditures on the upper portion of Obstruction Point Road, and reduce the likelihood of vehicles (and the public) being stranded on the wrong side of a future road failure.

CONSTRUCTION CONSIDERATIONS

Excavation Characteristics

Blasting is not allowed for this project.

Bedrock was encountered at 28 feet below the roadway in boring BH21-01. The strength and condition of the rock encountered is characterized in the boring log presented in Appendix C.

At Site 1, based on visual field observations in the existing roadway cuts along this portion of Hurricane Ridge Road and boring BH21-01, there is a high probability bedrock will be encountered within the RSS excavation and underdrain/utility trench excavation.

Based on visual observations at Site 2, the exposed bedrock is soft and fissile; however, the condition of the rock behind the exposed surface is not known.

Bedrock will be encountered during the construction at Site 2.

Suitability of Excavated Material for RSS and Deep Patches

It is anticipated that material from the cut at Site 2 and the excavation materials from the RSS and deep patch sites will not meet the select borrow, FP-14 Subsection 704.06, nor select granular backfill, FP-14 Subsection 704.08, material specifications.

REFERENCES

Brinker, R.W., Tufts, R.A., 1995 revised 2019, Forest Roads and Construction of Associated Water Diversion Devices, ANR-0912, ANR-0916, Alabama A&M and Auburn Universities, Alabama Cooperative Extension System.

FHWA, 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14, Federal Lands Highway, Federal Highway Administration, U.S.

Department of Transportation, Washington, DC.

Gerstel, W.J., Lingley, W.S., 2003, Geologic Map of Mount Olympus 1:100,000 quadrangle, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-4.

Quarternary Fault and Fold Database of the United States - Interactive Fault Map, United States Geologic Survey, Geologic Hazards Science Center, Golden, Colorado, https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aad f88412fcf, accessed June 22, 2021.

Copstead, R.L., Johansen, D.K., Moll, J., 1998, Water/Road Interaction: Introduction to Surface Cross Drains, Report 9877 1806-SDTDC, U.S. Forest Service, U.S. Department of Agriculture

U.S. Seismic Design Maps Web Service, 2009, AASHTO Guide Specifications -Design Ground Motions, Earthquake Hazards Program, United States Geologic Survey (USGS), U.S.

Department of the Interior, https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=47.9696&longitude=- 123.04900&siteClass=B&title=Hurricane Ridge Obstruction Point Roads, accessed July 26, 2021.

Washington Lidar Portal, Olympic Park 2014, Washington State Department of Natural Resources, Washington State Geology and Earth Resources, https://lidarportal.dnr.wa.gov/#47.96340:-123.49634:15 accessed June 8, 2021.

CLOSING

Please contact Diann Morehouse at (360) 619-7609 or Diann.Morehouse@dot.gov with any questions regarding this memorandum.

INITIALS

CC: Geotechnical File

Attachments: A – Project Photographs B – Figures C – Boring Log

D – Rock Core Photographs E – Laboratory Test Results F – Slope Stability Analyses https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=47.9696&longitude=-123.04900&siteClass=B&title=Hurricane%20Ridge%20Obstruction%20Point%20Roads https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=47.9696&longitude=-123.04900&siteClass=B&title=Hurricane%20Ridge%20Obstruction%20Point%20Roads https://lidarportal.dnr.wa.gov/#47.96340:-123.49634:15 mailto:Diann.Morehouse@dot.gov

ATTACHMENT A

Project Photographs

Photograph A-1: (2021) Marine rhythmites sedimentary rock in the cut slope along Hurricane Ridge Road across from Site 1.

Photograph A-2: (2018) An easterly view (towards Port Angeles) of pavement distress within the pullout on Hurricane Ridge Road; Site 1.

Photograph A-3: (2018) A westerly view (towards Hurricane Ridge Visitor Center) of pavement distress within the pullout on Hurricane Ridge Road; Site 1.

Photograph A-4: (2019) Westerly view of pavement distress within the pullout on Hurricane Ridge Road; Site 1. Note the distress is closer to the fog line but has not crossed it.

Photograph A-5: (2021) Westerly view of pavement distress in the pullout on Hurricane Ridge Road; Site 1. Note a tension crack was observed in the middle of the outside lane.

Photograph A-6: (2021) Easterly view of pavement distress in pullout and outside lane on Hurricane Ridge Road; Site 1. Note red arrows indicating tension crack in the pavement.

Photograph A-7: (2018) The location of outlet of a 24-inch culvert that crosses under Hurricane Ridge Road.

Photograph A-8: (2019) A westerly view of Sites 2A and 2B

Photograph A-9: (2018) Westerly view of Site 2A.

2A

2B

Photograph A-10: (2018) Southerly view of Site 2A. Note the bolts protruding from the concrete beam, near the top of the photograph, are very close to the edge of the void.

Photograph A-11: (2019) Southerly view of Site 2A. Note the distance between the protruding bolts and the surrounding ground surface, on the right side of the photograph, has increased from Photograph A-10.

Photograph A-12: (2021) The length of exposed concrete beam is greater than in Photographs A-10 and A-11.

Photograph A-13: (2021) An easterly view of the back of the concrete beam. Note the red arrow shows the end of the beam.

Photograph A-14: (2021) Site 2A and 2B from above Obstruction Point Road.

Photograph A-15: (2018) Pavement sawcut approximately 400 feet east of the beginning of Hurricane Ridge parking area.

Site 2A Site 2B

Photograph A-16: (2018) West about 40 feet from Photograph A-15. Note the slight drop of the road profile between the two red arrows. At this time the saw cut had a skin patch over it with no observed separations.

Photograph A-17: (2021) Westerly view, the red arrows show the location of the discontinuity of the pavement surface.

Photograph A-18: (2018) Differential pavement settlement shown in Photographs A-15, A-16, and A-17, in line with possible prehistoric landslide scarp. Hurricane Ridge and Obstruction Point Roads are located at the left and right side of the photograph, respectively.

ATTACHMENT B

Figures

Figure B-1: Project Location Map (excerpt from Google Earth)

N

Figure B-2: Google Earth satellite view of the project vicinity map

Sites 2A & 2B

Site 1

Tension crack

Obstruction Point Road

Interpreted rupture surface extent

Boring BH21-01

Possible realignment area

Hurricane Ridge Visitor Center parking area

Pavement saw cut

24-inch culvert

Concrete screen blocks

Settlement and patching Hurricane Ridge Road

Figure B-3: Excerpt from Geologic Map of the Mount Olympus 1:100,000 Quadrangle (Gerstel and Lingley). Refer to subsection Regional Geology.

1 Mile

Project Site

Figure B-4: Regional map excerpt showing the limits of the Juan de Fuca glaciation lobe. This figure is from Washington Department of Natural Resources Geology Portal. Solid blue line is the continental ice sheet limit from the 1:250,000 scale geologic mapping data. Blue hatched line delineates the Cordilleran Continental Ice Sheet extent from 1:100,000 scale geologic mapping data.

Hurricane Ridge Road

Obstruction Point Road

Figure B-5: Quaternary-aged faults within the vicinity of the project are shown by an orange line. Solid and dashed lines describe the faults as either constrained or moderately constrained, respectively. The Unnamed fault along Barnes Creek is mapped as an undifferentiated Quaternary-age fault. (excerpt from USGS Quarternary Fault - Interactive Fault Map)

6 miles N

Project Area

Unnamed fault along Barnes Creek

Calawah Fault

Lake Creek/Boundary Creek Fault

Figure B-6: LiDAR image of the project area.

Hurricane Ridge Visitor Center Prehistoric landslide head scarp

B

Tension crack surveyed in 2020

Tension crack noted in 2021 Tension crack noted in 2019 and 2021

Obstruction Point Road

Differential settlement noted during 2020 survey

Site 2A Site 2B

'O

OJ

u

OJ

.c u

'O

OJ

C:

en 'iii

OJ

Geocomposite sheet drain, type 1 [21

Separation geotextile, class 1, type A

Top geogrid reinforcement lap [lJ

Geogrid reinforcement lap min. 4'

[JJ

Finished face

9" (typ.J

::�------�- Reinforcement geogrid, type IV , Separation geotextile, 1 ' 50' Reinforcement Jen th seeno class 1, type A

. o .. O

.... 0 •

8" (2') solid outlet drain pipe, 2% min. slope See Sheet I.3 for outlet pipe details [4]

- Slope to drain, 2% (min.) Object marker post, white, plastic

Saw cut pavement

6+90

Select granular backfill Toe elevation

REINFORCED SOIL SLOPE

6+90.00 to 9+50.00

Existing ground

Reinforced soil slope

I : Access ramp

• 1

Saw cut pavement

7+10

________ L

Existing roadway

Reinforcement geogrid, type IV

Varies Toe elevation

REINFORCED SOIL SLOPE PROFILE

6+90.00 to 9+50.00

(PAY LIMITS)

9+50

NO SCALE

NPS PMIS No. 308387 NPS Drwg No. 149/177791

STATE PROJECT

SHEET

NUMBE

WA

NPS OLYM 12(7)

NOTE:

1. Slope or shore excavation according to Section 208.

2. Extend each course of reinforcements into access ramp areas as reinforced soil slope construction proceeds.

3. Back of temporary excavation to extend 1-foot behind back of primary reinforcements.

4. Place stabilization geogrid within 1-foot of slope face.

5. 7+10 to 9+00: 50' reinforcement length or inboard edge of pavement, whichever is shorter distance.

6.

Between 6+90 to 7+10 and 9+00 to 9+50:

10' reinforcement length.

Standard underdrain is located on the north side of Hurricane Ridge and runs independently of the RSS drainage.

See Sheet I.3 for standard underdrain details.

FOOTNOTE:

[1] Extend the top geogrid reinforcement lap to the back excavation slope.

[2] Extend Sheet drain from the bottom of excavation up to ½ the height of excavation.

[3] Approximate slope ratio. Match existing slope angle.

[4] Place outlets every 50 feet.

FIGURE B-9

REINFORCED SOIL SLOPE (RSS)

SHEET 1 OF 2

C.2 u

N N ro

C ro cl

4'-0" Reinforcement lap

Reinforcement geogrid type IV

Select granular backfill

Separation geotextile class 1, type A

Welded wire form facing

Tie transverse bars on facing and grid, 3 per panel (typ.)

Hardware cloth

Top soil

VEGETATED FACING DETAIL

Note: Select granular backfill between geogrid reinforcement is omitted for clarity.

Wall face ________.., Backside overlap acceptable

Reinforcement length L

CURVE LEFT WALL FACE

Wal/face -

Butt adjacent geogrids at backside without overlap

Reinforcement length (L)

TANGENT WALL FACE

(PLAN VIEWS)

GEOGRID BUTT DETAIL

I

Machine (roll direction)

NPS PMIS No. 308387

STATE

NPS Drw No. 149 177791

Place edge of wire inside of adjoining wire form for support, 4"

WA

PROJECT

NPS OLYM 12(7)

Support struts spaced maximum 24" C to C or as needed.

Inset end struts 6"

4x4 W5x WS

WELDED WIRE FORM l l

Wall face -------.1 Butt adjacent geogrids at I wall face without overlap ■

Backside separation acceptable

Reinforcement length L

CURVE RIGHT WALL FACE

NO SCALE

Mat…