GR 05-22 Icicle Cr Rd Geotech Report WA_USFS_OkWen617_76(1) FINAL UPDATED.pdf

Icicle Creek Road Rockfall Mitigation Project August 2022

Table of Contents 1-i

Geotechnical Report No. 05-22

Icicle Creek Road Rockfall Mitigation Project

FINAL GEOTECHNICAL REPORT

WA USFS OkWen617 76(1)

Chelan County, Washington

August 2022

This page intentionally left blank

Table of Contents i

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Project Description

2 GEOTECHNICAL ASSESSMENT

2.1 Previous Geotechnical Assessment (2017)

2.2 Current Geotechnical Assessment (2020 – 2021)

3 GEOLOGY AND CLIMATE

3.1 Climate

3.2 Vegetation

3.3 Regional Geology

3.4 Geologic Hazards

4 PROJECT SITE CONDITIONS

4.1 Unstable Slope US33

4.2 Unstable Slope US36

4.3 Unstable Slope US38

5 ROCKFALL ANALYSIS

6 DESIGN RECOMMENDATIONS

6.1 Slope US33; Type 1 Gabion Rockfall Barrier

6.2 Slope US36; Type 1 and Type 2 Gabion Rockfall Barriers

6.3 Slope US38; Draped Rockfall Protection, Cable Net

7 CONSTRUCTION CONSIDERATIONS

7.1 Excavation

7.2 Select Vegetation Removal

7.3 Safety Scaling and Hazard Tree Removal

7.4 Scaling Material Reuse and Disposal

7.5 Geotechnical Construction Support

7.6 Maintenance Recommendations

8 LIMITATIONS

9 SIGNATURES

10 REFERENCES

List of Figures ii

LIST OF FIGURES

Figure 1-1. Project location map

Figure 1-2. Vicinity topographic and aerial photographic maps

Figure 3-1. Blewett Pass median snow water equivalency graph

Figure 3-2. Geologic map of the project area

Figure 3-3. Photograph of site US33

Figure 3-4. Photograph of site US33

Figure 4-1. Topographic map of site US33

Figure 4-2. Photograph of site US36

Figure 4-3. Topographic map of site US36

Figure 4-4. Representative photograph of slope conditions at site US38

Figure 4-5. Photograph of slope US38

Figure 4-6. Map of Site US38

Figure 6-1. Lidar derived topographic and slope map of site US38

LIST OF TABLES

Table 2-1 Unstable slopes total Unstable Slope Management Program score

Table 3-1 Climate data summary

Table 5-1 RocFall slope parameters and slope material value inputs

Table 5-2 RocFall rock input values

Table 5-3 RocFall US38 modeling results for catchment

Table 5-4 RocFall US36 and US33 modeling results for catchment

Table 5-5 RocFall US 38 modeling results of total kinetic energy and bounce height

Table 5-6 RocFall US36 modeling results of total kinetic energy and bounce height

Table 5-7 RocFall US33 modeling results of total kinetic energy and bounce height

Appendices iii

APPENDICES

APPENDIX A…………………………………………………………………………………...38

Site Photographs (12 pages)

APPENDIX B…………………………………………………………………………………...51

Rockfall Protection Details:

Gabion Rockfall Barrier and Concrete Barrier Details (1 page)

Schematic Detail of Draped Rockfall Protection Limits at Site US38 (4 pages)

Draped Rockfall Protection, Cable Net Mesh with Double Wire Twist Mesh Backing (3 pages)

APPENDIX C…………………………………………………………………………………...60

Rockfall trajectory modeling results (16 pages)

4 Introduction

1 INTRODUCTION

This report provides geotechnical assessment results, analysis, and final rockfall risk reduction recommendations for the Icicle Creek Road Rockfall Mitigation project. The project is located approximately four miles southwest of Leavenworth, Washington off US Highway 2, as shown in

Figure 1-1, and Figure 1-2. Icicle Creek Road is a major recreational access corridor to the

Okanogan-Wenatchee National Forest (OWNF). The corridor is also known as United States

Forest Service Road (USFS) 7600. For ease of delineating between Chelan County and USFS maintained portions of Icicle Creek Road, the USFS maintained section of the roadway will be referred to as Road 7600 in this report.

Figure 1-1. Project location map.

1.1 PROJECT DESCRIPTION

Unstable slopes were assessed by Federal Highway Administration – Western Federal Lands

Highway Division (WFL) and rated using the Unstable Slope Management Program for Federal

Land Management Agencies (USMP) in 2017 (FHWA, 2019). Based on USMP survey results, OWNF maintenance records, and OWNF management input, three highly rated slopes were chosen by OWNF for further risk reduction work requiring more detailed geotechnical assessment including design recommendations for rockfall risk reduction measures.

Introduction 5

The project involves providing rockfall risk reduction recommendations for the three highly rated unstable slopes from approximate milepost 4.3 through milepost 4.8 on Road 7600. The following sections provide a summary of our collected field data, rockfall analyses, final design assumptions and considerations, and construction considerations.

6 Introduction

Figure 1-2. Vicinity topographic and aerial photographic maps. Unstable slopes assessed in this report are highlighted in yellow. Contours are at 40 foot intervals in the topo map. Lidar and aerial imagery are both from

2015.

Geotechnical Assessment 7

2 GEOTECHNICAL ASSESSMENT

2.1 PREVIOUS GEOTECHNICAL ASSESSMENT (2017)

Unstable slopes along Road 7600, from milepost 4.2 through milepost 10.7, were evaluated and rated using the USMP in 2017 by WFL engineering geologists. The USMP is designed to give land managers a decision support tool for increasing understanding of their slope assets performance, help to proactively plan preventative maintenance and funding requests, and provide support for response to litigation or claims related to unstable slopes. The USMP assigns a total score to an unstable slope based on the slopes’ hazard- and risk-related factors. A high total score typically indicates a slope posing a relatively higher unstable slope hazard and risk to transportation facility users than a lower total rating does.

The WFL geotechnical report GR 08-17 was prepared and submitted to provide USMP survey results, conceptual rockfall risk reduction design options, and associated geotechnical design cost estimates for the highest rated slopes for OWNF (FHWA, 2017). Thirty-nine unstable slopes were documented in the report and conceptual designs for 13 of the highly rated slopes were developed and provided to OWNF for consideration. This geotechnical report supersedes GR 08-17.

2.2 CURRENT GEOTECHNICAL ASSESSMENT (2020 – 2021)

After reviewing information in GR 08-17 and discussions with OWNF, eight highly rated USMP slopes were selected by the OWNF for further rockfall risk reduction design work and cost estimating: US23, US27, US32, US33, US34, US36, US38, and US39 (FHWA, 2017). A more detailed and focused field site assessment of those slopes were conducted by Nicholas Farny and

Orion George, WFL engineering geologists, between 9/28/2020 through 9/30/2020 and another site visit by Orion George on October 29, 2021. Site investigations from 2017 through 2021 provided field observations and detailed measurements informing rockfall analyses, interpretations, and recommendations detailed in this report. No subsurface investigation was conducted as part of this investigation.

After initial geotechnical cost estimates were developed, US33, US36, and US38 were selected for final rockfall risk reduction design, and are the focus of this report. Table 2-1 tabulates slopes discussed in this report with the beginning and ending GPS coordinates, mile posts, and project stationing along with their total USMP score.

8 Geotechnical Assessment

Table 2-1. Unstable slopes discussed in this report with GPS coordinates, mile post, project stationing and total

Unstable Slope Management Program score.

USMP1

Slope

ID

Coordinates2 Mile Post2 Stationing3 Total

USMP1

Score

Begin End Begin End Begin End

Latitude Longitude Latitude Longitude

US33 47.54173 -120.72046 47.54179 -120.72093 4.78 4.80 528+20 529+40 682

US36 47.54395 -120.71567 47.54309 -120.71715 4.50 4.60 512+25 517+50 716

US38 47.54461 -120.71217 47.54449 -120.71358 4.33 4.4 503+60 507+50 893

Notes:

1. USMP = Unstable Slope Management Program for Federal Land Management Agencies

2. Coordinates and mile posts are approximate and may differ from GR 08-17.

3. Project stationing begins at the east side of the project increasing to the west. Stationing is approximate. Note that the begin and end stationing of the slope may differ from begin and end of the recommended rockfall risk reduction design.

Geology and Climate 9

3 GEOLOGY AND CLIMATE

The project is located in the southwestern portion of Chelan County, Washington ( Figure 1-1).

This area is on the eastern side of the North Cascades physiographic province and denoted by highly dissected, glaciated mountain terrain up to 7,000 feet elevation with composite volcanoes rising above 10,000 feet. Valleys are often glacially carved U-shaped valleys with steeply sloping valley walls. The project is located along the bottom of Icicle Creek Canyon at an elevation of approximately 1,400 feet. Icicle Creek flows east, and to the south, immediately below Road 7600.

Field site investigation photos of unstable slope sites US33, US36, and US38 are provided in report sections below, and in Appendix A.

3.1 CLIMATE

The North Cascades physiographic province is marked by the maritime climate of the Pacific

Northwest; coupled with large vertical relief, an orographic effect punctuates differences in climate and vegetation on the drier east side versus the wetter west side. Summers are typically warm and dry with winters typically cool and wet. From December to March the project area is most susceptible to rain on snow events. The project area typically experiences an annual average of 24 inches precipitation and 96 inches of snow fall.

Table 3-1 shows the average maximum and minimum temperatures, average monthly precipitation, average monthly snowfall, and average monthly snow depth data for the period from

1914 to 2016 for the Leavenworth 3 S, Washington (454572) weather station (Western Region

Climate Center, 2021 [accessed 9/22/2021]).

Table 3-1. Climate data summary.

Period of record monthly climate summary for NOAA Cooperative Station: Leavenworth 3 S, Washington (454572)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Average Max.

Temperature (F)

33.4 41.90 52.5 62.9 72.1 78.8 87.8 87.2 78.0 63.1 44.2 33.9 61.3

Average Min.

Temperature (F)

17.3 21.4 27.8 33.9 40.3 46.3 50.8 50 45.7 34.6 28 20.4 34.5

Average Total

Precipitation (in.)

4.35 2.77 2.07 1.01 0.85 0.85 0.34 0.44 0.71 2.01 4.01 4.82 24.22

Average Total Snow

Fall (in.)

30.4 14.4 6.4 0.3 0.0 0.0 0.0 0.0 0.0 0.5 11.7 31.3 94.9

Average Snow

Depth (in.)

18 18 7 0 0 0 0 0 0 0 2 10 5

Notes:

Percent of possible observations for period of record; 05/01/1914 to 05/12/2016.

Max. Temp.: 84.6% Min. Temp.: 84.5% Precipitation: 88.5% Snowfall: 86.5% Snow Depth: 84.9%

The Blewett Pass (station 352: latitude, longitude coordinates 47.35, -120.68) snow telemetry station (SNOTEL) is approximately 11 miles south of the project site at an elevation of 4,240 feet

10 Geology and Climate above sea level. This site is at a similar longitude of the project site and is considered the best proxy for anticipated snow water equivalency (SWE) estimates for the project. Combined with average snow depths anticipated at the site from the Leavenworth 3S, Washington NOAA

Cooperative weather station the median SWE can be used together to estimate anticipated snow loads at the project site (Figure 3-1).

Figure 3-1. Graph displaying median snow water equivalency (SWE) in inches for the Blewett Pass snow telemetry station ID 352 over the period of 1991 through 2020; https://wcc.sc.egov.usda.gov/nwcc/site?sitenum=352 (NRCS, 2022). Data are organized in two week increments by the water year. The two highest median SWE for this period are labeled by quantity and date; both are in March.

3.2 VEGETATION

Vegetation around the project site is dominated by ponderosa pine (Pinus ponderosa), Douglas fir

(Pseudotsuga menziesii), and lodgepole pine (Pinus contorta var. latifolia) with big leaf maple

(Acer macrophyllum), mountain dogwood (Cornus nuttalii), and other deciduous tree species interspersed. Vine maple (Acer circinatum), Oregon grape (Mohania aquifolium), along with other shrub species and bunch grasses make up the understory.

3.3 REGIONAL GEOLOGY

Geology of the area is marked by subduction of the Juan de Fuca plate beneath the North American plate and alpine glaciation. Ongoing subduction is actively uplifting the region and causing volcanism. In the past, subduction led to intrusive activity to form the Mount Stuart batholith, 10-Mar, 13.8 21-Mar, 13.7

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

-O ct

-O ct

-N o v

-D ec

-J an

-F eb

-M ar

-A p r

-M ay

-J u n

-J u l

-A u g

-S ep

-S ep

S n o w

W at er E q u iv al en cy in

Water Year Date (2 week increments)

Blewett Pass Snow Telemetry Site Median Snow Water Equivalency

1991 - 2020

Geology and Climate 11 which is the core of the high elevation Stuart Range in the North Cascade Mountains. The Stuart

Range has experienced repeated periods of alpine glaciation, causing high levels of erosion within

Icicle Creek canyon. Figure 3-2 is a geologic map of the project area.

Figure 3-2. Geologic map of the project area, adapted from Tabor et al. (1987). Geologic units within map are Qa – alluvium, Qgd – alpine glacial drift, Kmst – tonalite and granodiorite, and Kmsd – diorite and gabbro.

Bedrock at the project site is mapped primarily as diorite and gabbro with isolated locations of foliation and metamorphic gneiss (mapped unit Kmsd; Tabor et al., 1987). Bedrock is aged Late

Cretaceous (100.5 – 66 million years ago) and is fairly resistant to erosion allowing for high elevations and steep valley walls.

Periods of Pleistocene aged (2.6 million – 11,700 years ago) alpine glaciation has carved the valley deeper and oversteepened the canyon walls. Past glaciers have also left numerous deposits within the valley, mostly glacial drift at the project locations (Tabor et al., 1987).

Surface soils are mapped around the project as Pleistocene to Holocene aged (11,700 years ago – present) alluvium (Qa) and glacial drift (Qgd) by Tabor et al. (1987). Alluvium (Qa) is associated with Icicle Creek and its steep tributaries, described as cobbles and gravel along drainages that transition to poorly graded sand on tributary fans. Glacial drift (Qgd) is described as

12 Geology and Climate overconsolidated and normally consolidated alpine glacial soils from past alpine glacial activity, and consists of outwash gravels, glacial till, and minor lacustrine deposits (Tabor et al., 1987).

3.4 GEOLOGIC HAZARDS

Rockfall and landslide hazards are common along Road 7600 and are caused by many different processes.

Exposed bedrock outcrops in the cut slope along Road 7600, whether they are igneous or metamorphic rock, are heavily jointed from exfoliation and tectonic stresses. These joints lead to a high occurrence of structurally controlled (i.e. stability is controlled by discontinuities/joints in the rock mass) slope failures, including planar, wedge (Figure 3-3), and toppling failure modes.

There are also areas of localized weak rock, where failures occur within the rock mass rather than as rockfall along structurally controlled joints. This is from a combination of past uncontrolled road blasting damage in rock cut slopes and zones of decomposing igneous rocks. Decomposed igneous rocks have weathered to the point where rock separates into gravel- to sand-sized particles known as grus (Figure 3-4). Large piles of grus can migrate down the slopes in debris slide events and accumulate as large debris fans, blocking the ditches and filling rockfall catchment ditches. In some cases, these debris fans eventually fill the ditch and continue to deposit materials into the road creating driving hazards.

Figure 3-3. Photograph of site US33 looking west. The red lines and shapes highlight discontinuity/joint faces, some dilated, that potentially allow for planar, wedge, and topple type rockfall failure modes in the adjacent slopes US32 and US34. Photograph dated October 20, 2021.

Geology and Climate 13

Exposed rocky soils in cut slopes also pose rockfall risks as fairly unconsolidated and loose dry soils differentially erode around boulders nested in and on top of soils (Figure 3-4). Some slopes are oversteepened and materials ravel downward undermining the upper natural slopes. The same differential erosion process occurs in zones of decomposing igneous rocks, with zones of grus eroding and exposing cobble to boulder-sized pieces of rock that eventually fail out of the slope.

Precipitation events and spring time thawing of snow and ice provide ongoing erosion of these loose soils and decomposing rock as melt water and precipitation drain down the slopes.

Maintenance indicates rock slope failures produce blocks of material up to four feet in diameter that fall into ditches and enter the roadway at slopes US33, US36, and US38. These rockfalls result in damaged pavement and periodically travel lane obstructions. They also report areas of soil and decomposing rock produce debris slide failures up to 15 cubic yards in volume. These debris slides often fill the catchment ditch and spill onto the roadway. We are not aware of any injuries or fatalities related to these unstable slopes.

Large landslides are mapped along Road 7600 by Tabor et al. (1987), however these features are not in proximity to the three slopes under consideration for this report. No field observations from this investigation suggest deep-seated landslides associated with unstable slopes US33, US36, or

US38.

Figure 3-4. Photograph of site US33 looking northeast. Arrows highlight areas of differential erosion of loose, silty sand soils eroding at the top of the slope, while the red polygon delineates an area of decomposing igneous rock.

Note the large piles of grus at the base of the slope in the ditch. Photograph dated October 20, 2021.

14 Project Site Conditions

4 PROJECT SITE CONDITIONS

Road 7600 is the sole vehicular access to the OWNF lands and private property inside the Icicle

Creek drainage. Roadway and slope characteristics vary between the three sites considered in this report: US33, US36, and US38. Each sites’ characteristics are described below.

4.1 UNSTABLE SLOPE US33

Unstable slope US33 is located at approximate project stationing 528+20 to 529+40 and has a total

USMP score of 682. The slope ranked as the seventh highest overall rating in the USMP survey.

At this site, Road 7600 consists of 27-foot wide paved roadway with two 11.5-foot wide lanes and approximately 2-foot wide paved shoulders. In addition there is an unpaved 7- to 11-foot wide shoulder on the fill-side of the roadway, and an unpaved 15- to 18-foot wide by 1-foot deep ditch on the cut-slope side. At the time of the 2020 site visit, construction material (from near site US38) had been stockpiled within the ditch. Figure 3-3 and Figure 3-4 above are photographs showing the general conditions at US33. Further representative site photographs can be reviewed in

Appendix A. Figure 4-1 is a map with lidar derived topography of the site.

Utilities should be expected at this site. Junction boxes and telephone pedestals were observed along the road and private property is located up river and appear to have underground utilities.

The OWNF communicated underground utilities are underneath the ditch line within limits of slope US33.

Maintenance reports this site as a nuisance with continual production of debris and rock blocks falling and rolling out onto the travel lanes. It has been reported that the ditch typically needs to be cleaned out twice a year at this site. Increased failure frequency coincides with spring snowmelt and after heavy precipitation events.

The cut slope is approximately 70 feet high with existing slope angles up to nearly 60 degrees.

Two distinct slope morphologies are represented along this rockfall prone section. The first is exposed diorite bedrock that is fairly fractured and foliated, causing structurally controlled planar and wedge type failures. Zones of this bedrock are also decomposing and producing grus. The second morphology is the overlying soils, which consist of rounded to angular boulders nested in the fairly unconsolidated, loose rocky soils. These are interpreted to be alpine glacial drift deposits.

The bedrock in this section acts as a chute for soil and rock debris from these deposits to deliver to the roadway.

Project Site Conditions 15

Figure 4-1. Topographic map of site US33. The critical cross section used for rockfall analyses (Section 5) is located in white. Base map is derived from aerial lidar acquired in 2015.

4.2 UNSTABLE SLOPE US36

Unstable slope US36 is located at approximate project stationing 512+25 to 517+50 and has a total

USMP score of 716. The slope is the ninth highest overall rating in the USMP survey. At this site

Road 7600 consists of 26-foot wide paved roadway with two 11-foot wide lanes and approximately

2-foot wide paved shoulders. It also has a paved 250 foot long and up to 36 foot wide fill-side pullout and an unpaved 10- to 12-foot wide by 2.5-foot deep ditch on the cut slope side. The fill-side pullout has an OWNF interpretive information kiosk. Near station 513+30 a dirt access road for the City of Leavenworth’s and the Icicle Peshastin Irrigation District’s water supply diversion

(located below Road 7600) enters the pullout (Appendix A). Approximate stationing of the pullout is 513+30 to 516+00. Figure 4-2 is a photograph showing the general conditions at US36. Further representative site photographs can be reviewed in Appendix A. Figure 4-3 is a map with lidar derived topography of the site.

16 Project Site Conditions

Figure 4-2. Photograph of site US36 looking west showing the weathered and jointed crystalline bedrock with differential erosion. Note the pavement damage from rockfall repaired with asphalt patches. Photograph is dated

October 20, 2021.

Utilities should be expected at this site. An unknown utility line was exposed within the ditch line and junction boxes and telephone pedestals were observed along the road with private property located up river appearing to have underground utilities. The OWNF communicated underground utilities are underneath the ditch line within limits of slope US36. An 18-inch corrugated metal culvert (CMP) transmits water underneath the pullout at approximate station 515+30 (Figure 4-3).

Maintenance reports this site requires seasonal patrols to keep the travel lanes open and clear of debris. It is common to have a maintenance callout at this site from debris and rock entering the road. Numerous asphalt patches are observed in the field and from historical aerial photographs suggesting pavement damage from rockfall events is common. During site visits in 2017 and 2020 the ditch had numerous boulders up to 3 feet in dimension within it. Maintenance reported that the ditch typically needs to be cleaned out twice a year and increased frequency of failures coincides with spring snowmelt and after heavy precipitation events.

Two distinct slope morphologies are represented in this rockfall prone section with the slope being characterized into two separate sections based on the size range of rock blocks. Area 1 was designated on the southwest portion of the slope, approximate station 515+00 to 517+50, and

Project Site Conditions 17 contains rock blocks up to 4 feet in dimension. Area 2 is the northeastern portion of the slope, approximate station 512+25 to 515+00, and contains boulders up to 3 feet in dimension (Figure

4-3).

Figure 4-3. Topographic map of site US36. An 18-inch culvert drains water from the ditch at this slope and is located on the map. Section 1 of site US36 contains larger block sizes, up to 4 feet in dimension. Section 2 of US36 had block sizes up to 3 feet in dimension observed. Critical cross sections A and B, used for rockfall analyses, are labeled and located in white. The Irrigation District access road enters road 7600 within site US36 near Station

513+30. Base map is derived from aerial lidar acquired in 2015.

Overall the slope is at least 195 feet high with the cut slope extending up to approximately 90 feet above the road. The existing cut slope angles range from approximately 55 degrees with bedrock outcroppings on the natural slope above the road cut nearing 70 degrees. Decomposing and fractured diorite bedrock, with some foliation, is producing differential erosion hazards. Eroding, unconsolidated soil is also exposing weathered bedrock and causing raveling and undermining failure types on the slope. Numerous discontinuities were observed during field investigations, but with an indeterminant pattern of joints and fractures.

18 Project Site Conditions

4.3 UNSTABLE SLOPE US38

Unstable slope US38 is located at approximate project stationing 503+60 to 507+50 and has a total

USMP score of 893. The slope has the highest overall rating in the USMP survey. At this site Road

7600 consists of a 28-foot wide paved roadway with two 14-foot wide lanes and approximately

2.5-foot wide paved shoulders. The ditch on the cut side of the road is 5- to 7-foot wide and typically 1-foot deep. An unpaved, about 8 feet wide, shoulder extends approximately 250 feet along the fill side of the road. Although large boulders are placed along this shoulder it is common for overflow traffic from the nearby popular Snow Lakes trail head to park along this section of the roadway on the fill side.

Additional nearby infrastructure adds complexity to this site. A large parking lot, with an entrance at approximately station 500+00, serves recreational visitors to the Snow Lakes trail. Furthermore, a service road accessing the City of Leavenworth’s and the Icicle Peshastin Irrigation District’s water supply diversion and mainline also enters Road 7600 at this trailhead parking entrance. This service road is not open to the public and is located below Road 7600, going upstream towards slope US36.

Figure 4-4. Representative photograph of slope conditions at site US38. Viewshed is looking west.

Jointed bedrock exposed with loose alpine glacial drift soils differentially eroding and raveling from the cut slope brow highlighted by arrows. Photograph is dated October 29, 2021.

Project Site Conditions 19

Figure 4-4 and Figure 4-5 are photographs showing the general conditions at US38. Further representative site photographs can be reviewed in Appendix A. Figure 4-6 is a map with lidar derived topography of the site.

Figure 4-5. Photograph looking east and taken below Road 7600 from the Irrigation District’s infrastructure access road. Slope US38 is labeled and arrow shows access road entrance at the Snow Lakes Trailhead.

Utilities should be expected at this site. Junction boxes and telephone pedestals were observed along the road and private property is located up river and appear to have underground utilities.

The OWNF communicated underground utilities are underneath the ditch line within limits of slope US33.

Maintenance reports this site is the worst rockfall prone slope along Road 7600 with year round call-outs and rock removal required. Typically, rockfall events occur two to five times a year, with large events averaging every 3 years. Larger events require blasting to reduce boulders to manageable sizes on the roadway and within the ditch for removal. Numerous asphalt patches are observed on the roadway and from historical aerial photographs, confirming the Maintenance reported high rockfall occurrence at this site. During 2017, 2020, and 2021 site visits the ditch had numerous boulders up to 3 feet in dimension within it.

Again, two distinct slope morphologies are represented at this rockfall section. The cut slope is approximately 90 feet high and extends about 390 feet along the road. The cut slope angle ranges

US38

oad

Irrigation District access road

20 Project Site Conditions from about 40 to 70 degrees with bedrock outcroppings exhibiting the steepest areas. Overall the average existing slope angle is approximately 45 to 50 degrees. Decomposing and fractured diorite bedrock, with some foliation, is producing differential erosion failures along with undermining and raveling failures of boulders out of the overlying, loose and unconsolidated soils. Blocks up to 6 feet in dimension have been reported, but sizes of 4 feet are more common based on field observations. During all three site visits the ditch was nearly filled with rockfall debris, which limits the ditch’s capacity to retain additional rockfall (Figure 4-5).

Figure 4-6. Map of Site US38 and nearby infrastructure. Location of Figure 4-5 is on the Irrigation District access road below the western portion of site US38. Critical cross sections A and B, used for rockfall analyses, are labeled and located in white. The base map is derived from aerial lidar acquired in 2015.

RockFall Analyses 21

5 ROCKFALL ANALYSES

Field visits consisting of taking photographs and measuring and developing critical cross-sections were required for geotechnical rockfall analyses to inform design recommendations (Figure 4-1, Figure 4-3, and Figure 4-6). We also reviewed and referenced 2015 light detection and ranging

(lidar) derived elevation data to inform critical cross-sections (WA-DNR, 2022).

Rockfall trajectory analyses were completed using Rocscience’s software program, RocFall, version 8.006. RocFall is a statistical analysis program designed to model different shapes of rock falling from, and along, slopes to estimate energies, velocities, and bounce heights of tumbling/falling rocks. RocFall input values are provided in Table 5-1 and Table 5-2 (Hoek, 2007).

Model results were compared to maintenance input and field observations. Appendix C contains images of rockfall trajectory models completed for this investigation.

Table 5-1. RocFall slope parameters and slope material values input for each slope, including normal coefficient of friction, tangential coefficient of friction, dynamic friction angle, rolling friction, and surface roughness.

Slope ID Material

Name

Normal

Coefficient of Friction

Tangential

Coefficient of

Friction

Dynamic

Friction

Angle

[tan(phi)]

Rolling

Friction Surface Roughness

Mean Std.

Dev.

Mean Mean Mean

Std.

Dev.

Mean

Std.

Dev.

Spacing

(ft)

Amplitude

(ft)

US38

Diorite

Outcrops

0.16 0.04 0.85 0.04 0.55 0.13 0.4 0.02 3 1

Colluvium

Cover

0.12 0.04 0.8 0.04 0.55 0.04 0.6 0.04 3 1

Asphalt 0.2 0.04 0.9 0.03 0.55 0.04 0.1 0.01 - -

US36 -

Section 1

Diorite

Outcrops

0.16 0.04 0.85 0.04 0.55 0.13 0.4 0.02 3 1

Colluvium

Cover

0.12 0.04 0.8 0.04 0.55 0.04 0.6 0.04 5 0.5

Asphalt 0.2 0.04 0.9 0.03 0.55 0.04 0.1 0.01 - -

US36 -

Section 2

Diorite

Outcrops

0.16 0.04 0.85 0.04 0.55 0.13 0.4 0.02 4 1

Colluvium

Cover

0.12 0.04 0.8 0.04 0.55 0.04 0.6 0.04 4 1

Asphalt 0.2 0.04 0.9 0.03 0.55 0.04 0.1 0.01 - -

US33

Diorite

Outcrops

0.16 0.04 0.85 0.04 0.55 0.13 0.4 0.02 4.5 0.5

Colluvium

Cover

0.12 0.04 0.8 0.04 0.55 0.04 0.6 0.04 4.5 0.5

Asphalt 0.2 0.04 0.9 0.03 0.55 0.04 0.1 0.01 - -

22 RockFall Analyses

Table 5-2. RocFall rock input values used for each modeling, including density, range of rock size

(dimensions) for rocks at each slope, mass, and rock shape.

Slope ID Rock

Type

Density

(lbs/ft3)

Dimensions

(ft)

Mass

(lb) Rock Shape

US38,

US36 -

Section 2

Diorite

Rock

3 x 3 x 4 6,480 Square, Super Ellipse^4 (1:1), Super

Ellipse^4 (5:6), Super Ellipse^4 (2:3), Super

Ellipse^4 (1:2), Super Ellipse^6 (1:1), Super

Ellipse^6 (5:6), Super Ellipse^6 (2:3), Super

Ellipse^6 (1:2) 1 x 1 x 1 180

US36 -

Section 1

Diorite

Rock

2 x 2 x 3 2,160 Square, Super Ellipse^4 (1:1), Super

Ellipse^4 (5:6), Super Ellipse^4 (2:3), Super

Ellipse^4 (1:2), Super Ellipse^6 (1:1), Super

Ellipse^6 (5:6), Super Ellipse^6 (2:3), Super

Ellipse^6 (1:2) 1 x 1 x 1 180

US33

Diorite

Rock

1 x 2 x 3 1,080 Square, Super Ellipse^4 (1:1), Super

Ellipse^4 (5:6), Super Ellipse^4 (2:3), Super

Ellipse^4 (1:2), Super Ellipse^6 (1:1), Super

Ellipse^6 (5:6), Super Ellipse^6 (2:3), Super

Ellipse^6 (1:2) 1 x 1 x 1 180

Critical design output values used for design parameters at proposed rockfall risk reduction barrier locations included the minimum, maximum, and average kinetic energy of rockfall, the minimum, maximum, and average rockfall bounce height, and the total number of rocks passing modeled barriers. Rockfall catchment was calculated for individual rockfall risk reduction barriers considered at each site:

• Gabion wall barrier o 5.5 feet high with 4.5 feet wide base (Type 2) o 2.5 feet high with 3 feet wide base (Type 1)

• Concrete barrier, 2.5 feet high with 2 feet wide base

Each gabion barrier was embedded 6 inches below ground surface, and therefore heights above are representative of the barrier above the ground surface. Concrete barriers were assumed to be placed directly on pavement surfaces. All barriers were placed approximately eight feet from the base of the cut slope. Eight feet was chosen to allow access behind the barrier with maintenance equipment for rockfall catchment area cleaning operations. This minimum width was based on our understanding of the available OWNF maintenance equipment and their communication with us during design.

Based on the slope complexity and site limitations, one critical cross-section was used for slope

US33 analyses and two critical cross sections were used at slopes US36 and US38 analyses (Figure

4-1, Figure 4-3, and Figure 4-6). Each critical cross section model had an upper seeder (rock initiation location) and a lower seeder. The upper seeder was located relatively high on the natural portion of the slope and the lower seeder was located at the top of the engineered road cut slope.

It is our engineering geology judgement that most rockfall is initiating from the road cut slope, but the upper seeder was modeled to estimate maximum kinetic energies barriers may experience.

RockFall Analyses 23

RocFall’s statistical modeling of rockfall attempts to replicate rockfall conditions, but a 100 percent confidence level is not achievable due to variable field conditions and mathematical approximations used for modeling complex trajectory paths of rockfall on different substrates and complex surfaces.

RocFall modeling catchment results for each critical cross section are tabulated in Table 5-3 and

Table 5-4 and modeled energies and rock bounce heights in Table 5-5, Table 5-6, and Table 5-7.

Table 5-3. RocFall modeling results of catchment for rockfall barrier types considered at slope

US38 for the maximum and typical rock sizes observed and anticipated. Results for each critical cross section is provided. The barrier type, height, design option, rock block dimensions and shape, percent of catchment by each type of barrier for the upper and lower seeders, and a total catchment of all rock sizes for each barrier type for both the upper and lower seeders. Models and rockfall trajectories are provided in Appendix C.

US38 - Cross Section A

Barrier

Type

Barrier

Height

(ft)

Design

Option1,2,3,4

Rock

Dimensions

(ft)

% Upper

Seed

Catchment5,6

% Lower

Seed

Catchment5,6

% Total

Rockfall

Catchment7

Gabion

Wall

5.5 Type 2

3 x 3 x 4 96 99

1 x 1 x 1 100 99

2.5 Type 1

3 x 3 x 4 64 65

1 x 1 x 1 93 87

Concrete

Barrier 2.5

3 x 3 x 4 57 58

1 x 1 x 1 79 84

US38 - Cross Section B

Gabion

Wall

5.5 Type 2

3 x 3 x 4 90 99

1 x 1 x 1 95 99

2.5 Type 1

3 x 3 x 4 41 45

1 x 1 x 1 76 90

Concrete

Barrier4 2.5 3 x 3 x 4 36 46

1 x 1 x 1 62 91

Notes:

1. All uphill side of barriers are distanced approximately 8 feet from the base of the cut slope.

2. Option 2 is a 5.5' high gabion barrier with a 4.5' wide base.

3. Option 1 is a 2.5' high gabion barrier with a 3' wide base.

4. The concrete barrier has a base width of 18-inches.

5. Cross section A at US38 model coordinates, upper seeder 1, 1551; lower seeder 93, 1461.

6. Cross section B at US38 model coordinates, 0, 1546; lower seeder model coordinates 78, 1502.

7. Total catchment percentages are averaged for upper and lower seeds for all rock sizes.

24 RockFall Analyses

Table 5-4. RocFall modeling results of catchment for rockfall barrier types considered at slopes

US36 and US33 for the maximum and typical rock sizes anticipated. Results for each critical cross section is provided. The barrier type, height, design option, rock dimensions, percent of catchment per barrier for each the upper and lower seeders, and a total catchment of all rock sizes for each barrier type for both the upper and lower seeders. Models and rockfall trajectories are located in

Appendix C.

US36 – Cross Section A - Area 2

Barrier

Type

Barrier

Height

(ft)

Design

Option1,2,3

Rock

Dimensions

(ft)

% Upper

Seed

Catchment4,5,6

% Lower

Seed

Catchment4,5,6

% Total

Rockfall

Catchment7

Gabion

Wall

5.5 2 2 x 2 x 3 91 98

1 x 1 x 1 94 100

2.5 1 2 x 2 x 3 62 58

1 x 1 x 1 72 88

US36 – Cross Section B - Area 1

Gabion

Wall

5.5 2 3 x 3 x 4 88 100

1 x 1 x 1 88 100

2.5 1 3 x 3 x 4 58 79

1 x 1 x 1 68 91

US33

Gabion

Wall

5.5 2 3 x 3 x 4 93 99

1 x 1 x 1 94 100

2.5 1 3 x 3 x 4 72 87

1 x 1 x 1 68 92

Notes:

1. All uphill side of barriers are distanced approximately 8 feet from the slope toe.

2. A 5.5' high gabion option 2 barrier has a 4.5' wide base.

3. A 2.5' high gabion option 1 barrier has a 3' wide base.

4. Cross section A at US36 model coordinates: upper seeder 150, 1619; lower seeder 294, 1520.

5. Cross section B at US36 model coordinates: upper seeder 46, 1630; lower seeder 150, 1524.

6. Cross section at US33 model coordinates: upper seeder 63, 1619; lower seeder 195, 1546.

7. Total catchment percentages are averaged for upper and lower seeds for all rock sizes.

RockFall Analyses 25

Table 5-5. RocFall modeling results of total kinetic energy and bounce height of anticipated rock sizes from the upper and lower seeder locations at slope US38. Models and rockfall trajectories are located in

Appendix C.

US38 - Critical Cross Section A

Seeder1

Rock

Dimensions

(ft)

Total Kinetic Energy (ft-lb)3

Total Kinetic

Energy (kJ)3

Bounce Height

(ft)3

Avg Min Max Avg Min Max Avg Min Max

Upper 3 x 3 x 4 345,967 45,853 675,086 469.1 62.2 915.3 3.3 1.3 7.6

1 x 1 x 1 10,986 423 18,853 14.9 0.6 25.6 2.0 0.6 5.9

Lower 3 x 3 x 4 190,074 17,627 476,442 257.7 23.9 646.0 3.0 1.3 5.6

1 x 1 x 1 5,611 1,073 13,469 7.6 1.5 18.3 1.7 0.4 5.2

US38 - Critical Cross Section B

Upper 3 x 3 x 4 327,347 6,122 746,045 443.8 8.3 1011.5 3.9 1.3 17.9

1 x 1 x 1 9395 65 21,243 12.7 0.1 28.8 2.8 0.5 15.0

Lower 3 x 3 x 4 114,925 3,025 262,914 155.8 4 356 3.0 1.3 5.0

1 x 1 x 1 3,622 16 8,023 4.9 0.02 10.9 1.7 0.4 4.9

Notes:

1. Cross Section A model coordinates: upper seeder 0, 1546; lower seeder 78, 1502.

2. Cross section B model coordinates: upper seeder 1, 1551; lower seeder 93, 1461.

3. Measured from a collector at the cut slope side edge of the barrier option, approximately 8 feet from the slope toe.

Table 5-6. RocFall modeling results of total kinetic energy and bounce height of anticipated rock sizes from the upper and lower seeder locations at slope US36. Models and rockfall trajectories are located in

Appendix C.

US36 - A - Section 2

Seeder1

Rock

Dimensions

(ft)

Total Kinetic Energy (ft-lb)2

Total Kinetic

Energy (kJ)2

Bounce Height

(ft)2

Avg Min Max Avg Min Max Avg Min Max

Upper 2 x 2 x 3 426,822 17,341 958,879 578.7 23.5 1300.1 3.8 1.4 15.0

1 x 1 x 1 12,862 424 25,786 17.4 0.6 35.0 2.7 0.5 11.3

Lower 2 x 2 x 3 154,758 2,561 416,391 209.8 3.5 564.6 2.9 1.4 7.3

1 x 1 x 1 5,147 739 12,372 7.0 1.0 16.8 1.7 0.4 4.1

US36 - B - Section 1

Upper 3 x 3 x 4 167,130 1,111 311,033 226.6 1.5 421.7 4.0 0.9 17.1

1 x 1 x 1 13,378 800 27,620 18.1 1.1 37.4 3.1 0.4 17.6

Lower 3 x 3 x 4 61,274 4,483 147,786 83.1 6.1 200.4 2.2 0.9 5.6

1 x 1 x 1 5,333 271 12,237 7.2 0.4 16.6 1.4 0.4 3.8

Notes:

1. Cross Section A model coordinates: upper seeder 150, 1619; lower seeder 294, 1520.

2. Cross Section B model coordinates: upper seeder 46,1630; lower seeder 150,1524.

3. Measured from a collector at the cut slope side edge of the barrier option, approximately 8 feet from the slope toe.

26 RockFall Analyses

Table 5-7. RocFall modeling results of total kinetic energy and bounce height of anticipated rock sizes from the upper and lower seeder locations at slope US33. Models and rockfall trajectories are located in Appendix C.

US33

Seeder1

Rock

Dimensions

(ft)

Total Kinetic Energy

(ft-lb)2

Total Kinetic

Energy (kJ)2

Bounce Height

(ft)2

Avg Min Max Avg Min Max Avg Min Max

Upper 3 x 3 x 4 45,736 1,983 114,473 62.0 2.7 155.2 3.4 0.8 11.1

1 x 1 x 1 9,752 318 20,468 13.2 0.4 27.8 2.8 0.4 11.0

Lower 3 x 3 x 4 25,244 278 55,706 34.2 0.4 75.5 2.2 0.7 6.0

1 x 1 x 1 4,426 92 9,620 6.0 0.1 13.0 1.7 0.4 5.5

Notes:

1. Cross Section model coordinates: upper seeder 63, 1619; lower seeder 195, 1546.

2. Measured from a collector at the cut slope side edge of the barrier option, approximately 8 feet from the slope toe.

Design Recommendations 27

6 DESIGN RECOMMENDATIONS

Final design for slopes US33, US36, and US38 was developed based on field investigations, interviews with OWNF maintenance, rockfall modeling, and input and final site selection provided from OWNF management. Details of these final geotechnical designs are summarized below by slope.

Two different types of gabion rockfall barriers, which are stackable rock-filled wire baskets with an ability to withstand high-energy impacts (Badger et al., 2012), are proposed to reduce rockfall risk for this project. Type 1 is a one tier gabion barrier and Type 2 is a two tier gabion barrier. The

Type 1 gabion rockfall barrier base width of 3 feet and 9 cubic feet (CF) of volume per linear foot of length (LNFT), or 1 cubic yard (CY) per LNFT. The Type 2 gabion rockfall barrier has a base width of 4.5 feet and 22.5 CF of volume per LNFT, or 2.5 CY per LNFT. Typical designs for the barriers are provided in Appendix B. Follow Section 654 for site preparation, assembly, and filling of gabion barriers.

A draped rockfall protection, cable net with double twist wire mesh system is also proposed to reduce rockfall risk for this project. This draped rockfall protection is a system with a row of anchors above the top of the road cut slope brow, on the natural slope, where wire rope cable tag lines then connect to a horizontal support cable which holds up the draped rockfall fabric. The cable nets with a double-twist wire mesh backing, are suspended and draped down the slope. The cable net (with double twist wire mesh backing) is not designed to stop rockfall, but rather to control and contain rockfall descent to the base of the slope for removal in the ditch line periodically by Maintenance personnel. Typical details for the draped rockfall protection, cable net with double twist wire mesh backing are provided in Appendix B. Follow Section 651 for site preparation and construction of the draped rockfall protection, draped rockfall protection.

To blend in the gabion baskets and draped rockfall protection, the cable net system with the surrounding environment, exposed steel components can be treated with weathering agent per

Section 563 (provided to the roadway designer under separate communication) to help blend into the surrounding landscape. The final color of steel components with weathering agent applied will be a marbled, rusty brown. Photos of this weathering agent treatment on other WFLHD projects can be shared with our partner agencies upon request.

6.1 SLOPE US33; TYPE 1 GABION ROCKFALL BARRIER

We recommend building a Type 1, one tier, gabion barrier a minimum of 8 feet from the toe of the cut slope from station 528+30 to 529+40 (110 LNFT, 110 CY). An 8-foot clearance distance between the base of the cut slope and ditch side of the gabion barrier is needed to provide appropriate space for maintenance equipment to access the rockfall catchment area behind the barrier for clean out.

28 Design Recommendations

Concrete barriers should be added on the roadway-side of the gabion barrier to address safety of potential vehicular impact to the gabion rockfall barriers. A concrete barrier flare at the beginning of the site, station 527+55 to 528+30 (75 LNFT), and at the end of the site, station 529+40 to

530+15 (75 LNFT), will protect the gabion barrier ends. Appendix B has typical details of Type 1 and Type 2 gabion rockfall barriers and concrete barriers.

It is our understanding shoulder width adjustments will be needed at this site. A reduction of the shoulder width will accommodate placement of the narrow base width concrete barrier with the gabion rockfall barrier and allow for as large of a rockfall catchment area as possible.

Furthermore, a reduction of the lane widths will help accommodate the needed space.

6.2 SLOPE US36; TYPE 1 AND TYPE 2 GABION ROCKFALL BARRIERS

We recommend building a gabion barrier along the ditch side of this slope from station 512+55 to

517+00. Due to space constraints and slope surface geometry differences we propose a Type 1 gabion barrier from station 512+55 to 513+40 (85 LNFT, 85 CY), transitioning to a Type 2 gabion barrier from station 513+40 to 516+20 (280 LNFT, 700 CY), and another transition to a Type 1 gabion barrier from 516+20 to 517+00 (80 LNFT, 80 CY). Again, 8-feet of width between the ditch side (back) of the gabion barrier to the base of the slope is needed to facilitate equipment clean out and maintenance of the rockfall catchment area. This is particularly important due to the

CMP located at about station 515+30 (Figure 4-3).

Concrete barriers should be added on the roadway-side of the gabion barrier to address safety of potential vehicular impact to gabion rockfall barriers. A concrete barrier flare at the beginning of the site, station 511+80 to 512+55 (75 LNFT), and at the end of the site, station 517+00 to 517+75

(75 LNFT), will protect the gabion barrier ends. Appendix B has typical details of Type 1 and

Type 2 gabion rockfall barriers and concrete barriers.

It is our understanding that roadway lane and shoulder adjustments will be needed to accommodate the need for rockfall catchment, gabion rockfall barrier placement, and roadside concrete barrier placement. This is particularly true where the Type 2 gabion rockfall barrier is recommended as the base width is 4.5 feet. Additional space will be needed for the added narrow base concrete barrier as well. Also access for CMP inlet maintenance will be critical at this site (Figure 4-3).

Additional consideration will need to be given at site US36 due to transitions between Type 1 and

Type 2 gabion rockfall barriers as the base widths are different. The roadside face of gabion barriers will need to be flush across the entire length of the barrier to accommodate the addition of roadside narrow base concrete barriers and maintain the rockfall ditch catchment for debris clean out and culvert maintenance.

6.3 SLOPE US38; DRAPED ROCKFALL PROTECTION, CABLE NET

Due to space constraints associated with site US38, we recommend construction of a draped rockfall protection, cable net system, from approximate station 503+65 to 506+80. The purpose of

Design Recommendations 29 a draped rockfall protection system is to provide long-term protection of the roadway and it’s users by controlling and containing the majority of rockfall and depositing it into the available rockfall catchment area at the base of the slope. We would like to highlight our recommended draped rockfall protection limits does not cover the cut slope end from approximately station 506+80 to

507+20. This decision was made based on the lower slope height of the cut slope in this location.

The draped rockfall protection anchor spacing and standard design elements were designed using

Washington State Department of Transportation’s guidelines (Muhunthan et al., 2005). It should be noted this rockfall protection system is not designed to reduce risk from rockfall initiating from above the engineered rock cuts where this installation will be installed. Figure 6-1 is a map displaying estimated areal coverage of the design, which is estimated at approximately 5,650 square yards (SY), when considering the slope length and anticipated slope roughness. Typical design details are located in Appendix B.

It is recommended a cable net with a double-twist wire mesh backing be used as the fabric for the draped rockfall protection. The fabric was chosen based on block sizes anticipated to initiate from the cut slope based on field observations and maintenance interviews. The cable net also has a higher tensile strength than a typical high-tensile-strength steel wire mesh, another deciding factor considering the large block sizes and their angularity anticipated at site US38. The double-twist wire mesh backing will keep a majority of smaller dimensioned rockfall from exiting the draped rockfall protection system. The double-twist wire mesh backing should be placed on the top of the cable net element.

The top of the draped rockfall protection fabric is recommended to extend 20 feet beyond the road cut slope brow to avoid being undermined over time by headward erosion of the upper raveling edge of the cut slope. It should be understood that the cable net fabric comes in panels typically

10 feet to 12 feet wide. As such the top and the bottom of the installation will likely be stepped or straight across instead of contoured to mimic the brow of the cut slope (Figure 6-1). Additionally, we recommend the fabric bottom should be a minimum of 5 feet from ditch elevation to avoid snow and ice accumulation during the winter months and rockfall debris from accumulating at the bottom of the fabric and pulling on the installation until it fails Conversely, the bottom of the fabric should be no more than 8 feet above ditch elevation to reduce the potential for rockfall to bounce or roll out of the limited rockfall catchment area.

Vegetation will need to be removed before the installation of the draped rockfall protection. Trees and brush will need to be cut flush with the ground to avoid the draped rockfall protection fabrics from draping over the vegetation and snagging. Tree and brush stumps must not be removed from the ground, but rather kept in place to maintain root cohesion of the surrounding soils and slope.

Considering limited space available from Icicle Creek and the Irrigation District’s infrastructure below the site, concrete barriers are recommended to be placed along the edge of the road from station 502+90 to 507+95. Placement of the concrete barriers is designed to keep rocks from reaching the roadway after exiting the draped rockfall protection system. Concrete barriers should

30 Design Recommendations be placed as far from the slope toe as possible to provide the greatest possible rockfall catchment and to facilitate maintenance cleaning the ditch section.